Comparing dynamic and static test results of bored piles

Owing to increasing time, cost and transportation difficulties, Irish contractors are seeking an alternative to conventional static pile load tests. As a result several firms have adopted dynamic testing techniques to supplement and in some cases replace conventional static tests. In order to assess the reliability of the systems and to address the concerns of owners and consulting engineers, a database comprising 43 pairs of static and dynamic tests on piles from 24 sites around Ireland has been assembled. The database was limited to medium- to large-diameter continuous flight auger (CFA) piles, typically 450 mm to 600 mm in diameter. Comparisons between actual measured settlement in static tests and predicted settlement for dynamic tests showed a clear pattern of underestimation of settlement by the dynamic tests, with the ratio between the two values typically varying between 0·4 and 1·2, with an average of about 0·8. This is due to the limited energy being imparted to the pile in the dynamic test. The results were best for piles in rock and worst for piles in clay. Scatter in the results increased with increasing pile load. However, as all of the measured settlements were modest, the absolute numerical difference between the actual and predicted values was typically less than 3 mm, which may not be significant in many practical cases. For piles supporting non-sensitive structures, in areas where the ground conditions are well known, and provided the ratio of the drop weight to the pile weight and to the pile specified working load is at least 50% and 1·5% respectively, it seems possible that static tests can be replaced by dynamic ones. However, for structures sensitive to settlement or for those in areas where ground conditions are uncertain, then it is suggested that the dynamic test should be calibrated by at least one static test.