GEOG M109 Lecture Notes - Lecture 6: Pore Water Pressure, Compressive Stress, San Joaquin Valley
Document Summary
Background: land subsidence is the surface sign, and the last step, of a variety of subsurface displacement processes. Common natural causes: tectonic movement, compaction, drying-up of lacustrine basins, oxidation of highly organic soils, karst cavern collapse, thermokarst depression. The most common man-made subsidence can be caused by: Los angeles has a very hard, rocky underground cannot sink. The mechanisms of subsidence after the transfer of subterranean fluid: Land subsidence will occur only under certain geological conditions, where the deposits involved are mostly composed of unconsolidated sediments of high initial porosity. Almost all the subsiding areas are characterized by underlying semi-confined/confined aquifers made up of sand/gravel of high permeability and low compressibility, inter-bedded with layers of clay/silt of low vertical permeability and high compressibility (aquitards). After pumping begins, the hydrostatic pressure (p) of the water percolating is reduced. This effective stress is the forces, which the grains exchange among themselves on contact.
