OPINION

Castle Rock Edinvar Housing Association Limited and Others v The Coal Authority

Introduction and Summary

[1] These are applications under Section 40 of the Coal Mining Subsidence Act 1991 (“the Act”) by the owners of around 20 dwellinghouses on either side of a street in Rosewell, Midlothian. They seek a determination that the damage was “subsidence damage” within the meaning of Section 1 of the Act, together with damages under Section 40(3)(b) in respect of certain costs and expenses. The Tribunal had previously considered an issue of competency and decided, for reasons given in an Opinion dated 11 June 2010, that the applications should proceed.

[2] Dwellinghouses on either side of a street in Rosewell, Midlothian, sustained varying degrees of structural damage which was first noticed in late 2003 and spread during 2004. The extent of the damage is agreed, and it is also agreed that the main cause related to ground movement, a substantial area of ground level subsidence having been found at the site. The ground in the area comprises natural superficial deposits underlain by solid geology comprising sandstones, mudstones, siltstones and coal measures. Coal has been mined during various periods and at various depths.

[3] Following a lot of investigation of various kinds, parties are agreed that this subsidence is not the type of subsidence caused by more modern deep mining by total extraction; that this was not one type of subsidence, ‘crown hole subsidence’, which sometimes results from shallower mine workings; and that all but one of the past mining activities in the area can be eliminated as possible causes. The applicants’ case is that the subsidence was ‘areal subsidence’ caused by a degree of collapse, by one mechanism or another, within one particular coal seam, the Whitehill Great Seam, which was worked some considerable time in the past by the ‘room and pillar’ (or ‘bord and pillar’), partial extraction, method at a depth of very approximately 30 metres below ground level. The evidence is clear, and not in dispute, that this coal seam was worked by that method at about that depth beneath the site. The respondents’ case is that this subsidence was not ‘areal subsidence’ and did not originate from this coal mining operation. They maintain that it may have been caused by the movement of wet sands (“running sands”) within the superficials, there being evidence of damaged drains in the street and other possible contributory factors. The respondents refer to some earlier subsidence claims history in respect of one of the houses, together with satellite evidence of ground levels at this location over several years, as indicating that the subsidence took place over a longer period of time, thus, they argue, weakening the theory that this episode of damage was caused by ‘areal subsidence’.

[4] The principal issue in this hearing was whether the damage was caused by coal-mining operations. The particular issues for decision were:-

(i) Whether, in terms of section 1(1) of the Act, the damage to the houses was “subsidence damage”, i.e. “any damage … caused by the withdrawal of support from land in connection with lawful coal-mining operations”. On this issue, the respondents ultimately accepted that Section 40(2) of the Act applied so that the onus was on them.

(ii) As a quite separate preliminary legal issue in relation to damages, whether, as a result of the exclusion in Section 1(4)(b), the respondents could not be liable under the Act’s provisions for the expense of substructure works which had been carried out at the applicants’ expense, involving injecting a cement mix grout into the disused mine workings.

[5] No other question as to the amount of damages, if applicable, was considered at this hearing.

[6] In summary, the Tribunal has decided:-

(i) The damage to the houses was “subsidence damage” within the meaning of the Act; and

(ii) Section 1(4)(b) does not have the effect of excluding the expense of works within the underground mineworkings from these claims for damages under section 40(3)(b) of the Act.

We have accordingly continued the applications for a hearing, if required, on quantum of damages.

Procedure

[7] The applicants were represented at an oral hearing by Ronald Clancy QC and Euan Duthie, Advocate, instructed by Messrs Simpson & Marwick, Edinburgh, on behalf of the insurers of the applicants’ properties. The respondents were represented by Malcolm Thomson QC, instructed by DLA Piper UK, Sheffield, on behalf of the Coal Authority. The respondents led evidence first. They called George Hogg, MRICS, the respondents’ project manager, a mines surveyor with extensive experience of subsidence assessment on the respondents’ behalf; and John Ottoway, BSc(Hons), MICE, MIME, of Messrs White Young Green Environmental. Mr Ottoway has considerable experience of mining engineering and geological issues, including, in particular, in relation to abandoned mineworkings, and spoke to his independent expert report, appended drawings and DVD. The applicants led oral evidence from Steven Rule, BSc, an experienced geotechnical engineer, of M. & J. Drilling Services Ltd, who had carried out certain investigative drilling and grouting works in 2007 and reported on it; and from David Charles Wilshaw, FRICS, IEng, FIMMM, MQB Mine Surveyor, of Messrs Wardell Armstrong LLP, also a very experienced mine surveyor, who spoke to his independent expert report with appendices. Extensive documentary and DVD evidence reflecting various types of investigation into the cause of the subsidence, reports and illustrative drawings prepared for the hearing, was lodged by both sides. Parties entered into a helpful Joint Minute of Agreement. The Tribunal also received three affidavits into evidence, one from a Mr Moffat and two from a Mr Donnelly, each in the employment of Midlothian Council, all referring to such information as they were able to give about, in particular, road drainage in and about Carnethie Street, Rosewell. That evidence was not agreed. At a hearing on submissions, counsel spoke to, and also orally supplemented, written submissions which had been exchanged. The Tribunal visited the site unaccompanied.

Authority referred to:-

[8] Wolverhampton MBC v The Coal Authority, LCA/156/1996, 7 May 1998

The Facts

[9] There was substantial, but not quite complete, agreement on the very extensive primary factual evidence, mainly data recording the results of the various forms of investigation, but sharp disagreement on material factual inferences. We shall summarise the factual position rather than attempt comprehensive exposition.

[10] The Applicants’ properties. The respondents’ “Drawing No 001” contains location and site plans locating the applicants’ properties. Rosewell is a small historical mining village approximately 500 metres NNE of the long closed Whitehill Colliery in the Midlothian mining area. The applicants’ houses are situated in a central stretch, approximately 75 metres long, of Carnethie Street which runs in an approximate SSW/NNE direction on the east side of the village. The houses are Nos. 92 to 116 on the west side (but not including No 114 in respect of which there is no claim), and Nos. 93 to 111 on the east side (not including No 97). The ground slopes gently from south to north and also from west to east, down towards Rosewell Primary School, which is beside No.93 but was not affected by the subsidence. The houses on the east side were built around 100 years ago in three separate terraces, with small gaps between Nos. 101 and 103 and Nos. 107 and 109. The houses on the west side were built around 130 years ago in one long terrace.

[11] Geology. The solid geology below the site consists of the Lower Coal Measures of the Upper Carboniferous Series of Strata. The coal measures comprise varying layers of sandstones, siltstones and mudstones interspersed with thin coal seams. Outcrops of five coal seams lie between 300 and 900 metres to the north-west, the seams dipping generally to the south-east but with some local variations.

[12] Mining History. A total of eight coal seams have been mined in or near to the location. Mining can be divided into two categories, the older and shallower workings and the more recent deeper workings. Whitehill Colliery worked coal throughout the 1800’s from some of the shallower seams. Bilston Glen Colliery worked coal from three further and deeper seams.

[13] The Whitehill Great Seam. Whitehill Great Seam T.C.M. (Top Coal Measures) is the shallowest of the seams worked from Whitehill Colliery. At this location underneath Carnethie Street, it was worked during the nineteenth century at depths in the range of about 27 to 37.5 metres below ground level, [probably in the late 1880s]. Abandonment plans apparently show the area as having been worked by the ‘room and pillar’ method, whereby substantial pillars of coal were left in place to support the overlying rock strata. The plans suggested a seam thickness of 1.55 metres overlain by thick sandstone. The borehole investigations in the area of the damaged houses generally confirm the position, as some show solid coal around this depth while others show voids, i.e. the worked mine areas, but the seam thickness appears mostly slightly greater and the immediate overlying and underlying strata are often mudstone, sometimes weak mudstone, referred to as ‘Seat Earth’.

[14] Superficials. Made ground deposits are present at the site up to a maximum thickness of around 1.9 metres. There is no suggestion of any problem with the made ground. It is underlain by natural superficial deposits of around 13 metres in thickness. These consist of boulder clay comprising gravelly clay, loose to medium dense gravelly sand and occasional loose to medium dense clayey sand horizons.

[15] Damage Sequence from late 2003 onwards. In December 2003, the damage which is the subject of these claims was first noticed. Damage was noticed at Nos. 107 and 109 Carnethie Street on or around December 2003. Thereafter neighbouring properties to the north on the east side of the street began to notice damage. Damage to No. 93 was noticed in March 2004. Damage was noticed at the front of both Nos. 106 and 108 in February 2004, migrating to the rear of those properties by May 2004. Damage to properties in the street progressed in severity during 2004. The main areas of damage were focussed around Nos. 107 and 109. The main cause of the damage to the properties was ground movement. The damage was compressive towards the centre of the subsidence area and tensile towards the edges. The type and distribution of the damage was consistent with a mining subsidence damage profile.

