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Geotechnical Engineering and Geological Hazard Study Lincoln Elementary School Modemization Project Manteca, California Page 13 8.2 SHALLOW r OUINDATIONS We recommend that shallow foundations (i.e., conventional spread foundations) supported on engineered fill should be designed to support dead 'loads plus normal duration live loads using allowable bearing capacity of 3,000 pounds per square foot (psf) for footings with a minimum embedment depth of 18 inches below compacted soil subgrade for the exterior subgrade, or top of soil subgrade for interior subgrade, whichever is lower. Landscape fills cannot be included toward embedment depth. The allowable bearing capacities may be increased by one-third (1,'3) when considering short -tens wind and seismic loads. Static differential settlement is predicted to be less than Il4 inches over 20 feet. 8.3 LATERAL RESISTANCE Resistance to lateral loads (including those due to wind or seismic forces) may be determined using the friction between the bottom of concrete foundations and the underlying soil and the passive soil pressure acting against the vertical face of the footings. These two modes of resistance can be combined. Sliding resistance to lateral forces may be calculated using a coefficient of friction of 0.30. The passive pressures available in engineered fill and undisturbed native soil may be taken as equivalent to pressures exerted by fluids weighing 350 pounds per cubic foot (pcf), assuming that the ground adjacent the foundation is level. These allowable values include a reduction factor of 1.5 to limit the foundation movement required to mobilize the ultimate passive resistance. Passive resistance contributed by soils within 1 foot of the ground surface should be neglected unless the ground is covered and confined by a slab -on -grade or pavement. To mobilize passive pressure, gaps between the footing and adjacent ground should be completely backfilled using engineered fill, concrete, or lean cement sand slurry with a 28 -day unconfined compressive strength of at least 500 psi. 8.4 CONSTRUCTION CONSIDERATIONS Where footing excavations loosen or disturb soils that have been compacted when graded per Section 7.2, excavations should be tamped using a gas powered wacker or similar equipment. We recommend that a representative of the Geotechnical_ Engineer of Record or Project Inspector observe all foundation excavations prior to the placing of reinforcing steel. This inspection should be conducted to ensure that the bottoms and sides of all foundation excavations are level or suitably benched and are free of loose or soft soil, ponded water, and debris. If any loose pockets are encountered in the bottotn of the foundation excavations, they should be over -excavated, and the base of the excavation should be recompacted or backfilled with lean concrete. It is important that foundation excavations be clean and free of loose or soft soils, water, or other debris at the time concrete is placed. 9.0 SLABS -ON -GRADE 9.1 INTERIOR CONCRETE SLABS Concrete floor slabs should be supported on at least 24 inches of engineered fill. The zone of engineered fill should extend at least 5 feet outside the perimeter of the building. Concrete slabs should be constructed on a surface prepared as described in Section 7.2. Where dampness of floor slabs is to be minimized, the slabs should be constructed on a minimum 4 -inch -thick layer of capillary break material covered with a high quality vapor retarder. The capillary break material should be free -draining, clean gravel or rock such as leo. 4 by 3/ -inch pea gravel or permeable aggregate complying with Caltrans Standard Specifications, Section 68, Class 1, Type B. A 2 -inch -thick protective cover (blotter) of clean sand should be placed over the vapor retarder. The vapor retarder should have a M