Static Load Testing of Instrumented Screw Piles in Soft Soil Deposits

With the rapid development across major cities, low-capacity screw piles are adopted by builders as a viable economical option in managing risk involving settlement in soft soil deposits. Although the required installation torque and the capacity of a screw pile can be correlated to the soil shearing resistance at the interface of its shaft and helical plates, the correlated ultimate capacity of the pile is specific only to undrained conditions. Therefore, if the water table fluctuates within the embedment length of the pile, the correlated ultimate strength is not valid. This poses a serious design concern in over-consolidated fills. Therefore, due to the uncertainty associated with the compressive capacity of installed screw piles in soft saturated deposits, it is advantageous to perform a static load test to verify the serviceability and ultimate loads. In this study, four static load tests were carried out on screw piles at four different construction sites in the city of Melbourne, to study the load transfer mechanism at various levels of axial loading and subsequent unloading/reloading stages. In one of the sites, the screw pile was equipped with miniature transducers to monitor the generated total stress and pore-water pressure during the installation and post-installation. The results of this study indicated that a static load test can accurately estimate the real bearing capacity of a screw pile which differs significantly from the design geotechnical strength calculated using theoretical equations. It was concluded that in the absence of a pile load test, it is rational to adopt a geotechnical reduction factor of 0.4 and neglect the skin friction capacity of the screw pile to provide a safe foundation design.