AN INSTRUMENTED DRIVEN PILE IN DUBLIN BOULDER CLAY

Precast concrete piles, driven to a set in Dublin black boulder clay, have been used extensively in Dublin where depths of fill or soft soil make shallow foundations uneconomical. This paper describes the resul ts of a test pile which was instrumented with vibrating wire gauges an d pore pressure probes at various levels along the pile shaft. Three c ompression tests were carried out on the test pile: one within 2.4 h o f its installation, one after 1.8 days and one after 17 days. Finally, a tension (pull-out) test was carried out after 24 days. The loading tests showed that there was a significant increase in both the ultimat e shaft and base capacity with time and that about 60% of the total pi le capacity was achieved in end-bearing in all three compression tests . The last compression test on the pile, which was carried out when al l of the excess pore pressure set up by driving had dissipated, indica ted an apparent N-c value of over 50 when applied to the in situ undra ined shear strength of the boulder clay prior to the test. An analysis of the base displacement and pore pressures during load indicated tha t this high N-c value is likely to be due to an increase in the mean e ffective stress at the pile tip due to its insertion.