7.6 Axially loaded piles
220.127.116.11 Limit state design
(1)P The design shall demonstrate that exceeding the following limit states is sufficiently improbable:
- ultimate limit states of compressive or tensile resistance failure of a single pile;
- ultimate limit states of compressive or tensile resistance failure of the pile foundation as a whole;
- ultimate limit states of collapse or severe damage to a supported structure caused by excessive displacement or differential displacements of the pile foundation;
- serviceability limit states in the supported structure caused by displacement of the piles.
(2) Normally the design should consider the margin of safety with respect to compressive or tensile resistance failure, which is the state in which the pile foundation displaces significantly downwards or upwards with negligible increase or decrease of resistance (see 7.6.2 and 7.6.3).
(3) For piles in compression it is often difficult to define an ultimate limit state from a load settlement plot showing a continuous curvature. In these cases, settlement of the pile top equal to 10% of the pile base diameter should be adopted as the "failure" criterion.
(4)P For piles that undergo significant settlements, ultimate limit states may occur in supported structures before the resistance of the piles is fully mobilised. In these cases a cautious estimate of the possible range of the settlements shall be adopted in design.
NOTE Settlement of piles is considered in 7.6.4
18.104.22.168 Overall stability
(1)P Failure due to loss of overall stability of foundations involving piles in compression shall be considered in accordance with Section 11.
(2) Where there is a possibility of instability, failure surfaces both passing below the piles and intersecting the piles should be considered.
(3)P Failure due to uplift of a block of soil containing piles shall be checked in accordance with 22.214.171.124 (4)P.
7.6.2 Compressive ground resistance
(1)P To demonstrate that the pile foundation will support the design load with adequate safety against compressive failure, the following inequality shall be satisfied for all ultimate limit state load cases and load combinations:
(2) In principle Fc;d should include the weight of the pile itself and Rc;d should include the overburden pressure of the soil at the foundation base. However these two items may be disregarded if they cancel approximately. They need not cancel if:
- downdrag is significant;
- the soil is very light,
- the pile extends above the surface of the ground.
(3)P For piles in groups, two failure mechanisms shall be taken into account:
- compressive resistance failure of the piles individually;
- compressive resistance failure of the piles and the soil contained between them acting as a block.
The design resistance shall be taken as the lower value caused by these two mechanisms.
(4) The compressive resistance of the pile group acting as a block may be calculated by treating the block as a single pile of large diameter.
(5)P The stiffness and strength of the structure connecting the piles in the group shall be considered when deriving the design resistance of the foundation.
(6) If the piles support a stiff structure, advantage may be taken of the ability of the structure to redistribute load between the piles. A limit state will occur only if a significant number of piles fail together; therefore a failure mode involving only one pile need not be considered.
(7) If the piles support a flexible structure, it should be assumed that the compressive resistance of the weakest pile governs the occurrence of a limit state.
(8) Special attention should be given to possible failure of edge piles caused by inclined or eccentric loads from the supported structure.
(9)P If the layer in which the piles bear overlies a layer of weak soil, the effect of the weak layer on the compressive resistance of the foundation shall be considered.
(10)P The strength of a zone of ground above and below the pile base shall be taken into account when calculating the pile base resistance.
NOTE This zone may extend several diameters above and below the pile base. Any weak ground in this zone has a relatively large influence on the base resistance.
(11) Punching failure should be considered if weak ground is present at a depth of less than 4 times the base diameter below the base of the pile.
(12)P Where the pile base diameter exceeds the shaft diameter, the possible adverse effect shall be considered.
(13) For open-ended driven tube or box-section piles with openings of more than 500 mm in any direction, and without special devices inside the pile to induce plugging, the base resistance should be limited to the smaller of:
- the shearing resistance between the soil plug and the inside face of the pile;
- the base resistance derived using the cross-sectional area of the base.