Geology topics
William B. Bull
Publications and source records attributed to William B. Bull.
Geologic factors affecting compaction of deposits in a land-subsidence area
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.
Appraisal of near-surface subsidence on the Panoche Creek fan, Fresno County, California
Near-surface subsidence results chiefly from the compaction of deposits by an overburden load as the clay bond supporting the deposits is weakened by water percolating through the deposits for the first time since burial. About 100 square miles of alluvial-fan deposits in western Fresno County, California, have been affected by manmade near-surface subsidence within the past few decades. Subsidence of 3-5 feet is common and 10-15 feet of subsidence has occurred within small areas. Oren cracks form between areas undergoing different amounts of settlement, and are a characteristic feature of compaction due to wetting. Differential settlement occurring within short distances has destroyed or damaged ditches, canals, roads, pipelines, electric-transmission towers, and buildings, and has made the irrigation of crops difficult. The areas of known near-surface subsidence along the alinement of the San Luis Canal-California Aqueduct were compacted by ponding before construction of the canal. The 11 miles of canal alinement that cross the fan of Panoche Creek were not ponded before construction. ,Although a few reports of local residents to the California Department of Water Resources indicated that some very small areas of settlement had occurred after irrigation, it was generally considered that the fan would not be susceptible to widespread near-surface subsidence. The irrigated fields did not show the pronounced hummocky relief and subsidence hollows that were typical of the areas of known near-surface subsidence. Nit as strong was the evidence from core holes that showed that the deposits, even in areas that had never been irrigated, were roughly at field capacity instead of being moisture deficient. This seemed logical because Panoche Creek had been observed to flow for 1-5 months during years of heavy rainfall, in contrast to the ephemeral flow of streams on the Moreno Gulch fan to the northwest and on the Tumey Gulch fan to the south. It was assumed that if similar flows had occurred in the past, the deposits would have been wetted thoroughly from the land surface to the water table with each major period of flow and deposition.
Land subsidence due to artesian-head decline, 1943-59, Los Banos-Kettleman City area, California
Explore the source record for details and available documents.