Assessment of Sinkhole Risk in Shallow Coal Mining
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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).