Herdsman Business Park, WA

Client: Herdsman Business Park
Location: Herdsman Lake, Osborne Park, Western Australia
Completion Date: 1982

This project was extremely challenging as apart from the dredging, it involved complex dewatering and materials handling including separation techniques.

At commencement, the whole project area was swamp land with up to 9 metres of peat overlying the structural sand base and the dredging borrow pit areas. The surface had typhus reeds growing over it which were up to 3 metres high and extremely thick. In winter and spring the surface was covered with up to 1 metre of water.

As the site had mining leases pegged over it and there were environmental groups trying to stop mining on the lake, our development permits required that no materials were allowed to be removed from the site.

CGC Group was both the developer and contractor on this site. This meant that we were also heavily involved in the initial design and approval processes. The biggest challenge initially was that the design had to cater for production and use of all materials from within the project boundaries.

The first process involved peat stripping from the structural land development areas of approximately 1.0 million cubic metres of peat. We used special low ground pressure earthmoving equipment and also special dewatering equipment and techniques.

Most of the peat materials were solar dried and used to form public open space. Other peat was used to backfill the lake floor where sand was excavated by the dredge. The created reserves in turn shielded out the lake water to allow dewatering for the peat removal. This reciprocal necessity created many challenges early on in the project life.

The second process involved filling the structural areas by dredging approximately 2.0 million m3 of sand from underneath the peat layers overlying the surface of the lake. Challenges included gravity separation of the peat and reeds from the dredge outflow. Engineering design criteria required deleterious materials had to be kept below 2% of the final dredge spoil makeup and this had to be in any sample taken.

Reed blockages in the pumps and cutter head were also a major problem which was solved with innovations generated within the CGC Group. As the peat stripping area was up to 9 metres below lake surface water level, the dredge spoil flowed to the bottom of a reclamation sump where a large sump pump was used to return the wash water to the dredge pond. This pump had to handle peat lumps and reeds from the dredging operation.

The third process involved dredging thirty hectares of shallow lakes into the completed sand extraction voids of the lake. The thick typhus reeds and heavy grasses overlying the peat were the main challenge and these began to again block the cutter head and pumps as well as the anchors, sheaves and wires. A further round of re-engineering overcame these challenges and the lakes were completed on time.

Civil services were then installed and the blocks were then top soiled with material generated from the lake excavations.

The project was generated and released in three stages and is now a fully established estate.