Bioengineering and Landscaping to Slopes in the HKSAR Government's Landslip Preventive Measures Programme
(ACE Paper 28/99)
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INTRODUCTION
This paper explains the HKSAR Government's approach to the use of bioengineering and landscaping in the Landslip Preventive Measures (LPM) Programme managed by the Geotechnical Engineering Office (GEO) of the Civil Engineering Department (CED).
BACKGROUND
2.Since 1977, the GEO has implemented a long-term LPM Programme to investigate pre-GEO man-made slopes and retaining walls, and to carry out upgrading works on Government features which do not meet current safety standards. The work is done on behalf of other Government departments who then take back the features for long-term maintenance. The LPM Programme deals with three main types of man-made features: cut slopes, fill slopes and retaining walls (Figure 1). In designing slope upgrading works, the GEO is not only concerned with the stability aspects. Attention is also paid to the appearance of the finished slopes and retaining walls.
3.The work done in the LPM Programme has increased rapidly in recent years. From the late 1970s to the mid 1990s about 35 features were upgraded each year at an average annual cost of HK$60M. Under the 5-year Accelerated LPM Project from 1995-2000, about 900 features have been or will be treated (approximately 180 per year) at a total expenditure of about HK$2.6 Billion at MOD prices. Under the new 10-year Extended LPM Project from 2000-2010 an average of 250 slopes will be upgraded annually at a total estimated expenditure of about HK$9 Billion at MOD prices.
GREEN POLICY
4.Works Bureau Technical Circular No. 25/93 on the Control of Visual Impact of Slopes states that shotcrete (sprayed concrete) or other hard slope surface covers should only be used as a last resort. It is the GEO's policy to make finished upgraded slopes look as natural as possible in order to reduce their visual impact and improve the environment. The use of shotcrete or other hard surfacing is considered only after other techniques have been explored and found not practical or considered inadequate on safety grounds. Where the use of a hard surface cover is unavoidable, mitigation measures are introduced, such as the use of paint on the surface or colour pigments in the concrete, and addition of planter boxes and creepers.
INFLUENCE OF VEGETATION ON SLOPE STABILITY
5.Vegetation has both beneficial and adverse effects on slope stability, see Figure 2 and Table 1. Appreciation of the various effects by the design geotechnical engineer is important when deciding on the choice of surface cover for LPM upgrading works. Many of the effects are reasonably well understood but are not well quantified. Detailed investigations involving field monitoring and measurement are difficult to establish and replicate, and involve many uncertainties. Traditionally a conservative approach has been taken in LPM works, with a shotcrete cover often preferred to vegetation on the grounds of limiting direct infiltration of rain water and surface water into the slope.
INFLUENCE OF VEGETATION ON SURFACE EROSION
6.Apart from its effects on slope stability, a vegetated surface usually has a positive influence on limiting erosion of the slope surface in comparison with a bare surface. Many typical Hong Kong soils are relatively resistant to erosion but steep slopes cut into the weaker local soils can be highly susceptible to gully erosion. If left untreated, this can lead to large washouts of soil and trigger landslides. Growth of a densely-rooted grass and/or shrub surface is necessary for erosion control when a vegetated surface cover is selected in LPM works.
USE OF BIOENGINEERING & LANDSCAPING IN THE LPM PROGRAMME
7.Bioengineering, or the use of living vegetation to protect slopes and improve their appearance, has been used in the LPM Programme since the late 1970s. The role of vegetation was promoted in 1984 when the first technical guidelines on the use of grass, shrubs and trees were published in the second edition of the Geotechnical Manual for Slopes (GEO, 1984). Over the last 20 years the use of bioengineering and hard landscaping mitigation measures in the LPM Programme can be summarised as follows:
Fill Slopes
8.Improperly compacted old fill slopes are usually upgraded by excavating and recompacting the fill to a denser soil. Excavation normally involves complete removal of the existing vegetation cover. A grass surface applied by hydroseeding is the normal surface treatment for newly-recompacted fill slopes up to an angle of about 33o (Plates 1 and 2). Occasionally a hard cover (commonly shotcrete but sometimes pitched stone blocks) is used if there are special concerns over infiltration or susceptibility to surface erosion. The existence of mature deep-rooting trees has occasionally been used to justify not excavating or recompacting the slope, but this is only possible in rare cases where a sufficient extent, depth and strength of the root system can be fully demonstrated by ground investigation and testing. By their nature, fill slopes tend to be located on the downslope side of roads or building platforms and are generally not as visually obvious or intrusive as cut slopes when viewed from the road or platform level (Figure 1).
Soil Cut Slopes
9.A hydroseeded grass surface is the normal treatment for upgraded existing or cut-back soil slopes up to an angle of about 45o. Existing old soil cut slopes were often cut to steeper angles (up to 60o) and there are often constraints on cutting them back to shallower angles. Erosion control is a problem on very steep soil cuts. A variety of erosion protection mats in combination with grass surfaces were tried since the mid-1980s, with mixed success for slopes with an angle of 45o-60o (Plates 3 and 4). Shotcrete is still the most popular treatment for very steep soil cuts (Plates 5 and 6) but the use of a green cover has increased in recent years (Plates 7 and 8). To mitigate the effects of shotcrete, since the mid-1990s there has been an increasing trend to use slope toe, bench and weephole planters and coloured pigments or painted concrete surfaces (Plates 9 and 10).
