Lessons learnt from dewatering challenges in Christchurch

Foundation and infrastructure construction in Christchurch often involves groundwater management which is executed as dewatering. By virtue of these works being temporary the design responsibility often falls to the contractor, and on many occasions for smaller projects dewatering systems are specified and installed without input from experienced engineers or personnel, and without close oversight from designers. This practice evolved due to the varying plant and equipment that contractors have available, and uncertainties involved in specifying dewatering systems. Depending on the chosen dewatering method and the quality of installation and operation, dewatering can pose significant risks to the surrounding buildings and infrastructure.

This paper outlines typical dewatering methods for horizontal and vertical infrastructure construction in Christchurch. Based on the author’s experience as a designer and while undertaking construction observations the common issues with dewatering will be discussed through two case studies, where improper dewatering installation led to ground subsidence and instability, and caused significant damage to adjacent infrastructure.

A review of each case study indicated that each share common organisational contributions and construction issues, which in turn have led to the poor performance. The paper will specifically discuss the timing of the dewatering drawdown period and the knowledge and background of the persons typically responsible for dewatering. Lessons learned are drawn for engineers designing and supervising projects involving dewatering. These lessons include appropriate limitations on dewatering for specifications, and the key stages of dewatering setup where site supervision is required.