Back Analysis of Rock-Socketed Pile in Bromsgrove Sandstone of Central Birmingham

The rapid development in central Birmingham in the past decades has led to the construction of several high rises founded on large diameter rock-socketed piled foundations. The piles are typically socketed into the Bromsgrove Sandstone unit of the Sherwood Sandstone group. A key reference used for the pile design in the Birmingham area is Whitworth & Turner (1989). The empirical approach based methods employed in normal practice for rock-socketed pile design has been discussed. These methods though based on calibration with load test found to be conservative. With the availability of more load test data in the area and efficient computational tools, the back-analysis could be used to reduce conservatism associated with the empirical approach. The back-analysis is particularly helpful in the mixed ground conditions where the estimation of contribution from weak overlying deposits becomes challenging.

A method for estimating ground response and pile capacity using a routine Finite Element (FE) tool is presented and discussed. The back-analyses were performed to reproduce the observed behaviour by iteratively adjusting the strength and stiffness parameters of each geological units to obtain a best fit load-settlement profile. A reliable model to estimate ground parameters and pile capacity was thus obtained. An estimate of the unit shaft resistance and end bearing for rock-socketed piles in Bromsgrove Sandstone has been provided which could be supplemented to update the Whitworth & Turner paper. A brief discussion on Phase 2 development at Paradise Circus is presented. The project required the construction of large diameter bored piles adjacent to the existing tunnel. The imposed load on the tunnel was a key concern, particularly under the mixed ground conditions. The forward analysis was carried out using the estimated strength and stiffness parameters, and mitigation measures were proposed.