[16] On the east side, the damage ranged from slight at the north and south ends to severe in the centre, particularly around the gap between Nos. 107 and 109, where the two gable walls were found to be leaning towards each other by as much as 100mm. Distinct distortion was evident at the front elevation of Nos. 109 and 111. Damage was evident to both elevations but more prevalent to the front, generally taking the form of horizontal cracking extending from the corners of the window and door openings, with more vertical/diagonal cracking noted at Nos. 107, 109 and 111. There was also lateral displacement of the walls around the fractures. Internally the fractures generally reflected the external damage.

[17] On the west side, the damage was slight to the north and south ends, increasing in severity towards the centre. The more serious damage was again evident to the front elevations, with only isolated appreciable damage evident to the rear. The damage generally took the form of horizontal fracturing at the corners of door and window openings and was at its most severe between Nos. 96 to 112. There was minimal vertical/diagonal fracturing. Again, fractures noted internally generally reflected the external damage. Damage was also noted to the party walls, particularly at junctions of ceilings and walls. Some properties have loft conversions but generally there was little damage evident at first floor level.

[18] The damage was not caused by ‘crown hole collapse’, i.e. collapse of the mining roof migrating vertically upwards from shallow mine workings, or by movement associated with current or at least recent longwall extraction methods in deeper mining operations.

[19] Borehole Investigations. More than 50 boreholes were drilled in and around the site between April 2004 and July 2007. A borehole location plan, the respondents’ “Drawing No 002”, shows the locations of the various boreholes (apart from one early series which did not go as far as the Great Seam). Many of the boreholes were cored so that, in addition to the record of the drillers’ observations at the time, further examination of the recoveries could be made. In several boreholes, at various locations around the site, there were gaps of varying sizes in the recoveries from the rock strata above the Great Seam.

[20] After receiving the first four damage notices, the respondents engaged specialist geotechnical contractors, Groundshire, to drill a series of boreholes. This firm drilled one air-flush borehole, to a depth of 24 metres, immediately outside the rear garden fence to the east of No 107, on 19 April 2004. A quantity of wet sand or sandy clay was blown out of this borehole, causing a noticeable depression at the surface. This borehole was recorded as revealing ash and rubble (made ground) to a depth of 1.5 metres, then sandy boulder clay to 5.0 , then damp running sand to 5.6 , clay to 5.8, and a further layer of wet sand to 7.6 and sandy boulder clay to the rockhead at 15.4.

[21] Thereafter, the respondents used a different company, Van Elle, to undertake drilling using a different method which involved casing the superficial deposits during drilling. In June and July 2004 Van Elle drilled 16 boreholes (not shown on “Drawing No 002”), 13 in a line outside the rear gardens on the east side of Nos. 101 to 109, mostly in the close vicinity of the Groundshires borehole, the other 3 in the east pavement of Carnethie Street outside Nos. 101, 109 and 113. These boreholes were taken to around 22 to 25 metres depth so that, like the Groundshires borehole, they did not reach the Great Seam. These borehole investigations excluded any mining operations shallower than the Great Seam as a potential cause of the subsidence. The respondents also arranged a ground level survey at the surface of each of these boreholes and trial pits to examine the foundations of the damaged property.

[22] In August 2004 the respondents issued letters of rejection in relation to the claims.

[23] Further investigations were commissioned by the applicants’ insurers in late 2004. In November and December 2004 Raeburn Drilling and Geotechnical Limited (“Raeburn”) sunk 7 boreholes (BH101-107) by cable percussion methods. Nos. 101 to 106 were continued below rockhead by rotary core drilling, to depths varying from about 34 to 40 metres below ground level. Laboratory geological examination and testing of the cores confirmed the geological structure at the site to be as shown on sheet NT26SE published by the British Geological Survey in 1964. These boreholes were sited as follows:-

BH101 – pavement outside No. 107

BH102 – pavement outside No. 94

BH103, BH104, BH107 – rear lane outside gardens to east of Nos. 101, 113, 117

BH105, BH106 – rear lane outside gardens to west of Nos. 106, 116

[24] This investigation also established that the Great Seam was around 2.1 metres thick and was worked at the site by ‘room and pillar’. Three Boreholes found intact coal (pillars) at the seam; two found voids (open workings); and BH 101 found:-

“DRILLER RECORDS WASTE: 20 cms recovered broken crushed brownish mudstone, some pieces of coal with sulphurous staining”.

[25] The recovered core strata was found not to have been fissured or broken or apparently under stress.

[26] Between May and July 2005 Van Elle drilled 4 boreholes (EXT 1,2,3,4), outside Nos. 94-96 and 107, in which extensometers were installed in an attempt to determine the horizon at which strata movement was occurring by monitoring the level of magnets installed in a central access tube. Installation of EXT2 failed and there were some difficulties with the working of the other extensometers.

[27] Between May and September 2005 another company instructed by the respondents, Allied Exploration and Geotechnics (AEG), drilled 28 boreholes (BH201, etc) to varying depths in 4 rows, to the rear of the gardens behind the houses on the west and east sides and on the pavements on both sides of Carnethie Street. They installed extensometers in two of these (EXT5, EXT6). The respondents’ “Drawings Nos 003 to 009” comprise 7 approximate cross-sections representing the drilling results (including two of the Raeburn boreholes) in 4 rows following the direction of the street and 3 rows across the site. Within the superficials, much of which was different types of clays, lenses of sand of varying descriptions were recorded in some boreholes, but not in others. The Great Seam was encountered at depths varying between 27.70m and 37.10m below ground level, with a combination of voids and pillars of coal. The thickness of the voids encountered by the drillers was typically recorded at around 2.5m but ranged from 3.40m to 1.90m. The rock strata above the Great Seam ranged from very weak mudstone to strong sandstone. Two further extensometers were installed, one within BH211, outside No 111 (EXT6), and another beside BH210, outside No 105 (EXT5).

[28] In April 2007 a further 11 investigatory boreholes were drilled by M & J Drilling, a company employed by the applicants’ insurers, prior to treating the area of the Whitehill Great Seam under the area most affected by subsidence by injecting grouting material. Several of these boreholes encountered mining voids at the level of the seam and in these boreholes plastic casing was inserted to allow access for a CCTV camera. Within the superficials, no drilling conditions were encountered to suggest disturbed ground.

[29] Downhole Geophysical Investigation. Downhole geophysical investigation of most of the AEG boreholes was undertaken by Robertson’s Geologging Limited. This included optical tele-viewer imaging and high resolution acoustic tele-viewer imaging (providing continuous 360 degree views of the rock strata from most of the boreholes); CCTV imaging from two boreholes; and natural gamma, high resolution density and caliper logs of most of the boreholes. Some caliper log data suggested smaller voids, e.g. at BH215 (1.2m).

[30] Recovering minewater is monitored as a matter of course in this area. From this, and also from piezometer investigation and the various CCTV viewings inside the mine, it was established that the Great Seam at this location was dry.

[31] Ground Level Monitoring. Ground level monitoring at the site, by Messrs Gleeds on behalf of the respondents, commenced in July 2005. Points were established along eight north-south longitudinal sections, the westmost being at the rear of the houses, Nos. 92 to 116 and the east most in the lane behind the gardens of Nos. 73 to 76, with a bench-mark to which the measurements were related established some 40 metres to the west of the site. Readings were taken at various irregular dates between July 2005 and November 2010, with some points being lost or abandoned over that period. The lack of monitoring points extending far enough on the west side and the small number of points, and of measurements from these points, to the rear of the gardens on the east side, made accurate representation of the extent and shape of the settlement area difficult.

[32] The level monitoring data demonstrates the occurrence, during the period from January 2005 to November 2010, of subsidence affecting the area with an epicentre of measured movement of about 88 millimetres over that period at the front wall of 105 Carnethie Street. The general pattern is of a reduction in the depth of subsidence radiating out from the epicentre. The general pattern of measured movement coincided with the pattern of damage to the individual properties. At least a similar amount of settlement occurred between the period from mid to late 2003 until the commencement of monitoring. The area affected by the subsidence was roughly oval shaped and measured at least 95 metres in an approximately north to south direction and at least 55 metres, and apparently some distance further, in an approximately east to west direction.

[33] Mine Stabilisation Works. Between April and September 2007, M & J Drilling, on the instructions of the applicants’ insurers following the advice of Mr Wilshaw, and with the consent of the respondents, undertook a programme of works to inject grout into the mine workings in the Great Seam under an area, measuring approximately 40 metres by 40 metres, approximately underneath the houses Nos. 102 to 110 and 103 to 109 and their front gardens, pavements and the road in between, for the purpose of stabilising this area of the mine. Grouting was injected into the mine workings through a grid pattern of 93 boreholes, a uniform pattern being maintained by expanding the investigation trial pits so that several boreholes could be ‘fanned out’ from each, i.e. using angled drilling. A cementitious grout mixture was injected into the abandoned mine horizon. The grout mixture is a bulk filler that fills the voids at the mine horizon and is a traditionally accepted means of infilling old mine workings to provide surface stability. Greater grout takes were encountered on the perimeter of the core area.

