Effective Overburden Pressure Effect on Probabilistic Immediate Settlement Analysis of Granular Soil for Raft Foundation

Abstract

This study conducted a probabilistic evaluation of immediate settlement at various foundation depths under effective overburden pressures of 100, 125, 150, 175, and 200 kPa at the center of foundations in Ahoada town, Rivers State, Nigeria. The assessment utilized deterministic immediate settlement methods proposed by Harr, Steinbrenner and Fox, and Terzaghi and Peck. The geotechnical and elastic parameters for the reliability analysis were obtained from soil samples collected during the geotechnical investigation at the site. Computer programs were developed using the First Order Reliability Method (FORM) coded in MATLAB to perform the reliability estimates. The findings indicated that for Ahoada region, the reliability index values for Steinbrenner and Fox was between 3.0 to 5.3, however, for foundation depth of 0.5 to 1.5 m, the reliability index was between 3.0 to 4.0 and these fell within the recommended target reliability index value range of 2.5  to 4.0  for the raft foundation considered, followed by those from Harr, and then Terzaghi and Peck. However, using the Harr method, the reliability index values ranged between 4.7 and 5.7, overestimating the target reliability index, while the Terzaghi and Peck method (0.2 to 2.8) underestimated the target reliability index. Therefore, for the Ahoada region, the Steinbrenner and Fox method is the most reliable model for the prediction of the immediate settlement of raft foundation at foundation depths of 0.5 to 1.5 m. These results agree with the results of the immediate settlement using the deterministic method, implying that as the value of the estimated settlement increases beyond the maximum allowable settlement, it is expected that the reliability index should reduce and vice versa. The MATLAB program developed in this study is user-friendly and is therefore suitable for geotechnical applications.