Client: Highgate Commercial Ltd

Location: The site is located in Silverdale, running parallel with State Highway 1 – Auckland's Northern Motorway

The site initially had a stream running through the middle with 14 m cuts on either side, thereby making environmental controls challenging.

Highgate is exposed to a prevailing south westerly wind. This made dust suppression a difficult task to mitigate. Ross Reid had to have two water carts running full time during the peak season.

Two slip planes had been identified on the batters running parral to SH1 which needed to be broken by Shear key excavations.

Over 3 kilometres of subsoil trenching needed to be placed on the ground to link to the SH1 culvert and to pick up any underground springs.

1,100,000 cubic meters of material needed to be shifted to be able to commence the civil infrastructure.

A dam needed to be constructed to withstand the head from 63,000 cubic meters of water. This was located directly above the SH1 culverts, making environmental control extremely technical.

Material

The Northland Allochthon Bedrock is described as having highly sheared and crushed mudstones, sandstones, muddy limestones and siltstones with comparatively hard sandstone concretions. The Northland Allochthon can have deep-seated shear planes due to the internal structure of the rock mass, and can deteriorate rapidly when left exposed to weathering.
Geotechnical ground improvement methods comprised of:

• Combination of toe buttressing, shear keys, drainage and construction of MSE walls to increase deep seated / global slope stability;
• Placement of an impermeable clay cap layer at finished level surfaces where the Northland Allochthon bedrock was exposed to prevent the ingress of water.

Earthworks

• Shear Key excavations – The shear keys were designed to run parallel with SH1 and varied in depth from 7 to 11m at an average width of 6 m. A lot of planning and risk management was put in place in order for the programme to be met. As the keys were in unstable ground they needed to be made safe. This was achieved by battering away from the motorway and site at a gradient of 1 in 1 and installing megaflow drainage. See picture below.
• MSE Wall Construction – Op top of the Shear Keys sat a 1:1 and a 2.75:1 (70 degrees) mechanically stabilised earth walls. The wall sat at a maximum height of 14m above SH1, and it consisted of GX200 geogrids that were strategically placed and wrapped around to a maximum height of 14m above SH1. The grids ranged from lengths of 14m through to 30m. Each layer had 600mm of engineered fill placed on top compacted to 140kpa and a maximum allowance of 6% air voids. This proved complex and pre-mixing of the material needed to take place so that the moisture content was perfect before being placed. At the wall’s highest point 24 lifts were required.