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LAND SUBSIDENCE

LAND SUBSIDENCE. Introduction. Subsidence is widespread in California 30 feet of subsidence has occurred in the San Joaquin Valley in 30 years 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

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LAND SUBSIDENCE

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  1. LAND SUBSIDENCE

  2. Introduction • Subsidence is widespread in California • 30 feet of subsidence has occurred in the San Joaquin Valley in 30 years • 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

  3. Causes And Occurrences Of Land Subsidence • Tectonic Activity • Easily recognized in coastal areas - gradual inundation • Vertical offset in Owens Valley 1872 Earthquake was 23 feet • Tectonic subsidence is generally slow compared to other causes

  4. Causes And Occurrences Of Land Subsidence • 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

  5. Causes And Occurrences Of Land Subsidence • 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

  6. Causes And Occurrences Of Land Subsidence • 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 CaCO3 or CaMg(CO3)2.both of which are easily dissolved in water

  7. Subsidence depressions filled with water in central Polk County, Florida.

  8. Causes And Occurrences Of Land Subsidence • Winter Park, Florida 1981: • Cause: karst topography • 100 m depression 13 m deep • $2 million in damage.

  9. Causes And Occurrences Of Land Subsidence • 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 km2 • 2-7 m in diameter, 8 m deep. • No new sinkholes after pumping stopped

  10. Causes And Occurrences Of Land Subsidence • 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

  11. Causes And Occurrences Of Land Subsidence • Yucatan Peninsula - The Cenotes of Chichén Itzá • Stages in the Formation of a Cenote (say-NO-tay) • Sacred Cenote • Used for ritual offerings

  12. Causes And Occurrences Of Land Subsidence • Application of Water to Soils of Low Water Content • Irrigation water applied to low density alluvial sediments may cause subsidence

  13. Causes And Occurrences Of Land Subsidence • This is most spectacular on the west side of the San Joaquin Valley • Test plots subside 10.5 feet in 27 months • Flat surfaces become very irregular • Extensive damage to structures occurs • P. Gas & Electric transmission lines • Gas and oil pipelines • Drainage ditches • Freeway requires special protection • Solution: Use sprinklers to get more even irrigation

  14. 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)

  15. Causes And Occurrences Of Land Subsidence • 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 • Solutions: • Reduction of pumping rate to equal recharge rate • Injection of water in oil and gas fields or even artesian aquifers

  16. Long Beach Subsidence - A Special Case History • 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 • 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 $100,000,000

  17. Long Beach Subsidence - A Special Case History • Postulated Causes • Oil reservoir compaction due to oil field pumping • Surface loading by buildings • Vibrations due to land use • Tectonic movement

  18. Long Beach Subsidence - A Special Case History • 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

  19. Long Beach Subsidence - A Special Case History • 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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