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|Title:||Geophysical investigation, Prado Dam and Mentone Damsite, California|
|Authors:||United States. Army. Corps of Engineers. Los Angeles District.|
Curro, Joseph R.
Skoglund, George R.
Ballard, Robert F.
|Publisher:||Soils and Pavements Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: A geophysical site investigation consisting of surface-refraction seismic and crosshole tests was conducted at the Prado Dam and the Mentone Damsite near Los Angeles, California, and a surface-refraction seismic line was run in an effort to confirm the location of the Chino Fault. Eight locations at Prado Dam and three locations at Mentone Damsite were investigated by geophysical methods. P- and S-wave velocities were determined as a function of depth below the ground surface. At Prado Dam, P-wave velocities obtained from surface-refraction seismic and crosshole tests were collapsed into 9 velocity zones. These velocities ranged from a low of 850 fps to approximately 5650 fps. A low-velocity zone approximately 20 ft thick was found to exist in the foundation materials underlying Prado Dam. The S-wave velocities at Prado Dam were contoured into 11 velocity zones. These velocities varied from 350 to 950 fps. Again, a low-velocity zone was found to exist in the same area as that determined by P-wave velocities in the foundation materials underlying Prado Dam. Data obtained from the 1600-ft long refraction seismic line run at the probable location of the Chino Fault in the vicinity of Prado Dam showed the existence of three separate velocity zones. From this data, existence of the Chino Fault could not be confirmed in this location, which was approximately a mile east of the dam. At Mentone Damsite, seismic tests were performed at three sites. At site G-1:, P-wave velocities suggested the existence of three separate zones. Velocities ranged from slightly less than 1600 fps to about 7200 fps at a depth of 146 ft. The S-wave data showed a general increase with depth, varying from about 700 to 1600 fps. At site G-2, the refraction seismic test indicated the presence of four velocity zones. Near-surface materials exhibited velocities of about 2000 fps, increasing to a high of 9250 fps at a depth of 241 ft. P-wave crosshole data ranged from a low of 963 fps to a high of 1918 fps at a depth of 80 ft. S-wave velocities varied from 500 to approximately 1000 fps. At G-3, the only test conducted was a surface-refraction seismic line. Results showed the presence of three P-wave velocity zones. Near-surface materials showed a velocity of approximately 2000 fps, increasing to approximately 7600 fps at a depth of 173 ft.
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