Introduction

In March 1987, a large area of steeply sloping ground was regraded to 45????? in order to improve stability. The newly exposed face required erosion protection to prevent the formation of deep gullies and to improve the long-term appearance.

3,015m2 of “Tensar” Mat was installed on the slope. Grass was quickly established and the performance during the first year has been satisfactory.

Site Description

The site is located adjacent to Wong Nai Chung Road on the west side of Leighton Hill in the northern part of Hong Kong Island. The original slope angle varied from 60????? to 85????? supported at the toe by a 2m high masonry retaining wall. The slope comprises decomposed granitic material, with some slope-wash deposits. The surface was very uneven, reflecting the numerous small slip failures that had taken place.

As part of their on-going investigation of slopes in Hong Kong, the G.C.O. carried out an extensive site investigation and determined that the slope stability was below the required standard. Remedial measures were therefore proposed.

Design Proposals for Remedial Works

A 14m high retaining wall was proposed to support the lower part of the slope. Above this, the slope was to be re-profiled to a gradient of 1:1 and surface drainage would be installed.

Remedial Works

The slope at the toe was cut back to an angle of 62????? and a 14m high retaining wall was constructed against the newly cut face. Above this the slope was re-profiled to an angle of 45?????. The exposure of very loose fill close to the top led to the upper part of the slope being reduced to 35?????.

Erosion Protection to 45????? and 35????? Slopes

The original slope had been well vegetated with a good tree covering and it was proposed that the slope should be replanted if possible. G.C.O. proposed to hydroseed the slope to provide erosion protection and to follow this with the planting of shrubs.

“Tensar” Mat was finally chosen on the basis of its low cost and ease of installation compared to the concrete systems, its long-term durability and very important in this case, the natural appearance of the finished slope. The high strength of the mat would enable it to be installed on the steep slope without risk of tearing. The flexibility or drapability of the mat would ensure that it maintained intimate contact with the re-profiled slope since close contact with the slope face over the entire area is critical to prevent wash out of soil beneath the mat.

Installation of the “Tensar’ Mat

For this project G.C.O. chose to apply a double spray of hydroseed. The first spray was applied to the re-profiled slope prior to placing the “Tensar” Mat.

The mat was unrolled down the slope face. Steel staples were used to fix the mat at the top of the slope and at 1.5m vertical and horizontal centers. This work was made easier by the use of ropes and ladders for access. Owing to the soft nature of the soil, G.C.O. specified 1.0m long staples fabricated from 10mm diameter ribbed reinforcing bar.

The “Tensar” Mat was continued under the concrete support to the surface drains installed across the slope face.

After fixing the mat in position, topsoil was tipped down the slope and brushed into the mat, filling it to its full depth.

The second spraying of hydroseed was then carried out. Each application of hydroseed consisted of:

Most of the “Tensar” Mat was installed in March 1987. A good lush grass growth was established by the end of April when the rainy season commenced.

The quilted mesh structure of “Tensar” Mat succeeded in retaining the topsoil and newly rooted grass throughout the period of heavy rain in April/May.

One section of the 45????? slope was left without “Tensar” Mat during this period to allow for additional profiling. By the end of May, this section had no grass growth and was highly eroded, with deep scouring. By comparison, the section with “Tensar” Mat had achieved good grass growth with negligible soil loss.

The entire slope has now been completed using “Tensar” Mat and is fully grassed.