2400 33RD ST GEOTECHNICAL INVESTIGATION 2018-01-02 MF Import

SC 130 Milbor-Pita aim, N c feet) has a significant head and can produce "heaving" sand conditions, therefore it is not desirable to drill into i,;is formation. Difficulties associated with high groundwater should be addressed in the contract documents. • Drilled Shaft Installations: Drilled shafts typically consist of large diameter boreholes that are filled with concrete and steel reinforcing. The anticipated subsurface drilling conditions will affect the method of shaft installation used. Sand below the perched water table will tend to slough and cave if unsupported and will require casing; the native clay, till, and sand above the perched zone is not anticipated to slough as readily. If casing is used to prevent sloughing during shaft construction, the casing should be filled with water or drilling fluid at all times to prevent bottom heave and minimize disturbance of bearing soils at/near the base of the shaft. Potential construction difficulties associated with groundwater and sloughing may be avoided, as well as the higher costs, by installing shortened drilled shafts that do not extend below the perched groundwater, or by using driven piles. The contractor should take appropriate precautions to limit the amount of ground loss or disturbance to soils around drilled shaft excavations. Care should be taken to limit disturbance of the bearing soils as the shafts near the target tip elevation. Drilled shaft bottoms should be cleaned to extent practical using air -lifting or other methods. After the shaft bottoms are cleaned, concrete should be tremied in the drilled shafts. Drilled shaft capacities presented in Table I incorporate both end bearing and skin friction resistance. Skin friction resistance was estimated assuming a concrete/soil interface. Therefore, steel casing used to advance the shafts should be withdrawn during concrete placement. • Auger -Cast Piles: Auger -cast concrete piles consist of cast -in -place reinforced drilled shafts. Auger -cast piles are installed by rotating a continuous -flight hollow -stem auger to a pre -determined depth, and pumping a sand -cement grout under controlled pressure through the center of the shaft as the auger is slowly withdrawn. By maintaining pressure in the grout line and slowly extracting the auger, a continuous column of concrete is formed. A single reinforcing rod can be installed for the full pile length through the hollow stem of the auger, and/or, a reinforcing cage can be placed in the column. This construction method is feasible for installing up to 2-foot diameter piles. Because auger -cast pile installation does not require an open hole to be maintained or a casing, groundwater should not adversely affect construction. A potential impact on auger -cast pile Geotechnical Tunnel and Fnvimmmtal Fii&eers 3080 1250 Ave NE Pho (425) 969.5778 lrcllcvue, WA 98005 Page 12 - Fu (425) 861-06T7