Tirau SH1 Geotextile Stabilisation

A cut batter at the base of a steep hillside on State Highway 1 west of Tirau had been collapsing into the road for several years. The substrate was ancient alluvial silts associated with the Waikato River.

This material was very soft and prone to wind erosion and slumping. The steep slope above the cut batter meant that benching was not an option.

The design called for hydroseeding followed by the application of 450 gram per square metre (gsm) coir (coconut fibre) matting.  This was held in place using 20mm galvanized wire ground staples.  The coir matting protected the soil surface from wind and rain impact until grass vegetation had become established.

The slope was then covered with high density polyethylene (HDPE) bi-axial (30 kilo-newton) geogrid anchored to the slope with 650mm long steel pins at one metre centres. The bi-axial geogrid provided the structural strength, ensuring that the surface layer of vegetation root mass will not shear off in the future.

We also not only fenced along the top with a conventional farm fence but added a single wire electric fence above the cut so cattle wouldn’t damage and start erosion.

Challenging at times with SH1 traffic and the steepness of the slope but with correct procedures and the use of abseiling along with a cherry picker the job was a success.