Environmental Engineering Reference
In-Depth Information
Ground Subsidence
In the 100 years prior to 1955, a large number of water wells were installed between depths
of 50 and 500 m in the sand and sand and gravel layers. Subsequent to the 1930s, ground-
water withdrawal began to exceed natural recharge. The wells caused a large reduction in
piezometric head, especially below 28 m, but the surface water table remained unaltered
because of the impervious shallow clay formation. A downward hydraulic gradient was
induced because of the difference in piezometric levels between the ground surface and
the water-bearing layers at greater depths. The flow of the descending water across the
highly compressible silty clay deposits increased the effective stresses, produced consoli-
dation of the weak clays, and thus caused surface subsidence (Zeevaert, 1972).
In some places, as much as 7.6 m of subsidence had occurred in 90 years, with about 5 m
occurring between 1940 and 1970. The maximum rate, with respect to a reference sand
stratum at a depth of 48 m, was 35 cm/year, and was reached in 1949. About 80 to 85% of
the subsidence is attributed to the soils above the 50 m depth. In 1955, the mayor of the
city passed a decree prohibiting all pumping from beneath the city, and the rate of lower-
ing of the piezometric levels and the corresponding rate of subsidence decreased consid-
erably. As of 2001, however, subsidence was still continuing at rates of about 3 to 7
cm/year (Rudolph, 2001), and groundwater was being pumped. Even after pumping
ceases, the compressible soils will continue consolidating under the increased effective
stresses. Attempts are being made to bring water in from other watersheds. Subsidence
patterns in Mexico City are being monitored with InSAR (Section 10.2.2).
Foundation Problems
Before the well shutdown program, many of the wells that were poorly sealed through the
upper thin sand strata drew water from these strata, causing dish-shaped depressions to
form around them . The result was the development of severe differential settlements,
causing tilting of adjacent buildings and breakage of underground utilities. Flooding has
become a problem as the city now lies 2 m below nearby Lake Texcoco (Rudolph, 2001).
A different problem occurs around the perimeter of the basin. As the water table drops,
the weak bentonitic clays shrink by desiccation, resulting in surface cracks opening to as
much as a meter in width and 15 m in depth. When the cracks open beneath structures,
serious damage results.
The major problems, however, have been encountered within the city. Buildings supported
on piles can be particularly troublesome. End-bearing piles are usually driven into the sand
stratum at 33 m for support. As the ground surface tends to settle away from the building
because of the subsidence, the load of the overburden soils is transferred to the piles through
“negative skin friction” or “downdrag.” If the piles do not have sufficient capacity to support
both the building load and the downdrag load, settlements of the structure result. On the
other hand, if the pile capacities are adequate, the structure will not settle but the subsiding
adjacent ground will settle away from the building. When this is anticipated, utilities are
installed with flexible connections and allowances are made in the first-floor design to permit
the sidewalks and roadways to move downward with respect to the building.
Modern design attempts to provide foundations that enable a structure to settle at about
the same rate as the ground subsides (Zeevaert, 1972). The “friction-pile compensated
foundation” is designed such that downdrag and consolidation will cause the building to
settle at the same rate as the ground subsidence. The Tower Latino Americano, 43 stories
high, is supported on a combination of end-bearing piles and a compensated raft founda-
tion (Zeevaert, 1957). The piles were driven into the sand stratum at 33 m, and a 13-m-deep
excavation was made for the raft which had the effect of removing a substantial overbur-
den load, subsequently replaced by the building load. The building was completed in 1951
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