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The phenomenon of subsidence has plagued the mining industry for centuries. Although subsidence is directly caused by withdrawal of ore from underground workings, the particular combination of rock type, geologic structure, and method of mining at the site is responsible for the physical character of subsidence appearing at the surface.

Continued mining of porphyry copper ores at the San Manuel Mine for ten years has resulted in the formation of three large subsidence pits in a wedge-shaped caprock of Tertiary conglomerates. The pits have a total surface area exceeding 160 acres. Peripheral growth of the pits is controlled by the geologic structure and engineering properties of the caprock and of the ore body, as well as by the relative location of caved blocks and rate of withdrawal of ore.

Specific items of interest at the site are numerous subparallel faults, a definite pattern of advancing ground fractures, and well-defined units of mass wastage within the pits. The study has shown that there are definite relationships between geologic structure in the area and the growth and configuration of the subsidence pits.

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