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SOIL SLOPE STABILISATION METHODS

John Oliphant1, Robert McCafferty

2& Mr Richard Apted

3

ABSTRACT

This paper focuses on the selection of soil slope stabilisation techniques. It considers the

factors which influence the choice of technique through the development and use of a web-

based decision support system and the examination of a landslide in Edinburgh, Scotland.

The paper highlights the potential benefits of the integrated use of bio-engineering techniques

and conventional stabilising methods

INTRODUCTION

The number of alternatives for soil slope stabilisation is large; ranging from simple

drainage measures, through the use of bio-engineering techniques to the more traditional use

of gravity and embedded retaining structures. The analysis of these alternative remedial

measures for soil slope problems requires experience and sound judgement on the part of the

engineer. In evaluating the alternatives, the engineer will be influenced by factors such as:

nature of failure; ground & groundwater conditions; ground topography; environmental

impact; availability of materials, labour and equipment; design life and maintenance

requirements; adjacent and underground structures; confidence in design and construction;

time constraints; and costs. The final decision will not normally be straightforward as it will

be based on a number of these and other inter-related factors and will often have considerable

cost implications and degree of success in terms of obtaining a practical and meaningful

result. Furthermore, the problem is compounded by incomplete, imprecise and uncertain

information and the engineer may therefore have to make decisions using empirical rules that

have been established from experience.

The purpose of this paper is to outline the selection of soil slope stabilisation methods and

to discuss the recent developments of a web-based decision support system. Also presented is

the integrated use of geotechnical and bio-engineering techniques which allow cost-effective

and environmentally acceptable solutions without recourse to large and expensive

geotechnical measures alone. Finally, an examination is carried out on a number of different

methods for stabilising a landslide in Edinburgh, Scotland. Here, four proposals are

described and examined to highlight the factors which influenced the choice of the adopted

approach.

STABILISATION TECHNIQUES

The strategy of slope remediation or stabilisation is a common treatment of landslides.

The principal categories of stabilisation methods are: 1) alteration of slope geometry; 2)

improvement of soil strength; and 3) provision of force systems to resist instability. Category

1 techniques can involve re-grading, head unloading, toe weighting (e.g. berm) and digging

out. The techniques normally associated with Category 2 include soil improvement through

1Heriot-Watt University, Department of Civil & Offshore Engineering, Riccarton, Edinburgh EH14

4AS, Scotland, UK.2

The City of Edinburgh Council, City Development Department, Bridges & Structures, 1 CockburnStreet, Edinburgh EH1 1BJ, Scotland, UK.3 Carl Bro Aquaterra, 7-15 Dean Bank Lane, Edinburgh EH3 5BS, Scotland, UK.

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grouting, shallow and deep drainage schemes and the use of vegetation. Category 3 systems

include the use of piles, piers and retaining walls. Bromhead (1997) has reviewed the wide

range of options available under these categories.

Howell (1999) has given greater emphasis to the use of vegetation under Category 2

through the general term of bio-engineering. Bio-engineering makes use of living plants for

engineering purposes and can be utilised for slope protection, reinforcement and, to a certainextent, stabilisation of shallow failures. Bio-engineering is not a substitute for civil

engineering but offers engineers an alternative set of tools which complement existing

techniques under the categories given above. It is best used in conjunction with the hard

geotechnical engineering structures such as earth retaining walls to offer a more effective

solution to problems. This integrated approach has been used successfully for soil slope

stabilisation in Nepal using the different combination of techniques provided in Table 1

(Howell, 1999).

Table 1 : Combination of slope stabilisation techniques (Howell,1999)

GEOTECHNICAL

ENGINEERINGTECHNIQUE

BIO-ENGINEERING

TECHNIQUE

COMBINATION OF BOTH

Reinforced Soil Densely rooting grasses, shrubs

& trees

Wire bolster cylinders and planted

shrubs or trees

Soil Nailing Most vegetation structures Jute netting with planted grasses

Soil Anchors Deeply rooting trees Soil anchors and deeply rooting

trees

Retaining Walls Large trees and large bamboo

clumps

Retaining wall with a line of large

bamboo clumps planted above

A case study will be presented later which discusses the combined use of geotechnical

engineering and bio-engineering techniques for a landslide in Edinburgh.

