I REFER to the letter by Mr Graeme Roberts headlined, 'Add some colour to the slopes' (South China Morning Post, November 28) concerning the use of shotcrete on slopes. Shotcrete is normally used only when a quick but effective treatment is required to prevent landslip and erosion of potentially hazardous slopes. Trials with colouring shotcrete have been carried out for some time with varied success. Basically, it amounts to a camouflage approach. The experience so far is that it is difficult to select colours which achieve a natural appearance all year round. For example, unlike colour finishes, the adjacent vegetation changes colour seasonally with predominantly yellow/brown tones in winter-time and more lush green tones in summer. In many cases the coloured concrete can become a greater eyesore as the paint or concrete colouring soon fades, stains or flakes off creating a shabby, tacky appearance. Adding colour to shotcrete inevitably costs more. When such additional cost is warranted for slope treatment in sensitive areas, this department, drawing upon past and current experience, considers that the additional money is better spent in adopting a more natural, durable and therefore cost effective treatment wherever possible. These methods include re-vegetation using grass with shrubs and tree planting where the nature of the slope and geotechnical safety factors permit. Where this is not possible, for example on rock slopes or very steep slopes, then either the cut rock face is retained or where space permits, planters are added at the bottom of the slope and on any intermediate berms. Screen planting and self-clinging vines are then provided to eventually clothe the slope. The shotcrete slope is a necessary evil for emergency slope work and is unlikely to disappear in the foreseeable future. Trials with colour finishes continue but fighting the 'grey fungus' with a 'green vine' is still considered the best long-term solution. HO SAU-PING, LUCY Senior Engineer/Highways Complaints Highways Department
