International Journal of Rock Mechanics and Mining Sciences
Technical NoteThe influence of proximate fault morphology on ground subsidence due to extraction
Introduction
It is well known that underground coal mining usually cause serious damage to underground structures and surface buildings. The research results show that subsidence due to extraction, which is a process ranging from a gentle movement to a sudden one, is influenced by factors such as the inclination angle of a proximate fault, the fill material and the distance between a fault and the workface, etc.1, 2. In all of these factors, a fault is the most important factor influencing the surface displacement. The strength of a fault is much lower than the strength of surrounding rocks making it easy for a fault to produce sliding along the fault plane. This kind of sliding of a fault causes a discontinuous displacements on the ground surface. As a result, the surface buildings across the exposure of a fault may be damaged seriously.
In this case, investigation of the influence of a fault on the ground surface subsidence is important for protecting surface buildings. In general, the reason for movement of a fault is the stress redistribution caused by underground coal mining. Additionally, the plane morphology of a fault is an important factor influencing the ground surface subsidence. The fault plane has often been assumed to be smooth but the actual morphology of fault plane is rough. So, to describe quantitatively the roughness of a fault is the first step in studying the influence of faults on surface subsidence.
To take the influence of roughness into account, JRC is one of the most effective method to describe quantitatively the rough degree of fault plane. However, there are some difficulties in estimating the JRC value. Some investigations employ fractal geometry3, 4 as an alternative tool to characterize the roughness of fault plane. Many research results indicate that the geological fractures with all scales from small scale (the micro-fractures of rock samples) to large scale (the faults) display statistically self-similar and/or self-affine fractal behaviors5, 6, 7, 8, 9, 10. Some investigations found the fractal dimension strongly correlated with the value of JRC and established an empirical relation between the fractal dimension and JRC[6].
In recent research, the fault plane is simplified as an one-dimensional profile. In this case, the degree of roughness of a fault plane can be quantitatively described by fractal dimension. In this paper, based on the Weaerstrass–Mandelbrot function, five self-affine fractal curves are constructed to simulate the surface morphology of a fault. These self-affine fractal curves are then produced in physical models to systematically investigate the effects of fault morphology on displacement due to extraction. By measuring displacement in physical models, the fractal effects of a fault, especially the influence of surface morphology on the subsidence due to extraction are analyzed in depth.
Section snippets
Simulation of morphology of a fault
As mentioned above, the faults display statistically self-affine behavior. This leads to the simulation of a fault profile on the basis of the Weaerstrass–Mandelbrot function expressed by the following form:where b is a real number greater than 1, ∅n is an arbitrary angle. Let b=1.5, ∅n=0 and suppose Eq. (1)is a sine function, then Eq. (1)can be given by:where D is self-affine fractal dimension, D∈(1, 2).
Based on the function shown in Eq.
Experimental results and analysis
According to experimental results (as shown in Table 2 and Fig. 4), it has been shown that the effects of coal mining does not extend to the fault as the coal face advances only 40 m, so, the surface displacements of the five physical models are almost same (as shown in Table 2). All of these five physical models display the same mechanical behavior. Before reaching 40 m, the overburden rocks appear to be descending, fracturing and bending when the coal face advances a certain distance (about 24
Conclusions
A fault is an important factor influencing the ground surface subsidence. Nevertheless, it is difficult to investigate the influence of faults on the surface subsidence in actual rock strata and at the actual scale. However, this problem can be investigated with physical models. In the present research, physical models are employed to investigate the effect of fault morphology on mining displacement. The research results indicated that,
(1) the movement of rock strata is more evident as the
Acknowledgements
Work for this paper was supported by the National Distinguished Youth's Science Foundation of China, and the Key Project (596 34030) of the National Natural Science Foundation of China.
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