Item 2: angle of draw
67 The next aspect of evidence the Board contended was not dealt with by his Honour related to the concept known as the angle of draw. This concept was first referred to in Professor Galvin's report dated 21 June 2007, which Professor Galvin produced after the conclave of experts held on 19 June 2007. It is necessary to deal with that report in greater detail than merely focussing upon the angle of draw, so as to attempt to put that analysis in context.
68 In the report of 21 June 2007, Professor Galvin noted that after the first conclave, Dr de Ambrosis withdrew as a technical expert regarding mine subsidence and underground behaviour and that Dr Thomas was retained by the respondent to provide that aspect of expert evidence in her case. Professor Galvin observed that Dr Thomas had produced a report, dated 17 January 2007, that not only rejected the reasons Dr de Ambrosis had at that stage put forward as indicating that the damage to the respondent's property was caused by mine subsidence, but produced a totally new thesis to establish that mine subsidence was the cause of the damage. Professor Galvin observed that this new theory was outside his 30 years of international experience in geotechnical and mine subsidence engineering. Nonetheless, he considered that as geomechanics was an inexact science, due diligence required that careful consideration should be given to Dr Thomas' view.
69 Professor Galvin noted that at the second conclave, held on 13 March 2007, it had become apparent that there were inconsistencies and errors in the survey data collected up to that point and that there was confusion as to which plan of the shape of the respondent's house was accurate. It was after the second conclave that the new surveys were obtained. Those surveys were completed in March 2007. Thereafter, the meeting of experts was held at the offices of Craven, Elliston & Hayes on 8 May 2007 to which reference is made above. Professor Galvin reported that the first opportunity for the experts to meet in respect of the new survey data was on 19 June 2007, which constituted the third conclave. Professor Galvin stated that as a result of that conclave, he could finalise his conclusions, which are set out in the report of 21 July 2007. In that report, he stated that he remained of the firm opinion that the probability of mine subsidence being the cause of damage to the respondent's house was extremely remote. He advanced four reasons for that opinion: first, the mine workings were in pristine condition; secondly, theoretical considerations and analysis produced outcomes consistent with the mine workings having a very high level of stability; thirdly, there were a number of more probable causes for the structural damage to the respondent's house; and fourthly, there was factual evidence to demonstrate that those probable causes were active. He then gave reasons indicating why the mine workings were in a pristine and stable state.
70 Professor Galvin next dealt with surface subsidence. He said first, at para 4 of his report, that most of the survey measurements could not be considered precise measurements of ground movement, because the survey stations were located on surface structures, rather than in the ground. He stated reliance had to be placed on those survey stations installed at the base of structures. He stated this was not considered to introduce significant error. In this regard, it should again be noted that a stable datum point was established after the meeting of 8 May 2007.
71 Professor Galvin, at para 5 of this report, then said that the surveyed ground movements were not absolute, because measurement only commenced in 1998, after damage to the respondent's house was reported to the Board. He rejected a proposition advanced by Dr Thomas at the third conclave, that perhaps 15 mm of additional subsidence had occurred prior to the establishment of the survey stations. He advanced a number of reasons for this, including that as the respondent's house was displaying significant signs of structural damage as at May 1998, it is inconceivable and unheard of that such damage would result from only 15 mm of subsidence. He said:
"As noted in the next point, the consequences of this level of movement are so negligible that this level of movement is ignored when surface subsidence [sic]."
72 The "next point" is, as I understand it, the point he makes in para 6 of the report in relation to the angle of draw. I will turn to that in a moment. I should record, however, that Professor Galvin then reiterated what was central to his thesis, namely, the survey results demonstrated that the survey stations were in a period of uplift, whilst other survey stations had displayed cycles of uplift and settlement. His firm opinion was that mine subsidence does not result in fluctuating ground levels.
