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Shotcreting of slopes acceptable if vegetation is also used

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Private developers are using shotcrete as an easy and low-maintenance solution for their slopes, and they avidly avoid greenery.

Shotcrete has an unwarranted bad reputation as it does not have to be an eyesore or result in destroyed vegetation ('Shotcreting often needless, critics charge', January 3). It can be an efficient, cost-effective and environmentally friendly method of fixing slopes. Unfortunately, Hong Kong's contractors normally spray an impervious layer of concrete over the whole area, which prohibits any moisture from penetrating the soil and therefore vegetation cannot be sustained.

There is a misconception that any surface cracking or plant growth will weaken the effectiveness of shotcrete and will harm slope stability. Slopes do not become unstable in normal wet conditions. It is only in deluge situations when the extreme amount of water is unable to evacuate the slope fast enough to stop two situations developing that will potentially destabilise a slope. Firstly, the water saturates the soil and emulsifies the soil particles, and secondly the increased weight causes the land to slide over the rock base.

The effectiveness of shotcrete is that it prevents surface soil erosion and acts as a raincoat to stop water inundation. If narrow horizontal cracks are made across the slope, some rainwater will permeate into the soil and encourage plant growth. Such cracking is no problem to the cover itself as the wire mesh holds the concrete in place, and plant roots help stabilise slopes. Indeed, potted seedlings can be inserted into the shotcrete cover. However, where there is a cloudburst and a downpour, the shotcrete will still perform its job of clearing storm water from the surface of the slope quickly.

In a short time, vegetation sprouts, mosses flourish, leaf mould covers and the cement weathers. Then, shotcrete blends into the natural environment far better than nylon matting and exposed plastic covered wire mesh.

Frank Lee, Mid-Levels

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