An instrumented driven pile in Dublin boulder clay

Abstract

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 results of a test pile which was instrumented with vibrating wire gauges and pore pressure probes at various levels along the pile shaft. Three compression tests were carried out on the test pile: one within 2.4 h of 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 ultimate shaft and base capacity with time and that about 60% of the total pile capacity was achieved in end-bearing in all three compression tests. The last compression test on the pile, which was carried out when all of the excess pore pressure set up by driving had dissipated, indicated an apparent Nc value of over 50 when applied to the in situ undrained shear strength of the boulder clay prior to the test. An analysis of the base displacement and pore pressures during load indicated that this high Nc value is likely to be due to an increase in the mean effective stress at the pile tip due to its insertion. Show more