[34] The grouting work apparently caused some ground movement while it was being carried out. The available settlement data demonstrates that during the grouting period there was something of a small spike in ground levels, i.e. raised levels, in the area outside the grouting area; and inside the grouting area, a mixed picture with raised levels at some points but most continuing, at varying rates, the decline in levels. Differential settlement continued or resumed for a short period after the grouting work, as is normal following such operations. Thereafter, measurements between January 2008 (the first date after the grouting works on which levelling data was collected) and November 2010 demonstrated some very slight continuing settlement, relative to the nearby benchmark, across the whole area monitored, at a considerably reduced rate at the points most affected by the settlement. There was generally up to around 5 millimetres of settlement over that period, the pattern of continuing very slight settlement being shown outside, as well as inside, the grouting area.

[35] The houses suffered no further damage caused by ground subsidence following the mine stabilisation works.

[36] Surface Water and Drainage. A combined road and surface water drain runs down the west side of Carnethie Street collecting surface water from the street and roof drainage from houses on the west side of Carnethie Street. This was discovered, its base being around 0.7 metres below ground level, after a water main was accidentally fractured during preparations for drilling one of the extensometers. Substantial excavation had been required to enable the burst to be repaired. After this was repaired, water continued to flow around the drain. A CCTV survey of the drain was undertaken by the Coal Authority. It revealed that the upper section of drain is a fireclay pipe which leads into a stone culvert or cundy which is approximately 300 mm square in section. The junction of the two pipes lies close to 100 Carnethie Street with the stone culvert running approximately 35 m northwards from no. 100 to 92. The culvert required to be cleaned before the CCTV survey could be undertaken and this revealed its age was about 100 years, its joints were open and there was a blockage formed by sand and other detritus. The extent to which water could pass through the drain, prior to the CCTV survey being carried out, is not known.

[37] Speed Humps. In January 2003 speed Humps were installed on Carnethie Street approximately opposite numbers 111 and 112 and 99 and 98. As part of the respondents’ investigation, seismic monitoring took place in June 2005 at a number of locations to assess the potential impact of vibration caused by passing traffic on the structure of the houses affected by settlement. The intensity of vibrations recorded in Carnethie Street during the period of monitoring did not exceed the thresholds normally expected to induce structural damage to properties that were initially structurally sound.

[38] Satellite Ground Level Measurements. The respondents also obtained a specialist report based on satellite measurements (“Insar” surveys), based on ‘reflecting points’ on the ground, carried out by Fugro NPA Ltd.over longer periods of time. This showed that generally slow subsiding ground movement had occurred in the Rosewell and surrounding area over the period from 1992 to 2000. At Carnethie Street, the average annual settlement recorded ranged from about 0.3 millimetres to 2.4 millimetres, the latter being recorded at No 107 with the immediately surrounding properties also recording higher levels. No data were available for the period 2000 to 2002, because there was a gap between satellites. A second Insar Survey report was obtained in respect of the period 2002 to 2008 (using different ‘reflection points’, which however were not so closely related to the points of greatest subsidence around the middle of this part of Carnethie Street). This showed generally, and apparently fairly uniform, small increases in the ground level, a suggested explanation being that “heave” was taking place due to mine water recovery within the deeper Bilston Glen workings which had been abandoned. It was also noted that the base stations used for the surface level monitoring were located within the zone of heave. Accordingly the absolute levels recorded by the survey cannot be relied on and only the relative differential levels can be used. All this data was subject to a small margin of error.

[39] Previous Claims History. There was a history of past mining subsidence claims and correspondence in respect of 107 Carnethie Street. The National Coal Board carried out repairs to an exterior crack above the front door and window in 1981. In September 1982, inspection revealed an exterior crack in the same location. The owner was informed that that the property was not being affected by the then current mining and had not been affected since a “possible slight fringe effect” in 1980. In August 1987, a damage claim was accepted following the appearance of cracking on the front elevation and on the party wall with number 109. In August 1993, a damage notice was submitted by the property owner and the claim, being made within 6 years of the previous claim, was accepted.

[40] In June 1999, a further damage notice was submitted. This claim was rejected in 2000 on the recommendation of Gleeds, who were working under contract for the Coal Authority. This followed correspondence with loss adjusters and reference to other possible causes such as a localised drainage problem, with some repair work to drainage around the house having been carried out. The owner was then advised that there had been no coal mining which could affect her property since 1989 and any movement resulting from that would have occurred at the time. No mention was made of the possibility of unrecorded shallow or other older mineworkings. The claim was not pursued further.

[41] No details of any ground investigations in connection with these claims were provided.

Expert Opinion Evidence: Mr Ottoway

[42] Mr Ottoway is a very experienced civil engineer who has had considerable involvement in coal mining hazards, investigation in respect of historic mining liabilities, etc. The company of which he is presently a director, White Young Green (“WYG”), has an ongoing contract with the respondents in relation to historic liabilities, including, amongst other things, subsidence. He had prepared an expert report dated 3 March 2011. He referred to a number of drawings, etc. prepared for the hearing as well as the earlier documentary materials which mostly comprised factual data gathered during the extensive investigation by both sides although perhaps mainly by or on behalf of the respondents.

[43] In his report and his oral evidence, Mr Ottoway reviewed the information obtained in the investigation. His report is available for reference, and we shall refer in more detail later to his opinions on contentious issues. Briefly, he described the location, the mining history and the history of these claims. The Great Seam was above the rest mine water level and the workings were dry. He described the respondents’ various investigations and related them to the issues. In relation to the extensometer monitoring, he accepted that this had limitations but considered that as there was no indication of widening of the distance between the magnets in the solid strata and those in the superficial strata relative movements in the superficials indicated that movement was limited to the superficials. He said that the geophysical investigations (downhole CCTV imaging, Optical televiewer imaging, Natural Gamma logs, High Resolution Density logs and Caliper logs) clearly demonstrated that there was no bed separation and no voiding in the overlying rock strata. He referred to the 1992-2000 Insar data as demonstrating differential settlement during that period at Nos. 107 and 109, and related this to the history of claims in respect of No. 107. He said that the surface level monitoring survey had shown continuing settlement after the grouting works. He referred to local reported events including the installation of speed bumps. He said that the ground was very subject to wash out and cavitation when water was present. He then explained the discovery of a drainage problem after a water main had been fractured. Plans of the drainage system had been obtained and the drain on the west side of the street found to be in poor condition, but no drain had been found on the east side. He referred to the earlier reports supplied by the claimants, highlighting Messrs Ross & Co’s view that no indication had been found of deterioration of the overlying strata which might be expected as the result of the collapse of underlying workings. He referred to assumed angles of draw and repose in the strata and superficials as requiring a minimum width of settlement of 57 metres plus the width of the failure zone itself for there to have been failure at a depth of 35 metres. He said that substantial waste may have been left in the mine, as it was progressed, rather than being removed, there being no indication where roadways were.

[44] Mr Ottoway summarised his opinion that the subsidence had not been caused by ‘areal subsidence’ as follows:-

[45] Mr Ottoway considered that there was “a sizeable body of evidence which strongly suggests” that the subsidence had resulted from lateral movement of fine material in the superficial deposits via a defective drainage system, summarised as follows:-

In conclusion, Mr Ottoway said that he had not found evidence to support the applicants’ theory, but had found evidence which, he said, established his theory on a balance of probabilities.

Expert Opinion Evidence: Mr Wilshaw

[46] Mr Wilshaw is a mineral surveyor with extensive experience in the coal mining industry. Like Mr Ottoway, his career has included earlier experience with British Coal. He too has extensive experience in relation to mining subsidence and instability problems associated with old mine workings of various kinds. He has no formal geological qualifications.

[47] Again, reference may be made to Mr Wilshaw’s written report, and aspects of his evidence will be considered in more detail later. His report may be summarised as follows. In 2005 he concluded on the information then available that the damage to the houses was associated with ground movement of a common related cause, the most probable cause being areal collapse in the Whitehill Greeat Seam. He had investigated, without the benefit of knowing the respondents’ exact theory (which had not been disclosed), possible causes within the superficial deposits. No sewer had been encountered on the east side of Carnethie Street. The natural superficial deposits were reasonably competent, despite the presence of made ground deposits and lenses of loose sands, no reason having been identified as to why these deposits should move in the absence of any influencing factor. Leaking drains could in certain circumstances cause ground movement, but Mr Wilshaw discounted this cause even if (as had not been established) the condition of the drain on the west side of the street had in 2003/4 been as it was subsequently discovered to be. There was no means of any significant quantities of loose deposits being able to be washed into the drain. Flows into the drain would be localised to the drain run. Similar comments applied to water egress from the drain. Mr Wilshaw noted various shortcomings with the data obtained from the extensometers, but also opined that they tended to suggest extension within the superficials, whereas compression would be expected if deposits had been washed away. Ground movement had ceased after the mine stabilisation works were carried out.

