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Rehabilitating Cut Slopes

Lyttleton Port Company, LPC
Area: 1000m2
Date: August 2017

Backstory

Benches had been cut during the Haul Road realignment which was subject to erosion. To remediate the loess clay cut faces, a geotextile erosion control blanket was installed and then hydroseeded.

In this case, the hydroseeding methodology achieved little vegetative success. Overland sheet flow from the slope above continued to blow out and erode the cut faces with every rainfall. The protection of the harbor against sedimentation also remained a concern.

Challenges

The challenge was to design and safely install a cost competitive land remediation package to protect and stabilize the loess clay cut faces above the Haul Road. Our health and safety policy and planning needed to interface with two contractors as Sumner Rd above was being remediated by Mc Connell. Dowell and the Haul Road cut face remediation was being managed by Fulton Hogan.

To install the Enviroberms, we needed to work on steep terrain with a gradient of 1:2. A major factor was the risk of falling from height due to the gradient, this required rope assisted access. As we were exposed to the elements of cold and high wind on this project, we maintained constant monitoring and active management to ensure everyone's personal safety.

The Solution

Our solution was twofold. First we addressed the source of the water flow at the top of the slope by installing a series of Enviroberm running laterally across the catchment area. In doing so, the slope was bisected, reducing the velocity of overland runoff and channeling water away from the cut faces below. Secondly, we installed a seeded vertical Enviroblanket™ on the loess clay face to establish vegetative cover to effectively bind and stabilize the face.

Red Tree partnered with Canterbury Woodchip employing state of the art pneumatic blowing technology. Canterbury Woodchip has the capacity to bulk haul 20 m3 of engineered compost blend to the site and install the product 200 meters horizontally, and lift 60 meters vertically. Using customised fines and a calibrated seed distribution mechanism the seeded Enviroblanket™ was blown into the existing geotextile matrix which contained it against the loess face.

Our Outcome

Although the Enviroberm are only 600 mm in diameter and 300 mm in height, the compost matrix absorbs five times its weight in water. Enviroberm have performed to specification on the slope with water temporarily pooling behind them, filtering through and slowing the sheet flow. Water has been channeled away from the loess clay cut faces and no further erosion or blow outs have occurred. Subsequently, the berms have become permanently vegetated and are now a long-term land stabilisation asset.

Photo Below Left: Enviroblanket application to vertical face, over geotextile.

Photo Below Middle: Vegetative germination after 6 weeks.

Photo Below Right: Vegetated cover after 6 months. Clover/Rye blend left and Hydroseed right.