In order to successfully complete this project the sewer had to cross a major state highway.
Due to expansion of Johnsonville the main trunk sewer serving the suburb has become undersized leading to surcharging of wastewater back into local properties under certain conditions.
Maunsell was asked to carry out an investigation and design a sewer upgrade to remedy this issue.
The design consisted of laying a new sewer along Johnsonville Road, under State Highway 1 and alongside the highway to connect to the existing trunk sewer.
Open trenching across the highway was not an option due to the need to maintain full use of Wellington’s major arterial route in and out of the city.
Directional drilling was initially considered but the existence of possible culverts, the current trunk sewer line, the former centennial highway and uncertain backfill materials under the existing highway in the proposed alignment meant this option was abandoned.
An initial geotechnical investigation identified engineered fill, moderately to highly weathered greywacke and silt deposits and this together with the known obstacles that could be encountered meant that pipe jacking was chosen as the best methodology to insert the new sewer under State Highway 1.
In order to ensure the pipe jacking was not affected by any obstacles and to allow a person to work at the jacking face as necessary, an oversized 1,350 mm internal diameter pipe was designed to be thrust with the new sewer line inserted into this at the completion of the works.
The contract was tendered as a complete package including both trenched portions and the pipe jacking works. The successful tenderer, HEB Smithbridge engaged D Paku Ltd to undertake the pipe jacking portion of the works. D Paku is a specialist pipe jacking company that works throughout New Zealand.
Pipe jacking launch and receiving pits were installed using sheet piling and pipe jacking was undertaken conventionally using a hydraulic jacking rig. Due to the expectation of meeting the existing trunk sewer, the excavation was carried out using a hand driven technique rather than a mechanical borer.
During the pipe jacking operation a number of obstacles were encountered both running across and following the direction of jacking, including the current trunk sewer (flow was diverted through a pumped bypass), steel telecom cable ducts, mass concrete culverts, engineered fill and hard rock. These were all expertly managed by D Paku who maintained the design grade throughout the 65 m length of the pipe jacking works.
Maunsell had worked with HEB Smithbridge on a previous project in the Wellington region, but had no previous experience with D Paku. A strong working relationship was swiftly built up between the parties involved leading to successful delivery of the project.