Can uplift be estimated from dynamic testing?
CAPWAP® estimates the total resistance (Capacity), shaft resistance, and end bearing. It is important to perform careful evaluation of the shaft and end bearing components when considering uplift. During evaluation, ensure that the bottom shaft element is reasonable in comparison to the soil profile and the element above. Uplift tests ignore the end bearing results. The soil's resistance during uplift is assumed to be similar to that of compression. This assumption is reasonable as long as the pile is not short.
Alternatively, some engineers discount shaft resistance on the lowest shaft element because some, or even all, might actually be end bearing. An engineer assigns a reduced total safety factor, of approximately 80%, to the remaining or all of the shaft resistance. Occasionally, a higher safety factor than in compression might be assigned. This is because of uncertainty, as well as because an uplift failure might be more severe.