Ashburton Wastewater Pipeline River Crossing

Scope:

• Construction of 500m long DN1000 GRP gravity main at depths of up to 6.8m and at least 4m below the bed across the Ashburton River.
• Construction of 180m long DN800 GRP gravity main. 
• Complex Tie-in of new DN 800 GRP to existing wastewater main coming from the town centre 
• Large diameter 5m deep Polyethylene manhole.
• Management and over pumping of significant wastewater flows. 
• Sheet piling and extensive dewatering.

Project Description:

This project was to construct a new river crossing pipeline to connect the existing DN750 RCRRJ wastewater main from Ashburton at the upstream end to a new Pump Station (Wilkins Road PS) on the downstream end. 

The new pipeline runs under the 500m wide flood plain and main channel of the Ashburton River. This required installation of the new pipeline at depths of up to 6.8m, and at least 4m below the bed of the river. 

A large 5m deep PE manhole was installed to allow for future expansion of the wastewater network, and a 180m long DN800 GRP gravity main was tied into the existing pipeline coming from the town centre, allowing the existing syphon across the river to be abandoned. 

Working within a flood plain meant significant volumes of groundwater coursed through the gravels, as well as in the main channel of the river. We successfully managed this, using sheet-piles combined with extensive dewatering carried out by experienced in-house teams. 

Works were undertaken in a highly sensitive river environment. There were requirements to ensure tree and ground nesting birds were not disturbed, and interfaces with aquatic wildlife were carefully managed. We worked closely with a specialist aquatic subcontractor to move the thalweg from the river in a controlled and environmentally considerate way, to achieve safe relocation of the fish. All works were conducted in liaison with, and to the satisfaction of, ECan. As a result, the Seipp team was awarded a certificate for its exemplary attitude towards compliance and collaboration from ECan.  

Start Date: November 2019

Completion Date: November 2020