Assessment of Sinkhole Risk in Shallow Coal Mining

I Canbulat, School of Mining Engineering, University of New South Wales C Zhang, School of Mining Engineering, University of New South Wales K Black, Subsidence Advisory NSW J Johnston, Subsidence Advisory NSW S McDonald, Subsidence Advisory NSW

Summary

Shallow bord and pillar coal mining is often associated with extensive subsidence and/or sinkholes on the surface due to high percentage extraction from mines operating at shallow depth. Sinkholes can occur during mining or many years after mining. Types of collapses range from single immature sinkholes to multiple mature sinkholes, all of which may pose a significant risk to the public. Most sinkholes develop due to the progressive collapse of roof strata or a formation of a chimney to the surface. Often a sinkhole takes the appearance of a small but deep hole on the surface, which then enlarges into a conical shaped depression as unconsolidated surface deposits erode into the depression (Galvin, 2016).

In NSW, particularly in the Newcastle, Hunter Valley and Lake Macquarie regions, there are many abandoned coal mines located at shallow depth, which have been responsible for a significant number of sinkholes in the region. A representative database was developed from the digital archives of SA NSW. This database includes sinkholes located in the Newcastle, Lake Macquarie and Singleton Mine Subsidence Districts over a variety of seams and mining conditions. Analysis of this database indicated that the cover depth of the sinkholes varied from 4m to 50m. The diameter of sinkholes ranged from 0.1m to 25m with an average of 3m and the majority of sinkholes were less than 1m in depth.

Bulking controlled failure type analysis is recommended using the stochastic modelling technique to quantify the risks associated with sinkholes. A summary of this approach together with a case study is presented in the paper.

1. Introduction Pillars located at a shallow depth are usually smaller in size compared to Shallow bord and pillar coal mining is pillars located at a greater depth. often associated with extensive Therefore, they tend to fail over time subsidence and/or sinkholes on the due to spalling of pillars and/or the roof. surface due to high percentage extractions from mines operating at Weathering can adversely influence the shallow depth (Hill, 1996). strength of coal and the rock mass in the roof, floor and overburden. In general, the effect of weathering is more

Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 331 pronounced at shallower depths than at collapses range from single immature greater depths. Minor variations on the sinkholes to multiple mature sinkholes, surface, such as humidity, can be all of which may pose a significant risk transferred underground over a short or to the public. long period of time resulting in weakening of the overburden rock This paper firstly analyses the sinkhole masses and eventually the formation of database that has been collected by sinkholes. Subsidence Advisory NSW (formerly the Mine Subsidence Board) to assess the Due to the lack of confining stresses relationship between sinkhole (i.e., the horizontal stress), the roof and dimensions and then proposes a overburden at shallow depths are in method to quantify the risk of sinkhole tension rather than compression. The occurrences. tensile zone above mining operations can extend to the surface, particularly in 2. Overview of sinkhole failures secondary extraction areas. Since the rock mass is approximately 10 times Sinkholes are a common type of weaker in tension than compression, subsidence associated with bord and this stress state can result in different pillar mining. A sinkhole is a localised behaviours from those observed at phenomenon occurring due to sudden greater depths. In addition, the or progressive collapse of overburden existence of a continuous zone of into the openings. Sinkhole occurrences tensile stresses from the surface have been widely reported in mining through to the underground workings areas in South Africa, the United States, creates potential paths for the inflow of Australia, Europe and India (Gutierrez et surface water (Galvin, 2016). al. 2008; Hill, 1996; Singh, 2007; Singh and Dhar, 1997; Tharp, 1999; Whittaker, Sinkholes, also referred to in the 1985; Whittaker and Reddish, 1989). literature as potholes, chimney caves, This section presents a preliminary piping or funnelling, can occur during review of sinkhole features, contributing mining or many years after mining. The factors and engineering controls. pillar factor of safety formulae developed for bord and pillar mining 2.1 Sinkhole geometry give no indication of the stability of Sinkholes are commonly formed at the intersections or roadways. Most intersections (3-way and/or 4-way) of sinkholes develop due to the shallow bord and pillar mining progressive collapse of roof strata or a operations, and the geometry of the formation of a chimney to the surface. sinkhole is governed by the mining Often a sinkhole takes the appearance characteristics (Gray and Bruhn, 1984; of a small but deep hole on the surface, Singh, 2007). The sinkholes are usually which then enlarges into a conical a cylindrical or conical depression, with shaped depression as unconsolidated the surface shape being circular or surface deposits erode into the elliptical. The diameter of a sinkhole is depression (Galvin, 2016). Types of typically less than 10m and depth is less

