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NAIL FILE

SLOPE ENGINEERING - Alan Phear and Jim Johnson report on progress of the new CIRIA best practice guide to soil nailing.

Soil nailing has become a well established and relatively common method of slope stabilisation and retention in the UK.

There has been rapid expansion in the use of the technique in the last five to 20 years - by about twenty-fold in the decade up to 2003 according to case study data.

Future investment in transport infrastructure is likely to see a further increase in popularity. The principal uses of soil nailing in the UK are stabilisation of retaining walls or unstable slopes; and construction of new steep slopes or new retaining walls.

However, anecdotal evidence exists of problems and failures during construction, due to misapplication of the method, for example poor specification, inappropriate ground conditions or poor detailing of flexible facings.

Sometimes there has been a lack of understanding that soil nailing is a reinforcement technique, usually needing at least two and generally more rows of nails.

Martin Pedley argued in Ground Engineering (Talking Point, February 01) that a comprehensive UK design and performance manual for soil nailing was needed to enhance the technique's stature and dispel the mystique surrounding it. Nearly two years of lobbying resulted in the Construction Industry Research & Information Association (CIRIA) initiating Research Project 674 in early 2003.

Soil Nailing: Best Practice Guidance, was completed in December 2004, compiled by a partnership of Arup, Bachy Soletanche, Keller Ground Engineering and the Transport Research Laboratory (TRL).

The report is now available to CIRIA's core funders and will be published this summer.

A key objective was to draw together relevant information from the many research reports, codes, specifications, standards and procedures to create more confidence in the use of soil nailing in the UK.

Until now, designers and contractors have had to refer to the work undertaken in France reported in Recommendations Clouterre (1991), to the extensive US Federal Highways Administration Manual for design and construction monitoring of soil nail walls (1998) and to the more recent Geotechnical Circular No 7 - soil nail walls (2003).

But although these documents were written specifically for soil nailing and are user-friendly for practitioners, they reflect local practice. The draft European Standard prEN 14490 Execution of special geotechnical works - soil nailing (issued for public comment in 2002) helps the industry to identify key areas and to select potential solutions, but focuses mainly on construction and implementation.

The new CIRIA report provides best practice guidance for the design, installation, testing and maintenance of soil nailing as a slope stabilisation system for new and remedial works in the UK, with particular emphasis on the latter.

Its purpose is to present best practice; provide a guide for practice in design, construction and maintenance; disseminate specialist knowledge of soil nailing; and encourage confident, appropriate and effective use of soil nailing.

The report significantly improves on previously available published advice in many areas:

l the design and detailing of flexible and soft facings l soil nail and facing materials and design and detailing for durability l the design of soil nailing using a Eurocode 7 approach l the use of soil nailing for stabilising existing slopes and walls.

Health and safety is stressed throughout.The UK health and safety legislation most relevant to soil nailing is reviewed, and planning for health and safety during construction and maintenance is emphasised. This includes safe access for construction and maintenance, safe working procedures on slopes, safe use of drilling rigs and safety during testing of soil nails.

The report adopts a risk-based approach.

The scale of the works and the consequences of a failure are key considerations when deciding the degree of rigour required in site investigation, design, selection of soil nail materials, detailing for durability, testing, site supervision during installation, monitoring and maintenance. This is consistent with the approach given in BS 8006, the Highways Agency's document HD 22/02 Management of geotechnical risk, and the design approach given in Eurocode 7.

The report summarises the mechanisms of behaviour of soil nailed slopes and structures and then covers conceptual and detailed design.

Movement of the slope crest and the slope or wall face has to occur to induce load on the soil nails. The amount that a soil-nailed slope deforms not only depends on the nail lengths and spacing but also on the construction process.

For steep slopes and walls constructed by excavation, the nails are progressively loaded as each stage of excavation is performed.

However, for existing slopes stabilised using soil nails, the nails will remain untensioned (and therefore will have little stabilising effect) until relative movement of the active and resistant zones of the slope occurs. This may take some years (or may never happen).

The role of the facing is discussed. Flexible and soft facings have been used much more extensively in the UK than abroad. This reflects the relative importance of the application of the technique to stabilising existing slopes and is also driven by sustainability issues.

When soil nails are used to stabilise an existing slope, or to construct a new slope or wall, they do not stabilise the surface soil. This is done by head plates and/or a facing.

Separate measures to retain the surface (and near-surface) soil must be adopted and integrated with the soil nail system. The selection and detailing of an appropriate facing is fundamental to the performance of the soil-nailed slope.

Design of soil-nailed slopes or walls involves much more than just designing the soil nails themselves, the report emphasises. It needs to include consideration of ground and groundwater conditions; construction sequence and buildability; site constraints;deformations; design life; facing and head plates; and drainage.

The detailed design chapter is set out step by step, cross-referenced to two worked design examples and based on the latest draft of Eurocode 7 (September 2003).

Methods of determining and/or estimating the pull-out resistance of soil nails are outlined, and typical values of ultimate bond stress from test nail programmes for UK soils are given. The relevance of short-term pull-out tests to long term performance is discussed and a very conservative approach is recommended for soil nails in high plasticity clays.

The internal and overall stability of soil nailed slopes are usually designed using limit equilibrium design methods. The main shortcoming of these is that they do not give a prediction of deformations. Nor do they consider the deformation required to mobilise the resisting forces in the soil and soil nails.

The chapter on durability and degradation covers the assessment of degradation risk - based on the results of site investigation.

Turning to service environments for soil nail tendons and nail heads, the report notes that the conditions encountered by the buried components are often different from exposed parts.

The materials commonly used for soil nails are uncoated steel, coated steels (galvanised steel, epoxy and other coatings), stainless steel, and fibre reinforced plastic (glass fibre, carbon fibre and Aramid fibre).

Systems are discussed by increasing degree of protection against degradation, starting with uncoated materials directly within the ground, moving on to materials with cement grout surround, materials surrounded by grouted impermeable ducting, and finally materials with enhanced corrosion protection.

Guidelines are given for commonly available soil nailing systems in the UK in relation to different risk categories, whether the application is permanent or temporary, and whether the ground is non-aggressive, mildly aggressive, aggressive or highly aggressive.

The advice on soil nail testing in the report follows that in the draft European execution standard prEN 14490.The need for good records of construction and maintenance works is emphasised.

There are several areas where further work and research will be of value. These include the need for a national soil nailing specification for the UK; further research into flexible facings to develop a simple but not overly conservative design method; understanding of the way in which bond reduces with time (especially in high plasticity clays); and methods to predict deformations accurately.

Alan Phear is associate and Jim Johnson is GE business leader at Arup.

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