KNOWLEDGE-BASED SYSTEM APPROACH

A knowledge-based system (KBS) is a piece of software used to solve problems in a

particular domain. Within the system there is an artificial representation of the knowledge

possessed by the experts in the particular field and it is this artificial intelligence that is used

for problem solving. It is called a system rather than a program because it contains both a

problem solving component, the inference mechanism, and a support component. The

support component enables, for example, the user to interface with the main program and for

output to be directed to an output device. The inference mechanism contains problem solving

expert knowledge. The system can then be used, in the absence of experts, to represent the

problem solving and decision making qualities of the experts. The applications of KBSs hasbeen extensive, and several have been developed in the field of geotechnical design (e.g.

Oliphant et al, 1996; Smith et al, 1998).

An early prototype web-based system has been developed by the first author (Oliphant &

Kontoulis, 1999) on the selection of soil slope stabilisation techniques. The system can be

accessed online at

http://www.civ.hw.ac.uk/slopes/index.htm

The prototype has been separated into two sub-systems Biostable and SlopeStable and

designed as integrated Web browser applications (applets) in Java with Symantec Visual Caf

2.5 and will only work under the latest Netscape browsers (versions 4.06 or higher). The

Web site was designed in HTML with Microsoft FrontPage 98. Biostable covers the

selection of bio-engineering techniques alone while SlopeStable offers advice on the selectionof both bio-engineering and simple geotechnical structures.

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Biostable was designed to evaluate the user provided input and then give a

recommendation of a specific set of suitable bio-engineering techniques. This is an

innovation in terms of KBS design since most conventional systems give a general

assessment on suitability for a wide range of methods. Biostable incorporates user interface

and expert knowledge (Howell, 1999) in a single application thus reducing download times

on the Web. The sub-system supports grass planting, turfing, shrub and tree planting, bolstercylinders and jute netting.

The operation of SlopeStable is rather traditional by providing general advice on the

suitability of all the techniques covered. These techniques include the bio-engineering

techniques of Biostable in combination with retaining walls such as gabions, crib walls and

propped walls.

The research work has demonstrated that the development of Web-based KBSs for the

selection of slope stabilization techniques is feasible. However, both Biostable and

SlopeStable would benefit from further development in relation to: a) more comprehensive

selection criteria; b) optimization of user interfaces; c) incorporation of other stabilization

techniques; and d) integration to offer advice on a specific set of techniques.

CASE STUDY

Background

Gillespie Road is located in Colinton, south-west Edinburgh. Following the landslide of

the valley soil slope adjacent to Gillespie Road in April 1998, emergency works were carried

out to close the road. (Figure 1 Gillespie Road is at the top of the slope and the river Water

of Leith is at the toe of the slope). On closure of the road, The City of Edinburgh Council

arranged for ground investigation work and commissioned Heriot-Watt University to carry

out a preliminary investigation. This work suggested that the soil slope may have failed

through an increase in porewater pressure most probably from an ingress of water. The shape

of the failure surface would seem to indicate that there were two possible sources of this

water but these could not be confirmed. The average depth to the failure surface from theoriginal ground profile was estimated at 2m.

Figure 1 : Landslide at Gillespie Road, Edinburgh

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The stabilisation works were offered to 4 consortia on a design/build form of contract with

a contract period of 35 days. This form of contract was chosen in order to allow contractors

and consultants to consider the most economic method of stabilisation given the obvious

health and safety implications of working under a failed soil slope. Ground investigation,

land survey and public utility information was given to each consortium.

Four construction options were submitted (Figure 2):1. Contiguous Bored Pile Wall supporting Gillespie Road.

2. Soil Nailed Slope

3. Reinforced Gabion Wall

4. Combination of Gabion support to Gillespie Road and Rockfill Toe at river.

Each option is now described in terms of the use of geotechnical engineering structures at

both road and river levels and the measures for re-vegetation of the slope.

Option 1

An anchored contiguous bored pile embedded wall was proposed as the main geotechnical

structure at road level. The 600mm diameter piles alternate in length from 10m to 5m with tie

rod anchors spaced at 2.4m centres. Gabion baskets were used at the toe of the slope forstabilisation and river training purposes.

Re-vegetation took the form of re-grading the slope with topsoil and grass seeding, and the

construction of wattle fences made out of bundles of live cuttings from the surrounding area

placed across the slope to form broken coverage and help re-establish woodland growth.

Option 2

The technique of soil nailing was proposed throughout the slope with the construction of a

Reno mattress at river level for toe protection.

Re-vegetation consisted of re-grading with topsoil and then fixing a Tensar Mat net across

the face of the slope. Groundcover would then be established by planting low growing native

species.

Option 3

The use of the Box Teramesh system (5m x 2m x 1m units) was proposed with 5m long

reinforcement tails to stabilise the toe of the slope. The Boxes were stepped up to height of

5m above the river bed level and the remaining upper slope was to be re-graded using suitable

fill and reinforced as necessary.