73 Professor Galvin then turned his attention to the angle of draw. He stated at para 6 of his report that surface ground movement due to factors other than mine subsidence was a well-known phenomenon. He said:
"The angle of draw is a term used in subsidence engineering to define the lateral extent of surface subsidence due to mine workings in an attempt to address this problem. It is the angle between the vertical and the line joining the edge of the mining void with the subsidence trough. Holla and Barclay (2000) report that:
Kratzsch (1983) defines the trough margin as the point where a clear subsidence of 10 or 20mm can be found by levelling, provided 'there is no question of ground settlement through non-mining causes'. An estimate of non-mining subsidence movements in the proposed mining area can be obtained either by monitoring movements in an area located close to but outside the mining area or by monitoring movement over a period of time well before the commencement of mining. Measurement of surface movement in areas outside underground mining in New South Wales has shown that movements of up to 20mm can occur in response to such factors as soil moisture movement, variations in water table and other climatic fluctuations. Therefore, a cut-off subsidence of 20mm can be taken for fixing the limit of mining influence . " (Citations omitted; emphasis added)
74 Professor Galvin then analysed the latest survey results, which I understand to be the 2007 survey undertaken by Craven, Elliston & Hayes in March 2007. He considered the survey results demonstrated that, as compared to the May 1995 baseline survey, settlement exceeding 20 mm only occurred at seven of the survey stations. Of those seven, three exceeded 20 mm by 1, 2 and 3 mm respectively. He considered that those differences fell within the range of "survey error" which he said was 2 mm. Of the remaining four, he considered that two, survey stations 21 and 22, initially rose, before settling back to elevations of minus 32 mm and minus 30 mm respectively as at March 2007. The remaining two survey stations (stations 23 and 25) indicated movements of minus 51 mm and minus 40 mm respectively.
75 In questioning from the trial judge, Professor Galvin stated that the survey markers at stations 21 and 22, which revealed a subsidence of minus 32 mm and minus 30 mm, related to the movement of the slab of the floor of a lean-to on the side of the respondent's property. Professor Galvin noted that the lean-to was not keyed into the house and stated, "it's broken away from the house". He observed that it had been referred to in the conclave as an "anomalous slab".
76 The remaining two survey stations, stations 23 and 25, that showed movements of minus 51 mm and minus 40 mm, were located on the verandah extension of the house. In that regard, Professor Galvin considered the foundation design was problematic and the cause for its deficiencies was in the realm of the structural engineers. Professor Galvin expanded upon this in his oral evidence. He said:
"So the point being that the two points that exceed the angle of draw, the criteria, the two points that are 40 to 50, are in the extension of the house which has been the subject of discussion amongst the structural engineers as to the nature of the foundations and their compliance with Australian design standards."
77 Professor Galvin said that this had been the subject of comment in the joint report and that others were better qualified to comment upon it. Professor Galvin concluded that the fact that the survey stations which exceeded 20 mm were consistent with his experience, that the measured movements at the residence were not unique to areas overlaying mine workings.
78 The relevance of the angle of draw was raised by the Board's senior counsel in Professor Galvin's oral evidence. Professor Galvin was asked to describe what the angle of draw meant, how it was applied and what its relevance was to the circumstances here.
79 Professor Galvin explained that any type of mining could result in some form of surface movement. He said the extent that the mine affects the surface is defined by the angle of draw. He explained it was calculated by going to the edge of the mine workings and drawing a vertical line to the surface. He said that surveyors measure the subsidence over the mine workings and continue to do so until they get down to a value of 20 mm or less. He said the surveyors stop at that point and draw a line to the edge of the workings. He explained that this point was over the solid, that is, the edge of the mine workings, not over the mine workings themselves.
