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

SERS Public Safet Q-1-1 Communication Tower SI Report No 704-05224-1 December 26, 2000 Page 9 5.3.2 Drilled Pier Alternative The tower structure may be supported on straight-shaft, cast-in-place, reinforced concrete drilled piers extending into the very dense silly sand with gravel stratum. The piers should extend at least 15 feet into the very dense stratum and should have a minimum diameter of 24 inches to facilitate observation and cleaning of the excavation. Drilled piers should be spaced a minimum of four pier diameters apart (measured center to center) in order to be considered as acting independently and not as a group. The piers may be designed using a maximum net allowable end bearing pressure of 28,000 pounds per square foot (psf) for Vie first 15 feet of embedment in the very dense stratum, and may be increased 1 900 psf for each additional foot of embedment. An allowable skin friction value of 157 psf may be used for the first 15 feet of embedment in the very dense stratum, and may be increased 10 psf for each additional foot of embedment. The increases in end bearing pressure and skin friction for each additional foot of embedment beyond 15 feet are limited to a total pile length of 20B, where B equals the pile diameter. Uplift skin friction values equal to two-thirds of the allowable skin friction values may be used for uplift design capacity. The allowable end bearing and skin friction values incorporate factors of safety of 3 and 2, respectively. It should be noted that the above bearing capacity and skin friction values are somewhat conservative (due to the lack of subsurface information within the footprint of the proposed tower) and should be re-evaluated once an additional boring is performed Due to the presence of free groundwater at the site and cohesionless overburden soils, it may be necessary to case the drilled pier excavation, depending upon the pier design depth. Because we were not able to drill within the footprint of the proposed construction and the geology appears that it may vary from the soil conditions in the single boring we performed, it is difficult to accurately predict the need for casing. Any water and loose materials which accumulate at the bottom of the pier excavation should be removed prior to concrete placement. If water is present in the bottom of the excavation, a pump should be used to remove the water. If it is not feasible to remove the water in its entirety (i.e. the pump cannot keep up with the water seepage) then the concrete should be tremied or pumped. The tremie, or pump, should be placed at the bottom of the excavation so that the concrete pushes the free water up and out of the excavation. The vibrator should not be placed in the fresh concrete until all of the free water is pushed out of the hole The intent of this placement method is to prevent the free water from mixing with the fresh concrete, which would potentially reduce the concrete strength. The pier excavation should be filled with concrete immediately following approval by the geotechnical engineer or his representative Excavations should in no case be allowed to stand (L