Prediction of the settlement associated with construction on soft clay improved using rigid inclusions considering the installation effect

Abstract

One of the most common methods in soil improvement of soft soils is using rigid inclusions (RI) that enhance the stiffness of the soil surrounding and beneath the inclusions. This paper aims to develop simple and user-friendly charts to estimate the settlement improvement factor (N) that correlates the settlement values without soil improvement to the settlement values after improvement, considering the installation effect of the rigid inclusions in soft clay formations. The cavity expansion method is used to model the installation process of the rigid inclusions. A parametric study is carried out using numerical modelling to predict the variation in the improvement factor with the variation of the treated soil stiffness and geometry of the RI system. The analyses are performed using a verified numerical model for a case study of a zone loading test performed at New Mansoura city in Egypt, employing the finite element software (PLAXIS 3D). The cavity expansion approach is first utilized for an axisymmetric model by imposing a prescribed displacement equivalent to the inclusion radius to simulate the construction effect of the rigid inclusion. From the axisymmetric model, the enhancement in the deformation modulus and the over consolidation ratio of the soft clay, resulting from the construction of the rigid inclusions, are determined. In the second stage of the analysis, the improved soil parameters are then implemented in the 3D finite element model, in which the external stresses due to superstructure are applied over the improved soil. The settlement associated with loading both the improved and unimproved soils is then compared for each studied condition in the parametric study to determine the improvement factor (N). The performed analyses clearly showed the high effect of the rigid inclusions in reducing the settlement associated with the construction on the improved soft clay. The achieved improvement factor varies between 2.88 to 58.10, with an average value of about 12.00. Based on the conducted parametric study, design charts are developed to provide a simple tool for predicting the settlement values associated with construction on improved soft clay formations with rigid inclusions under different conditions.