Deep Foundation Axial Capacity (Driven Piles & Drilled Shafts)

Method summary (drilled shaft)

Clay (α method): \( f_s = \alpha\, s_u \); \( \alpha = 0.55 \) for \( s_u/p_a \le 1.5 \), \( \alpha = 0.55 - 0.1\,(s_u/p_a - 1.5) \) for \( 1.5 < s_u/p_a \le 2.5 \).
Sand (β method): \( f_s = \beta\, \sigma'_v \le 200 \) kPa, with \( \beta = 1.5 - 0.245\sqrt{z} \), bounded to \( 0.25 \le \beta \le 1.20 \).
Cohesive IGM: \( f_s = 0.65\, p_a \sqrt{q_u/p_a} \).
Rock socket: \( f_s = C\,\alpha_E\,p_a\sqrt{q_u/p_a} \), \( C \approx 0.65 \) (up to about 1.0 for rough sockets); set C = 0 to neglect socket friction.
Rock base: GEC-10 \( q_b = 2.5\,q_u \), or CFEM \( q_b = 3\,k_s\,d_f\,q_u \) with \( k_s = \dfrac{3 + s/D}{10\sqrt{1 + 300\,\delta/s}} \), \( d_f = 1 + 0.4\,L_s/D \le 3.4 \).
Base: clay \( q_b = \psi N_c s_u \) with \( N_c = \min(9,\; 6[1 + 0.2 L/D_b]) \); sand \( q_b = N_q \sigma'_{v,tip} \le 4800 \) kPa; IGM / rock \( q_b = 2.5\, q_u \).
\( Q_u = Q_s + Q_b \), \( Q_{all} = Q_u / FS \), with \( p_a = 101.325 \) kPa.