Trying to be more environmentally friendly
I refer to the letter by Alex Tam headlined, 'Spare the hills from concrete' (South China Morning Post, November 25), regarding the use of sprayed concrete as part of the slope improvement works carried out by this office along Wong Nai Chung Gap Road.
To protect the public from landslide danger, we upgrade substandard man-made slopes under the Landslip Preventive Measures (LPM) Programme. Three slopes along Wong Nai Chung Gap Road have recently been upgraded by installation of soil nails under the LPM Programme.
The slope surface covering has also been improved from a soil cement plaster, commonly called chunam, to sprayed concrete, that is, from one type of hard surfacing to another which has better maintenance performance. I agree that the new surfaces are not visually appealing, however, in just a few months the new colour will begin to fade to a more natural look. As part of our effort to reduce the visual impact of the hard surfaces, tree rings, planter toe walls, and holes in the surface for creeping plants have been installed on these slopes.
Your correspondent also raises the wider issue of more environmentally friendly ways to stabilise slopes.
With the increased attention given to slope maintenance and accelerated Landslip Preventive Measures, there has been some criticism that the Government is pushing ahead with safety measures without sufficient concern for environmental impact, in particular regarding the use of sprayed concrete for slope surfacing.
It is our policy that a soft 'green' cover is given priority over a rigid slope surfacing for upgrading works under the LPM Programme and we have achieved many successful cases in this regard. However, it is important to realise that this is not always feasible.
There are real concerns on safety grounds for cases where there is a need to minimise infiltration of water into a slope face, or where site constraints prevent cutting back the slope to a relatively gentle angle suitable for establishing vegetation.
Mature trees are preserved wherever practicable. Where hard surfacing is unavoidable, the introduction of more natural colouring into the sprayed concrete is now commonly used to soften the appearance and where possible tree rings are provided to accommodate existing trees or new planting. However, it takes time for existing vegetation and new planting to become well established.
The Geotechnical Engineering Office continues to carry out research and development on the use of vegetation on slopes, including methods of erosion control to help establish vegetation on steep slopes and the effects of vegetation on slope stability. While I acknowledge that vegetation removal and stark sprayed concrete are undesirable, this effect is relatively short term. In just a few months the sprayed concrete colours usually fade to a more natural look, and in just a few years vegetation is re-established on other parts of the slope. The reduction in landslide risk to the public is however immediate.
We share the concern of the community about loss of vegetation to slopes just as we share their greater concerns with regard to slope safety. We will continue to strive to meet the expectations of the community for the former while achieving the latter.
R.K.S. CHAN Principal Government Geotechnical Engineer