332 Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 than 15m (Gray et al., 1977, Singh and Dhar, 1997). The shape of the sinkhole is also affected by the depth and erosion of surface features. The diameter of the sinkhole increases with depth and the profiles may appear like bottles without caps (Singh, 2007). The diameter may increase at the ground surface due to the erosion of soil and eventually forms an hourglass shape, as shown in Figure 1 (Singh and Dhar, 1997).

Hill (1996) reviewed sinkholes in South African coalfields and concluded the following:

 Sinkholes are unlikely to occur when the depth exceeds 40m.

 Failure occurs where sandstone layers account for less than 30% Figure 1 Shallow-sided (a) and of the overburden. steep-sided (b) sinkhole formations (Singh and  Extraction height determines the Dhar, 1997) height of caving before bulking arrests upward migration. Canbulat and Ryder (2002) stated that sinkhole development is initiated by the  Sinkhole development may occur failure of the immediate roof layer, and decades after mining. that this failure is either tensile or shear.  Sinkholes are usually circular in Once the immediate layer fails, the shape, 5-10m in diameter, with failure propagates and the broken vertical sides. material spills into roadways. The amount of material going into the  Erosion may cause funnelling at workings is determined by the bulking the surface. factor, mining height and the roadway width.  Most sinkholes are formed above intersections and occasionally the Van der Merwe and Madden (2010) connecting roadways collapse to stated that the formation of sinkholes form a trough-like subsidence appears to be especially prevalent at feature. mining depths of less than approximately 50 m. It is rare for sinkholes and pillar collapse to occur in

Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 333 the same area, as the effect of cases (Johnson, 2013). As the depth of sinkholes is to decrease the load acting extraction increases, the risk for on the pillars. However, the two sinkhole formation decreases as the phenomena may be linked by tensile zone within the overburden underground fires. When sinkholes first strata tends to decrease (Joel, 2015). appear, they let fresh air into the underground workings potentially Mining height does not directly affect the supplying the necessary oxygen for initial failure of a sinkhole, but it underground fires, which will then result determines the height of caving before in pillar failure. Van der Merwe and bulking arrests upward migration (Joel, Madden (2010) also indicated that, 2015). Similarly, the roadway width based on limited data, it appears that determines the effective span affecting the maximum sinkhole width is about the roof fall characteristics, which in turn half the thickness of the soil layer. influences sinkhole formations.

2.3 Contributing factors The presence of weak rock is another critical factor affecting sinkhole A range of factors contributes to the development. Brady and Brown (2004) formation of sinkholes including mining concluded that one of the mechanisms depth, mining height, the composition of sinkhole formation is a progressive and properties of overburden, geological mechanism starting with failure of the discontinuities, void space, bulking stope roof or hanging wall on inclined factor, the nature of overburden, surfaces. A sinkhole may not propagate presence of water and weathering. to the surface if the upward propagation of the collapse is halted by competent The shallow depth of cover is identified strata or if the natural bulking of the as an important causative factor in the caved material prevents further strata development of sinkholes. Sinkholes failure (Gray and Bruhn, 1984). form as a result of collapse of roof strata Waltham et al. (2005) stated that the which propagates to the surface, integrity and capacity of a rock mass therefore the depth of cover needs to be unit in the overburden, referred to as sufficiently shallow to allow the voussoir arch, significantly affects progressive collapse. Based on sinkhole development, and the shear extensive statistical analysis on sinkhole strength of the rock mass particularly formations (Gray et al., 1977; Hill, 1996; affects sinkhole formation. Most Hunt, 1980), the depth of cover for sinkholes occur where the ratio of the sinkhole occurrences typically ranges soft rock thickness to the strong strata in from 10m to 101.5m, with a large the overburden is 0.1 to 0.6 (Singh, proportion of sinkholes formed at 2007). Similarly, according to statistical shallow depth less than 50m in South analysis in Coronation Colliery, South Africa and India. In the United States, Africa, 80% of the known sinkholes most sinkholes formed at the depth of occurred when the depth of weathering cover less than 15m, based on was greater than 50% of the overburden statistical analysis of 354 subsidence (Hill, 1996).

