Erosion & Sediment

/Erosion & Sediment
Erosion & Sediment2019-03-25T03:55:55+00:00

Silt Fence

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Permathene Silt Fence Fabric complies with the minimum requirements of ADM GD05 and TP90.
  • Grab tensile strength: >440N (ASTM D4632)
  • Tensile modulus: 0.140 pa (minimum)
  • Apparent opening Size: 0.1 – 0.5 mm (ASTM D4751).


Silt Fence is used to help prevent contaminants from sediments entering streams, lakes, coastal waters. Construction run-off into drainage systems can severely impact New Zealand’s ecology unless contained. Silt Fence is only part of the solution.


The preferred method of installation is to trench the material using a silt fence trenching machine. In absence of the proper machinery several other methods are used. Silt fences must be installed to prevent run-off from passing around them, ends should be installed up slope.

A trench is constructed, to a width of 100mm x 200mm depth, minimum. Proper installation requires the Silt Fence be anchored in the trench preferably with the fabric covering the bottom of the trench. Backfill using earth moving equipment, followed by compacting.

Stakes should be either tanalised timber of 50mm square or steel star stakes of 1.5m length. Drive the stakes into the ground to a minimum depth of 400mm, or 600mm on a slope of 3:1 or greater.

Spacing of stakes is at a minimum distance of 2m apart. Always install the silt fence with the stakes behind the flow.

Support fence is recommended to be installed along the entire length along the upslope side of the fence from the top to the bottom of the silt fence. A support wire (2.5mm galvanised) is attached along the top of the fence.

Coir Mat

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Coir Mat is an erosion control material designed to either bio-degrade or photo-degrade. bio-degradable coir matting is 100% natural and contains no poly reinforcing. A photo-degradable coir mat does contain a poly netting to give the product a longer lifespan.

These products are designed for exposed slopes to prevent soil loss and are temporary (12 to 48 months depending upon the type and grade of material selected). These products will enhance water retention and allow good flow rate of water and nutrients into the soil until vegetation takes hold. The fully bio-degradable coir mat will naturally degrade over time and will leave no chemicals or plastic in the soil.

Coir mat


  • Over steepened slopes, road, rail embankments, industrial, mining restoration, earth dams
  • Drainage channels, culvert outlets, washout drains and emergency water courses
  • Highly erodible soils (embankments and sandy / silty soil areas, high gradient slopes)
  • Badly compacted areas, junctions with civil engineering works such as bridge abutments and inaccessible areas
  • Unpredictable or low rainfall areas
  • Heavy rainfall areas
  • Irrigated areas
  • Channels, coastal and sea shores – shore protection Public and private parks, terraces, dams


Geocell is an effective cellular confinement system providing a cost-effective solution for ground stabilisation, erosion control and earth retention on slopes. An innovative geosynthetic product Geocell is a lightweight and flexible cellular structure made from connected polyethylene strips. By ultrasonically bonding the strips together an extremely strong and flexible configuration is produced. A variety infill materials can be filled into the geocell system; aggregate, concrete, sand, soil, etc.



  • Slope Protection: Prevents sliding by providing sufficient weight at the foot and along the face of the slope
  • Earth Protection: Prevents erosion actions of wind and water on exposed soils
  • Channel Protection: Protects channel slopes from soil erosion and confines the infill materials of channel bed and banks

Slope Protection Applications

Most slope protection applications utilize 10 cm (4 in) depth, standard cell sections. However, this is a function of (1) the potential minimum angle of repose (fiction angle) of the infill material, (2) the slope inclination, (3) the slope length of the individual cell, (4) the depth of the cell, and (5) the characteristics of the infill material.

Cell size is governed by slope geometry and design cover thickness.

Steep slopes must have global stability or be internally reinforced before application of the geocell system. The single-layer geocell system has been used on slopes as steep as 75 degrees. The controlling factors are the angle of repose (friction angle) of the infill material and slope inclination. Steep slope applications will require additional anchoring mechanisms such as polymeric tendons with the anchor system.

Anchor methods used to secure the geocell system to a slope include wood stakes, metal J-pins, and other earth / tension anchors. Stakes bear against the top of the cell wall or against tendons passing through the cell. Anchor array spacing is determined through static analysis methods.

A variety of polymeric tendons covering a range of tensile strengths are available to meet anchorage requirements when anchor stakes cannot be used for the following reasons (1) they become too numerous, (2) geomembrane liner cannot be punctured, or (3) rock is present in the embankment. Integral polymeric tendons are tied to restraint pins to transfer sliding forces to the tendons. The tendons are then attached to an anchoring system at the top of the slope. Spacing and quantity of tendons are determined through static analysis methods.

Generally sections are connected to prevent relative movement of sections during the infilling operation using rivets or heavy-duty metal staples applied with a pneumatic stapler. A variety of rivets/ staples are available to meet site environmental conditions.

Advantages of the Perforated Cells

  • In load support applications, a high degree of frictional interaction is developed between the aggregate infill and the cell wall that directly increases the stiffness of the system by reducing the ability of a shear plane developing between the infill and the cell wall.
  • The perforations allows water to move from cell to cell reducing undesirable cell ponding and providing lateral drainage in all application areas.
  • In vegetated slope and channel protection systems, roots can grow through the perforations increasing the stability of the vegetated cover when subjected to gravitational and hydrodynamic forces.