Pile capacity estimation of existing deep foundations by CPT methods

Abstract

The reuse of existing deep foundation elements such as piles is possible if a reliable estimation of the pile capacity can be determined. Without an estimate of capacity, either the risk of failure is unknown or an overly conservative estimate of capacity will be used adding cost to the foundation. The only exact method to determine in place capacity is to perform a full-scale axial load test. This method is often impractical due to cost and the likelihood of destroying a needed foundation element in the process. An alternative method to estimate pile capacity is needed. Cone Penetration Test (CPT) methods can be used to determine the load capacity of piles before construction. However, it is unknown if these methods can be used on existing deep foundations. A research study was conducted on piles of two existing bridges in southeast Missouri within the Mississippi flood plain. CPT soundings were performed adjacent to the bridges and the pile capacity was estimated by three CPT analysis methods. Later, select bridge piles were tested for capacity by loading the select foundation elements to failure. After load tests were completed the results were compared to the static load capacities that were calculated using CPT methods. Two of the CPT methods were within either 21 percent conservative (under predicted capacity) or 37 percent unconservative (over predicted capacity) of the static load capacity for the two bridges studied. The last method was 10 percent conservative for one bridge and 11 percent conservative for the other bridge of static load capacity. Overall, there did not appear to be a drastic difference in the estimated load capacity between initial setup of the piles and long-term static load capacity. Therefore, this research indicates these CPT methods can be used to estimate pile capacity without modification for existing deep foundation elements in drained, coarse-grained (drained sandy) soil conditions.