Environmental Engineering Reference
In-Depth Information
Chapter 7
Subsidence Induced by Fossil Fuel Extraction
7.1. Introduction
In Chapter 5, the problem of subsidence related to the extraction of water
contained in deep layers between aquitards was addressed. In this case, the ground is
always saturated, the subsidence simply corresponds to a decrease in pore water
pressure, a phenomenon well-described in saturated soil mechanics through the
concept of effective stress.
This way of evaluating subsidence can be extended to a more general case: water
extraction from limited depths that results in drawdown of the water table. In this
case, there is increase in effective stress in the zone above the water table, thus
causing a reduction in soil volume or subsidence on the surface of the ground.
It should be stressed that, from a phenomenological point of view, a drawdown
of the water table results not only in an increase in effective stress, but also an
increase in suction in the zone above the water table. This increase in suction gives
rise to an additional soil settlement. Thus, if we want to deal with this problem in a
rigorous way, it is necessary to consider the effect of non-saturation. Nevertheless,
in practice this effect is often included in the total effect of the “effective stress”,
which is possible because the deformation generated by suction is of the same nature
(compression) as that of effective stress.
