Warkworth Wastewater Pump Station and Transmission Pipeline

Project Update

This design-build project for Watercare Services Ltd., undertaken with McConnell Dowell as the construction partner, is a new wastewater pump station and transmission pipeline to service the growing population of Warkworth Township, 65 kilometers (40 mi) north of Auckland. The 5-kilometer (3.1 mi) pipeline will convey up to 545 liters/second (144 gal/s) of wastewater to the new Wastewater Treatment Plant (WWTP) at Snells Beach.

Delve Underground as the design partner designed all permanent works for the pump station and pipeline and provided design support to the contractor’s temporary works for the Direct Pipe® installation and temporary shafts. This is our fourth Direct Pipe design collaboration with McConnell Dowell in New Zealand. Other collaborations include Army Bay Outfall (the first Direct Pipe installation in New Zealand) and Snells Algies Outfall (with a single drive extending 2.2 kilometers [1.4 mi] and claiming a new world record).

Design Overview

Design originally consisted of three pump stations and 8 kilometers (5 mi) of trenched pipelines, predominantly along roadways between the proposed pump stations and the new WWTP. Delve Underground worked with McConnell Dowell to produce a successful alternative design that provided the following benefits, resulting in operational and construction cost savings:

• Removed the need for two pump stations and optimized the transmission pipeline length from 5 kilometers (3.1 mi) of pump rising mains to 1.4 kilometers (1.9 mi), and 3 kilometers (1.9 mi) of gravity pipeline to 3.6 kilometers (2.2 mi).
• Mitigated the requirement for any air valves or scour points along the transmission pipeline, and limited the number of access chambers to one at the transition point from pumped to gravity flow.
• Allowed for long trenchless drives using the Direct Pipe method, avoiding excavation in public roads, limiting the number of temporary shafts required within private properties, and enabling the alignment to be deep enough to minimize settlement risk.

Transmission Pipeline

Transmission pipeline construction is due to commence later this month. The pipeline is being constructed inside a DN 1200 steel casing pipe installed entirely by Direct Pipe®. The casing pipe will be installed in three drives—the longest being approximately 2 kilometers (1.2 mi). The large-diameter casing pipe enables twin DN 500 polyethylene (PE) carrier pipes to be installed for the pumped rising main component of the transmission pipeline, followed by a single DN 900 PE carrier pipe for the gravity flow section. This arrangement means that one rising main will cater to population growth for the next 35 years, and the second can be commissioned later to cater to population growth over the following 35 to 50 years. The gravity section has been sized for the predicted 50-year population growth.

Pump Station Construction Currently Underway

Currently construction is occurring both aboveground and belowground:

• Belowground inlet chamber, wet well, and dry well, designed as a 12-meter-deep (39 ft) rectangular (17 m x 11 m [56 x 36 ft]) caisson structure
• Belowground emergency storage tank designed as an 8-meter-deep x 15-meter-diameter (26 ft x 49 ft) circular caisson structure
• Aboveground building for the electrical room

The emergency storage tank will provide up to 830 m³ (29,311 ft³) of wastewater storage should the pump station need to be taken out of operation for maintenance. The circular tank will facilitate gravity drainage of the stored water back to the pump station. The tank has been designed with an automated vacuum flushing system such that as the tank fills, a column of water is held under vacuum, ready for flushing the tank clean once it has been emptied. This design facilitates maintenance without the need for personnel entry or a potable water source.

Ancillary pump station equipment such as air valves, surge vessels, and biofilter units were designed to be aboveground but within a fenced compound. The pump station is located within a public park, so the design focused on minimizing the station’s footprint and visual impact while maintaining safe operational access.

Construction of Caisson Structures Recently Completed

The caisson method involved casting concrete walls at the surface in approximately 3.5-meter (11.5 ft) heights and sinking these units into position using a cutting shoe to excavate the ground under the weight of the units and pumping bentonite behind the walls. Once the desired depth was reached for each caisson, the base slab was cast and the caisson structures grouted into place.

The benefits realized from using this method­­­­ology include a compact construction footprint and construction cost savings by eliminating the requirement for piling or other ground support systems for the underground structures.

With the caisson structures successfully constructed, installation of three submersible wastewater pumps, pipework installation, and construction of the roof slabs and aboveground structures are underway. Cold commissioning of the pump station is expected to be completed in February 2024. Delve Underground continues to work with McConnell Dowell during the construction phase, providing technical support and construction monitoring.