McLeans Pit Landfill – Cell 3a

Client: Grey District Council
Contract value: 1.3m
Contract period: April 2018 – January 2019

Scope:
• Construction of a new landfill cell, groundwater drainage system, leachate collection network and new pumped leachate rising main.

Technical Difficulty:

Cell 3a is constructed adjacent to the currently operational cell 2. Some 60,000m3 of gravel was extracted and stockpiled from the Cell 3a site to expose the underlying siltstone.

A groundwater drainage system was constructed around the perimeter of the footprint of the new cell. Once installed this allowed for the winning and processing of siltstone bunds and sub-liner. A system of sequential ripping, track rolling and padfoot rolling was developed, tested and employed to construct the impermeable sub-liner.

Meticulous preparation of the sub-liner was required to lay Geosynthetic Clay Liner (GCL), which was over laid with a 100mm sand protection layer, and a 300mm leachate collection layer. Progress of each stage of this process was staged to work with the notoriously wet West Coast winter climate. This required sealing, compressing and ballasting of all GCL each night prior to leaving site. No traffic was permitted to travel on the cell floor without 600mm of cover over GCL. A series of elevated tracks were constructed to allow for construction of the leachate collection network.

All aggregates, including drainage stone, sand and leachate collection stone was site won, reducing transport costs, and providing a saving to the Principal.

The Leachate collection system was constructed using a network of perforated and non-perforated PE pipe. Tees and bends were custom built using extrusion welding techniques. A submersible pump and PE rising main was installed to transfer leachate from the new cell to the treatment ponds some 330m away. Additional power reticulation and control was installed around the site to service and control the new leachate collection system