Shaft friction from instrumented displacement piles in an uncemented calcareous sand

The behaviour of displacement piles in uncemented calcareous sand is investigated using field piles instrumented with a sensor that simultaneously records the radial stress and shear stress at specific locations on the pile shafts. These tests are interpreted with the assistance of data from adjacent self‐boring pressuremeter tests and from monotonic and cyclic direct shear interface tests performed on reconstituted samples. The existence of extremely low radial stresses on the pile shafts is verified. Although dilation during shear is seen to compensate for such low radial stresses, short term shaft capacities are much lower than capacities of equivalent piles in siliceous sands. The development of a ‘bonded’ or ‘welded’ crust to the pile shaft was seen to be the primary contributor to the set‐up observed at the test site; this crust forced failure to take place at a sand‐sand rather than a sand‐steel interface and also gave rise to higher levels of dilation during monotonic loading. The ‘welded’ sand crust did not, however, give rise to a higher long term cyclic capacity.