Geology Reference
In-Depth Information
EXERCISE
9
Subsidence
INTRODUCTION
Subsidence, or sinking of the land surface, is a haz-
ard commonly associated with resource extraction.
Subsidence has been attributed to several factors,
including decline of hydrostatic pressure in oil- and
water-producing zones due to withdrawal of fluids,
underground mining, solution and compaction of soil
by irrigation water, oxidation of organic materials,
and formation of sinkholes by collapse of bedrock in
karst regions. Well-known examples of subsidence
include sinkholes in Winter Park, Florida; collapse into
abandoned underground coal mines in eastern Ohio,
Colorado, Pennsylvania, Utah and Wyoming; and
groundwater and petroleum withdrawal in California.
Specific areas of subsidence due to fluid with-
drawals include oil fields in Goose Creek, Texas,
Long Beach, California, and Lake Maricaibo,
Venezuela; gas fields in Niigata, Japan, and the Po
Delta, Italy; and groundwater reservoirs near Houston-
Galveston, Phoenix, Las Vegas, Mexico City, Tokyo,
and the San Joaquin and Santa Clara valleys in
California. The magnitude of the problem is illus-
trated by the following statement:
The objective of the subsidence exercise is to
investigate the causes and hazards of land subsi-
dence associated with fluid withdrawals and with
mining. After an overview of the nature and occurrence
of subsidence due to groundwater extraction (Part A),
this exercise examines three cases of subsidence:
Part B, groundwater removal in Santa Clara Valley,
California; Part C, sulfur extraction from the Orchard
Salt Dome, Texas; and Part D, coal mining in Illinois
and Colorado.
PART A. LAND SUBSIDENCE DUE
TO GROUNDWATER EXTRACTION
The causes of subsidence by water withdrawals are
related to the compressibility of rocks and sediments
and their contained water. When the water level is
lowered by pumping wells, water not only drains
from the aquifer but is also squeezed from the fine-
grained confining beds. The withdrawal of water,
which may be under considerable pressure, may lead
to subsidence because the reduction of hydrostatic
pressure results in compaction of the compressible
aquifer and the confining beds. The volumetric reduc-
tion is exemplified by subsidence (Figure 9.1). General
characteristics of major areas of subsidence due to
groundwater extraction are given in Table 9.1.
When wells tapping a confined aquifer remove
water from the sand and gravel of the aquifer, a wide-
spread reduction in the water pressure in the aquifer
and an increase in the load on the aquifer's frame-
work commonly results (Figure 9.1). The increased
load on the aquifer framework of sand and pebble
grains causes the decrease in volume (compaction),
but this decrease may be stopped or even slightly
reversed if the fluid pressure is increased enough to
cause the aquifer to return to its original shape. Thus,
the aquifer portion of the unit behaves in a nearly
elastic manner.
Subsidence in the San Joaquin Valley probably
represents one of the greatest single manmade
alterations in the configuration of the Earth's sur-
face in the history of man. It has caused serious
and costly problems in construction and mainte-
nance of water-transport structures, highways,
and highway structures; also many millions of dol-
lars have been spent on the repair or replacement
of deep water wells. Subsidence, besides chang-
ing the gradient and course of valley creeks and
streams, has caused unexpected flooding, costing
farmers many hundreds of thousands of dollars in
recurrent land leveling. (Ireland, et al., 1984)
=3
With increased need for the resources extracted
from the subsurface and increased human use of the
surface, the problem of land subsidence can be
expected to increase.
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