Pot-hole subsidence in Son-Mahanadi Master Coal Basin
Introduction
Subsidence is a time-dependent deformation of the sub-surface ground, which is created by readjustment of the overburden above voids created by underground mining. Surface manifestation of underground coal extraction occurs in two forms, i. e., trough and pot-hole subsidence. Trough subsidence is a gentle and large area depression of the surface terrain, which is a very common form of subsidence. Pot-hole subsidence is an abrupt depression of local ground surface, which occurs due to sudden collapse of the overburden into an underground void. Pot-holing phenomenon generally occurs following the failure of mine roof which propagates through the overlying strata until intercepts the unconsolidated overburden. The pot-hole subsidence is very dangerous to lives and properties, as it does not give any prior warning. The geometry of most of the pot-hole subsidence is governed by its origin, the thickness and character of the overburden, the dimension of excavation, the state of stress existing in the roof, etc. (Karfakis, 1993).
Pot-hole subsidence is a very common form of subsidence in shallow underground workings of Son-Mahanadi Master Coal Basin (Fig. 1). Average depth of extraction in most of the Indian coalfields has gone beyond 100–150 m. A few areas in Son-Mahanadi Master Coal Basin are still operating at a depth of around 100 m or less wherein pot-hole subsidence is a very common phenomenon. Therefore, only those areas have been covered for pot-hole study. Thus, this study was confined to Jamuna and Kotma, Hasdeo and Bisrampur areas of Shohagpur coalfield lying in the Son-Mahanadi Master Coal Basin. Pot-holing phenomenon is very common at Meera Incline, Govinda, Jamuna, Kotma and Kotma West collieries of Jamuna and Kotma area. Bijuri colliery of Hasdeo and New Jainagar Incline in Bisrampur areas also witnessed pot-holing phenomenon. This paper covers experiences of causes, mechanism, prediction based on data analysis of 34 cases of pot-holes and suggested remedial measures.
Section snippets
Pot-hole subsidence
Cases of 34 pot-hole subsidence were collected from Shohagpur coalfield. The geographic distribution of 28, 4 and 2 pot-holes were in Jamuna and Kotma, Hasdeo and Bisrampur areas respectively (Table 1). Shape of the most of the pot-holes were circular. The diameter and depth of the pot-holes were 1.5–30.0 m and 2.0–26.0 m respectively. All the pot-holes occurred over bord and pillar development and depillared workings except two over Incline number 9 (case No. 7) and 3 (case No. 11) at Jamuna
Causes of pot-hole subsidence
The main causative factors of pot-hole subsidence in Son-Mahanadi Master Coal Basin are discussed below:
Mechanism of pot-hole subsidence
The mechanism of only one pot-hole subsidence was studied in detail at Meera Incline of Jamuna and Kotma area. The overburden close to pot-hole site comprises of saturated, weak and weathered sandstone and alluvium (Fig. 5). The compressive and tensile strengths of overburden sandstones were 2.85–5.32 MPa and 0.30–0.97 MPa respectively (Table 4). When the Main Dip (MD) was advanced 10 m ahead from the Main Dip-11 Level junction in the Upper Kotma (top section) seam, an up throw fault was
Prediction of pot-hole
Pot-hole subsidence can be quite hazardous to life and property as it does not give any prior indication before its occurrence. Therefore, it is necessary to develop methodology to predict the pot-hole subsidence in advance to avoid any damage. The causative factors for the occurrence of pot-holing phenomenon in Son-Mahanadi Master Coal Basin are shallow depth, weak overburden, i.e., Uniaxial compressive strength, geological discontinuity and water seepage from the saturated overburden. In
Mitigation measures
A number of pot-hole control methods have been developed in response to safety of lives and structures as it does not give any prior indication prior to the occurrence. Selection of a particular mitigative measure requires evaluation of many factors, viz., economics, technology availability, void size, surface features, public opinion, moisture condition, stability of overburden, etc. Various remedial measures for bord and pillar development workings as well as depillared abandoned workings are
Conclusions
Analysis and interpretation of 34 pot-hole cases collected from Jamuna and Kotma, Hasdeo and Bisrampur areas of Sohagpur coalfield in Son-Mahanadi Master Coal Basin led to the following conclusions:
- 1.
Most of the pot-holes are circular in shape with 1.5–30.0 m and 2.0–26.0 m their diameter and depth respectively.
- 2.
Pot-holes occurred between 16 and 63 m depth cover towards the in-crop side of the upper seam. Most of the pot-holes occurred when the soft/strong rock ratio in the overburden was from
Acknowledgements
Financial support provided by the Chairman-cum-Managing Director, South Eastern Coalfields Limited to undertake this study is sincerely acknowledged. The author is grateful to the Director, Central Mining Research Institute, Dhanbad for his kind permission to publish this paper. Physico-mechanical testing of core samples done by Sri Santosh K. Singh, scientist is also sincerely acknowledged. Thanks are also due to personnel of Jamuna and Kotma, Hasdeo and Bisrampur areas of Shohagpur coalfield
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