O'Neill Dam and Reservoir are located on the west side of the San Joaquin valley, California, and on the east flank of the Diablo Range, one of the numerous northwest-trending California Coast Ranges. Bedrock underlying the dam and reservoir is composed of the sedimentary marine Cretaceous Panoche formation having a fairly uniform northwest-strike and northeast dip. At the damsite, formation rock is directly encountered only in the isolated hill forming the left abutment; elsewhere it is covered by Pleistocene to Recent unconsolidated to poorly consolidated sediments. In the left abutment, the bedrock typically consists of beds, from one to 60 feet thick, of poorly consolidated clayey and silty shales and fine-grained sandstone having an average bedding-plane dip of 45 degrees northeast.
Except for the left abutment shales and sandstones, the dam embankment lies on Pleistocene and Recent soils. Pleistocene alluvial and lacustrine-related deposits directly underlie the embankment from approximately dam stations 20+00 to 65+00 and adjacent to the left abutment where they form a narrow aprons against the bedrock. Recent alluvial deposits form the embankment foundation from station 65+00 to the Pleistocene deposits adjacent to the left abutment.
The Pleistocene deposits are mostly silts, clays and clayey sands from stations 20+00 to 65+00. Against the left abutment, where they appear to be a strandline or beach of Pleistocene "Lake Corcoran", the deposits are flat-lying, clean to clayey sands with minor interbedded clays. There is also a deposit of talus-breccia, made up of slumped fragments of sand and shale in a sand matrix. This talus-breccia possibly indicates deposition at the base of a wave-cut escarpment, such as might occur if there was wave erosion of a segment of shoreline of Lake Corcoran.
The Recent alluvial deposits from station 65+00 to the left abutment are capped by a layer of lean clay except in the area of the former San Luis Creek where clean to silty and clayey sands were at the surface. The Recent alluvial deposits contain numerous and variable beds and lenses of sands, silts, clays and gravels. The capping layer of lean clay varies from 5 to 10 feet thick and is mostly a flood plain deposit. This layer is critical to the reservoir water-tightness, and a 5-foot thick Zone 3 blanket was placed on top of the forebay foundation where the capping layer was naturally missing, such as along San Luis Creek channel, or removed, such as in borrow areas.
Early investigations for a site for the pumping plant had located a gypsum-free foundation with a relatively high percentage of sandstone beds. This site remained the foundation of preference although later investigations determined that a large bedding plane fault was present directly under the pumping plant. This bedding plane fault was determined to be continuous from one edge of the extensive, combined pumping plant and spillway excavation to the other. There was no obvious evidence of, or further investigation into, the nature of its potential activity at the time of original construction. A section that follows, Earthquake Loading, discusses post-construction activities which may relate to this fault under the pumping plant.
The high cut slope above the pumping plant has been the source of stability concerns. In August 1970, a 1/2-inch-wide crack was observed extending from the top of the cut slope to the northwest of the plant (El. 315 feet) to the first berm above the siphon crest road (EL. 269 feet). The crack, in sandstone, dips about 500 E northeast, parallel to the bedrock bedding. By October 1970, the crack had enlarged considerably, widening from 0.1 to 0.4 foot. A It had short splays in several places and extended down almost to the pumping plant access road at elevation 180 feet. Apparently, not much more cracking has occurred since 1970 and slope stability concerns have diminished.
Four inclinometers, SI-1 through -4, were installed in the cut slope northwest of the pumping plant early in 1971, presumably to monitor the crack and adjacent slope stability. However, these inclinometers were installed upstream of the northeasterly-dipping crack, thus none of them, intersected the crack plane. The cause of this crack has not been determined; rebound as a result of the large excavation, shrinkage caused by changed ground water conditions, and stress relief along the thrust fault parallel and close to the crack are some possibilities.
Ref: Above taken from August 1997 Technical Memorandum No. FT-8312-4