Learn · worked example

Pile capacity from soil layers

A pile carries its load two ways: by friction along its shaft in each soil layer, and by end bearing under its toe. Each layer adds friction according to its type (in clay from the undrained strength, in sand from the effective stress and the friction angle). The ultimate capacity is divided by a factor of safety to get the working load, or factored for a limit-state design. The pile itself must also be strong enough as a column. Every number below comes from the free STATIX pile capacity calculator with the inputs in step 1.

1. The problem

Input	Value
Type	Bored (cast in place)
Pile section	Round mm
Section diameter or side	600 mm
Length below the cut-off	12 m
Concrete fcu	30 MPa
Bars number × size, fy	6
Bar size	Y20
Steel yield	450
Design basis	SANS 10160-5 Table B.2
Working FoS compression / uplift (practice)	2.5
Factor of safety uplift	3
N per pile working; blank = capacity only	500 kN
Layer 1 soil	clay
Layer 1 bottom depth m	4
Layer 1 strength	50
Layer 1 unit weight	18
Layer 1 saturated unit weight	19
Layer 2 soil	sand
Layer 2 bottom depth m	20
Layer 2 strength	34
Layer 2 unit weight	19
Layer 2 saturated unit weight	20

2. The pile

The shaft area per metre (the perimeter) collects the friction; the base area collects the end bearing.

bored pile, circular 600 mm, length 12.00 m (toe at 12.00 m) perimeter = 1.885 m, base area = 0.283 m²

3. Shaft friction, layer by layer

Along each layer the pile picks up friction. In clay it is a fraction (α) of the undrained shear strength. In sand it grows with depth, from the effective vertical stress times a friction factor (β, from the friction angle). Below the water table the effective stress is lower. The friction of all layers is added up.

Q_s = Σ (friction × perimeter × layer thickness) = 833.2 kN

4. End bearing at the toe

The soil under the toe takes a bearing pressure q_b over the base area. In clay it is about nine times the undrained strength; in sand it grows with the effective stress, up to a cap.

toe in layer 2 (sand): q_b = 4,000 kPa (at the cap) Q_b = q_b × A_base = 1,131.0 kN

5. Ultimate and working capacity

The ultimate capacity is the shaft plus the base. Dividing by a factor of safety gives the working (allowable) load; a limit-state design uses partial factors instead. The working load on the pile must not exceed it.

Q_ult = Q_s + Q_b = 833.2 + 1,131.0 = 1,964.2 kN working (allowable) compression = 704.2 kN ULS design resistance R_d = 737.0 kN

6. Tension (uplift)

Pulled upward, a pile has no end bearing: only the shaft friction and the pile's own weight resist.

shaft 833.2 kN + pile weight 81.4 kN working tension = 359.2 kN, ULS R_d = 400.0 kN

7. The pile as a structural member

The concrete and steel of the pile must carry the load too, as a short column in compression and with the bars alone in tension.

A_s = 1,885 mm², f_cu = 30 MPa compression: 3,939 kN (ULS), 2,626 kN (working) tension: 738 kN (ULS) governs: compression geotechnical, tension geotechnical

8. The checks

The design load against each capacity: soil and pile, compression and tension, working and ultimate.

Geotechnical compression (working): [FoS 2.5 / 3 (practice value (source not on disk) - user can change it)] 500.0 ≤ 704.2 kN OK Geotechnical tension (working): [FoS 2.5 / 3 (practice value (source not on disk) - user can change it)] INFO Geotechnical compression (ULS): [SANS 10160-5 B.3, Table B.2 GEO] INFO Geotechnical tension (ULS): [SANS 10160-5 B.3, Table B.2 GEO] INFO Pile shaft compression (ULS): [SANS 10100-1 4.7.4.3] 750.0 ≤ 3,938.6 kN OK Pile shaft tension (ULS): [SANS 10100-1 3.3.3.2 a)] INFO Settlement / pile head movement: [-] NOT-CHECKED Negative skin friction (down drag): [SANS 10160-5 5.4.2, B.3.6] NOT-CHECKED Lateral load / bending in the pile: [-] NOT-CHECKED

Pile groups add an efficiency factor and block failure; the calc sheet shows them when a group is given.

Run this example yourselfOpen the free STATIX pile capacity calculator and press Load the example (600 mm bored pile, 12 m, clay over sand) under "How it is calculated". Change any value and every step updates, with a calc sheet.
Open the pile capacity calculator

A worked example for learning, not a design. Check every value against the current code and your own project, and have the design checked and signed by a registered professional engineer.

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