Deep Pile Axial Capacity in Clay (Alpha Method)

in clay the alpha method takes Qult = alpha cu (pi D L) skin friction plus 9 cu (pi D^2/4) end bearing. A 16 in pile 40 ft into cu = 1 ksf clay carries 105 kip ultimate (88% in skin friction) and 35 kip allowable at FS 3 - and doubling the length nearly doubles it while the tip term stays put, which is why a friction pile is lengthened, not fattened. Single pile, uniform clay, total-stress method. A design aid; the geotechnical engineer and a load test govern.

As = pi D L; Ap = pi D^2/4; Qs = alpha cu As; Qp = 9 cu Ap; Qult = Qs + Qp; Qall = Qult / FS.

The alpha (total-stress) method for the static axial capacity of a pile in clay -- skin friction alpha cu over the shaft plus end bearing Nc cu (Nc = 9) at the tip -- with the adhesion factor alpha (~1.0 soft to ~0.5 stiff) as compiled in the FHWA driven-pile manual and Das (Principles of Foundation Engineering), and the customary factor of safety of 2 to 3 without a load test, by name.

The FHWA driven-pile design manuals are free at fhwa.dot.gov; the alpha-method relations are public in the standard foundation texts.

Estimate. AHJ and licensed professional govern.

Field names used by the API: d_ft, l_ft, cu_ksf, alpha, fs, qs_kip, qp_kip, qult_kip, qall_kip

• Skin friction fs = alpha cu over the shaft surface pi D L; alpha from clay stiffness (~0.55 medium stiff)FHWA / Das alpha method
• End bearing qp = 9 cu over the tip area (Nc = 9 for a deep foundation in clay)Skempton / FHWA
• Safety factor the customary 2 to 3 on ultimate without a static load testFHWA driven-pile practice