[48] Mr Wilshaw said that a subsidence trough in the order of 100m by 40m had been identified in 2005, with a damage pattern consistent with areal subsidence originating in the Great Seam. Movement had progressed from an epicentre outside Nos. 107/109. The depth of the identified mine workings was consistent with depths at which areal subsidence had been known to have occurred elsewhere. Published literature confirmed the phenomenon of areal subsidence consistent with the situation in Carnethie Street. The pattern of movement recorded in the ground level monitoring was consistent with residual movements associated with areal collapse. The subsidence had stabilised after the mine stabilisation works. The microgravity survey had not recorded any ground conditions consistent with voiding within the superficials or the solid strata. The report of the seismic vibration monitoring had not identified significant ground vibrations and the position of the speed humps was not coincident with the most severely damaged properties. The optical televiewer inspection had confirmed the presence of intact strata with little or no bed separation, and had shown the mining horizon in varied states of degradation. The height of the voiding appeared to reduce within the core area. The CCTV survey of the drainage had not shown significant quantities of debris/silt and the drains had consistently held water and not been shown to be completely blocked. Downhole CCTV surveys in May 2007 had confirmed the presence of partially collapsed workings and connectivity between boreholes BH4 and BH5. Mr Wilshaw considered the type and distribution of property damage was entirely consistent with a typical mining subsidence damage profile.

[49] In his oral evidence given after a break of some days in the hearing, Mr Wilshaw spoke to some CCTV imaging, first in passages, which had not been put to Mr Ottoway in cross-examination, from the AEG downhole geophysical investigation. Mr Wilshaw explained that he had been asked to detail comments to the effect that there were some signs of fracturing within the strata. He pointed to dark lines within some images, not reflected in borehole logs. He also pointed to signs of possible fractured coal and coal debris, and also spalling of pillars, from imaging within the mine, where Mr Ottoway had found no such signs of degradation. He would not have expected large lumps of coal to have been left at the time of working the mine, where they would form obstructions. He estimated the total settlement, i.e. including that measured after monitoring started, at 200-300mm, consistently with the extent of damage to the houses. He had experienced localised ground loss, but nothing on this scale, resulting from drainage problems. He had not seen any report of a drain causing subsidence on this kind of scale. Mr Wilshaw was clear that he had examined the possibility of the drains and, being aware of the presence of the cundy and the lack of any drain on the east side, discounted it as unrealistic. He said that on the basis of the borehole information on its own, it would probably not be possible to reach any firm conclusion.

Closing Submissions: Whether “subsidence damage”

[50] Counsel each spoke to written submissions. In his written submission, Mr Thomson for the respondents first referred to the effect of Section 40(2) of the Act. He submitted that once the evidence had been heard, the question was whether it was more likely than not that the damage was “subsidence damage”, Section 40(2) only being relevant, to the applicants’ benefit, if the Tribunal found it equally likely that the damage was subsidence damage or not. The respondents were not required to prove what the material cause of movement was – only what it was not. In this case there was no direct evidence as to the cause of the admitted subsidence. The opinion evidence might be sub-divided into opinion as to causation and opinion as to the existence of facts on the basis of which causation might be determined, the latter being as much opinion evidence as the former. Mr Thomson summarised the established facts. Turning to the expert evidence, he invited the Tribunal to treat Mr Ottoway as a credible and reliable witness and to find his final position, in relation to competing causes each of which might reasonably be regarded as rare, fair and reasonable.

[51] Mr Thomson attacked Mr Wilshaw’s evidence as partisan. There was no “true evidence” declaration in his report and he had not appeared to understand the conflict of interest point put to him arising out of the fact that the grouting works had been carried out on the basis of his opinion: he had a clear interest in defending his position. He appeared to have proceeded on the basis of second hand evidence as to the existence of debris in the voids and the lack of any other possible cause, and his mind was closed to the possibility of another cause of movement. He had failed to investigate matters such as the presence or absence of water in the void and the drainage difficulties apparent in Carnethie Street. He had wrongly dismissed Messrs Ross & Co’s reference to the absence of evidence of deterioration of the overlying strata as irrelevant. He had increasingly attempted to bolster his position and justify his opinion. He had prepared evidence about alleged stress in the upper rock strata only after Mr Ottoway’s evidence. He referred to two unnamed geologists as supporting his position on that. He had been loath to admit the simple fact that where areal collapse occurred there would be greater vertical movement at the roof of the seam than at the ground surface. The evidence of void reduction had not matched the extent of subsidence at the surface, forcing Mr Wilshaw to change his evidence about the extent of subsidence before monitoring began. He had failed to address two further issues: the INSAR evidence of slow and progressive movement, and the inconsistency that if BH5 and BH8 showed signs of collapse, the grouting had not been carried out there. There could be no confidence in Mr Wilshaw’s theory, and Mr Ottoway’s evidence as to the cause of the ground movement was either correct or more likely to be correct.

[52] At the hearing of oral submissions, Mr Thomson first submitted that Section 40(2) had no bearing on the evidential burden. He invited the Tribunal to find areal subsidence not established, on all the evidence including the respondents’ evidence of another possible explanation. He drew attention to the circumstances in which Mr Ottoway had not been able to address Mr Wilshaw’s evidence on signs of stress in the optival televiewer images, Mr Wilshaw having in his report referred to ‘very little or no bed separation’. Mr Ottoway’s opinion, supported by a common sense interpretation of Mr Wilshaw’s diagrams, was that as the sagging at the top would always be less than at the bottom, there had to be bed separation. There were other examples of Mr Wilshaw’s increased “scrabbling around” for support for his theory, in relation to oxidisation and water, matters either new or at least not in his written report. He had spoken about things not foreshadowed in his report, giving rise to anxiety about his reliability. Mr Ottoway had been trying to assist the Tribunal in a way not possible for Mr Wilshaw whose clients had acted, incurring expense, on the basis of his advice. Mr Wilshaw’s “closure summary” report on the grouting works could not be regarded as contemporaneous, and support from the M & J borehole records was not based on much material. The CCTV evidence as to the position within the mine should be viewed cautiously. Mr Wilshaw needed to find at least half a metre of void reduction. Mr Thomson then reviewed the borehole evidence about BH101, a matter on which there had been conflicting evidence: Mr Wilshaw had not reliably found evidence of collapse. The respondents’ cross-section drawings were more reliable. The evidence was of varying degrees of waste rather than reduced voids, of which Mr Ottoway had reasonably found none. The Tribunal had to reach a view about the nature of Mr Ottoway’s evidence and Mr Wilshaw’s, and about the actual evidence. There were issues of opinion on whether anything could be seen, as well as on what could be seen. Similar questions about objectivity of evidence arose on the matter of what would have been left by the miners on the floor. Mr Thomson referred also to the nature and pattern of the damage, and to the evidence about the extent of settlement before monitoring started. Mr Wilshaw had sidestepped the literature, in relation to the initial phase of the subsidence. Mr Thomson conceded that there was some coincidence between the grouting and reduced movement, but that did not create a causal link if there was another mechanism at work, nor did it explain the subsequent movement. In relation to the washout theory, Mr Ottoway had been using the mains water supply incident merely to illustrate the mechanism. Upstream from the cundy blockage was close to the centre of depression. There was a history of water-related issues. The absence of voids in the superficials did not matter once the sand had gone: water and particles could move through so long as there was permeable material. Mr Thomson accepted that there was no evidence about the likely shape and pattern of subsidence from ‘running sands’, that not having been investigated in detail, and that Mr Ottoway had been unable to refer to any reported case affecting an area of similar extent. Mr Wilshaw had, however, been wrong to say that there was no other possible explanation.

[53] Mr Clancy’s written submission took a different form from that of Mr Thomson, consisting of proposed findings in fact, at some length and supported where necessary by commentary on the evidence.Reference may be made to this, and we have endeavoured to deal with contentious submissions on crucial matters in our consideration below.

[54] At the oral hearing, Mr Clancy challenged Mr Thomson’s approach to Section 40(2). This, he said, arose after the triggering evidence showing that the nature of the damage may be subsidence damage had been led, so that the question was, how had the respondents set about proving that it was not subsidence damage? In this case, that was by trying to prove that is was caused by surface water drainage. This was an inherently implausible theory of a speculative nature. If they had not discharged the onus of proving it, they were left without any case. The appellants had sought to prove their theory and, it was submitted, had done so on a balance of probabilities, but they did not need to go that far. Rejection of one piece of evidence did not prove the contrary.

