Design and Construction Monitoring of a Road Embankment and an Underpass Founded on Soils Susceptible to High Static Settlement and Liquefaction

The Waikato Expressway is one of the New Zealand Transport Agency’s (NZTA), now Waka Kotahi, seven Roads of National Significance. Tetra Tech Coffey is part of the City Edge Alliance (CEA) engaged by Waka Kotahi to design and construct the Hamilton Section of the Waikato Expressway project. As part of the project, the CEA was required to design and construct an 8 m high embankment founded on soil consisting of highly compressible soft Peat, Clay, and liquefiable embedded Sand layers. Preloading the subsoil with a high surcharge was recommended to minimise the post-construction settlement at the expressway embankment and underpass concrete box structure at Powells Road. Contiguous flight Auger (CFA) lattice was utilised post-surcharge to improve the ground further at the underpass location. The CFA ground improvement was not utilised in the earthwork’s embankment portion on either side of the underpass to minimise the construction costs. Planning and designing a new 8 m high soil embankment close to a CFA soil improvement zone increased the need for accuracy in differential settlement and complexity of the work. Limited laboratory testing and underestimated compressible soil parameters caused delay in the settlement rate in the initial stage and additional surcharge was placed to speed up the consolidation period. Several settlement monitoring instruments were installed and several settlements back analysis methods using finite element modelling and empirical methods were used to observe the settlement rate and estimate the additional surcharge requirement. This paper presents the design and monitoring of an expressway embankment for static and dynamic loads, action taken to stabilise the surcharge slope failure and other difficulties faced during construction.