Environmental Engineering Reference
In-Depth Information
Surcharging
Surcharging of weak compressible layers is a positive application of construction dewa-
tering. If a clay stratum, for example, lies beneath a thickness of sands adequate to apply
significant load when dewatered, substantial prestress can be achieved if the water table
remains lowered for a long enough time. Placing a preload on the surface adds to the sys-
tem's effectiveness.
10.3
Subsurface Mining
10.3.1
Subsidence Occurrence
General
Extraction of materials such as coal, salt, sulfur, and gypsum from “soft” rocks often
results in ground subsidence during the mining operation or, at times, many years after
operations have ceased. Subsidence can also occur during hard rock mining and tunnel-
ing operations.
In the United States, ground subsidence from mining operations has occurred in about
30 states, with the major areas located in Pennsylvania, Kansas, Missouri, Oklahoma,
Montana, New Jersey, and Washington (Civil Engineering, 1978). Especially troublesome
in terms of damage to surface structures are the Scranton-Wilkes-Barre and Pittsburgh
areas of Pennsylvania. The approximate extent of coal fields in the eastern United States
in the Pennsylvanian Formations is given in
Figure 10.5.
Other troublesome areas: The midlands of England where coal has been, and is
still being, mined. Paris, France, and surrounding towns have suffered surface collapse,
which at times has swallowed houses, over the old underground limestone and gyp-
sum quarries that were the source of building stone for the city in the 18th century
(Arnould, 1970). In the Paris area, collapse has been intensified by groundwater pumping
(see
Section 10.4.2).
Metal Mining
During the 1700s and 1800s various ores, including iron (magnetite), copper, zinc, and
lead, were extracted from relatively shallow underground mines in New Jersey and
Pennsylvania. In recent years, the deterioration of these workings has resulted in the for-
mation of surface sinkholes. Over 400 abandoned iron mines in Northern New Jersey have
been identified (Cohen et al., 1996). A collapse resulting in a 30-m-diameter sinkhole jeop-
ardizing a major road was reported. The Schuyler Copper Mine in North Arlington, New
Jersey, is estimated to contain up to 55 shafts located in a 20-acre area within the town
(Trevits and Cohen, 1996). In 1989, a collapse occurred and a large hole opened in the back-
yard of a home, and depressions formed at other shaft sites.
In Chester County, Pennsylvania, numerous shallow zinc and lead mines were devel-
oped in the 1800s. In general, the metals were deposited in thin, near-vertical formations.
Mining proceeded along drifts that, in many cases, were extended upward toward the sur-
face leaving shallow caps of overburden. The area periodically suffers the development of
small sinkholes, particularly after heavy rains. The use of electrical conductivity to locate
an old mine shaft, where a housing development was proposed, is described in
Section
2.3.2
and the results are shown in
Figure 2.37.
