3A Public Hearing 2006 1204 - Laserfiche WebLink

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GEOTECNIA Project Number: 060805 2888 Darius Way, San Leandro October 27, 2006 Page 2 include excavations into the cut slope and other retaining walls as needed to provide the required grade separations. No other project details are known at this time. FINDINGS Site Description The subject property is a hillside lot located east of the Darius Way cul-de-sac in San Leandro, California, at the approximate location shown on Plates 1 and 2. Existing improvements at the site include an unpaved access road, two concrete-lined V -ditches; and drain inlets conveying runoff from the upper V -ditch and access road to the lower V -ditch through an underground pipe (see Plate 2). The cut slope at the site slopes down to the south at an inclination of about 2: 1 (H: V). The western half of the access road slopes down to the west at an inclination of about 6.3:1 to 7.1:1, and the eastern half of the access road includes a large, relatively level area between approximately elevations 285 and 290 feet. The fill slope along the southern portion of the site is also inclined at 2: 1 like the cut slope. Geoloe:ic Conditions The site is within the Coast Ranges Geomorphic Province, which includes the San Francisco Bay and the northwest-trending mountains that parallel the coast of California. These features were formed by tectonic forces resulting in extensive folding and faulting of the area. The oldest rocks in the area include sedimentary, volcanic, and metamorphic rocks of the Franciscan Complex, and sandstone, shale, and conglomerate of the Great Valley Sequence. These units are Jurassic to Cretaceous in age and form the basement rocks in the region. The site geology was previously evaluated by Terrasearch, Inc. (1989). The majority of the cut slope at the site is underlain by gabbro bedrock, while the access road is underlain by clay fill and native clay soils overlying the gabbro bedrock. The nearest active fault is the Type-A Hayward Fault, located approximately 1f4 mile (~kilometer) east ofthe site. Earth Materials The eight borings drilled for this study encountered gabbro bedrock on the cut slope, and fat clay fill soils overlying both native clay soils and the gabbro bedrock on the access road. The subsurface conditions are summarized below stratigraphically starting with the fill and native soil layers. Detailed descriptions of the materials encountered as well as the field and laboratory test results are shown on Plates 3-13. The majority of the soils in the upper portions of Borings B-1, B-3, B-4, B-6, and B-8 consisted of stiff to hard fat clay, sandy fat clay, and lean clay (both fill and native), although the upper 18 inches of Boring B-1 consisted of medium dense clayey sand. Samples of the clay soils tested had plasticity indices (PIs) ranging from 26 to 38, pocket penetrometer shear strengths ranging