334 Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 Geological discontinuities within the Dynamic loading can also accelerate overburden also affect sinkhole sinkhole formation. Blasting can trigger formation. Closely spaced joints and sinkhole formation (Benson and faults create planes of weakness for the LaFountain, 1984; Iqbal, 1995; Tharp, roof strata failure and fracture 1999) and other dynamic sources such propagation up to the surface. Apart as vibrating equipment, vibro- from the discontinued rock mass, compaction and earthquakes can also sinkhole and plug failures are cause sinkhole occurrences (Ferreli et sometimes controlled by one or more al., 2004; Tharp, 1999). major structural features which provide low shear strength surfaces (Brady and 3. Analysis of the sinkhole Brown, 2004). This type of failure is database characterised by the sliding of the overburden strata along the structure. Sinkholes occur in a variety of Singh (2007) reported that nearly a third conditions and environments, even with of sinkhole cases occurred in the close slight changes in dimensions of vicinity of fault planes intersected in the workings, overburden geology, jointing underground workings in Shohagpur and surface conditions. Coalfield in India. As a part of ongoing research into The risk of sinkholes is also increased pothole development and assessment, by the presence of surface and ground SA NSW developed two databases of water. Many researchers have reported pothole occurrence. A limited, colliery that the frequency of sinkholes specific database of approximately 650 significantly increased during rainy potholes in the Borehole and Victoria seasons (Bruhn et al., 1978; Esaki et Tunnel Seams located in the Newcastle al., 1989; Wildanger et al., 1980). The region (Johnston et al. 2017), and a increased pore pressure due to rainfall second more general database could accelerate roof fall, which then examining approximately 450 sinkholes propagates upwards to form a sinkhole located in 23 seams between 1973 and under certain geological conditions. 2017, analysed in this paper. This Furthermore, heavy water seepage was database has been analysed to monitored through the fault along with evaluate relationships between sinkhole sand and soil prior to the formation of depth, diameter and cover depth of sinkholes (Singh, 2007). On the other workings. A total of 426 cases have hand, dewatering or lowering the water been deemed to be reliable and table can also contribute to sinkhole included in this analysis. A summary of occurrences. As the cavern helped to the data is presented in Table 1. support the cavern roof, the dewatering process may remove this support from The cover depth is presented as a the roof (Sahu and Lokhande, 2015; range in the database. Considering the Singh and Dhar, 1997). inaccuracies in data collection and in old mine plans, the maximum cover depth has been used in this analysis.

Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 335 Table 1 Summary of sinkhole data collated in the Newcastle, Lake macquarie and Hunter Valley regions

Minimum Maximum Average Standard Total Number (m) (m) (m) Deviation of Cases Sinkhole diameter 0.1 25.0 3.0 4.0 426 Sinkhole depth 0.2 15.0 2.5 2.6 224 Cover depth 4.0 50.0 15.9 8.7 313

The cover depth of the sinkholes varied sinkholes depths greater than 5m from 4m to 50m with an average of (Figure 4). 15.9m. While the majority of the sinkholes occurred at depths of 10 to Figure 5 shows the relationship between 15m, a total of 23 cases were recorded the sinkhole depth and the sinkhole at depths greater than 30m, as shown in diameter over 220 cases. This figure Figure 2. indicates that there is no apparent correlation between these two Sinkhole diameters ranged from 0.1m to dimensions. Similarly, neither sinkhole 25m with an average of 3m. Figure 3 diameter (Figure 6) nor sinkhole depth shows that more than 40% of the cases (Figure 7) appear to have any significant involved diameters of up to 1m. correlation with the cover depth of Although, the majority of the sinkholes sinkholes occurring over old mine also had depths of less than 1m, workings. approximately 12% of the cases had

40% 100%

90% 35% 80% 30% 70% 25% 60% Frequency 20% Cumulative frequency 50%

Frequency 40% 15%

30% frequency Cumulative 10% 20% 5% 10%

0% 0% 0 5 10 15 20 25 30 35 40 45 50 Cover depth (m)

Figure 2 Distribution of the sinkhole cover depth

336 Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 45% 100%

40% 90%

35% 80% 70% 30% 60% 25% 50% 20% Frequency Frequency Cumulative Frequency 40% 15% 30% frequency Cumulative

