• Timber: Economical, susceptible to decay.
• Concrete: Precast or Cast-in-place.
• Steel: H-piles or Pipe piles.

• End-bearing piles: Transfer load primarily to a hard stratum (rock/dense sand) at the tip.
• Friction piles: Transfer load primarily through skin friction along the shaft.

$Q_p = A_p q' N_q^*$ $Q_s = \sum p \Delta L f_s$

• $q'$: Effective vertical stress at tip.
• $f_s = K \sigma'_v \tan \delta$: Unit friction resistance.

For skin friction in clay: $f_s = \alpha c_u$ $Q_s = \sum p \Delta L (\alpha c_u)$

• $\alpha$: Adhesion factor (empirical).