The North Surrey Interceptor (NSI) forms part of a large trunk sanitary sewer system which is owned and operated by Metro Vancouver. Municipal sanitary sewer systems located in North Surrey, Maple Ridge and Pitt Meadows flow into the NSI, which delivers the wastewater to the Annacis Island Wastewater Treatment Plant for treatment.

A condition assessment of the NSI section between Scott Road and Grace Road indicated that it is in a deteriorated condition and that rehabilitation of the existing sewer is needed to sustain ongoing safe operation. Projected population growth in the catchment area also indicates that a significant increase in the existing NSI capacity is needed to meet future projected flows.

The existing NSI section located between Scott Road and Grace Road is a cast-in-place 1.4 m wide x 1.7 m high reinforced box culvert support by piles and which was constructed in 1974 using open cut methods.
Metro Vancouver intends duplicating the existing NSI between Scott Road and Grace Road to allow for bypassing of the existing NSI during rehabilitation of the existing box culvert and to increase the capacity of this section of the NSI to carry future projected flows. Options for twinning the existing conduit by typical trench installation were reviewed by Metro Vancouver, but this methodology presented technical and constructability challenges, significant environmental and social impacts, as well as high cost. Metro Vancouver has therefore proposed to twin the existing conduit using an alternative alignment along River Road with construction of the new sewer pipeline using microtunnelling methods.

GHD has undertaken the preliminary design for a new 830m long 2.7 m diameter sewer tunnel coupled with a 50 m long 2.4 m x 2.4 m box culvert that is designed to carry a future design flow of 5,000 L/s.

GHD performed preliminary design for the new sanitary sewer system, including a review of system hydraulics, geotechnical and geo-environmental field investigations, environmental studies, permitting, alignment selections and documentation of the preliminary design. The pipe alignment was selected with consideration of operations & hydraulics, land matters, and geotechnical, environmental, & social considerations. Hydraulic modelling was undertaken to inform the pipe size and elevation. Shafts and maintenance holes were sized for operational and construction requirements.

Partnering with Thurber, a geotechnical field program was undertaken to investigate subsurface conditions. This investigation found that both ends of the pipe will be excavated in soft ground conditions such as clays, silt and organic materials, and the middle section is comprised of sand and gravel layers. The tunnel horizon is below the water table and traverses a section with artesian conditions.

Seismic performance requirements were nominated, soil units with potentially liquefiable materials determined, and conceptual ground improvement requirements described.

An environmental and social impact assessment was undertaken to identify applicable constraints and permitting requirements. This included a review of regulatory requirements, field studies, and determination of permitting requirements.

Microtunnelling construction methods are proposed, coupled with open cut construction for the box culvert.

This project improves the resiliency of critical infrastructure serving North Surrey, Maple Ridge and Pitt Meadows communities for the next 50 years by replacing a degraded and undersized sewer.
The challenging design prepared for Metro Vancouver included 830m long 2.7m diameter tunnel with a 2.4m x 2.4m box culvert supported on piles. The design addressed soils with liquefaction potential, artesian groundwater pressures, in a dense urban setting with environmentally sensitive areas.