Risk vs Cost - Protecting Hutt City Council's Vital Wastewater River Crossing

The Hutt City Council is responsible for the conveyance of wastewater from Upper Hutt to Lower Hutt through a 24km gravity network. This includes a section of pipeline crossing the Hutt River near Silverstream. This section of pipeline is encased in a reinforced concrete structure which is anchored to the Hutt River bedrock.

During some maintenance work on the reinforced concrete structure a large scour hole was observed developing in the river bed approximately 15m downstream from the pipeline structure. There was a risk the scour hole would in time undermine the pipeline structure. If the structure were to fail, it could lead to sewage discharging into the Hutt River.

Optioneering quickly discounted re-routing of the pipeline due to cost and time constraints and so, with consideration of the whole of life financial impacts, the design focussed towards an ‘in river’ solution.

The solution needed to prevent further bed degradation, counteract the river’s energy and allow movement from the earthquake fault running underneath the river. Through the optioneering and design process a ‘rock ramp’ structure was selected as the preferred solution.

This paper will outline some of the design issues and discuss some construction features of the rock protection and illustrate durability post construction.