Can uplift be estimated from dynamic testing?
In CAPWAP®, the total resistance (Capacity) is estimated, including shaft resistance and end bearing. It is very important to perform a very careful evaluation of shaft and end bearing components when considering uplift – make sure the bottom shaft element seems reasonable compared to the soil profile and the element above. For uplift, the end bearing is then ignored. It is then assumed that the soil will resist similarly in uplift as it would be in compression. This might be reasonable if the pile is not short. Some engineers might discount the shaft resistance on the lowest shaft element (since some, or even all, might really be end bearing). The remaining shaft resistance (or all of shaft resistance) is then usually assigned a slightly reduced total (typically 80%), and sometimes a higher safety factor than in compression (due to the uncertainty and also because an uplift failure might be more severe).