FABRIMAT erosion control protects large floodway drainage system
General Project Description
The Drouin by-pass section of the Princes Freeway under construction in Gippsland, Victoria, passes through rich agricultural land. It crosses a long narrow floodplain at Longwarry where it became necessary to not only increase the capacity of the existing Thompson’s Drain to eliminate future flooding of the Freeway at this point, but also to accommodate the increased run-off expected from the Freeway itself and have its discharge channelled under the Freeway by means of a large cuI vert.
To achieve this and prior to the commencement of final design, Vicroads carried out geological investigations of this particular area, which indicated the material to consist of 56% clay and 44% silt and extensive Hydrology Studies and Computer Model Analysis. This resulted in the acceptance of a design discharge of 25 cumecs along the open trapezoidal channel corresponding to a stream velocity of approximately 1.6 m/s.
Inlet and Outlet Fans
It was decided to design the Thompson’s Drain on the Constant Energy Principle which requires the flow to pass through inlet and outlet fans without significant form loss, i.e. the overland approach velocity needed to increase gradually from approximately 0.6 m/s to 1.6 m/s in the trapezoidal channel. This was achieved with a 40 m long fan and inlet crest of 60 m length.
Similarly the outlet fan required a 90 m long fan with a 60 m long outlet crest to allow a gradual transition from the 1.6 m/s channel flow in the 0.9 mls velocity entering the floodplain.
Erosion Protection
In view of the hydrological and geological conditions of the site it became evident that both inlet and outlet fans had to be protected from erosion during high flow conditions. Vicroads recommended the use of FABRIMAT Erosion Protection to Denbo Pty Ltd, the main contractor on this project.
FABRIMAT
This Erosion Control System of concrete filled nylon mattress construction is supplied and installed in Australia by Structural Systems Limited, Melbourne and Perth, as well as by authorised local FABRIMAT installers in NSW and Queensland.
The system consists of 100% nylon mattress fabric with filter points (acting as weep holes) woven into it at either 125 mm or 200 mm centres. The fabric is at either 125 mm or 200 mm centres. The fabric is generally supplied on 1.8 m wide rolls, each containing approximately 100 linear metres and is then cut and tailored to suit the particular area to be protected.
This can be done on site or alternatively large—200 m²—pre-assembled panels can be produced off-site and then finally joined together on-site using an electrically operated hand-held sewing machine.
The FABRIMAT style selected for the Thompson’s Drain Project was of the heavier 200 mm filter point type providing an erosion control mattress of a mass equivalent to approximately 160 kg/m².
The concrete grout of an approved FABRIMAT mix is pumped into the fabric from low points. In order to carefully control the flow of grout and contain it within certain boundaries the FABRIMAT revetment is divided into panels of approximately 100 m² each by means of “grout stops” sewn into the panels.
To retain FABRIMAT revetments at the outer perimeter—particularly at the high points—anchor trenches of approximate 0.5 m in depth are required. These anchor trenches are later backfilled with compacted soil or 20 MPa concrete where recommended.
Correct, neat and well compacted ground preparation is absolutely essential for any areas to be protected with FABRIMAT Erosion Control Mattresses, as the finished, installed FABRIMAT revetment follows the ground surface and thus reflects any ground irregularities.
A neat and very pleasing appearance was achieved with the FABRIMAT lining of the inlet and outlet fans of Thompson’s Drain due to the level of ground preparation carried out by the main contractor and subsequent “hand-trimming” of certain areas immediately prior to the installation of the FABRIMAT by Structural Systems Limited.
A total of 2400 m² of FABRIMAT erosion protection using the 200 mm filter point fabric was installed on this project at an overall cost of approximately $95,000.
FABRIMAT is supplied and installed by Structural Systems Limited, Melbourne (ph: 03-309 0055) and certain authorised interstate installers.
