Performance of cast-in-place piles in dynamically compacted soil-rock mixtures

The cast-in-place (CIP) pile is a type of deep foundation that has been widely used in various soils. However, the performance of CIP pile in dynamically compacted (DC) soil-rock mixtures has been rarely investigated. This study presents a field investigation on the performance of CIP piles in DC treated soil-rock mixtures. First, a 30-meter-thick soil-rock mixture layer on site was improved using DC (maximum energy level =10,000 kN·m) on grouting area and non-grouting area. Second, two groups of six CIP piles with a diameter of 1.2 m and a length of 34 m were constructed. After that, static pile load tests (SPLT) were carried out to evaluate the performance of CIP piles. The testing results suggest that the compressive bearing capacity of CIP pile in grouting area is identical to that installed in non-grouting area. The pile displacement in grouting area, however, was reduced over 40% under an applied load of 12,000 kN compared to non-grouting case. The DC treatment mobilized the shaft resistance in upper part of test piles (0 ~ 10 m). The applied load shared by shaft resistance mobilized at depth of 0 ~ 10 m is beyond 77% regardless of grouting, and the smaller the applied load, the higher the shared ratio. It indicated that the DC can significantly eliminate collapsibility within the main influence depth from water injection test. The grouting can eliminate the potential collapsibility and negative frictional resistance of pile side, which is especially beneficial in reducing pile displacement, improving the strength of thick soil-rock mixtures, and increasing bearing capacity of the CIP piles.