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Tip top condition

An observational approach proved to be a good solution for an embankment at a former tip in the West Midlands.

A 4.2ha parcel of elevated land situated just off the A444 (Phoenix Way) on the outskirts of Coventry, West Midlands, was for many years regarded as being uneconomical to develop.

A former tip, the site comprised a narrow, elongated mound of made ground – placed there during construction of the A444 – increasing by up to 14m above the surrounding areas.

The land was considered unsuitable for economic development because of the perceived need to remove substantial quantities of spoil, with its associated high costs and environmental impact.

Client Wigley's proposed to develop the land for commercial use, including a new operational command unit for the West Midlands Police. This required the transformation of the shallow-sloped mound into a relatively level development platform. The earthworks contractor for the development was Trelanmex, with consultant Joynes Pike & Associates (JPA) acting as designer to Trelanmex.

Two key development criteria were sustainability and the issue of the made ground, which had to be dealt with and not just moved from one area to another.

This meant that the least amount of material removed from the site was the most desirable outcome for all parties. It presented JPA and Trelanmex with the challenge of incorporating the existing made ground, cost-effectively, within the site boundary.

The proposal involved a split-level development with flat benches at 7m and 14m above surrounding areas. Bordered by a dual carriageway to the east and Coventry Canal to the west led to slopes averaging between 45˚ and 50˚. This raised issues over their long-term stability. A scheme design identified that reinforced soil slopes would comprise the most economical solution.

Because of the tight time schedule JPA had less than a week to produce a satisfactory reinforced soil slope design, which also required approval from a British Waterways engineer. This meant that there was little scope for any site investigation and soil shear strength testing. To progress the works and achieve a cost-effective design, the following strategy was adopted.

Ground conditions were taken from an existing phase-two geoenvironmental assessment, which had identified up to a 14m depth of made ground, underlain by firm to stiff sandy gravelly clay of glacial till over very weak sandstone of the barren measures of the upper coal measures.

The made ground was identified as comprising materials associated with the site's use as a landfill for inert materials. They were typically highly variable, broadly classifiable as a soft to firm gravelly clay, becoming increasingly granular with depth.

In spite of its variable nature, the made ground was classified as predominantly suitable material for earthworks – category 2C, with the exception of some oversize concrete boulders and occasional wood, metal and glass – having neutral pH and low sulphate.

A site visit by a JPA engineer allowed trial pitting to generally confirm the reported characteristics of the made ground. After discussion with Trelanmex, it was agreed that during excavation for the wall installation, where necessary the material would be screened to remove unsuitable material.

This allowed JPA to assess design values for the shear strength of the made ground, based on the average plasticity index of clay component within it and well-published relationships with its critical state internal angle of friction. A key requirement was for the material's validation by an engineer during the works – an observational approach to the design.

The sections were designed to satisfy global and external stability for a 60-year design life in accordance with BS 8006:1995 and to the principles of BS EN ISO 14475:2006. The resulting design led to the requirement for over 56,900m2 of geogrid, with uniaxial ultimate tensile capacities ranging from 20kN to 100kN.

The construction of the 490 linear metres, 7m to 14m high reinforced soil slope was conducted over an 18-week working period from September 2007 to February 2008, (including a break over the winter months).

The installation involved excavation the existing mound to the surrounding ground level and then the basal 450mm of fill to be excavated and re-compacted in engineered layers for the footprint of the reinforced soil slope.

This subterranean re-compaction was important, given that rock-head was between 1m to 2m below the slope founding level; the design had identified that this was necessary to increase shear strength. It would also provide an adequate factor of safety on three-part wedge failure mechanisms undercutting the reinforced soil slope, which were identified as a critical global stability issue at design stage.

The philosophy behind the material selection was based on the Observational Method, advocated in Eurocode 7. Provided the proposed methodology and the design requirements could be satisfied, and perishable substances had been adequately screened out of the material, it would then be validated for use by a JPA engineer.

Occasional lenses of soft cohesive fill were identified during the excavations and given the potential for mattressing and inadequate drained shear strength, were segregated from the works and used for landscaping.

The Observational Method used by the design team was not simply a method of observing the construction. Clear criteria had to be met and a number of alarm levels adhered to where some materials might not have been suitable for use in a particular area.

A methodology was also required to deal with these materials. Everything that could be reasonably foreseen had to be taken into account and procedures put in place to deal with them.

An additional consideration within the screening approach was the section of salient boulders for incorporation into landscaping features, an aspect specifically requested by the project architect to reflect the site's history.

For the slope construction JPA and Trelanmex negated the requirement to use formwork or a wraparound geogrid facing by overfilling the compacted backfill and trimming the profile back to the leading edge of the geogrids.

Given that the long-term stability of the exposed face would lead to slumping / minor face instabilities, the design included a 2m length mid-layer geogrid to prevent excessive material loss at the face. A topsoil retention system with hydro-seeding to provide a green and cohesive resistance at the face for long-term aesthetics will also be installed as the development progresses.

This project demonstrates that through innovative design and co-operation between engineer and contractor, a sustainable, cost-effective design carried out by an observational approach can be used where limited time is available for laboratory testing. Last, but not least, this £500,000 project demonstrated the transformation of an unsightly former tip into a valuable commodity.

Beverley Chiang is marketing and business development manager with Joynes Pike & Associates.

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