Re-grading of topsoil on slope was to be protected by a soil blanket. Grass seeding and

the planting of a mixture of Willow, Sycamore, Ash, Dog Rose, Bramble and Ground Ivy was

proposed for the bio-engineering treatment.

Option 4

This proposal recommended a gabion basket retaining wall at road level and a rockfillberm at the toe of the slope. The entire slope was top-soiled to a minimum depth of 300mm.

Both grass seeding and the planting of indigenous trees and shrubs was to be carried out.

A selected seed mix was proposed to achieve strong growth within four to six weeks of

sowing. A maintenance schedule for the following 4 years was also included to ensure that

the vegetation became well established.

Discussion

Consideration was given to each submission in terms of speed of construction, cost, health

and safety and method of construction. Time-scale was important in that disruption to the

public and business had to be minimised. Gillespie Road is an important public transport link

and the diversions put in place were long routed. Table 2 provides a summary of evaluatingthe four proposals. The re-vegetation method alone did not influence the final choice but was

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viewed as an effective means of stabilising the slope for shallow failures and for minimising

maintenance costs on the slope surface.

Figure 2 : The four remediation measures

The contiguous bored pile wall was rejected on the grounds that security of existing gas

mains and stability of an existing masonry retaining wall adjacent could not be guaranteed.

The bid represented the second lowest cost that was achievable within the 35 days.

The soil nailed slope solution was rejected primarily on the basis of cost (it represented

the most expensive solution).

The reinforced gabion wall was rejected on the basis of time-scale. Although the solution

was economical the time-scale submitted to achieve the solution was 66 days.

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The combination of gabion basket support to Gillespie road, rockfill toe at river level and

vegetation cover to the slope was selected as an economic and satisfactory engineering

solution offering completion within the 35 day time-scale. The stabilisation works were

completed in 33 days and Gillespies Road fully operational by early August.

Table 2 Evaluation of Proposals

PROPOSAL Tender

Price

()

Cost of

Vegetation as

a % of

Tender Price

Comments

Bored Piled Wall

Soil Nailing

Reinforced Gabion

Wall

Gabion Wall &

Rockfill Toe

237,268

394,585

169.910

219,995

0.5

0.2

6.3

3.0

Not Feasible

Feasible but too expensive

Feasible and cost-effective

but

construction time too long

Feasible and cost-effective

CONCLUSIONS

The development of BioStable and SlopeStable has brought together the power of

knowledge-based programming and the enormous flexibility and system-independence of the

world wide web to provide a prototype system for the selection of soil slope stabilisation

techniques.

Although cost of any civil engineering project is important, the situation regarding the

Gillespie Road Landslide necessitated other factors to govern the ultimate selection of the

preferred bid. In this case, vegetation alone did not influence the final choice but was used to

help stabilise and thereby minimise maintenance costs of the slope surface. However, there is

a need to provide better understanding of vegetation as an engineering material. This can

only help to enhance and integrate the use of vegetation in soil slope stabilisation in the UK.

An improved understanding of the various remedial measures for soil slope stabilisation

will undoubtedly lead to a more informed choice and cost savings. Conventional soil slope

remediation techniques alone may not be sustainable in the long term due to high initial

capital expenditure and in some cases increasing maintenance requirements in the long term.

The integrated use of bio-engineering techniques and conventional methods may have

advantages in the form of cost savings and sustainable solutions.

REFERENCES

Bromhead, E. N. (1997). The treatment of landslides. Journal of Proc. Instn Civ. Engrs

Geotechnical Engineering, 125, April, 85-96.Howell, J. (1999). Roadside Bio-Engineering. Site Handbook, Department of Roads, His

Majestys Government of Nepal, ISBN 1 86192 170 5.

Oliphant, J., Ibrahim, J.A.R. & Jowitt, P.W. (1996). ASSIST: a Computer-based Advisory

System for Site Investigations. Proc. Inst. Civ. Engrs. Journal of Geotechnical Engineering,

119, 109-122.

Oliphant, J. & Kontoulis, L. D.(1999). Development of a Web-Based KBS for the Selection

of Soil Slope Stabilisation Techniques. Departmental Report, Heriot-Watt University,

Edinburgh, Scotland.

Smith, I.G.N., Oliver, A. & Oliphant, J. (1998). WallAid: A KBS for the Selection of Earth

Retaining Walls, 11th

Int. Conf. on Industrial and Engineering Applications of AI and Expert

Systems, 888-895, Castellon, Spain.