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 feet/yr
Santa Monica Mountains feet/yr
San Gabriel Mountains 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

7. 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

8.
Subsidence depressions filled with water in central Polk County, Florida.

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

11. 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

12. 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

13. 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
Stages in the Formation of a Cenote
SOLUTION CAVERN - Naturally acidic groundwater seeping through cracks in the limestone bedrock dissolves areas of softer rock lying beneath the hard surface crust. Over time, this process creates large undergound caverns roofed with only a thin layer of surface limestone.
YOUNG CENOTE As erosion continues, this thin roof eventually collapses, leaving an open, water-filled hole.
MATURE CENOTE Over thousands of years, erosion gradually fills the cenote with organic and mineral debris, reducing its depth. The Cenote of Sacrifice is currently in this stage.
DRY CENOTE As erosion continues, the cenote may completly fill, becoming a dry, shallow basin supporting trees and other vegetation.

15. 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

16. 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

17. 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)

18. 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 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

19. 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

20. 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

21. 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

22. 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