3A Public Hearing 2006 1204 - Laserfiche WebLink

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GEOTECNIA Project Number: 060805 2888 Darius Way, San Leandro October 27,2006 Page 10 If groundwater is encountered during pier shaft drilling, it should be removed by pumping, or the concrete must be placed by the tremie method. The tremie pipe should be extended to the bottom of the pier hole and kept below the top of the concrete in the hole as the hole is filled with concrete in order for the concrete to displace the water upward. If the pier holes are dry, the concrete should not be dropped more than 5 feet vertically to avoid segregation of the cement mix and the aggregate, which would weaken the concrete. Mushrooming of the concrete at the tops of the piers should be prevented. Shallow Footings The portions of the structures (including retaining walls) in level areas underlain by bedrock north of the access road may be supported on shallow spread and continuous footing foundations, provided the footings are at least 10 feet from any downslope. The spread footings should be at least 15 inches wide and extend at least 18 inches below the lowest adjacent finished grade or 12 inches into the bedrock, whichever is deeper. The geotechnical engineer should check the footing excavations, prior to placing any steel reinforcement, to evaluate the appropriate depth for. the actual earth materials encountered. Spread footings should be designed for maximum allowable soil contact pressures of 3,000 pounds per square foot (pst) for dead loads, 3,600 psffor dead plus sustained live loads, and 4,500 psffor total loads, including wind and seismic forces. We anticipate that footings designed and constructed in accordance with our recommendations will experience total settlements less than % inch and differential settlements less than Yz inch over a 25-foot span. Footing excavations without vertical sides, as well as the above-grade portions of the footings, should be formed to prevent overpours and mushrooming of concrete. The finished footings should have vertical sides. Resistance to lateral loads can be obtained using an allowable passive pressure equivalent to that provided by a fluid weighing 350 pounds per cubic foot (pct), and a base friction coefficient of 0.35 multiplied by the net vertical dead load. These values include a safety factor of 1.5, and may be used in combination without reduction. Passive pressure for any keys extending below the bottoms of footings may be calculated using a uniform passive pressure of 1,500 psf, provided they are excavated neatly into undisturbed bedrock. Retainine Walls Static Loads The lateral earth pressure criteria below assume that the backfill materials within the "active zone" are - ilnport. non-expansive granular fill material (select backfill) instead of the in-situ clays. The active zone includes the entire volume above an imaginary plane inclined at 60 degrees above horizontal from a point one foot behind the bottom of the back of the heel of the