2017-02-21_REVISION - M1988112 (3)

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2.2 Geotechnical Investigations Under a subcontract with MGC, Engineering Analytics conducted subsurface investigations in the area of the proposed new drop structure. The complete Geotechnical Investigation Report is provided as Appendix A with this Design Report. The scope of the investigation and key findings are summarized in the following sections. 2.2.1 Scope of Field Investigation Engineering Analytics conducted a field investigation between October 13 and 15, 2015 that included excavating three test pits (EA-DSTP1 through EA-DSTP3) to depths ranging from 4.5 to 8.5 feet below the ground surface (bgs) and drilling of five exploratory boreholes (EA-DSB 1 through EA-DSB5) to depths ranging from 34.7 to 44.8 feet bgs. The locations of the boreholes and test pits are shown on Figure 2.1, relative to the footprint of the proposed structure. Boring and test pit logs and photographs taken during the site investigation are presented in Appendix A. 2.2.2 Subsurface Conditions Observed Soil conditions generally consist of silty to clayey and gravelly sand alluvial deposits from the ground surface to depths ranging from 1 to 5 feet bgs. Poorly -cemented, highly weathered conglomerate was encountered below the alluvium and ranged in thickness from 1.5 to 3.5 feet. The weathered conglomerate was friable under hand pressure and easily excavated using a Caterpillar 420F backhoe loader. More competent conglomerate was encountered immediately below the weathered conglomerate. Although the conglomerate was not rippable using the Caterpillar 420F used to excavate the test pits, it was poorly cemented and could be broken by hand pressure using two hands. Figure 2.2 shows an interpretive geologic profile on the alignment of the proposed drop structure. The geologic profile depicts the current ground surface, and the interpreted interfaces between soil, weathered conglomerate and conglomerate bedrock, as encountered in the test pits and soil borings. The following summarizes the general site stratigraphy as interpreted by the subsurface exploration. All depths are relative to the ground surface at the time of the investigation. SAND FILL: Sand fill with varying amounts of silt, clay, and gravel was present from the ground surface to between 2 and 3 feet bgs at two boring locations. The fill material consisted of reworked and relocated native materials used to construct drill pads at EA- DS133 and EA-DSB5. • CLAYEY TO SILTY SAND, ALLUVIUM: Native silty to clayey sand, with gravel was generally encountered from at or near the ground surface to depths ranging from 4 feet to 9 feet bgs at each location except EA-DSB3 and EA-DSB5. The alluvium was brown to dark yellowish brown, dry to slightly moist, and medium dense to very dense. WEATHERED CONGLOMERATE: Weathered conglomerate was encountered below the alluvium at each soil boring location and was observed up to 14 feet bgs in the soil borings. The weathered conglomerate was poorly cemented and easily friable under ---------------------------- -- ------------------------ San Luis Project - South Diversion Ditch Drop Structure - Final Design Report