319 76TH PL SW 2016-01-01 MF Import

SERS Public Safety 9-1-1 Communication Tower 31 Report No. 704-05224-1 December 26, 2000 Page 8 5.3 Tower Foundation Support We understand that both a structural mat and drilled pier are being considered as foundation alternatives. Based on the soil conditions encountered, either alternative would be appropriate from a geotechnical perspective. The final decision on which alternative to select should be based on construction cost, availability of local contractors, difficulty to drill in the very dense silty sand, drilled pier casing requirements, and structural mat footprint size requirements versus available land. 5.3.1 Structural Mat Alternative Based on the results of our geotechnical investigation, it is our opinion that the proposed communication tower can be supported on a mat foundation designed for a maximum allowable soil bearing pressure of up to 4,000 psf, when founded on the undisturbed, very dense silty sand with gravel stratum, or on an engineered, granular structural fill placed overlying the very dense, native silty sand stratum. The allowable bearing pressure includes a safety factor of 3 and is intended for dead loads and sustained live loads and can be increased by one-third for the total of all loads, including short-term wind or seismic loads. The mat foundation should extend to a minimum depth of 18 inches beneath the lowest adjacent exterior grade to provide frost protection. The foundation should be constructed at least 10 feet laterally from the top of any slopes, which should be permanently graded no steeper than 2 horizontal: 1 vertical. When sizing the mat, the structural engineer should evaluate the structure for overturning. Allowable lateral frictional resistance between the base of footings and the subgrade can be expressed as the applied vertical load multiplied by a coefficient of fnction of 0.35 for undisturbed, very dense silty sand with grave:soil. In addition, lateral loads may be resisted by an passive earth pressure based on an equivalent fluid density of 300 pounds per cubic foot (pcf) on footings poured "neat' against in-situ soils or properly backfilled with granular structural fill, respectively. These recommended values include a factor of safety of approximately 1.5, which is appropriate due to the amount of movement required to develop full passive resistance. We estimate that foundations designed and constructed in accordance with the above recommendations will experience total settlemer ?s generally less than 1-inch with differential settlements between columns generally less than ;:-inch. If footings are constructed during wet weather, It may be necessary to protect the foundation excavation bottoms from disturbance during construction activities In this regard, we recommend that a 3 to 4-inch thickness of crushed rock be placed at the bottom of the footing excavations immediately after the excav ation is completed. If footings are constructed during the drier summer months, this crushed rock layer should not be required ll