7.3 Actions and design situations
(1) The actions listed in 2.4.2 (4) should be considered when selecting the design situations.
(2) Piles can be loaded axially and/or transversely.
(3)P Design situations shall be derived in accordance with 2.2.
(4) An analysis of the interaction between structure, pile foundation and ground can be necessary to prove that the limit state requirements are met.
7.3.2 Actions due to ground displacement
(1)P Ground in which piles are located may be subject to displacement caused by consolidation, swelling, adjacent loads, creeping soil, landslides or earthquakes. Consideration shall be given to these phenomena as they can affect the piles by causing downdrag (negative skin friction), heave, stretching, transverse loading and displacement.
(2) For these situations, the design values of the strength and stiffness of the moving ground should usually be upper values.
(3)P One of the two following approaches shall be adopted for design:
- the ground displacement is treated as an action. An interaction analysis is then carried out to determine the forces, displacements and strains in the pile;
- an upper bound to the force, which the ground could transmit to the pile shall be introduced as the design action. Evaluation of this force shall take account of the strength of the soil and the source of the load, represented by the weight or compression of the moving soil or the magnitude of disturbing actions.
22.214.171.124 Downdrag (negative skin friction)
(1)P If ultimate limit state design calculations are carried out with the downdrag load as an action, its value shall be the maximum, which could be generated by the downward movement of the ground relative to the pile.
(2) Calculation of maximum downdrag loads should take account of the shear resistance at the interface between the soil and the pile shaft and downward movement of the ground due to self-weight compression and any surface load around the pile.
(3) An upper bound to the downdrag load on a group of piles may be calculated from the weight of the surcharge causing the movement and taking into account any changes in ground-water pressure due to ground-water lowering, consolidation or pile driving.
(4) Where settlement of the ground after pile installation is expected to be small, an economic design may be obtained by treating the settlement of the ground as the action and carrying out an interaction analysis.
(5)P The design value of the settlement of the ground shall be derived taking account of material weight densities and compressibility in accordance with 2.4.3.
(6) Interaction calculations should take account of the displacement of the pile relative to the surrounding moving ground, the shear resistance of the soil along the shaft of the pile, the weight of the soil and the expected surface loads around each pile, which are the cause of the downdrag.
(7) Normally, downdrag and transient loading need not be considered simultaneously in load combinations.
(1)P In considering the effect of heave, or upward loads, which may be generated along the pile shaft, the movement of the ground shall generally be treated as an action.
NOTE 1 Expansion or heave of the ground can result from unloading, excavation, frost action or driving of adjacent piles. It can also be due to an increase of the ground-water content resulting from the removal of trees, cessation of abstraction from aquifers, prevention (by new construction) of evaporation and from accidents.
NOTE 2 Heave may take place during construction, before piles are loaded by the structure, and may cause unacceptable uplift or structural failure of the piles.
126.96.36.199 Transverse loading
(1)P Consideration shall be given to transverse actions originating from ground movements around a pile.
(2) Consideration should be given to the following list of design situations, which may result in transverse actions on a pile:
- different amounts of surcharge on either side of a pile foundation (e.g. in or near an embankment);
- different levels of excavation on either side of a pile foundation (e.g. in or near a cutting);
- a pile foundation constructed in a creeping slope;
- inclined piles in settling ground;
- piles in a seismic region.
(3) Transverse loading should normally be evaluated by considering the interaction between the piles, treated as stiff or flexible beams, and the moving soil mass. When the horizontal deformation of weak soil layers is large and the piles are widely spaced, the resulting transverse loading of the piles depends mainly on the shear strength of the weak soil layers.