Newsletter No. 202

CUHK Newsletter No. 202 4th May 2002 3 Safe and Green No More Slipping and Sliding of Slopes on Campus T he University campus contains some 300 o f Ho ng K o n g ' s 60 , 000 registered slopes. They comprise a substantial part o f the wo rk o f the Un i ve r s i t y 's Campus Development O f f i c e ( CDO ) , w h i c h c o n d u c ts stabilization works on an average of 12 slopes each year. To keep the campus green, the CDO not only avoids cutting down rare p l ant species, but also maximizes green coverage on slopes, i nc l ud i ng steep ones that are less conducive to plant growth. To this end, the CDO carries out systematic research and development on surface protection and green technology, exploring the use of bio-engineering, cultivation on fibred or reinforced planting soil topped with organic surface materials, and the growth of plants on a large and sustainable scale. The slopes on campus can be classified in terms of gradient into gentle (45 degrees or below, see Fig. 1), steep (between 45 and 70 degrees, see Fig. 2), and sub-vertical (70 degrees or above, see Fig. 3). Different greening technology is implemented on these slopes. Depend i ng on the t echno l ogy employed, slope landscaping can be c l as s i f i ed i n to soft or hard. So ft landscaping involves simpler greening technologies including hydro-seeding. i.e. spraying grass seeds on the slope, then covering it with a control mat to prevent erosion and protect the seeds from birds. It is often used for gentle slopes and certain steep slopes which require little or no surface protection. Hard landscaping is used for most steep and all sub-vertical slopes, which require more surface protection. Hard landscaping ranges from stone-pitching to cutting slopes to reduce the gradient, building retaining/planter walls, and installing planter holes. In the past, when engineering sense was focussed mainly on safety, stone pitching and sprayed concrete were the usual methods for protecting surface erosion. But these have been replaced or complemented by technologies that produce environment friendly and aesthetically pleasing results, such as hydro-mulching, the spraying of a sticky reinforcing material containing organic peat, then spraying grass seeds or planting shrubs on top of this material. The resulting adhesion also provides p r o t ec t i on to slope surface. Such technology has the dual effects o f greening the slope and protecting surface soil. Mr. Francis Wong, geotechnical engineer of the CDO, pointed out that, besides the slope's gradient, the decision to use soft or hard landscaping or a combination of b o t h a c t u a l l y depends also on m a n y o t h e r factors, such as the slope's height, its geology, groundwater t a b l e , l o c a t i o n a nd orientation. the properties of its surface soil and the local climate such as humidity, sunlight, and temperature changes. For example, for a relatively short (four-metre) and gentle (30-degree) slope (Fig. 4) extending from below the platform supporting the swimming pool to a natural stream course behind the Theology Building, both hard and soft landscaping are required. The slopes on either side of the stream course are hydro- seeded, whereas the entire length in the middle, serving almost as a storm water channel where water may flow through in the rainy seasons, is stone-pitched to prevent erosion. Sub-vertical slopes, consisting usually of a mixture of rock and soil, are not an ideal environment for the growth o f vegetation, except where there is underground water or in crevices between the rocks. A l t h o u gh greening and stabilization techniques for gentle and steep slopes can be employed for such slopes, the outcome is often less than satisfactory. More research is necessary to further improve on green coverage and to overcome the challenges of sub-vertical slopes. The CDO is currently studying, using the experiences of other places such as Taiwan and Japan as reference, how to achieve even growth of vegetation on sub- vertical slopes where underground water is irregularly distributed. It is also exploring how to grow diverse species of plants on different slopes. Is it possible, for example, to turn a stubborn sub-vertical slope sprayed with artificial engineering soil into a sloping flower-pot or herbal garden? The CDO has begun, in April, to adopt bioengineering technology for soft landscaping. In the process, young grass seedlings with very long roots are transplanted onto a slope in the hope of achieving surface stabilization (through their roots) and greening with one stone. This method has been used with success in Southeast Asia, Spain, and ma i n l and China. The CDO is also examining how to reduce maintenance and installation costs, and improve the quality of materials used. The continuous exploration of such technologies has important implications not only w i t h in but also beyond the University's gates. I f plants can be grown and their growth sustained on engineering soil on a slope, then perhaps flowers can be grown in Mongkok or Causeway Bay where they are most needed but least expected. Piera Chen Fig. 1 A gentle slope behind the Elisabeth Luce Moore Library on Chung Chi campus Fig. 2 A steep slope along Campus Circuit Road — once the highest slope on the campus. It has now been made more gentle in addition to being hydro-seeded and covered by an erosion control mat. Over 1,000 soil nails have been inserted into the slope to prevent erosion. Fig. 3 A sub-vertical slope to the south of the Tsang Shiu Tim Building on United College campus Fig. 4 Stream course behind the Theology Building and under the swimming pool, displaying features of both soft and hard landscaping.