Calculation of End Bearing Capacity and Shaft Resistance of Cast-in-Place Pile in Coral Reef Formations Considering Grout Penetration and Cementation

Existing methods for calculating the bearing capacity of cast-in-place piles in coral reef formations usually treat coral reef strata as ordinary sandy soil or conventional rock–soil media, without explicitly considering grout penetration and cementation in highly porous coral reef rock. This may lead to an incomplete evaluation of pile end bearing capacity and shaft resistance. To address this limitation, this study proposes a semi-empirical calculation framework that incorporates the contribution of the grout-induced cementation-enhanced zone. In the proposed model, the pile end bearing capacity is divided into three components: the intact coral reef rock contribution, the grout–coral reef rock cemented interface contribution, and the vertical effective stress term. The shaft resistance is divided into interface friction resistance and additional cementation-induced friction resistance. Key parameters were determined through an integrated procedure combining established laboratory and field techniques, including saturated weighing, mercury intrusion porosimetry, CT scanning, field coring, tracer observation, CT-based back-analysis, unconfined compression tests, and interface shear tests. For the investigated engineering case, the comprehensive porosity was 35%, the effective grout penetration radius was 0.118 m, the shear strength of intact coral reef rock was 2.5 MPa, and the shear strength of the cemented interface was 3.0 MPa. The calculated pile end bearing capacity was 1.76 MN, close to the field static load test value of 1.72 MN, with a relative difference of about 3%. The calculated total shaft resistance was 2.59 MN, compared with the measured value of 2.63 MN, with a relative difference of about 1.5%. The results suggest that considering the cementation-enhanced zone can better reflect the bearing response of cast-in-place piles in the investigated coral reef project. However, because the assessment is based on a single project and several parameters were obtained from local tests or back-analysis, the proposed method should be regarded as a site-specific semi-empirical framework. Further independent field tests are needed to examine its transferability and statistical reliability.