AG 1997 09 15 - Laserfiche WebLink

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Proposed Radio Tower Page 4 Cabarrus County Sheriffs Dept. Soil Load Capacity_ Evaluation and Recommendations Drilled Shqft or Pile Foundations Based upon the data from the borings and my past experience with similar soils, the residual soils in the vicinity of the bor'mgs should provide adequate bearing and uplift support for a system of drilled shaft (drilled piers) or driven pile foundations for the proposed tower. Drilled shaft foundations founded at a depth of 31 ft may be designed for a maximum allowable total end-bearing pressure of 25 ksf. For uplift resistance, a horizontal modulus of subgrade reaction (Kb) of 20z/D can be used for the upper 15 ft and 40z/D can be used below 15 ft, where z is the depth below grade and D is the pier diameter. The units of K~ are tons per cubic foot. The following values can be assumed: qb (phi) angle of 32°, Rankine passive earth pressure coefficient (Kp) of 3.3, Rankine at-rest lateral earth pressure coefficient (Ko) of 0.31. Assume a soil unit weight of about 120 pcf (lb/ft3)/ The shaft bottoms should be approved by the geotechnical engineer after cleaning, and before placing concrete. The engineer should also be retained to observe placement of the rebar cage and concrete. This is because improper placement of concrete in the rebar cage can cause segregation of the concrete which could potentially compromise the structural integrity of the pier. The borings encountered groundwater within the expected construction depths. During the wetter seasons of the year (the late winter and spring), the subsurface water levels may be higher than those at the time of this exploration. Groundwater will likely occur within the depths affected by construction. The fact that all the borings caved below the water table verifies that the soil in that portion of the boring is predominately sand, and indicates that there will potentially be some minor difficulty installing the piers. The shaft construction will require casing seated into the partially weathered rock which was encountered at roughly 25 to 30 ft. Water will likely infiltrate the excavation and will need to be continuously pumped prior to placing concrete. If the excavation is not seated into the auger refusal material, the excavation .bottom could potentially be unstable. It will likely be very difficult, if not impossible, to construct . 'abell in the event that the contractor elects to use a relatively small diameter shaft with a belled Based upon the data from the borings and my past experience with similar soils, the residual soils in the vicinity of the borings should provide adequate support for a system of shallow foundations for the proposed structure, subject to the criteria and site preparation recommendations that follow. File: 97-026R.sam