Land subsidence
G. Gambolati, M. Putti, P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
The increasing exploitation of subsurface fluids, such as
water, oil, or natural gas,
from basins filled with unconsolidated deposits of alluvial,
lacustrine, or shallow marine origin,
may cause the sinking or settlement of the land surface, a phenomenon
known as land subsidence.
Observed ground settlements of about 9 m have
been reported from Wilmington, Los Angeles County,
due to oil extraction, and Mexico City,
due to extensive groundwater withdrawal.
When land subsidence takes place in coastal areas,
huge damages to the natural
environment, and to man-made structures as well, can be experienced.
A pore pressure decline in a pumped aquifer, or reservoir, produces
an increase of the effective stress in that part of the overburden
supported by the depressurized sediments. As a major result, these
sediments shorten, or compact, and the land surface subsides.
Horizontal displacements may also occur, but in a lesser amount.
The flow and stress-strain fields, which develop because of
fluid withdrawals, are intimately connected and theoretically coupled.
However, coupling is weak in most real hydrogeological settings, and
uncoupled prediction models can be used.
Furthermore, in large aquifer or multi-aquifer systems subject to
distributed pumping, horizontal soil displacements may be
negligible, and one-dimensional vertical compaction can be assumed.
This chapter presents a summary of the most well-known subsiding
sites in the world, and provides a review of the basic linear theory
used to build the mathematical models for the prediction of land
subsidence due to fluid withdrawal.
The coupled and uncoupled approaches are compared and shown
to yield similar head drawdowns and land settlement in realistic
geological settings. Three well-known examples of land
subsidence due to water and gas extraction are reviewed and discussed,
together with the related models and model predictions.
The examples include the sites of Ravenna, Italy,
where extraction of both water and natural gas caused a ground
settlement of more than 1 m, and the site of Mexico City, where land
subsidence of around 9 m was caused by the extensive groundwater
withdrawal from a shallow, highly compressible, semi-confined
aquifer system.
