Near-surface land subsidence in western San Joaquin Valley, California

Abstract

Numerous evidences of land subsidence have been observed on unusual soils in the San Joaquin Valley immediately after the application of water. This subsidence results in extensive settling and cracking of soil along ditches, and in irregular, undulating topography in irrigated areas. Experiments on quarter-acre test plots are being made to determine the nature and magnitude of near-surface subsidence on different types of alluvial deposits. After 27 months of observation of test plot B, to December 29, 1958, surface bench marks had settled an average of 10.50 feet; a bench mark anchored at a depth of 25 feet had settled 9.44 feet; a bench mark at 50 feet, 8.13 feet; a bench mark at 75 feet, 6.13 feet; a bench mark at 100 feet, 3.93 feet; and a bench mark at 150 feet, 0.39 foot. Infiltration at a continuing rate of 0.20 foot of water per day for a 15-month period of inundation was more than sufficient to fill the pore space beneath the test plot to the wetted front, and considerable lateral spreading of the water occurred. Core samples collected before and during the spreading operation show the increase in density and moisture content of the subsurface deposits. The low field density and extreme dryness of the alluvial deposits apparently are two important factors in land subsidence due to application of water.