Rock Cut Slopes
10.A full vegetation cover can be applied to rock slopes but only at considerable expense. Thick soil mulches and grass are used occasionally to disguise these slopes (Plate 11). Normally vegetation efforts are limited to the use of bench and toe planter boxes (Plate 12).
Retaining Walls
11.Stone facings and patterned block finishes of various designs are in regular use to break up large plain concrete surfaces (Plates 13 and 14). Occasionally decorative artwork is also used on very prominent walls, but again at considerable expense (Plates 15 and 16). More frequent use of toe and crest planter boxes has been a trend since the mid 1990s.
SURFACE PROTECTION OF SLOPES BEFORE/AFTER UPGRADING WORKS
12.It is the responsibility of the slope owner, either a government department or a private party, to regularly inspect and maintain their slopes. Of the approximately 54,000 sizeable man-made slopes registered in the GEO's New Catalogue of Slopes, around 30% belong to private owners. The rest are maintained by various government parties such as the Highways Department, Agriculture & Fisheries Department, Water Supplies Department, Architectural Services Department, Urban Services Department and Housing Department. In line with the above green policy, these departments are encouraged to leave vegetation on their slopes and only to use shotcrete as a last resort as part of their routine maintenance works. The annual recurrent cost of maintaining the approximately 38,000 government slopes in the Catalogue is around HK$800-1000M.
13.In the case of private slopes, the owners are responsible for any slope upgrading works, which are subject to the approval of the Building Authority under the Buildings Ordinance, and for subsequent maintenance. The GEO checks the designs of any upgrading works but has no direct control over the surface cover applied in such works.
14.For emergency remedial works to a slope after a landslide or severe surface erosion, safety must always come first. Under such circumstances, shotcrete is generally used as a quick and secure method of sealing the slope surface to reduce the immediate danger posed to the public and to avoid prolonged closure of the affected area, which may cause public inconvenience. The GEO normally advises the relevant Government department on emergency slope surface protection.
PUBLIC CONCERNS
15.The growing public concern over slope appearance in recent years is well appreciated by GEO staff involved in slope design and construction. There is a common shared view that bare concrete surfaces are environmentally unfriendly, but it is important to be realistic about landslide risk and public safety. Often there is little space available to carry out slope stabilisation work. Buildings, roads or other facilities are frequently located close to the top or bottom of cut and fill slopes. There are often severe constraints in cutting back slopes to a flatter angle and creating a gentle grassy surface. Even where slopes can be readily cut back, such as along highways in rural areas, there is often a disadvantage in removing existing mature trees from a vegetated natural hillside above the cut slope (see Figure 1). In such cases the pros and cons need to be carefully balanced, i.e. is it better to remove mature natural vegetation in order to plant a larger area of new vegetation, which will not achieve a similar appearance for many years?
16.Sprayed concrete is very beneficial for slope stability because it virtually eliminates direct infiltration of water into a slope and it provides extremely durable and reliable protection against surface water erosion. Sprayed concrete will continue to be needed for safety reasons on many slopes, but the clear strategy for the LPM Programme is to only use shotcrete minmally, and where it has to be used effort will be made to disguise it.
17.The GEO relies on the cooperation of slope owners or their maintenance agents to willingly accept vegetated surface covers on upgraded slopes and to pledge their commitment to maintaining the vegetated surfaces in the long-term.
NEW INITIATIVES IN BIOENGINEERING AND LANDSCAPING
18.In view of the growing public concerns the GEO is not content with the status quo and recognises the need to take new initiatives on slope appearance in the LPM Programme. The following initiatives have recently been or are currently being established in the GEO:
(i) In October 1998, a consultant was appointed to review methods of integrating slopes and walls into their surroundings, especially for roadside slopes, as part of a project to develop a new Highway Slope Manual. The results of this review, together with proposed new technical guidelines, will be available by the end of 1999.
(ii) In January 1999, the GEO issued a set of interim guidelines for the prescriptive use of vegetation for soil cut slopes, i.e. measures that can be simply applied without the need for detailed studies or ground investigations. These guidelines aim to promote the use of a grass or turf cover on cut slopes up to 35o, grass and erosion protection mats for steeper cut slopes up to 55o, and the general use of shrubs, trees and planter boxes (Figure 3).
(iii) In June 1999, the GEO appointed for the first time a specialist landscape consultant to provide professional advice on the surface appearance of LPM works and to organise tailor-made training courses on landscape design for geotechnical engineers.
(iv) Beginning in June 1999, the slope maintenance audit team from the GEO's Slope Safety Division has commenced checking various government departments as regards the extent of their compliance with Works Bureau Technical Circular No. 25/93 on the Control of Visual Impact of Slopes.
(v) Starting from October 1999, a wider range of erosion control mats and alternatives to the conventional method of hydroseeding will be incorporated in new LPM slope works contracts. This will facilitate testing of some new products and should lead to an increased range and scope of bioengineering applications, especially for steep soil cut slopes.
(vi) In April 1999, a study group was set up within GEO to enhance the application of bioengineering and landscaping to slopes. The group is now overseeing a continuous improvement project whose vision is to provide landscaping/bioengineering at reasonable cost to all features upgraded under the LPM Programme with a view to blending them into the environment. Under this project, the various initiatives on developing new technical guidance will be drawn together and an updated set of guidelines on the use of landscaping and bioengineering on slopes will be provided by mid 2000.
Geotechnical Engineering Office
Civil Engineering Department
July 1999
