Subsidence Land Subsidence Subsidence is human caused in
Subsidence
Land Subsidence • Subsidence is human caused in 37 states • Subsidence affects 15, 000 sq. miles of the United States • The cost nation wide is $100 million/year • The world wide economic impact is astounding
Causes • • • Tectonic Activity Dewatering Oil Extraction Mining Cave Collapse / Sinkholes Unconsolidated sediments
Tectonic Activity • Easily recognized in coastal areas - gradual inundation • Tectonic subsidence is generally slow compared to other causes
Tectonic Activity • Vertical offset in Owens Valley 1872 Earthquake was 23 feet • Surveys in the LA area began in the 1930's and reveal that sedimentary basins are going down and hills are going up – San Fernando Valley -0. 045 feet/yr – Santa Monica Mountains +0. 013 feet/yr – San Gabriel Mountains +0. 02 feet/yr
Dewatering Sediments • Occurs where water table is lowered due to drainage or pumping • Peats have been drained in the Sacramento San Joaquin Delta area since 1850 – 12 to 14' subsidence – This is a problem when the areas are close to sea level • Solution: reduce pumping rate to equal recharge rate
Cave Collapse / Sinkholes • This is often caused by lowering of the water table and reduction in buoyant support of the rocks above the cave. • Occurs in regions underlain by carbonate rocks – limestone, dolomite, and marble – All contain Ca. CO 3 or Ca. Mg(CO 3)2. both of which are easily dissolved in water
Subsidence depressions filled with water in central Polk County, Florida.
Cave Collapse / Sinkholes • Winter Park, Florida 1981: – Cause: karst topography – 100 m depression 13 m deep – $2 million in damage.
Cave Collapse / Sinkholes • Hershey, Pennsylvania in 1949 – Cause: Dewatering at a limestone quarry created a cone of depression in water table – 100 sinkholes appeared in an area over 26 km 2 • 2 -7 m in diameter, 8 m deep. – No new sinkholes after pumping stopped
Cave Collapse / Sinkholes – near Johannesburg, S. Africa • Cause: Dewatering of a gold mine • 1962 – 1966: water table lowered 450 m • Eight sinkholes > 50 m in diameter x 30 m deep formed in cone of depression
Cave Collapse / Sinkholes • Yucatan Peninsula - The Cenotes of Chichén Itzá – Formation of a Cenote (say-NO-tay) – Sacred Cenote • Used for ritual offerings
Application of Water to Soils of Low Water Content • Irrigation water applied to low density alluvial sediments may cause subsidence
Application of Water to Soils of Low Water Content • San Joaquin Valley (west side) – Test plots • subside 10. 5 feet in 27 months • Flat surfaces become very irregular
Application of Water to Soils of Low Water Content • San Joaquin Valley – Extensive damage to structures • • P. Gas & Electric transmission lines Gas and oil pipelines Drainage ditches Freeway requires special protection
The marks on this utility pole indicate the level of the surrounding land in preceding years. Between 1925 and 1975 this part of the San Joaquin Valley subsided almost 9 meters because of the withdrawal of groundwater and the resulting compaction of sediments. (Photo courtesy of U. S. Geological Survey)
Application of Water to Soils of Low Water Content • San Joaquin Valley – Solution: Use sprinklers to get more even irrigation
Pressure Decline in Artesian Aquifers and Oil and Gas Fields • Occurs in partially consolidated sedimentary deposits • As much as 1 foot of subsidence for each 10 - 25 feet of water level decline – -2. 8 feet in Lancaster between 1926 & 1965
Pressure Decline in Artesian Aquifers and Oil and Gas Fields • Solutions: – Reduction of pumping rate to equal recharge rate – Injection of water in oil and gas fields or even artesian aquifers
Long Beach Subsidence - A Special Case History • General Character of Subsidence – Significant subsidence began with oil field development in 1938 & 1939 • East end of Terminal Island had subsided 4 feet by 1945 – Extensive diking and reconstruction has been necessary • Cost exceeded $100, 000
Long Beach Subsidence • General Character of Subsidence – It greatly affected the L. B. port and naval shipyard • Subsidence exceeded 29 feet • Also included 10 feet of horizontal shift • Caused very extensive damage
Long Beach Subsidence • Postulated Causes – Oil reservoir compaction due to oil field pumping – Surface loading by buildings – Vibrations due to land use – Tectonic movement
Long Beach Subsidence • Mechanics of Movement – Subsidence produced compression along axis of a bowl-shaped depression • Tension occurred on the flanks – Sudden slippages on clay beds 1750' deep set off small earthquakes • Oil wells were badly damaged • 5 such earthquakes occurred
Long Beach Subsidence • Subsidence Arrest – The field was repressured with water injection • Subsidence decreased drastically – Some areas even rebounded • Bench marks are now constantly watched
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