Design Challenges Of Road Widening In Soft Grounds: Characterisation To Numerical Analysis

This paper presents challenges involved in robust geotechnical design for upgrading an existing motorway, which is founded on soft soil. A critical review is presented on current approaches adopted for soft soil characterisation using results of laboratory and field tests commonly applied in Australian practice. Particular attention is given to assessment of undrained shear strength, overconsolidation ratio (OCR), primary and secondary compression (creep) indices and coefficient of consolidation. Some of the limitations of finite element analysis using 2D PLAXIS software, as a commonly used tool, and its ‘soft-soil creep model’ are also highlighted. Analytical approaches are presented which were used in a case study to overcome these limitations and help with deformation analysis of soft soils undergoing creep, as well as the design of rigid inclusions taking into account 3D effects.

The case study project upgrade works involved widening of the motorway embankment and in turn extension of existing culverts, which were located in a river floodplain. Preload and wick drains were considered for soft soil improvement to meet residual and differential settlement criteria over the design life of the new pavement. Rigid inclusions were also designed for the new culvert extension to reduce potential differential settlement between the proposed extension and existing culverts.