Environmental Engineering Reference
In-Depth Information
This simplification is however no longer valid for subsidence related to fossil
fuel extraction because in this case it is generally the reduction in suction that
generates the decrease in volume. When using the concept of effective stress, we
would have obtained the opposite, i.e. a swelling, because a reduction in suction
corresponds to a reduction in effective stress [BIS 59].
The study of subsidence related to the extraction of fossil fuel thus requires some
specific concepts developed in unsaturated soil mechanics.
Three examples are given in this chapter. The first relates to coal extraction, the
second to oil exploitation, and the third to gas exploitation. The first example
involves little from the aspect of non-saturation, however the two others correspond
to direct applications of unsaturated soil mechanics. As interpretations of the
phenomena observed are often related to the basic Barcelona model or BBM [ALO
90], a brief recap of this model will be made after the first example.
7.2. Subsidence due to coal extraction
Most coal basins in the world have been, and still are, affected by ground
subsidence or collapses related to the extraction of coal. The amount of subsidence
is proportional to the total volume of materials extracted, i.e. not only the fuel,
generally coal, but also part of the surrounding (country) rocks and galleries, shafts
and other mining elements.
We can get an idea of the extracted volume by examining the “spoil heaps” that
are characteristics of the landscapes in mining areas. Some of them could have a
volume of several millions of cubic meters.
The total vertical ground motion can reach some meters (up to 10 m) causing
considerable damage to buildings and roads. This subsidence sometimes affects
areas of tens or even hundreds of square kilometers, lowering the ground level under
rivers. This poses important problems, among others obliging the local authorities to
install special pumping station systems to collect the sewage and eject it in the river,
several meters higher.
“Mining subsidence” is a topic that has been studied in detail since the early 19
th
century (for example the Liege coal basin and the North of France). Many
approaches have been made since this time, such as empirical, mechanical and even
finite elements methods. The detailed description of these methods is beyond the
scope of this chapter and numerous publications on this topic exist [BAR 95, LAB
65, WHI 89]. Hereafter we present the global mechanism of subsidence induced by
