Comparing dynamic and static test results of bored piles

Due 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 to
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 know, and provided the ratio the drop weight to the pile
weight and to the pile SWL 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.