[55] Mr Clancy rejected the criticism of Mr Wilshaw’s evidence. It was for the Tribunal to judge the general attck on his credibility and reliability. He had not had any conflict of interest: rather, he had proposed a solution; the outcome of the grouting supported his position; and he had maintained it, relying on that and additional adminicles of evidence. The individual criticisms made were out of proportion to the general worth of his evidence. As regards the optical televiewer evidence not put to Mr Ottoway, while ideally that material should have been available sooner, it did not reflect badly on Mr Wilshaw and did not support any allegation of bad faith. It was open to the Tribunal to approach the evidence that he had spoken to two geologists, as hearsay evidence validating his evidence. In any event, there was substantial support for Mr Wilshaw’s position, particularly in regard to the shape and extent of the subsidence area and the extent of measured ground movement before and after the grouting. There was support from Mr Rule’s evidence. There was no other mechanism which might have brought the ground movement to an end. There was evidence that the grouting had successfully ended the movement, as part and parcel of the repairs, and also that movement of the extent subsequently recorded was of a normal and not a harmful magnitude. Criticism of Mr Ottoway’s theory was not centred on the burst main incident. Reference was made to Mr Donnelly’s affidavit evidence. If the Tribunal was not satisfied by the evidence from within the mine and strata, it did not show the opposite; there was enough other evidence; and the statutory onus would in any event, still apply.

Submissions: Whether substructure works recoverable

[56] On the second issue, Mr Thomson’s written submission referred to Section 1(4)(b) of the Act and, on the meaning of ‘mine’, Section 180(1) of the Mining and Quarries Act 1954. If the applicants’ theory were correct, the mine had been damaged and that damage, remediated by the grouting, was not ‘subsidence damage’. A claim in respect of grouting of mine workings had been disallowed by the Lands Tribunal in a reference between Wolverhampton Metropolitan Borough Council and the Coal Authority. The respondents’ duty under the Act did not extend to this work.

[57] Mr Clancy covered this issue in oral submissions. He argued firstly that injecting grout into old mineworkings was a recognised part of the standard method of repairing properties on the surface. It was artificial to the point of absurdity to say that it was repair of mineworkings, which it would have the effect of closing off. Section 1(4)(b) referred to damage underground in a mine, but the provisions of Section 2 showed that the property concerned was the solum and the buildings erected: the proviso excluded damage occurring within the mineworkings but not damage to properties on the surface. The correct meaning was that it excluded from the statutory duties repairs to only the mineworkings. Here, the repairs were not to or for the sake of the mineworkings. On a purposive consideration, it would be absurd to interpret this so as to exclude grouting of old mineworkings, a standard method of achieving repairs to the properties. For these reasons, the decision on this point in the Wolverhamptoncase was wrong. It had overlooked the importance of the limitation of the exclusion to damage ‘in a mine’ and the fact that repairs were required to deal with damage on the surface. Separately, this was not, at the material time, an ‘excavation … to which a common system of ventilation is provided’ (Section 180(1) of the 1954 Act).

[58] In oral submission in reply, Mr Thomson first dealt with the latter point: he could rely on the wider, primary definition. He explained that he was not maintaining one argument which had been taken in the Wolverhamptoncase, where the grouting had been criticised as preventive rather than remedial. However, and in response to the challenge of absurdity, reference to section 33 of the Act, on preventative works, showed that the Authority might wish to protect itself from the risk of future damage but also had the opportunity to confine itself to immediate repair of the properties. Any absurdity became fainter when it was remembered that most subsidence was crown hole collapse, where the void disappeared. The developer of a greenfield site would not have this opportunity to recoup this cost. This cost had been taken out of ‘subsidence damage’. The applicants could have gone to the Authority and said they were going to repair the building and it might be prudent to do the grouting.

Tribunal’s Consideration: (1) Whether “subsidence damage”

[59] Introduction

It being agreed that the damage to the houses in question was caused by ground subsidence, the main issue is whether the subsidence was “caused by the withdrawal of support from land in connection with lawful coal-mining operations” and therefore “subsidence damage” under the Act (Section 1(1)). This is a factual issue, dependent to a very large extent on expert opinion, and neither counsel referred to any legal precedent although similar issues have been considered, on their own facts, in a small number of cases.

[60] There were of course the two competing theories. Certain other mining subsidence possibilities were, by agreement, discounted and no other possible cause suggested. Section 40(2) of the Act provides :-

“(2) Where in any proceedings under this Act the question arises whether any damage to property is subsidence damage, and it is shown that the nature of the damage and the circumstances are such as to indicate that the damage may be subsidence damage, the onus shall be on the Corporation to show that the damage is not subsidence damage”.

[61] The respondents led in evidence although not at that stage accepting that this provision applied. In his closing submission, Mr Thomson did accept its application. He submitted that it did not alter the evidential burden and it was still a matter for the applicants to advance and endeavour to establish their theory. The provision, he said, only remained relevant if we found that it was equally likely that the damage was subsidence damage or not. We accept at least the latter part of that submission. The applicants have advanced their theory and indeed submitted that it was established on a balance of probabilities. We note that despite Mr Ottoway’s opinion Mr Thomson did not invite us to hold the respondents’ theory established. Rather, he submitted that we should keep in mind that the respondents had advanced a viable alternative theory. We can accept that approach, but we do also observe that where there has been so much investigation of this episode of subsidence and no other possible non-mining cause has really been canvassed at all, it is in practice difficult to avoid looking at the two theories beside each other and considering their respective likelihoods.

[62] We start with some brief general comments on the evidence. Firstly, some assertions made on both sides have seemed to us perhaps to go further than the evidence as presented to us warranted. Some of the productions which were prepared specifically for the hearing were, and were clearly acknowledged to be, purely schematic and illustrative, and as such were helpful. Other productions prepared for the hearing were to an extent ‘interpretative’ in that they sometimes went beyond simply presenting the primary data in an easy form. Although many of these, on both sides, were helpful and straightforward, it has been necessary in places to look beyond some of that evidence and see whether it really presented an accurate picture. As was explored to some extent in oral evidence, the very helpful geological cross-sections, drawings ‘003’ to ‘009’ did not at every point relate entirely clearly to the borehole logging records from which they were mainly prepared, leaving some difficulties in interpretation. They did not, perhaps provide answers, particularly on the important central issue about the extent (if any) of void reduction in the old mineworkings, as clearly as at first sight appeared. We have also to record that the respondents’ ‘Settlement Contours’ drawing (‘014’) can only be regarded as misleading in suggesting a rather different shape of the subsidence area – a shape to some extent relied on by Mr Ottoway, although it should also be said that when referring to this production he himself drew our attention to the problem. Settlement contouring, although no doubt produced by computerised graphics, is not a precise science, particularly where, as in this case, the levelling data was not spread consistently over the subsidence area. In fairness, the applicants’ attempt at settlement contouring on the basis of the level monitoring data was also problematic.

[63] In assessing the opinion evidence of Mr Ottoway and Mr Wilshaw, we were not persuaded by Mr Thomson’s quite strong attack on Mr Wilshaw’s reliability. We appreciate the point made about conflict of interest but do not think that his position was in practice much different from that of Mr Ottoway, insofar as each of these experts was apparently committed to opinions which they sought to advance persuasively. Mr Ottoway’s firm was apparently on contract with the respondents in relation to historic mining liabilities. We can, however, accept that it may not be unusual in this area to find experts who regularly appear on one side of the argument. Neither expert seemed to us either to overstep the mark or overlook his responsibility to the court. Each from time to time made assertions of opinion which we did not find adequately supported by the evidence. Mr Wilshaw seemed to us to be as ready to acknowledge limitations in his position as Mr Ottoway. If anything, we felt that the latter was slightly more ready to make over-positive assertions, for example that the CCTV footage inside the mineworkings revealed nothing about their state and in his assessment of the significance of the burst water main incident. We also felt that Mr Ottoway was somewhat reluctant to acknowledge the lack of support in other expert opinion, or in his own or others’ experience, for ‘running sands’ causing subsidence over such a large area. Generally, however, we are satisfied that both experts were doing their best to assist the Tribunal in this specialised area, and we have sought to test their competing opinions on their reasoning and on the evidence rather than on any overall impression or comparison.

[64] Mr Hogg clearly has experience of investigating subsidence but he was not advanced as an expert on the issue of areal subsidence, and we do not put any weight on such evidence as he gave about that.

[65] Mr Rule also was not called as an expert but clearly does have expertise in his field and appeared to us to be giving evidence from an independent point of view. We had a degree of difficulty with his evidence, insofar as the factual findings in his report were agreed in the Joint Minute to be true and accurate and, on one reading at least, appear supportive of the view, central to the applicants’ case and not accepted by the respondents, that there was degradation within the mineworkings underneath the core area of subsidence. Yet that aspect of his evidence was not explored with him at the hearing and his company’s investigative borehole logs take a rather more summary form than those of other boreholes. We have approached that part of his evidence with caution.

Areal Subsidence

[66] Turning to our consideration of the applicants’ theory that the subsidence was so-called ‘areal subsidence’ caused by degradation of or associated with the pillars within the old mineworkings, the starting point is parties’ agreement that the Whitehill Great Seam (“the mine”) was worked at the site of the subsidence by ‘bord and pillar’. This was clearly established by the borehole investigation.