80 He explained that before the mining starts, the surveyors run a line of pegs "over the solid and out over the proposed workings". He said that when the mining starts, the surveyors continue to measure the subsidence on the pegs and as they move back from the mine workings and back over the solid, the subsidence decreases until they get to a point where it is 20 mm. He said it is at that point, so far as subsidence engineering is concerned, that it is considered there is no distinction between subsidence from mine workings and subsidence from other natural ground movements. Accordingly, the surveyors draw a line from that point down to the edge of the mine workings. The angle between that line and a vertical line straight to the surface is called the "angle of draw".
81 Professor Galvin explained that when mining is undertaken and it is intended that structures on the surface be protected, the mine workings were designed so that they fell outside the angle of draw. By way of explanation, he said that if there was a power station near intended mine workings, the angle of draw would be established and once the workings get to within that angle, the mining is stopped, so that the subsidence from the mine workings does not affect the structure. He said that as a matter of mining practice in New South Wales, the angle of draw was 26.5 degrees. He further explained:
"… [therefore] the lateral extent of mining - the extent of mining extends for half the depth, so if the depth is 10 metres I would expect subsidence to continue to 5 metres outside the edge of the workings."
82 Professor Galvin was then asked to explain the way the angle of draw operated in this case. He said:
"In this case, beyond the abutment [that is, the barrier pillar] that runs diagonally across the house … [then, if] you were to go out another 5 metres you would then not expect under any type of mining with any type of circumstance, you would not expect to see effects of mining on the surface."
83 There were other references to the angle of draw in the evidence. On 18 April 2007, the respondent's experts, Dr Thomas, Dr de Ambrosis and Mr Kotze, prepared a position statement. The paper was stated to be in response to the position paper of the Board's experts dated 19 March 2007. The Board's position statement was not in evidence, although the respondent's position statement was. In that paper, the respondent's experts stated, at para 11.1:
"We … disagree that a location directly above the barrier pillar will exclude damage by mine subsidence. The house's location 'hinged' over the barrier pillar, places it in the most vulnerable position for damage. Further, subsidence is not confined to the edge of the barrier pillar, as the 'draw angle' phenomenon associated with underground subsidence, causes ground settlement to extend back beyond the edge of the pillar ." (Emphasis added)
84 There was also a reference to the angle of draw in the joint experts' report, but only by the respondent's experts. The Board's experts had stated that the most severely damaged portion of the respondent's house was located directly above a barrier pillar "which is in unmined ground". The respondent's experts disagreed, reiterating what they had stated in their report of 19 March set out above at [*83].
85 As will be apparent from the foregoing, the relevance of the angle of draw in this case was to explain that, as the survey evidence between 2005 and 2007 demonstrated there had only been downward movement greater than 20 mm in respect of four of the survey markers, when account was taken of "survey error" of 2 mm, it could not be concluded that the house was within the angle of draw. It followed, on the thesis advanced by Professor Galvin, that any downward movement of the ground was not referable to mine subsidence. However, the respondent's experts, who were not cross-examined on their report, or their comment in the joint report, refuted the Board's experts' reliance on the angle of draw and their contention that there could be no damage due to subsidence because of the location of the barrier pillar.
86 The trial judge, at [40], summarised the factors Professor Galvin took into account in reaching his opinion that mine subsidence was unlikely to be the cause of the damage to the respondent's house and in concluding that the most probable cause was reactive soil. In doing so, his Honour recorded Professor Galvin's evidence that movements of less than 20 mm were to be disregarded, because they could be attributed to and could not be distinguished from natural ground movement. His Honour then set out the evidence of the four measured survey points showing movements in excess of 20 mm. His Honour's short summary reflects the matters to which I have referred above. In setting out this information, his Honour did so without comment, merely summarising the evidence.
87 His Honour referred again to this part of Professor Galvin's evidence at [63], where he noted that Professor Galvin had agreed that two points at the rear of the house (survey stations 23 and 24) showed a trend of downward movement between March 2004 and March 2007. In the preceding paragraph, his Honour had referred to Professor Galvin's evidence comparing the first survey in 1998 with the last survey in 2007, where he had conceded that in absolute terms, the survey points had demonstrated a downward movement. Professor Galvin had qualified this by saying that in terms of trends, they were up and down.