10% 20%

5% 10%

0% 0% 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Sinkhole diameter (m)

Figure 3 Distribution of the sinkhole diameter

45% 100%

40% 90%

35% 80% 70% 30% 60% 25% 50% 20% Frequency Frequency Cumulative frequency 40% 15% 30% frequency Cumulative

10% 20%

5% 10%

0% 0% 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Sinkhole depth (m)

Figure 4 Distribution of the sinkhole depth

Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 337 16 221 cases 14

12

10

8

6 Sinkhole diameter (m) 4

2

0 0123456789 Sinkhole depth (m)

Figure 5 Relationship between sinkhole depth and diameter

30

25

20 305 cases

15

Sinkhole diameter Sinkhole diameter (m) 10

5

0 0 5 10 15 20 25 30 35 40 45 50 Cover depth (m)

Figure 6 Relationship between cover depth and sinkhole diameter

338 Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 16

14

12 160 cases 10

8

6 Sinkhole depth depth (m) Sinkhole

4

2

0 0 5 10 15 20 25 30 35 40 45 50 Cover depth (m)

Figure 7 Relationship between cover depth and sinkhole depth

4. Occurrence of sinkholes appropriate approach under critical surface structures is to assume that the When a competent unit exists in the competent layer will fail and to assess if overburden, it will arrest the failure and the failure will reach the surface or not. the development of sinkholes, provided that the unit is capable of carrying both Once the immediate strata fails, the itself and the surcharge load imposed on failure will propagate further up into the it by the overlying weak units. The overburden and the failed material will stability of the component layer can be spill into the bords. The amount of assessed by using reliable site data material spilling is controlled by the assuming that the unit is a crown pillar. bulking factor (defined as the ratio of There are numerous numerical and total volume to solid volume) of the analytical tools that can be used for this caved material, the mining height and assessment. However, these the roadway width. If there are no assessments determine the stability of competent strata units in the the competent layer at the time of data overburden, the failure will reach the collection. Therefore, they are useful in surface. This type of failure is referred to assessing the stability of competent as a bulking controlled failure, illustrated layers mostly for the medium to short in Figure 8. term. As the time-dependent behaviour The maximum height (z) of a potential of rock masses cannot be predicted, this collapse in small pillar widths above the method cannot be used to determine the roof line, based on an equilibrium long-term stability of the competent volume between the in situ rock in the layers in the overburden. Therefore, the “chimney” and the caved (and bulked)

Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 339 material in a four-way intersection, can where H is cover depth and te is the be expressed by the following equation: thickness of weathering.

2 w The depth of a sinkhole (h`) can also be 22tanhb b w h  2 [1] estimated using the following formulae: z   (1)Bb  2 When the height (z) of the goaf is less than H-te (i.e. less than the thickness of For larger pillars, where the toes of the weathering), the bulked goaf is stable collapsed material will be touching each and no sinkhole can form. other, the formula becomes: When the height of the goaf reaches a 2(hb22 2 bh cot ) z  depth equal to te below the surface (1)Bb  2 [2] (topsoil level), the topsoil will rapidly leach into the chimney and the depth of where w = pillar width (m) the sinkhole will be: h = mining height (m) If z=H-te, then h` = te [5]  = angle of repose (o) When the height of the goaf reaches a B = bulking factor value equal to, or greater than H-te, the depth of the sinkhole is given by: b = roadway width (m)

If z > H-te, then h` = (z-H)(B-1)+ Bte [6] When the pillar and roadway dimensions are smaller in one direction, the Another critical consideration in this type maximum height of potential collapse of failure is the groundwater, as there is can be calculated using a combination the potential for unconsolidated wet of the above two formulae. material to flow into the mine if the caving chimney intersects with an The analysis of toes either touching aquifer within the overburden. Also, the each other or not can be assessed using height of the goaf in the bulking factor the following formulae: analysis is only valid where water has not washed away or decomposed the 2cothw  toes will not touch fallen material. [3] 2cothw  toes will touch 5. Proposed assessment The factor of safety (FoS) for the methodology likelihood of sinkhole occurrence can then be calculated using the following As evident in the preceding sections, simple formula: sinkhole occurrence is a function of many parameters and the geological H and surface environments. Analysis of FoS  [4] s zt e the sinkhole data indicates that there is no single parameter that correlates with