[67] There is also apparently no dispute that old workings worked by this method may cause areal subsidence even although they might be excluded by an investigative rule of thumb, known as the ‘10 times’ rule, to the effect that mineworkings (other than those currently or at least very recently worked by a ‘longwall’ method) at a depth below rockhead of more than 10 times their own height do not produce ground surface subsidence. Although areal subsidence is a rarely encountered cause of ground subsidence, the literature clearly refers to established cases without any such depth limitation. Reference to degradation associated with the pillars distinguishes areal subsidence from subsidence caused by collapses of the mine roof, which might produce ‘crown hole’ subsidence, a more common form of subsidence for which the ‘10 times’ rule may well be applicable. In areal subsidence, the pillars may themselves be crushed, or they may ‘punch’ upwards into the mine ceiling or downwards into the mine floor. Mr Wilshaw spoke in evidence of all three mechanisms, to which reference can be found in the literature, and it appears that it would be possible for more than one of these mechanisms to occur at a location where areal subsidence was happening. He was criticised for failing to refer specifically to these in his written report, but we accept that they are aspects of areal subsidence to which he had referred.

[68] Continuing with the evidence which supports the theory before turning to more difficult aspects, the evidence as to both the dimensions of the subsidence area at the surface, and as to the shape of that area, was in our view positively consistent with the theory. Mr Ottoway referred to accepted angles of draw and repose to suggest that the subsidence area, being substantially less than 57 metres, could not be areal subsidence. Mr Wilshaw did not question that calculated, although approximate, minimum diameter of subsidence from degradation at one pillar at the depth of this mine. Our consideration of the evidence of the results of the surface level ground monitoring suggest not only that the width exceeded that distance but that it exceeded it by an amount approximately right for collapse or degradation of a few pillars. We accept Mr Wilshaw’s evidence that degradation of one pillar may increase pressure on others which may, in turn also experience problems until such time as the load is sufficiently spread to prevent further continuation. It seemed to us that the respondents’ settlement contour drawing, leading Mr Ottoway to assert a width of about 35 metres, was clearly incorrect. Apart from a basic issue about the lack of settlement points in an area to the west, apparently subject to the subsidence, it was clear that some data received from actual settlement points on the east side had, for whatever reason, been omitted. Considering also the need to add subsidence prior to the start of recording (which Mr Wilshaw said, without contradiction, would lead to at least some widening of the surface area affected), and also subsidence outside the 10 millimetre contour, the dimensions all round the subsidence area seemed to us to fit quite nicely the application of the calculation from depth and angles. For example, settlement points 55 and W11, at each of which cumulative settlement of around 20mm was recorded, were separated, broadly north to south, by 95 metres, and points 63 (20mm) and 74 (over 45mm) were, broadly east to west, 55 metres apart. Reliance on a small number of readings might be problematic, but our finding on this is based on all the evidence before us on level monitoring.

[69] We also reject Mr Ottoway’s argument that the shape of the subsidence area was lineal rather than oval or elliptical and therefore inconsistent with mining subsidence, on the basis of the same consideration of the settlement monitoring evidence. We find the shape also to be broadly consistent with areal subsidence.

[70] Again, the nature of the subsidence “saucer” and of the damage to the properties, tensile on the outside and compressive towards the centre, is consistent. However, as Mr Wilshaw acknowledged, this of itself does not exclude other possible causes of a subsidence area of this size and shape.

[71] We also accept the applicants’ contention that the subsidence substantially ceased following the grouting and, so far as the evidence goes, at a time after the grouting which is at least consistent with the grouting having caused the subsidence to cease. We shall refer in more detail later to the INSAR survey results, on which Mr Ottoway apparently placed some reliance in his contention that the grouting did not bring the subsidence to an end. As far as this matter is concerned, we found the surface level monitoring of the ground levels, under reference to a fixed datum point outside the subsidence area, far more persuasive than the satellite based evidence of absolute ground levels. Although it appears that there was some continuing subsidence after the grouting, the rate of this was minimal in comparison with the rate previously shown at the worst affected points and also the rate which, as Mr Ottoway accepted, must have occurred there prior to the start of the monitoring. There was also undisputed evidence that damage to the houses ceased. Further, the time line graph seems to us to show clearly enough that such small continuing settlement as did take place was fairly uniform across the monitored area and not simply in the central area of substantial subsidence. There was discussion of the period of time before which the grouting would have the desired effect. Unfortunately the first measurements after the grouting period were some 6 months later so that the time within that period when the rate of subsidence diminished is unclear, but, accepting also the evidence of Mr Rule on this, we did find it to be consistent. Again, that this could be coincidental, but in our view it is another pointer in favour of the applicants’ position.

[72] We now turn to the more difficult areas for the applicants’ theory. We of course require to consider whether there is any evidence which positively excludes areal subsidence as the cause. We look first at what can be said to have been found within the abandoned workings. Mr Thomson submitted that there was no direct evidence of the cause of the subsidence, but it seems to us that at least some evidence of what was found at the mining horizon would be direct evidence. The question is what the effect of that evidence was: was there evidence of areal subsidence, was there evidence that there had been none, or was the evidence simply unclear? As we understood it, reduction in pillar or void heights, or evidence of distress in the pillars, would support the theory, but increase in the height of voids (which might simply mean that roof material had come down) would be neutral. Debris on the floor of the mine, if it consisted of coal, might also support, unless we accepted Mr Ottoway’s view, to which Mr Hogg also spoke, that some coal, as well as other waste, may have been left there at the end of mining. Mr Wilshaw also suggested that one pillar viewed on CCTV showed signs of spalling.

[73] The applicants invited us to make positive findings on these matters. Mr Ottoway, on the other hand, asserted that he could see nothing to support the applicants’ theory. We have not found the position at all clear or straightforward. We did not have the advantage of being able to compare clearly intact mineworkings. As we have indicated, the geological cross-section drawings do not appear to us to provide the last word on this. For example, Mr Ottoway used them to indicate consistency in the height of the mine ceiling and floor, but our examination of, in particular, boreholes 214, 215, 216 and 217, did not fully support that. We have reached the view that some reduced pillar or void heights were probably established, at some boreholes. We think we can say with confidence that the borehole logs, together with some of the CCTV images from inside the mineworkings, did also reveal at least some form of debris in or around the core area of subsidence, but the evidence did not enable us to conclude that this included coal which would have come off pillars. Nor were we persuaded that the rough and irregular appearance of a pillar told us one way or the other whether there was spalling. So this chapter of evidence provides some weak support for the applicants’ theory, and does not exclude it.

[74] The respondents relied strongly on Mr Ottoway’s opinion that no bed separation or fissures were found in the strata and that this was fatal to areal subsidence because that could not occur without bed separation or fissures. Mr Wilshaw disagreed on both points, and claimed – although again not with any high degree of certainty – to have seen one or two signs of fracturing in optical televiewer images. These were the particular images on which Mr Ottoway did not have the opportunity to comment, although he had viewed, and opined on, the imaging as a whole. We do note that Mr Ottoway had some support from the March 2005 report by Ross & Co and from some of the other downhole geophysical investigation (gamma density), while Mr Wilshaw said that he had spoken to two geologists who had supported his view. Mr Wilshaw also sought support from some gaps in borehole core recoveries. On this evidence, we think, although again with no great certainty, firstly, that it should be possible to detect such signs if they were present where the borehole passed, and secondly, that no appreciable bed separation or fissures were found. That leaves the possibility that there was bed separation elsewhere in the strata, although there were a lot of boreholes in which it might have been revealed.

[75] It also raises the question whether bed separation is necessary, which brings us to some fundamental issues which were discussed about the the theory of areal subsidence. Could it occur gradually, rather than suddenly and catastrophically, and - this may be a closely related question - might it produce an overall sag of the strata rather than fractures and separations working their way up it? What relationship is required between the extent of the collapse within the mine and the extent of subsidence at the surface? Was Mr Wilshaw, as Mr Thomson suggested, desperately searching for support in the face of evidence which did not support the theory? Did the literature provide solid support beyond the views of colleagues of Mr Wilshaw with a similar approach to the theory?

[76] Although the distinction between ‘crown hole’ and areal subsidence has been clearly made, our impression in considering the experts’ views and the literature referred to is that it cannot be asserted that there is only one way in which areal subsidence manifests itself. Sudden, more violent events, as on at least two occasions at Bathgate, are documented, but there are references to more progressive events, and also to ‘sag’ of the overburden. We are thinking here of the ‘Arup’ papers and also the suggestion that this form of subsidence may often have gone undetected.

