 Band drains assist in the acceleration of the ground consolidation process.
Consolidation process
The construction of earth embankments and the general raising of site levels imposes an increased pressure on the subsoil. When the subsoil predominately consists of sands or gravels, the resulting settlements are generally small and occur rapidly. If the subsoil contains any cohesive, highly compressible layers such as soft clay, peat or loam, the increased pressures will cause consolidation settlements. These may take a considerable time to occur, particularly when the cohesive layers are thick.
Settlement times can be reduced by placing permeable vertical drains in the cohesive soil at relatively close centres - typically on a 1m to 3m grid. The drains reduce the length of the drainage path, thus accelerating consolidation.
Generally the COLBOND CX1000 vertical drain is used. This type of drain has been has been found to be particularly effective and robust. | Installation of prefabricated drains
Prefabricated drains are placed in the subsoil by means of a mandrel (steel tube of tubular sections), which is guided along the leader of a piling frame. Vibro Projects can typically install drains to depths up to 20m. A machine operated by a two man crew can install around 3000m of drain a day. The drains are installed from the top of a suitably thick, wholly granular, working platform drainage blanket, typically 300-600mm thick.
On extraction of the mandrel the band drain remains in the soil by way of an anchor plate - attached to the bottom end whilst the top will be cut off 150mm above working platform level.
A drainage blanket will be placed over the site surface before the drains are installed to provide a level working platform for the rig and subsequently to act as a drainage layer. |
How It Works - ANCHORING THE DRAIN AT BASE LEVEL. The COLBOND drain is anchored by a flexible steel or plastic shoe which remains in the soil when the mandrel is withdrawn.
- The drain is usually anchored to the shoe by staples. During installation the lower end of the mandrel is closed off by the shoe to prevent the ingress of water and sand.
- DISCHARGE OF THE EXPELLED PORE WATER.
The water passing through the drains must ultimately be discharged sideways at ground level. The drains must discharge into a layer of permeable granular material. A thin layer will serve the purpose in most cases. This, and the drainage blanket material above will not only serve as the working platform but also to drain pore waters away from the loaded area.
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