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NATURAL REACTION

SLOPE ENGINEERING

A slope stabilisation technique pioneered in Japan and successfully used in Hong Kong is being introduced into the UK. Peter Sunderland and Noriman Mak report.

Lack of land for development has for years forced Hong Kong to dig into slopes to create new sites and, increasingly, for road widening. So although it is much smaller than the UK, Hong Kong probably has more slopes needing protection.

Shotcrete was traditionally used as a protective cover to prevent surface erosion and hence provide slope stability, even under the territory's severe summer typhoons.

Its major drawback is that the supposedly impermeable and inflexible layer not only causes an increase in local water pressure, but tends to crack and deteriorate with time, thus causing the washing out of material behind.

As a shotcreted slope needs regular maintenance and has to be refurbished every five years, engineers are beginning to question the initial cost advantages of this approach.

Raised environmental awareness means that the sight of so many hillsides covered with concrete is now considered an unacceptable blight on the landscape.

Hong Kong's search for a better way of producing stable, vegetated steep slopes led three years ago to the adoption of a technique developed in Japan and used there since the early 1990s - continuous fibre soil reinforcement.

The Geofiber method has since been used in Hong Kong on more than 20 projects. Hong Kong Construction (Technology) has secured the worldwide rights to market the technique and is now introducing it to the British market.

In Hong Kong its use has been carried out with the support of the Highways Department and under the supervision of consultants such as Ove Arup & Partners Hong Kong, Maunsell Geotechnical Services and Atkins China.

The Geofiber concept is to simulate natural weathered ground conditions with established root systems, creating a good environment for re-establishing local vegetation - a stable soil zone. Natural roots form in the artificial layer and extend through it, creating a strong bond with the hillside. The inert artificial fibres remain in the soil, providing an extra safety factor for the slope.

The method consists of protecting the slope with a 200mm thick layer of sandy soil continuously reinforced with multi-filament polyester fibre. As the soil is sprayed on to the slope it is mixed with a continuous supply of fibre from a thread feeder, giving three-dimensional reinforcement directly in contact with the slope.

This is a significant improvement over the traditional use of geotextile which only provides two-dimensional reinforcement. The technique increases the soil's shear strength, enabling it to be placed on surfaces sloping at up to 70infinity.

In trials the reinforced soil has shown considerably better erosion resistance, making the method suitable for river bank protection. The permeability is about the same as the natural soil permeability, so it does not adversely affect natural drainage patterns.

Fibres on the surface give protection against erosion by run-off and the natural characteristics of the soil allow internal drainage to pre-installed drainage mats within the reinforced layer.

Installation begins by remedying any basic instability in the slope, for example removal of overburden, soil nailing or rock bolting, usually by roped access. Any loose material is also removed.

Reinforcing bars with a short right-angled length at the protruding end are driven into the face to integrate the continuous fibrereinforced soil body with the slope surface behind. These keep the artificial layer in place and limit deformations.

Drainage mats are laid down the slope which connect to a toe drain to allow groundwater discharge along the interface.

The sandy soil is sprayed on to the face, using pressurised air, by a wet sand sprayer, typically a machine that would be used for shotcreting. The fibres are pulled from their reels by high pressure water and sprayed through four nozzles of the ejector, together with the sand, at the foot of the slope.

The combined mixture is placed in layers a minimum of 200mm wide and about 500mm thick, sloping backwards to the slope at about 10infinity to 15infinity to ensure stability within the layer.

The material becomes stable very rapidly, sufficient for site workers to use it as their working platform immediately after it is placed. To enhance the nutritive properties of the Geofiber material, fertiliser blocks are placed at between 0.5m and 1m intervals during installation.

Other greening methods, such as hydroseeding; pots, tiles or gabions for turf; and cemented soil fibre mulch have shown additional longterm costs. Any irregularities on the underlying slope can result in voids and the thin plant base for grass is not able to withstand sharp slope gradients or strong surface run-off. Growth of trees and shrubs is impossible as there is limited room for any natural root system to develop further.

Reinforced soil is an ideal medium for establishing natural growth on the slope.

Traditionally such surfaces have been hydroseeded to give an almost instant greening, but the alternative now being favoured is to allow natural seeding from the surrounding vegetation.

This has been shown to be effective and results in the characteristics of local flora being maintained. Overall maintenance is therefore minimal as the ecological equilibrium is restored.

The Geofiber method offers a cost-effective solution for slope protection as it enhances the environment and very little maintenance is needed once installed.

Although the method is extensively used in Hong Kong for steep slope protection, the advantages of the method make it suitable for use in the UK for embankment and cutting widening, as well as for the greening and protection of slopes.

It will be particularly suitable where the natural soil is sandy, or where soil is being imported. Natural material can be used after it has been passed through a grille to remove gravel too large to be sprayed.

Where soil is being imported, the use of Geofiber allows a much steeper slope to be formed, creating more space, requiring less material, or reducing the need for other concrete-based retaining structures.

In all cases vegetation will thrive and the natural growth of indigenous species will be encouraged.

Noriman Mak is managing director of Hong Kong Construction (Civil Engineering) and Peter Sunderland is business development consultant for Hong Kong Construction.

Materials and characteristics

Sandy soil Maximum size 20mm, 50% of particles between 0.2mm to 1mm, uniformity coefficient 2 to 15, content of particle 75mm or smaller, 10% or below.

Fibre 100% polyester, 150 denier, tensile strength 4gf/denier, tensile strain 40% or below.

Composition The standard mix proportion is 3.3kg of fibre threads per m 3of soil.

The consistency of this mixture is gauged by a computer which measures the application rates.

Geofiber matrix Pseudo-cohesion, c'= 40-100 kN/m 2, Internal angle of friction, 33-35infinity, permeability coefficient k, around 0.1cm/sec

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