Doctor of Philosophy
Faculty of Engineering
Nemcik, Jan Anton, Floor failure mechanisms at underground longwall face, Doctor of Philosophy thesis, Faculty of Engineering, University of Wollongong, 2003. http://ro.uow.edu.au/theses/1826
Floor failure in coal mines has not been studied to the same extent as roof failure despite causing significant delays in production. Understanding floor failure mechanisms when mining tabular deposits is, therefore, extremely important. This thesis reviews current understanding of stone floor failure and also explains several other floor failure mechanisms that can occur at the longwall face. Key objective of this study is to provide geotechnical engineers with comprehensive guidelines for predicting floor failure, and outline possible solutions to this problem.
Most commonly encountered floor problems are associated with floor heave and floor puncturing below the hydraulic supports. Although, there are many variables that can contribute to these problems, identifying the causes is often complex and requires a considerable understanding of failure mechanisms. In this research study, two major floor failure mechanisms were identified, primary floor failure and secondary floor failure. Primary floor failure occurs in response to the triaxial stress state ahead of the coal face, where stresses high. Secondary floor failure can be attributed to post failure distribution of stress and subsequent floor displacements. Five mechanisms of secondary floor failure were identified. Typically, these failures cause floor heave that may interfere with the longwall operations or affect stability of the powered supports.
The puncture of weak stone floor that often occurs when load is applied onto the weak floor below the powered supports is the only failure mode that has been well understood for some time. This failure which has been researched in both soil mechanics and rock mechanics, resembles a foundation bearing capacity type of failure mechanism. Buckling of bedded floor strata that often occurs between the longwall face and the bases of the powered supports is associated with excessive yielding of the seam deposit. Failed seam moves towards excavated goaf, and if coupled with the stone floor, can shear the floor bedding planes and buckle upper floor strata. Floor failure induced by multiple blocks sliding within the floor can occur when weak bedding planes shear l-3m below floor level and the sub-vertical fractures split the floor at regular intervals. The floor blocks interact at the corners and can induce large lateral stress at the floor level. If stress exceeds floor strength, shear failure floor level can be expected, manifesting itself as floor heave, which may interfere with mining operations.
Stress concentrations at the longwall corners depend on the magnitude and direction of the pre-mining stress. At these corners, stone floor buckling is often experienced. Weak bedding planes within the floor can exacerbate this type of floor failure. Large goaf areas redistribute vertical stress that concentrates adjacent to goaf excavations. When the chain pillars that support the roof in the goaf are too small, their failure will transfer the vertical stress forward towards the longwall face where increased vertical load interacts with the floor, inducing complex floor failure.
A floor monitoring program was undertaken to investigate fracture formation and floor movement at the longwall face. The monitoring program indicated formation of near vertical fractures in the floor as well as bedding planes shearing well ahead of the face. These primary fractures were measured ahead of the face and provide the basis for the secondary floor failure mechanisms described in this thesis. Understanding fracture formation and their behaviour is very important in developing an analytical theory of floor failure. Analytical solutions were derived to calculate probable floor failure occurrence, which were then compared with modem numerical modelling to predict possible floor failure under a variety of conditions expected to occur sedimentary strata.
Each of the described floor failure mechanisms is investigated in detail using an analytical and numerical approach. Towards the end of the thesis, a risk assessment procedure has been prepared with the floor failure mechanisms included, for the practising engineer to follow when investigating the possibility of floor failure in an underground mine.