[77] It does seem obvious that the subsidence, over the wider area, at the ground surface, would be less than at the level of the mine, so that one would look for rather more void reduction at the mine than the amount of subsidence at the surface. It seemed to us, however, that it must be a matter of degree how much differential is required. This was one particular area in which Mr Wilshaw’s reliability was put in issue. It was submitted that, recognising that there was not enough evidence of void reduction, he had changed his tune on the extent of movement which would occur at the time of the main event and would therefore not have been recorded. In his report, it was “possibly as much as 80-90%”, but in oral evidence, when Mr Ottoway’s view on this was put to him, he suggested 200 to 300 (mm) in total, i.e. implying (roughly) 60% to 70% movement before recording started. Given the uncertainty of this area, and the rather vague way in which Mr Wilshaw gave the original figures (in the context only of making a point about potential survey constraints), we accept this as a reasonable development of his evidence after hearing the other side. We have referred already to the lack of definitive evidence on void reduction, but we do consider that there is evidence which, although weak, is consistent with the applicants’ theory.

[78] There do appear to be some pre-disposing factors which are either excluded or not shown to be present in this case. There was not such a steep dip as to pre-dispose this mine to this problem. The mine was not flooded, although this would seem not to exclude the possibility of some groundwater percolating into and trickling through the mine with some effect on the mine floor over a long period of time. Over-extraction, or ‘pillar robbing’, seems a fairly obvious factor which is not established here but nor is it excluded. The quality of the overburden, and indeed the mine floor, does not seem to us to preclude areal subsidence, this being apparently quite a complicated factor. There did seem to us to be references in some of the borehole findings to weaker strata – seat earth and weaker mudstones - on the floor or in the roof of the mine. Competent sandstone strata are actually referred to as a possible contributor to areal subsidence, because they might impede the natural process of consolidation of collapsed material gradually filling up the voids and reducing or removing the potential for subsidence.

[79] It was accepted that the extensometer installations had not really been successful and that only limited reliance could be placed. There were also conflicting views on what could be taken from the results even if accurate. We have not been able to take much from this evidence. We did not consider that it negated areal subsidence.

“Running Fines”

[80] Mr Ottoway told us that three factors were required for ‘running fines’ to produce ground subsidence: ability of the materials, here sand and other ‘fines’, to move; a vehicle to transport the materials, here the defective drain or drains; and water. As we understood it, according to the theory, water would ‘back up’ in the blocked drain, escape from it at faulty joints, pick up the materials which had the propensity to be moved and, finally, deposit the materials back in the drain at a lower level, so that the materials were transported away, Mr Ottoway thought to the local sewage works.

[81] We accept that this is a possible mechanism. There was sufficient reference to examples of localised subsidence around defective drains, which in any event appeared to us entirely credible, to support the possibility of water moving ‘fines’ in the right conditions. We have examined the degree of support for this as a possible cause of the subsidence which caused this particular damage in Carnethie Street, starting with the discovery of damage at Nos. 107 and 109 in around December 2003 and continuing to develop thereafter.

[82] Mr Ottoway’s rejection of areal subsidence as not fitting the dimensions and shape of the subsidence area was matched by his contention that the subsidence area was ‘linear’ and, further, that it ran along this street, thus supporting the theory that it emanated from defective old drains running along the two sides of the street. Again, and for the reasons already indicated, we take a different view of the settlement monitoring evidence. We have found the area of subsidence to be broadly oval or elliptical rather than linear. To the extent that it may have appeared to follow the line of the street, this appeared to us to reflect the location of the settlement points, there being none over to the west, behind the gardens and such a small number on the east side, and some failures there to obtain readings on some dates, as to make that impression misleading.

[83] Our central, strong impression is that the mechanism advanced is most unlikely to account for this area of subsidence, or this shape of subsidence area, within these superficials. We have accepted the mechanism as possible, and we can accept that the CCTV investigation of the drain showed that water could escape. We can appreciate that the running of the water from the road surface and from some houses into the drain, concentrates the water so as to differentiate it from rain simply falling into the ground and percolating through it. Even so, we find it very difficult to envisage this water flowing sufficiently far, and with sufficient force, from the drain to move these materials in the necessary quantity laterally over an area of this size, even over a substantial period of time.

[84] We find the theory essentially speculative and lacking evidential support. It is perhaps fair to say that the inversion of the legal onus under this statutory scheme reflects the difficulty (which can certainly be seen in the present case, despite the substantial resources which have been devoted to the investigation) in establishing whether or not subsidence has been caused by degradation within a worked coal seam – particularly a disused and inaccessible mine - in underground rock strata. We would not have thought that there ought to be so much difficulty in establishing a cause near the surface and within the superficials. At all events, the evidence in support of the respondents’ theory appeared to us weak.

[85] As regards the composition of the superficials, there clearly was fine material, including sand, within the deposits, and some of it was wet (although this was recorded in only a very small number of the borehole logs). It seems to us, however, agreeing with Mr Wilshaw, on the basis of the site investigations, that these were typically overlain and underlain by clays, which were the predominant materials, with no real indication of a propensity to migrate laterally to the extent required, even with the introduction of water. The investigation does not appear to us to have demonstrated anything in any way unusual about the composition of the superficials. No voids suggestive of such lateral migration were found. The sands were typically located rather lower in the ground than the drainage system into which they required to flow. Mr Ottoway agreed with Mr Wilshaw’s opinion that the relatively flat-lying profile precluded the type of rotational failure which could occur on a steeply inclined site. We were not persuaded that either of the events referred to by Mr Ottoway – the Groundshires borehole and the accidental bursting of the water main – demonstrated such a propensity. It seems to us that there must have been something about the Groundshires drilling method which brought wet sand to the surface, which is not suggested to have happened with any of the many other boreholes, but it does not tell us anything about the propensity of that material to move within the superficials. Likewise with the fractured water main, which would have produced water under considerable pressure: there is a world of difference between water scouring materials in the ground under mains pressure and water percolating (laterally) through superficial deposits. It may be added, as elicited in cross-examination, that the movement of materials on that occasion was not shown to have been caused by the water, as opposed to subsequent digging around the pipe.

[86] We do not ignore the presence of the old, damaged drain, contrasting apparently with modernised drains elsewhere in Rosewell, but, again, doubt whether there is anything particularly unusual about that in an older settlement or about such a build up of materials in the drain. There was also nothing to indicate how long the drain had been in that state.

[87] The secondhand reference to the possibility of an increase in such materials entering the local sewage works was quite unsubstantiated and, to us, unconvincing. A vague reference to replacement of the water main at some date was not followed through.

[88] There was, as we understand it, no attempt, on the basis of the ‘running sands’ theory, to explain the cessation of the subsidence and the damage to the houses in the period after the mine stabilisation works.

[89] We are also influenced by the complete lack of support in any literature, or, as it appeared to us, in the experience of Mr Ottoway or any other witness, for ground subsidence caused by movement of materials by water, through defective drainage, over such a large area, as opposed to much more localised subsidence around a defective drain.

Previous Claims History and Subsidence

[90] Previous claims history apparently revealed that there had been structural damage to No 107 in 1981, 1982, 1987, 1993 and 1999. Defective drainage at that address was repaired in 2000. Further, there being no reason to doubt Mr Donnelly’s affidavit on this, more work was done on drainage in August or September of 2008 after a report of ponding outside the front door. The respondents rely on at least the history of damage in the 1990s, together with the satellite ground level evidence showing a degree of differential settlement during the 9 year period from 1992 to 2000 at that address, as showing or at least suggesting that the subsidence event with which we are concerned was not caused by areal subsidence.

[91] Mr Wilshaw acknowledged that he had no reason to question the satellite data itself, although he referred, really anecdotally, to a level of margin for error rather higher than that mentioned, also without any clear basis, by Mr Ottoway. We do not need to be concerned by speculation about general reduction in ground levels measured by the first satellite, compared with general slight increases measured by the second. What the respondents can point to clearly enough is some average annual settlement at a higher rate outside No 107 and the immediate neigbouring houses over that 9 year period. Presumably, uniform settlement or indeed raising of levels would not cause damage to houses, so the history of damage at No 107 corroborates the satellite evidence of differential settlement, albeit at a much lower rate than during 2004 to 2007.

[92] It seems to us, however, firstly, that this was settlement of a quite different order than the episode with which this case is concerned, and, secondly, that this could be subsidence induced by defective drainage, there being evidence of particular drainage problems at this particular address. Counsel did not make detailed submissions about the affidavit evidence from officials of the roads (not water) authority, but the burden of that evidence was firstly that the search for an old drain or cundy running along the east side of the street, parallel to the one on the west, was fruitless, but secondly that outside No 107 there was evidence of at least some partial drainage and of problems with it. Whether seeking to advance their general theory or, which is all they require to do, to negative the applicants’ theory, the respondents require to link this particular situation to the general subsidence episode. It is no doubt possible that what started as a particular problem with drainage outside one house gradually developed into something else, but in our opinion that is very unlikely as it simply encounters the more general difficulties which we find with the ‘running fines’ theory.