340 Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 the occurrence or the dimensions of the the Lake Macquarie Region, New South sinkholes. Therefore, traditional Wales. The information in Table 2 has deterministic approaches, in which the been obtained from the mine Record input parameters are presented as Tracing and Subsidence Advisory NSW single values, may not be capable of records. accounting for uncertainties governing sinkhole occurrence. A stochastic Table 2 Case study mine modelling technique, which allows for information the randomness of the input parameters, Depth of cover 15 to 60m is considered to be more appropriate in Mining height 4.3m quantifying the risks associated with Pillar width 6.4m sinkholes. Pillar length 30m Roadway width for pillar 4.9m width (average) Roadway width for pillar 10.8m length (average)

The roadway dimensions have been extracted from the mine plan. Goodness of Fit Tests indicated that the best-fit distribution for roadway widths is Pearson5 distribution. Figure 9 shows the distribution of roadway width for Figure 8 Mechanism of sinkhole pillar length and Figure 10 shows the development above bord distribution of roadway width for pillar and pillar workings width. In stochastic modelling, the input To determine the bulking factor in coal parameters are expressed as probability strata, a number of references were distribution functions rather than single reviewed including Sweby (1997), Hill values. The design outputs (i.e. factor of (1996), Galvin (2016), Esterhuizen and safety) are also statistically distributed; Karacan (2007), Ofoegbu et al. (2008), thus the probability of sinkhole Richards et al. (2002), Das (2000), occurrence for a given layout and Konak et al. (2006), Palchik (2002), geological conditions can be calculated. Pappas and Mark (1993), Peng and As such, the associated risk from the Chiang (1984), Trueman (1990), Unrug varied inputs can be quantified, which in (1982) and Yavuz (2004). From these turn assists with risk-based decision- studies a total of 52 relevant bulking making. In the following section, an factors were extracted for broken coal, example of this application is presented. carbonaceous shale, carbonate rock, clay, clay shale, claystone, coal, coarse 6. Case study sandstone, fine sandstone, sandstone, The case study is an old coal mine sandstone and shale, sedimentary rock, located in the Great Northern Seam in shale, shaley sandstone, silt mudstone, strong sandstone, weak argillite, sandy

Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 341 shale, and weak sandstone. This data This data can be best represented by a indicated that the bulking factor of coal Pert distribution using the following measure rocks varies from 1.1 to 1.9, parameters (Table 3): with an average of approximately 1.42. 25% 100% 90% 20% 80% 70% 15% Frequency 60% Cumulative Frequency 50% 10% 40%

Frequency 30% 5% 20% Cumulative Frequency 10% 0% 0% 4 6 8 101214161820

Roadway width for pillar length (m)

Figure 9 Distribution of roadway width for pillar length

30% 100% 90% 25% 80% 70% 20% Frequency Cumulative Frequency 60% 15% 50%

Frequency 40% 10% 30% Cumulative Frequency 20% 5% 10% 0% 0% 024681012

Roadway width for pillar width (m)

Figure 10 Distribution of roadway width for pillar width

342 Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 Table 3 Parameters used for the The modelled distribution of the bulking statistical analysis factor is presented in Figure 11.

Continuous mode parameters 1.08 Boundary parameter 1 1.31 Boundary parameter 2 2.16

14% 100%

90% 12% 80%

10% 70%

60% 8% 50%

Frequency 6% 40% Frequency Cumulative Frequency Cumulative Frequency 4% 30%

20% 2% 10%

0% 0% 1.02 1.08 1.14 1.20 1.26 1.32 1.38 1.44 1.50 1.56 1.62 1.68 1.74 1.80 1.86 1.92 1.98 2.04 2.10 2.16 Bulking factor

Figure 11 Distribution of bulking factor data obtained from the literature

Using the methodology outlined above, presented above. It is important to note sinkhole occurrence has been assessed that the probability of sinkhole assuming a depth of weathering of 8.6m occurrence presented in Figure 12 is for (from the borehole logs) and an angle of the life of the workings. Therefore, the repose of 35o of the failed material. results should be normalised to annual Figure 12 shows the probability of probability of occurrence to quantify the sinkhole occurrence at different cover risks associated with sinkhole depths for four-way and three-way occurrence. intersections. Based on this figure, it can be concluded that the probability of Average sinkhole depths in four-way sinkhole occurrence is greater in 4-way and three-way intersections for different intersections than the 3-way depths are shown in Figure 13. As intersections. It is also evident in this expected, this figure shows that the figure that the probability of sinkhole average sinkhole depth increases with occurrence decreases when the cover decreasing cover depth for both depth is greater than approximately intersection types. This figure together 30m. This finding is consistent with the with Figure 4 can be used to estimate Subsidence Advisory NSW database the volume of sinkholes that can be expected in this particular site.

Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 343 1.0

0.9

0.8

0.7 4-way intersection 0.6 3-way intersection

0.5

0.4

0.3

0.2 Probability of Probability occurrence sinkhole 0.1

0.0 15 20 25 30 35 40 45 Cover depth (m)

Figure 12 Factor of Safety against sinkhole occurence

11 10 9 4-way intersection 8 3-way intersection 7 6 5 4 3 Average depth of sinkhole (m) (m) sinkhole of depth Average 2 1 0 15 20 25 30 35 40 45 50 55 60 Cover depth (m)

Figure 13 Average depth of sinkholes for intersection types

7. Conclusions approximately 450 sinkholes at abandoned coal mines in 23 seams This paper assessed the risks of located in the Newcastle, Lake sinkhole occurrences associated with Macquarie and Hunter Region was shallow bord and pillar mining. An analysed for this study. extensive sinkhole database of

344 Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 The relationship between sinkhole Conference on Sinkholes, Balkema, dimensions was assessed. The cover Rotterdam, 201-215. depth of the sinkholes analysed varied from 4m to 50m with an average of Brady BHG. and Brown ET. 2004 Rock 15.9m, the diameter of the sinkholes mechanics for underground mining, ranged from 0.1m to 25m with an 3rd Edition, Kluwer Academic average of 3m; and the majority of the Publishers. sinkholes had a depth of less than 1m. Bruhn RW., Magnuson MO. and Gray The statistical analysis showed that RE. 1978 “Subsidence over the mined there was no apparent correlation out coal”, ASCE Preprint 3293 (Am between these three parameters. The Soc Civil Eng Spring Convention, time-dependent stability of the Pittsburgh), 26–55. competent units in the overburden, bulking factor and presence of Canbulat I. and Ryder JA. 2002 groundwater are critical considerations “Prediction of surface subsidence and identified in this study, which sinkholes”, Re-defining boundaries, significantly affect sinkhole formations. SANIRE Symposium Vereeniging, 6 September. A stochastic modelling technique was then used to assess and quantify Das SK. 2000 “Observations and sinkhole occurrence. A case study was classification of roof strata behaviour presented examining the probability of over longwall coal mining panels in sinkhole occurrence at different cover India”, International Journal of Rock depths for four-way and three-way Mechanics and Mining Sciences, 37 intersections and the relationship (4): 585–597. between sinkhole volume and depth of cover. Esaki T., Kimura T. and Shikata K. 1989 “Subsidence and environmental Given the difficulties in assessing impacts in Japanese coal mining”, sinkhole risks, bulking controlled failure Proceedings 30th US Symposium on type analysis is recommended using a Rock Mechanics as a Guide for stochastic modelling technique to Efficient Utilisation of Natural quantify the risks associated with Resources. West Virginia University, sinkholes. The results can assist risk- 511–518. based decision-making on sinkhole management. Esterhuizen, GS. and Karacan CO. 2007 “A methodology for determining 8. References gob permeability distributions and its application to reservoir modelling of Benson RC. and LaFountain LJ. 1984 coal mine longwalls”, 2007 SME “Evaluation of subsidence or collapse Annual Meeting and Exhibit, Denver, potential due to subsurface cavities”, Colorado, February 25–28. Proceedings 1st Multidisciplinary

Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 345 Ferreli L., Guerrieri L., Nisio S., Vita L. Johnson CP. 2013 A guide to surface and Vittori E. 2004 “Relations among features related to underground coal seismogenic structures, earthquakes mining, Master thesis, University of and sinkhole phenomena: a Washington. methodological approach in the Apennines (Italy)”, Proceedings of Johnston J., Black K., Fityus S., Rigby 32nd International Geological R. and Canbulat I. 2017 “Pothole Congress, Firenze, Italy, 20–28 subsidence in the Newcastle region”, August, Volume Abstract, Part 1, 669. Tenth Conference on Coal Mine Subsidence, 5-7 November, Pokolbin, Galvin JM. 2016 Ground engineering NSW. (In Press). and management in coal mining, Springer. Konak G., Onargan T., Aksoy CO., Kose H., Tatar C. and Pamukcu C. 2006 Gray RE. and Bruhn RW. 1984 Coal “Determination of immediate roof at mine subsidence—eastern United mines of the Kutahya-Omerler Coal States, in Holzer TL (ed.) Man- Basin, Turkey”, Journal of Mining Induced Land Subsidence. Geological Science, 42 (2): 157–170. Society of America. Lokhande RD., Prakash A., Singh KB. Gray RE., Bruhn RW. and Turka RJ. and Singh KKK. 2005 “Subsidence 1977 Study and analysis of surface control measures in coal mines: A subsidence over mined Pittsburgh review”, Journal of Scientific and coalbed, US Bureau of Mines, Industrial Research, 64: 323-332. 366047. Ofoegbu GI., Read RS. and Ferrante F. Gutierrez F., Calaforra J. and Luch P. 2008 “Bulking factor of rock for 2008 “A genetic classification of underground openings”. Draft report sinkholes illustrated from evaporite prepared for U.S. Nuclear Regulatory paleokarst exposures in Spain”, Commission. Contract NRC 02–07– Environmental Geology, 53: 993- 006. 1006. Palchik V. 2002 “Influence of physical Hill RW. 1996 Safety and environmental characteristics of weak rock mass on risks associated with shallow bord and height of caved zone over abandoned pillar workings, Chamber of Mines subsurface coal mines”, Research Organisation. Final project Environmental Geology, 42: 92–101. report. Report No: CE 9401. Pappas DM. and Mark C. 1993 Behavior Joel F. 2015 Sinkhole risk management of simulated gob material, Report of process within thermal collieries: a Investigations RI 9458. practical approach thereof, Master thesis, University of the Peng SS. and Chiang HS. 1984 Witwatersrand. Longwall mining, New York: John Wiley & Sons.

346 Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 Richards L., Mazengarb C., Beetham Second Edition. ISBN 978-1-920410- D., Brathewaite B. and Smith W. 2002 06-3. Waihi underground mine workings, Stage ii investigations (Risk Waltham T., Bell FG. and Culshaw MG. assessment and mitigation). August, 2005 Sinkholes and Subsidence, Report by Institute of Geological and Springer Science & Business Media. Nuclear Sciences Ltd. Whittaker BN. 1985 Surface subsidence Sahu P. and Lokhande RD. 2015 “An aspects of room and pillar mining. investigation of sinkhole subsidence Mining Department Magazine, and its preventive measures in University of Nottingham, 37, 59–67. underground coal mining”, Procedia Whittaker BN. and Reddish DJ. 1989 Earth and Planetary Science, 11, 63- Subsidence: occurrence, prediction 75. and control, Amsterdam: Elsevier. Singh KB. 2007 “Pot-hole subsidence in Wildanger EG., Mahar J and Nieto A. Son-Mahanadi Master Coal Basin”, 1980 Sink-Hole Type Subsidence Engineering Geology, 89: 88-97. over Abandoned Coal Mines in St Sweby G. 1997 Review the caving David, Illinois, Abandoned Mined mechanisms around high extraction Lands Reclamation Council, systems and determine the effect of Springfield, Illinois, 88. the mechanisms on the safety of the Yavuz H. 2004 “An estimation method system, CSIR-Division of Mining for cover pressure re-establishment Technology. COL327. distance and pressure distribution in Trueman, RA. 1990 “Finite element the goaf of longwall coal mines”, analysis for the establishment of International of Journal Rock stress development in a coal mine Mechanics and Mining Sciences, 41: caved waste”, Mining Science of 193–2 Technology, 10: 247–252.

Unrug KF. 1982 “Roof falls and caving in longwall mining operations”, Proceedings of the 2nd Conference on Ground Control in Mining. Morgantown, West Virginia: Department of Mining Engineering, West Virginia University. pp. 45–52.

Van der Merwe JN. and Madden BJ. 2010 Rock engineering for coal mining. Safety in Mines Research Advisory Committee (SIMRAC). SAIMM Special Publications Series 7.

Proceedings of the 10th Triennial Conference on Mine Subsidence, 2017 347