Subsidence and Its Control<xref ref-type="fn" rid="ch08fn1"><sup>1</sup></xref>

Abstract

Land subsidence due to fluid withdrawal has been reported from many parts of the world. It has developed most commonly in overdrawn groundwater basins, but subsidence of serious proportions also has occurred in several oil and gas fields. Subsidence due to groundwater overdraft occurs in many places in Japan, where it has caused dangerous environmental conditions in several heavily populated areas. For example, in Tokyo, 2 million people in an area of 80 sq km now live below mean high-tide level; subsidence is only partially controlled, and the difficulties of achieving full control are great. The San Joaquin Valley in California is the area of the most intensive land subsidence in the United States. Subsidence, which affects 4,200 sq mi (10,875 sq km), reached 28 ft (8 m) in 1969. The total volume of subsidence to 1970 was about 15.5 million acre-ft. Surface-water imports to subsiding areas have reduced groundwater extractions and raised the artesian head, causing subsidence rates to decrease. In the Santa Clara Valley at the south end of San Francisco Bay, excessive pumping of groundwater between 1917 and 1967 caused as much as 180 ft (50 m) of artesian-head decline and maximum land subsidence of 13 ft (4 m). A fourfold increase in surface-water imports in 5 years has achieved a dramatic rise of artesian head--70 ft (20 m) in 4 years. Subsidence rates have decreased from as much as 1 ft (0.3 m) per year in 1961 to a few hundredths of a foot in 1970. Wilmington oil field, in the harbor area of Los Angeles and Long Beach, California, is not only the oil field of maximum subsidence (29 ft or 9 m) in the United States, but also the outstanding example of subsidence control by injection and repressuring. Large-scale repressuring was begun in 1958 by use of injection water obtained from shallow wells. Subsidence of some bench marks was stopped by 1960. By 1969, when 1.1 million bbl of water per day was being injected into the oil zones, the subsiding area had been reduced from 20 to 3 sq mi (52 to 8 sq km) and parts of the area had rebounded by as much as 1 ft (0.3 m). Methods employed to measure the change in thickness of sediments compacting or expanding in response to change in effective stress include (1) depth-bench-mark and counterweighted-cable or free-pipe extensometers with amplifying and recording equipment; (2) casing-collar logs run periodically in a cased well; and (3) radioactive bullets emplaced in the formation behind the casings at known depths and resurveyed by radioactive detector systems at a later time. In evaluation of potential land subsidence due to fluid withdrawal, an essential parameter is the compressibility of compactible beds. When effective (grain-to-grain) stress exceeds maximum prior (preconsolidation) stress, the compaction is primarily inelastic and nonrecoverable, and the virgin compressibility may be 50-100 times as large as the elastic compressibility in the stress range less than preconsolidation stress. If fluid pressures in a compacting, confined system are increased sufficiently to eliminate excess pore pressures in the fine-grained sediments, subsidence will stop. If fluid pressures continue to increase, the system will expand elastically and the land surface will rise.