Advances in sprayed fibre soil application for the sustainable landscapingof cut slopes in Japan and Hong Kong
Koan Ouchi1, Toshinori Matsuoka1, Norio Machida1, Brian K Hitchcock2
1
Raito Kogyo Co Ltd, Japan, 2 Bachy Soletanche Group Limited, Hong Kong
Synopsis
Sprayed fibre soil technology has been extensively used in Japan for over ten years and has been
recently applied in Hong Kong. Three methods will be presented which can be used for retaining,erosion control and successful landscaping for both slope remediation and major site formation
works. A sustainable approach to raw materials supply is described. Construction flow can be
modified to include re-cycled materials produced either during the site works or by integratingby-products from exiting industrial processes. The authors will outline a prescriptive approach todesign which can be used to assess required thickness using empirical design criteria of soil strength,
rainfall and slope angle.
Keywords: fibre soil, spray installation, vegetation base, slope greening, landscaping
1. Introduction
Slope greening technology based on the spray installation system in Japan was developed during thetime of high economic growth in 1960’s and 70’s in response to the increased environmental
awareness of the Japanese community. In Hong Kong, during the last 3 years, the local government
has also sought to provide an improved balance between the growing urbanisation and the localenvironment. Technical Guidelines have been published by the Government1 and a number oftechniques have undergone trials2. In this paper, we aim to introduce well-established slope
greening techniques to Hong Kong which are based on spray installation technology developed andimproved in Japan for over 2 decades. The systems are applicable to a variety of situations
including steep slopes, rock and concrete environments.
2. Applicable slope greening methods
2.1 Development history of spray installation system
The vegetation base installation technique using a mortar spray gun was first developed in the mid
70’s in Japan to improve landscaping and environmental conservation. The materials consisted
primarily of bark compost and peat moss which are installed using pressurized air to create acompacted vegetation base with a high water retention capability and erosion resistance. The

vegetation base spray installation technology became rapidly and widely applied across Japan, andestablished itself as a standard method in steep slope greening works.

Figure 1: The ECOCYCLE Process

In the early 1990’s, a new method was developed to promote the recycling and reuse of constructionwastes produced in site formation works as a replacement of the vegetation base materials suppliedon the market. Cut trees and shrubs are crushed and composted, then sprayed back to the surface
where these materials are originated (see Fig.1 The “Ecocycle process”). This recycling process ledto a number of other cottage industries emerging providing organic additives and fertilizers which
in turn solved problems of disposal of by-products from other industries.
Also in the early 1990’s, a new function was added to installation system to develop a new form of
reinforced spray soil. The soil structure was reinforced with a continuous synthetic fibre installed on
slope surface to protect from shallow slip failure (see Figure3 “Roving Wall”). The vegetation basecontaining grass seeds is applied on top of this fibre soil layer to produce a variable thick layer
system which is capable of providing a media for root development of larger shrubs. Care should betaken however installing these isolated systems against typhoon damage where the tree heightexceeds the stability provided by the root system and plant base.

Recently developed is a new type of short fibre technique currently being introduced to Hong Kong
(The All Greening Method see Figure 2 and 3). The hybrid system combines the recycling
advantages of the Ecocycle with the reinforcement advantages of the continuous fibre reinforced
soil system. Additionally, a simple one layer system provides a significant improvement inproduction rates of spraying with increased sustainable value.

Figure 2: Fully automatic spray system for short fibre “All Greening System”

2.2 Discussion on applicability of these systems in Hong Kong
The local slope conditions in Hong Kong make fibre reinforced soil an effective product to use due
to its high erosion resistance and capability to be installed in thick layers when required. The maintechnical benefits of continuous fibre reinforced soil (Roving Wall) are:
1. Continuous synthetic fibre (lightweight polypropylene) is mixed with sand by pressurized air toincrease the shear strength of the sprayed surface layer. Prescriptively shear strengths arecohesion 40-45 kPa (design range 40-100 kPa) and a friction angle of 42°(43-51°) giving thedesigner flexibility to provide the optimum strength per site.

2. Efficient bonding and compaction of the sprayed soil layer to the underlying surface alleviates
water erosion behind the applied layer which may often occur in undular slopes.
3. The natural hydrological cycle and existing vegetation is minimally disturbed.

All Greening 20 cmRoving Wall 20 cm

Spray Works

Spray Works

1 year after

1 year after

Schematic Cross Section

Schematic Cross Section

Figure 3: Standard structural detail of the fibre reinforced soil.

In addition, the All Greening Method (Single layer system see Figure 3) allows incorporation of theEcocycle process enabling the reuse of construction and industrial by-products, enhancing asustainable approach to slope landscaping within the community.

3. Application Examples in Hong Kong
Trial panels for both techniques, “All Greening” and “Roving Wall”, were installed at the Chinese
University in 2002 by the authors on shotcrete covered slopes. The first year’s performance isillustrated in Figure 3. Additional work includes the installation of continuous fibre reinforced soil
in Sai Kung for a private development and work undertaken for a roadside slopes on Hong Kong
Island. The former example in Sai Kung is one of the first slopes in Hong Kong where thereinforced soil was designed over a decomposed soil nailed slope without the use of any interceding
shotcrete.
4.1 Determination of required thickness of spray installed vegetation base layer
A prescriptive design criteria for the required thickness of installed vegetation base layer (applicableto short fibre methods) is well established. In this criteria, sustainable vegetation growth is regarded,from extensive installation and trials, to be controlled by 3 major parameters; 1) precipitation, 2)
slope gradient and, 3) surface conditions which determine how deep roots can penetrate (see Figure4 below).

Figure 4: Rainfall, hardness and surface condition vs thickness of sprayed vegetation bed.
Hardness index taken from portable tester by YAMANAKA.

In Hong Kong, the authors have adopted these criteria with good results to date. However it is anongoing process to develop and refine similar criteria specific to Hong Kong conditions based on
local experience and long term observation.
5. Conclusion
The use of the fibre reinforced sand and vegetation base spray systems is proving an effective
method for sustainable landscaping of a variety of slope surfaces including rock, soil and artificial
covers such as shotcrete or mass concrete surfaces. The major disadvantage of these techniques lie
principally in their perceived high cost.
In making a cost comparison with other methods however we should be careful to take full accountof the sustainable value of bioengineering methods. Especially those which offer to replace anumber of key cost centres such as structural performance, landscaping and aesthetic value,environmental purpose, cost of re-current maintenance, social value and promotion of a recycling.
Further research on the long-term of performance of sprayed vegetation bases for the effective
landscaping and erosion control of slopes in Hong Kong is ongoing. The development of aprescriptive design approach is presented and is currently under review for application to local slopeengineering and landscaping projects in Hong Kong.
5. Acknowledgements
The authors would like to thank the Estates Office of the Chinese University of Hong Kong for theircontributions and support.
Reference.
[1] GEO 2000: Technical Guidelines on Landscape Treatment and Bio-engineering for Man-madeSlopes and Retaining Walls in Hong Kong. GEO Publication 1/2000. GeotechnicalEngineering Office, Hong Kong. 146P
[2] Martin, R.P., “Panelist Report – Landscaping and bio-engineering of slopes in Hong Kong”,
Geotechnical Engineering: Meeting Societies Needs., Proceedings of the 14th Southeast Asian
Geotechnical Conference, Hong Kong., Eds. Ho K. K. S., K. S. Li. 2003, p661-670.
[3] Japan Slope Greening Association., Technical Information On Organic Vegetation Base Spray
Installation Works, 2000, p14-16 (in Japanese).