In the west-central San Joaquin Valley, California, pumping of ground water has changed water levels, thereby increasing the stresses that tend to compact alluvium by as much as 50 percent and creating a large area of intense land subsidence.
The estimated 1943–1960 specific unit compaction (compaction during a time period, per unit thickness, per unit applied-stress increase) of the deposits in a northern subarea is four times that of a southern subarea, which suggests marked differences in the compressibility of the deposits. A third of the compressibility difference is real and is due to less prior total applied stress in the northern than in the southern subarea.
The other two-thirds of the compressibility difference is only apparent and is attributed to different water-expulsion rates in the clayey beds of different depositional environments. In the northern subarea, the deposits are mainly flood-plain sediments that contain extensive sand beds and thin clayey beds that are dewatered relatively quickly under increasing effective stress. In the southern subarea, the sediments are mainly alluvial-fan deposits that contain thick clayey sequences adjacent to lensing sandy beds. These deposits are de-watered more slowly than those in the northern subarea.
Variations of mean lithology in the chief compacting zone could not be correlated with variations in the specific unit compaction of the deposits.