88 At [78], his Honour again referred to Professor Galvin's evidence relating to the vertical movement at the rear of the house, noting that the Professor would not agree that they showed the ground trending downwards. Rather, he had maintained his position that the surveyed points indicated up and down movement, which supported his contention that the damage to the house was due to a combination of factors, including soil reactivity.
89 His Honour considered that in order to clarify the survey evidence generally and to resolve the disputes over what the readings demonstrated, it was necessary to have regard to the evidence of the surveyor Mr Grosfeld: [85]-[87]. Having briefly made reference to Mr Grosfeld's surveys, his Honour dealt with the oral evidence of Mr Hawkins, the soil engineer retained by the Board. In the course of his oral evidence, Mr Hawkins expressed the view that slope instability or landslip were an unlikely cause of the damage to the respondent's residence, but were more likely than mine subsidence: [88]. He was asked to explain what he based that opinion on. In doing so, he noted there was "no stable datum that is not on top of the coal seam": [88]. He said that all the evidence showed was a differential movement. Importantly, however, Mr Hawkins agreed that in the surveys taken from 2005 through to 2007, a "downward trend" was shown at the rear of the house. He agreed that a downward trend was inconsistent with soil reactivity being the sole cause of the movement: [98].
90 It is also to be remembered that Professor Galvin's evidence was not the only evidence relating to the angle of draw. I have set out the respondent's experts' statement that subsidence was not confined to the edge of the barrier pillar, as the "draw angle phenomenon causes ground settlement to extend back beyond the edge of the pillar": see above at [*83]. This was a direct refutation of Professor Galvin's opinion, given in reports prior to the hearing and included in the joint experts' report. It was not the subject of a single question in cross-examination of the respondent's witnesses.
91 In my opinion, contrary to the submission advanced by the Board, his Honour clearly had in the forefront of his mind the evidence showing that there was a different rate of downward movement at different points on the house and that different witnesses placed different emphasis on this movement and proffered different reasons for it. The real question for his Honour's consideration, therefore, was not whether the house was located within the angle of draw, a matter which was never established in the evidence. Rather, the question was what was the significance of the extent of the movement, which was recorded as between 20 mm and 52 mm at different points of the property and whether what was being exhibited was a permanent downward movement, or as Professor Galvin considered, an up and down movement. His Honour explored these matters in his judgment. At the end of the day, his Honour did not accept the evidence of the Board's witnesses and he gave his reasons for not accepting their evidence.
92 In my opinion, it has not been established that the evidence relating to the angle of draw was critical to confounding Dr Thomas' theory. First, as I have said, it was never established that the respondent's residence fell within or without the angle of draw. Thus, although his Honour did not deal with the Board's submission as to the angle of draw head on, his approach to the evidence indicates that he clearly understood the basis of the challenge being made relating to the extent of movement of the land.
93 Secondly, Professor Galvin had used the angle of draw in a deductive sense, that is, because movements in the centre of the respondent's house had been recorded as being in the order of 20 mm, the house must not have been within the angle of draw.
94 Thirdly, the respondent's experts did not accept Professor Galvin's thesis relating to the angle of draw and they were not cross-examined. If the Board considered that Professor Galvin's evidence on this was critical, it has an obligation to cross-examine the respondent's experts. Its failure to do so meant that there was uncontradicted evidence that refuted this particular aspect of his evidence.
95 Further, other experts commented on the causes of the movements on the respondent's land, which was the essential matter for determination. In this regard, Mr Hawkins' evidence was particularly relevant. It was not consistent with Professor Galvin's evidence. Indeed, if anything, it confounded Professor Galvin's evidence, because he considered that there was no stable datum on top of the coal seam from which the angle of draw could have been established.
96 I would reject this ground of appeal.