Question 11.6: The subsoil conditions at a site are as shown in Fig. 8.13. ...
The subsoil conditions at a site are as shown in Fig. 8.13. Predrilled cast-in-place concrete piles that have a diameter of 1 ft will be installed to a depth of 50 ft. For pile adhesion and end bearing, use an average undrained shear strength s _{u} = 0.6 kg/cm² (1200 psf). Ignoring the soil adhesion in the upper 10 ft of the pile, calculate the allowable load capacity Q_{\text {all }} of the pile using a total stress analysis and a factor of safety of 3. If the site will be subjected to significant earthquake shaking, is a one-third increase in Q all acceptable for seismic loading conditions?
Based on Fig. 11.15, use c_{A} / c=0.67 (average curve for concrete piles) and therefore C_{A} = (0.67)(1200) = 800 psf.
Using Eq. (11.12):
Q_{ ult } = end bearing + side adhesion
=9 \pi c R^{2}+2 \pi c_{A} R z= 9 π (1200 psf)(1.0/2)² + 2 π (800 psf)(1.0/2)(40)
= 8000 + 100,000 = 108,000 lb = 108 kips
Q_{\text {all }}=Q_{\text {ult }} / F=108 / 3=36 kips