Reinjection of construction dewatering water at the University of Canterbury

The University of Canterbury is situated above a shallow semi-confined aquifer. Dewatering is required during construction of the basements of multi-storey buildings. Previously, dewatered water has been discharged to local waterways.

During the recent construction of the Biosciences building, concerns raised during the resource consent process meant it was not practicable to discharge all dewatering water to the local Okeover Stream. Alternative options evaluated included reinjecting the dewatering water to ground. The shallow aquifer is high yielding, which is a challenge for site dewatering due to the high pumping rates required, but also means reinjection can occur without raising groundwater levels excessively.

The results from testing of a trial bore indicated reinjection was feasible. Groundwater modelling was carried out to estimate where additional reinjection bores could be placed to minimise the mounding and the impact on the dewatering system.

Eight additional bores were drilled, tested and connected into a pipe network from the excavation site. The reinjection was successfully completed over the 6 month dewatering period while maintaining groundwater levels below ground level and minimising interference.

This paper discusses the investigations and the reinjection system, and illustrates the effectiveness of this solution through groundwater level monitoring data collected during construction.