[93] The respondents can argue that a low level of gradual subsidence arising from drainage was accelerated by some other factor. We think that the only such factor which, on the evidence, is properly before us is the introduction of the speed humps in January 2003. The timing of that could be seen to fit, particularly as it would presumably take some period of time for differential settlement to start to cause damage and the first damage was noted shortly before the end of 2003. Mr Ottoway accepted that the evidence of the seismic monitoring apparently excluded the installation of speed humps as a cause of the subsidence event which commenced later that year. That investigation suggested, however, that traffic over the humps could have exacerbated any weakness in the superficials. That would appear to leave this factor open for consideration along with the suggestion that this subsidence may have been a continuation of a much longer episode dating back to the 1990s. However, there was no evidence which would square this with the size and shape of the subsidence area, or the precise location – midway between the humps – of the greatest subsidence. It would also seem difficult to reconcile it with either the extent of acceleration of the subsidence from late 2003 or the extent of diminution of the subsidence after the mine stabilisation works.

Conclusion

[94] Having endeavoured to assess, with the considerable assistance of the expert witnesses, all the evidence, including that of the previous claims history and earlier subsidence, we have to reach a conclusion as to whether the applicants have persuaded us of their theory, or whether we are simply undecided, in either of which situations the applicants succeed, or whether the respondents have persuaded us, on a balance of probabilities on all the evidence, that this was not areal subsidence.

[95] For the reasons which we have given, areal subsidence is in our view supported by some of the chapters of evidence, although it is by no means clear and such direct evidence as there is in support is weak. We are unpersuaded by the “running fines” theory, which, although we accept it as possible, seems to us inherently unlikely in this case and not well supported by evidence. We do take the previous claims history and satellite evidence of ground levels into account. In our view on a balance of probabilities, areal subsidence has not been negatived. Indeed, the respondents’ theory being to our mind weak, and there being no other suggested cause, we think that this subsidence is likely to have been caused by areal subsidence of the Whitehill Great Seam. In these circumstances, we have decided that the respondents have failed to persuade us on a balance of probabilities that areal subsidence was not the cause.

[96] We have accordingly determined that the damage to these houses was “subsidence damage” within the meaning of the Act.

Tribunal’s Consideration: (2) Whether substructure works excluded by Section 1(4)(b)

[97] This is a specific, preliminary, legal issue (in court terms, a relevancy issue) as to whether the respondents can be liable, if the damage to the properties is found to be ‘subsidence damage’, for the cost of the substructure works involving injecting grouting into the mineworkings. The respondents’ argument is that, because damage occurring underground in a mine of coal is excluded from the definition of ‘subsidence damage’ in Section 1(4)(b) of the Act, they cannot be so liable. They rely on a decision of the Lands Tribunal for England and Wales, as it then was, in Wolverhampton Metropolitan Borough Council v The Coal Authority, to that effect.

[98] We regret that we are unable to agree with the reasoning of the Tribunal member in that case. In our opinion, once it is decided that there is ‘subsidence damage’ to the applicants’ houses, the issue is simply whether the works are ‘remedial works’, within the meaning of the Act, in relation to these houses.

[99] Section 1 of the Act defines the ‘subsidence damage’ to which the Act applies, and provides:-

“1(1) In this Act ‘subsidence damage’ means any damage-

(a) to land; or

(b) to any buildings, structures or works on, in or over land,

Caused by the withdrawal of support from land in connection with lawful coal-mining operations.

(4) References in this Act to subsidence damage shall not apply –

(b) to damage occurring underground in a mine of coal (being a mine within the meaning of the Mines and Quarries Act 1954).”

[100] Section 6(2) provides:-

“(2) A schedule of remedial works shall specify-

(a) the works which the Corporation consider to be remedial works in relation to the damage, that is to say, such works (including works of redecoration) as are necessary in order to make good the damage, so far as it is reasonably practicable to do so, to the reasonable satisfaction of the claimant and any other person interested; and

(b) in the case of each item of those works, the amount of the cost which the Corporation consider it would be reasonable for any person to incur in order to secure that the work is executed.”

[101] The Wolverhampton case followed a different procedural route, because the respondents in that case all along admitted liability for subsidence damage, whereas in this case the respondents denied liability following which the applicants proceeded with the works which they considered necessary and then brought this application for an award of damages under Section 40(3)(b) of the Act, all as considered in our earlier procedural decision. It was not, however, suggested that, as regards this particular legal issue, the question is any different. The claim, once it is established that there has been subsidence damage, is that the respondents have failed in their duty under Section 2 to take appropriate remedial action, i.e. (putting it shortly) either to carry out, or to pay for, ‘remedial works’. The question is therefore whether the works called into question can, as a matter of law, be ‘remedial works’. Section 52(1) provides that ‘remedial works’ has the meaning given by Section 6(2) (subject to Section 6(7), which has no application in the circumstances of this case).

[102] There may be an argument in a particular case whether work which can be seen as preventive does or does not also meet the test in Section 6(2) of being remedial. As Mr Thomson pointed out, that question was considered, following evidence on the matter, in the Wolverhamptoncase. The Tribunal member concluded on the evidence in that case that both the works of underpinning the house and the grouting of the mineworkings were remedial.

[103] The Tribunal member went on, however, to consider what he referred to as two subsidiary questions. The first was whether the grouting and underpinning were necessarily excluded because, although he had found them to be remedial, they were also preventive. After considering Sections 16-18 and Section 33 of the Act, he answered that question in the negative.

[104] The second subsidiary question was whether the grouting of the mineworkings were excluded under Section 1(4)(b). The member held that they were. He said:-

“In my view, the grouting of the shallow mineworkings under the subject-property was work to make good damage occurring underground in a mine of coal within the above provisions. It was work to make good the collapse, settlement or disturbance of those workings and to stabilise the ground under 10 Ormond Place. It is therefore excluded from “subsidence damage” as defined in Section 1(1) and (4)(b) of the 1991 Act. Under Section 2(1) of the Act the Authority have a duty ‘to take in respect of subsidence damage to any property remedial action … ’ This duty does not extend to damage which is not ‘subsidence damage’ as defined in section 1 and therefore does not extend to the grouting of shallow workings. The operation of Section 1(4)(b) excludes from the remedial work under section 6(2)(a) the grouting of the old mineworkings.”

[105] We agree that damage within the mine is excluded by Section 1(4)(b) from the definition of ‘subsidence damage’, and we reject Mr Clancy’s brief alternative submission that the definition of ‘mine’ was not satisfied in this case.

[106] With due respect, however, it does not in our opinion follow that this work could not qualify as remedial work in relation to the ‘subsidence damage’ which is the subject of this application. The question in relation to any works is whether they fall within the meaning of Section 6(2)(a) and in particular whether they “are necessary in order to make good the damage, so far as reasonably practicable to do so … ” ‘The damage’ referred to is the ‘subsidence damage’. This is the damage to the applicants’ houses. Their claim relates to remedial action in relation to damage to that property, not in relation to damage to the mine. Remedial works, i.e. works “necessary to make good” damage to a property might include work on other property. If a building is damaged by the withdrawal of support from another property, the remedial works might include work to stabilise that other property. So where a house is damaged by the withdrawal of support from some workings below, the remedial works might include work to stabilise those workings. Whether or not this work was ‘remedial’ in relation to the mine is in our view irrelevant. The argument confuses the question whether there was ‘subsidence damage’ with the question whether the works in question were ‘remedial works’ in relation to the identified subsidence damage.

[107] Both counsel deployed ‘purposive’ arguments. Mr Clancy argued that it would be absurd to exclude this type of work, a standard method of repairing properties damaged by mining subsidence. Mr Thomson countered this by suggesting that, areal subsidence being rare and crown hole subsidence producing a different result, where the void disappeared, this would rarely arise. While we have some sympathy with Mr Clancy’s position, we see this issue as primarily one of construction of the Act’s provisions.

[108] Mr Thomson pointed to Section 33 in relation to preventive work, which the respondents might at their option carry out but were not obliged to do so, suggesting that the structure of the Act gave the Authority a discretion whether to take such preventive action. This was essentially the first subsidiary question considered in the Wolverhamptoncase, and on it we respectfully agree with the member’s reasoning. This case is not concerned with works before subsidence damage has occurred for the purpose of preventing it. Works falling within the definition of ‘remedial works’ should not be excluded because they might also be described as preventive.

[109] For these reasons we reject this challenge to the relevancy of the claim so far as related to the cost of grouting the underground mineworkings. The issue, under Section 6(2), in relation to these costs in the circumstances of this particular case was not, as we understood it, addressed specifically, although it might be that there has been sufficient evidence to enable it to be decided. Quantum has also not been considered at this hearing. In these circumstances, we should, while rejecting the respondents’ specific preliminary plea based on Section 1(4)(b), continue the case for a hearing on quantum of damages. It is of course to be hoped that it may now be possible to agree damages without the need for a further hearing.


Certified a true copy of the statement of reasons for the decision of the Lands Tribunal for Scotland intimated to parties on 24 February 2012

Neil M Tainsh – Clerk to the Tribunal