Course · lesson 5 of 7

Piles: capacity

How much load one pile can carry: shaft friction in each layer and bearing under the toe, the working and ultimate checks, negative skin friction, tension piles and groups, with the STATIX pile capacity calculator.

1. Learning outcomes

After this lesson you can:

work out shaft friction in clay and sand and end bearing under the toe;
find the effective stress with a water table;
check a pile at working load and at ULS;
allow for negative skin friction above the neutral plane;
find a pile’s tension capacity, its group efficiency and its structural capacity.

2. Before you start

Lesson 1 (soils, c_u, φ and effective stress).
The free pile capacity calculator; lesson 6 uses its result.
A pocket calculator and about 50 minutes.

3. Shaft and base

How a pile carries load: friction along its shaft in every layer it passes through, and bearing under its toe. The ultimate capacity is the sum, Q_ult = Q_s + Q_b; the pile’s own weight is taken off in compression.

Shaft friction in clay: a fraction α of the undrained shear strength. Boring softens the clay at the shaft, so a bored pile takes a lower α than a driven one. The coefficients and caps are practice values, shown on the calc sheet and open to change.

Shaft friction in sand grows with the effective vertical stress, so the deeper part of the shaft does most of the work. The engine slices the shaft into short lengths that never straddle a layer boundary or the water table and adds them up.

End bearing: nine times c_u in clay, σ’_v N_q in sand (N_q from φ), and 2.5 q_u on rock. In sand the formula soon reaches the cap: a bored base mobilises only part of a driven one at working settlement.

The effective vertical stress grows with the unit weight above the water table and with the buoyant weight below it. A high water table lowers the friction and base resistance in sand: give the water depth.

Pile types. A driven pile pushes the soil aside and densifies it, so it gets more friction and a higher base cap than a bored or CFA pile cast in place. The calculator offers bored, CFA and driven.

4. The checks

Two checks of the same pile. The working check divides the ultimate capacity by a factor of safety (practice values in the calculator). The ultimate check divides it by the SANS 10160-5 resistance factor and model factor, or uses the EN 1997-1 R4 set; STATIX shows both and compares each with the matching load.

The SANS 10160-5 partial factors in this lesson (γ_R 1.6 in compression, 1.7 in tension, model factor γ_R,d 1.5) are as STATIX applies them; check them against your copy of SANS 10160-5.

5. Downdrag, tension and groups

Negative skin friction. Where fill or soft clay settles past the pile, the shaft above the neutral plane is dragged down: that friction is a load on the pile, not a resistance (SANS 10160-5 5.4.2). Give the calculator the neutral plane depth; it adds the drag to the compression load and counts only the shaft below.

Tension piles. In uplift only the shaft resists (no base), with the pile’s weight helping; a larger factor of safety is used. The bars must carry the tension too: 0.87 f_y A_s. Portal frame bases and towers often need them (Steel Design 2, lesson 1).

Pile groups. Closely spaced piles share the same soil, so each carries less than it would alone. The calculator applies the Converse-Labarre efficiency from the rows, columns, size and spacing, and warns below 2.5 diameters; block failure of the whole group is not checked.

What the calculator hands on: the pile’s capacity is the smaller of the soil and the pile itself (a short column, SANS 10100-1 4.7.4.3), times the group efficiency, as a working value and a ULS value. Those go into the pile cap calculator of lesson 6.