An Isotach Approach for Considering the Rate-Dependent Axial-Loading Behavior of Open Steel Piles Driven in Chalk

This paper analyzes the effects of loading rate on the axial shaft capacity and stiffness for piles driven in the low-to-medium density chalk, taking advantage of a series of field maintained load (ML) tests. As with sands and clays, the axial loading behavior of piles in chalk is significantly time- and rate-dependent once the creep-yield load is exceeded, which typically corresponds to approximately one-third of the piles’ shaft capacity. An isotach approach was employed to derive families of load–displacement curves for a range of constant displacement rates from the loading and creep stages of ML axial pile tests while recognizing a long-term load–displacement relationship that acts as a lower bound to the field behavior. A framework was developed for adjusting the load–displacement behavior to account for rate effects, in which empirical load modifiers are applied that depend on loading levels and pile head displacement rates. Overall, the impact of the loading rate amounts to an increase of load of around 7.0% axial shaft resistance per log cycle of displacement rate over the ranges applied in typical tests. These findings aid the assessment of rate effects when analyzing site tests on piles driven in chalk and designing the foundations to cope with a wide range of both extreme short-term events and longer-term maintained loading cases.