A practical design approach for piles with negative friction

Abstract

Misconceptions remain in the minds of some pile designers when negative friction effects have to be taken into account. This paper outlines some of these misconceptions, and then describes a relatively straightforward approach for designing piles subjected to negative friction. This approach relies on the consideration of the portion of the pile that is located in the ‘stable’ zone—that is, that part of the ground profile that is not subjected to ground settlements. By designing this portion of the pile to have adequate length and strength, the key design requirements in relation to geotechnical capacity, structural capacity and pile head settlement can be satisfied. The case where the ground settlements extend to a large depth is also described briefly, and it is shown that it may then be prudent to design the piles to settle with the ground, rather than attempt to restrain them from settlement. Some other issues that can affect the response of piles to ground settlements are examined, including the presence of residual stresses in the pile, live load application and group effects. It is demonstrated that preloading a pile has the potential to reduce the axial force induced in the pile by the ground settlements.