skin friction layer by layer, end bearing, safe load, group efficiency and structural check.

Tools · Foundations

Pile foundation:Guided step by step as per IS 2911 (Part 1/Sec 2): effective overburden, skin friction in each layer, end bearing, safe load, pile group efficiency and the structural check of the shaft.

Guided · step 1 of 8

Pile, soil profile and the method

A bored cast-in-situ pile carries load by friction along its shaft and by bearing at its tip. IS 2911 (Part 1/Sec 2) Annex B gives a static formula: add the shaft resistance of each layer to the end bearing, then divide by a factor of safety of 2.5. Use effective unit weights (submerged below the water table).

Pile 600 mm diameter, 15 m long

Layer 1: 6 m clay, γ_′ = 8 kN/m³, c = 30 kN/m²

Layer 2: 12 m sand, γ_′ = 9 kN/m³, φ = 32°

Pile

Diameter D (m)Length below cut-off L (m)Earth pressure K (sand)1 to 1.5 for bored pilesFactor of safetyNq overrideBlank = formula

Soil layers (top down)

Type	Thick (m)	γ′	c	φ°

Group and structure

Rows mPiles per row nSpacing s (m)Blank = 3DColumn load (kN)ConcreteSteelLongitudinal steel (%)IS 2911 minimum 0.4

Soil profile and load transfer

Shaft Qs

1123kN

End Qb

550kN

Safe load

669kN

Qu 1673 kN

Group

4377kN

η = 0.727

Pile group plan

Full solution, all steps at once

Pile, soil profile and the method
Pile 600 mm diameter, 15 m long
Layer 1: 6 m clay, γ_′ = 8 kN/m³, c = 30 kN/m²
Layer 2: 12 m sand, γ_′ = 9 kN/m³, φ = 32°
Effective overburden pressureσ_′_v = Σγ_′·zz_c = 15D to 20D
σ_′_v at the tip (15 m) = 129 kN/m²
Critical depth = 16 × 0.6 = 9.6 m → P_D used = 80.4 kN/m²
Shaft (skin) resistanceQ_s = α·c·πD·L_i (clay)Q_s = K·P̄_D·tan δ·πD·L_i (sand)
Layer 1 (0 to 6 m): 1 × 30 × π × 0.6 × 6 = 339.3 kN
Layer 2 (6 to 15 m): 1 × 73.9 × tan 32° × π × 0.6 × 9 = 783.6 kN
Q_s = 1122.9 kN
End bearing (tip in sand)Q_b = A_p·(½Dγ_′N_γ + P_DN_q)N_q = e^{π tan φ}·tan²(45° + φ2)
N_q = 23.18 (formula 23.18), N_γ = 30.21
Q_b = 0.2827 × (0.5 × 0.6 × 9 × 30.21 + 80.4 × 23.18) = 549.9 kN
Ultimate and safe loadQ_u = Q_s + Q_bQ_safe = Q_u2.5
Q_u = 1122.9 + 549.9 = 1672.8 kN
Q_safe = 1672.82.5 = 669.1 kN (68.2 t)
Confirm by an initial load test (IS 2911 Part 4) before working piles are cast.
Group of piles: efficiencyθ = tan⁻¹(Ds)η = 1 − θ·[n(m − 1) + m(n − 1)]90mn
s = 1.8 m (3D)
θ = tan⁻¹(0.61.8) = 18.43°
η = 1 − 18.43 × [3 × 2 + 3 × 2]90 × 3 × 3 = 0.727
Group capacity against the column loadQ_g = η·N·Q_safe
Q_g = 0.727 × 9 × 669.1 = 4377 kN
Column load 4500 kN → ✗ add piles, lengthen them or increase the diameter
In clay also check block failure of the whole group and consolidation settlement.
Structural capacity of the pileP_u = 0.4f_ckA_c + 0.67f_yA_scP_w = P_u1.5
A_sc = 0.4 % × 282743 = 1131 mm² (≈ 6 bars of 16 mm)
P_u = 0.4 × 25 × 281612 + 0.67 × 415 × 1131 = 3131 kN
P_w = 2087 kN ≥ load per pile 500 kN ✓