Subsidence Induced by Fossil Fuel Extraction - Environmental Geomechanics - page 174

Expression [7.2] governs the movement of LC with its hardening parameter p 0 * ,
whereas expression [7.3] governs the movement of SI with its hardening parameter
S 0 . It can be seen from the two hardening laws that a movement of SI defined by dS
>0 corresponds to a plastic strain d ε vs p >0. This contribution to the total volumetric
strain d ε v p (see equation [7.4]) will result in an increase in dp 0 * , which corresponds
to a movement of the LC curve.

Let us go back now to the case of coal extraction, to see how using the BBM
model we can explain the delayed subsidence observed after exploitation.

S

SI

1

LC i

LC f

Disturbance

2

3

P

Figure 7.5. Description of the delayed settlement using the BBM model

Let us suppose that the initial state of the soil that is not influenced by the
advance of the stope is represented by LC i and SI in Figure 7.5. As the figure
indicates, the coal extraction generates a zone of decompressed soil, which
corresponds mechanically to disturbed soils. As a result of this disturbance there
would be a reduction in the mechanical characteristics: higher compressibility, lower
shear strength, etc. This is represented in Figure 7.5 by a movement of the LC curve
to the left, from LC i to LC f .

In this configuration, let us now consider a saturation path 1-2-3, which could
correspond to precipitations or increase in the water table after extraction. It can be
observed that before the disturbance this path does not cross the LC curve, and after
the disturbance the same path crosses it at point 2. According to the BBM model,
only a small elastic strain (swelling) occurs in the first case, whereas in the second