Soakage Disposal in High Groundwater

The Christchurch Southern Motorway Stage 1 (CSM) is a $140 million, 10.5km motorway, currently being constructed between Barrington and Halswell. The stormwater design provides treatment and attenuation of stormwater prior to disposal to soakage or surface water, with the soakage disposal being a consent requirement in areas where soils allow, to enhance groundwater recharge.

This paper describes the design challenges with stormwater disposal to soakage for the CSM, and how these were resolved. Due to high groundwater levels, there is a risk of groundwater levels rising close to the base of the soakage disposal areas in extreme conditions. In these circumstances, conventional assumptions about unconstrained vertical discharge to ground no longer apply, and an understanding of horizontal groundwater movement and groundwater mounding was required. The combined probability of an extreme high groundwater level and a large design storm also needed to be understood, as well as the consequences of such events for design. This led to risk management decisions around the implications of low probability but high consequence events, and design of contingency measures to address these. This paper will examine the issues, the design approach, and the solutions adopted.