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The BGA Fleming Award 2005 for excellence in geotechnical design and construction was won by a team who carried out costal protection and slope stabilisation at Castlehaven on the Isle of Wight. GE presents summaries from this and the two other finalists.


Castlehaven coast protection and slope stabilisation scheme, Niton, Isle of Wight

An innovative geotechnical solution has reduced the risk of coastal landslide reactivation.

Castlehaven, on the southern most point of the Isle of Wight, has been under threat from coastal erosion and landslide reactivation.

It forms part of the Undercliff system, one of the largest developed landslide systems in western Europe, about 12km long and extending up to 1km inland. Reactivated by coastal erosion, high groundwater levels and susceptible geology, the landslide put at risk properties and infrastructure valued at £18M.

Consultant High-Point Rendel worked with the Isle of Wight Council to develop an innovative geotechnical solution to reduce the risk of landslide reactivation using deep and shallow drainage and coast protection measures (GE August 2004).

The site is covered by numerous designations including special area of conservation, site of special scientific interest, area of outstanding natural beauty and heritage coast.

Within this sensitive setting a unique engineering solution was sought to improve the stability of the developed landslide while mitigating the significant environmental impact.

Extensive investigations and the development of an accurate ground model confirmed the main landslide failure mechanisms as recession of the sea cliffs due to marine erosion and reactivation of the landslide due to high groundwater levels.

A 550m long revetment comprising two layers of 3t to 6t rock armour, delivered by barge, was placed at the toe of the sea cliffs to stop marine erosion.

A comprehensive system of instrumentation and monitoring allowed accurate landslide forecasting by correlating rainfall and groundwater levels that trigger movement.

An innovative system of groundwater control, developed in France by TP.Geo, was assessed and designed to limit the rise of groundwater to levels typical of summer months, thus preventing reaching landslide trigger levels.

The groundwater control system consisted of 35 electro-pneumatic pumps driven by compressed air and 121 gravity-fed siphon wells installed in 156 deep drainage wells at depths of up to 25m below ground level. A unique feature of the siphon pump was the automatic priming and no need for power. This was the first time in the UK these techniques had been used to stabilise landslides.

The performance of the drainage system to achieve the specified drawdown was assessed using the observational method and a comprehensive system of instrumentation, monitoring and reporting. Monitoring to date confirms the minimum drawdown of groundwater levels has been achieved and the drainage system is performing as predicted.

High-Point Rendel designed, procured and supervised the work, which started in October 2003, for a 52-week contract. It was completed on time and on budget by Team Van Oord for an outturn cost of £4.6M.

The scheme was unique in the way environmental issues were managed, how an innovative design solution was implemented and how the parties - including the council, consultant, specialist drainage subconsultant/contractor and main contractor - worked to build a scheme that has provided confidence and security to the residents of Castlehaven.

Stephen Fort, assistant director, HighPoint Rendel

Project team

High-Point Rendel, Van Oord UK, TP.Geo/Groupe Ress, Isle of Wight Council, Defra, WJ Groundwater


The UK's largest soil mixed reactive barrier

Soil mixed reactive barrier systems have given a new lease of life to two former landfill sites in Northampton.

Mowlem has installed the UK's largest soil mixed reactive barrier system at former landfills at Sixfields and Harvey Reeves Road in Northampton.

Work forms part of the £37M Northampton Brownfield Initiative between English Partnerships and Northampton Borough Council.

The initiative received £17.1M from the Office of the Deputy Prime Minister as part of the government's Sustainable Communities Plan.

The Northampton Brownfield Initiative was created in September 2002 to regenerate land at three key sites - Sixfields, Harvey Reeves Road and Ransome Road. All former landfills, they cover a total area of about 97ha.

The initiative aims to rediscover and enhance the Nene River Valley. End use will include new retail, leisure, commercial, residential and community developments.

The remediation strategy was devised by consultant Halcrow Group with the regulators. It involved installation of barrier systems around the Sixfields and Harvey Reeves Road sites, including two soil mixed reactive barrier systems using proprietary modified clay technology.

Mowlem Remediation was appointed by English Partnerships as principal contractor in October 2004.

Barrier designer Envirotreat was brought in by Mowlem Remediation to design the reactive barriers which provide an effective solution for the treatment of groundwater contaminated with polyaromatic hydrocarbons (PAH), diesel/petrol range organics, Btex (benzene, toluene, ethylbenzene and xzylene), ammonia, heavy metals and others.

Using its unique patented E-clay technology, Envirotreat developed a design to address pathway contamination issues, and to protect surface and groundwater receptors.

Mowlem Piling & Foundations, working in partnership with Mowlem Remediation, was engaged to install the barrier columns using CFA piling rigs.

It also developed a 'state of the art' batching process for slurry production and a computer system to monitor and record each barrier column installation to provide quality assurance from start to finish.

The batching plant was fully automated with calibrated delivery of dry materials (ordinary Portland cement, bentonite and zeolite) and liquid chemicals during the batching process.

Computer hardware on the piling rigs used to install the barrier columns recorded depth, torque and slurry addition as installation progressed to produce an 'as built' record for each of the 3300 columns.

At the 24ha Harvey Reeves Road site, a 972m long soil mixed reactive barrier system, comprising a 402m reactive section and a 570m low permeability section, was installed to a depth of up to 14m.

At the 31ha Sixfields site a 590m long soil mixed reactive barrier system, consisting of a 350m reactive section and total of 240m low permeability sections, was installed to a depth of up to 9m. Work was carried out under Envirotreat's mobile process licence.

Mowlem Remediation installed a total of more than 1500m of soil mixed reactive barrier systems, incorporating 34,500 vertical linear metres of overlapping barrier columns, the largest of its kind in the UK. Work was completed in 26 weeks.

Henry Crossley, project manager, Mowlem Remediation

Project team

Northampton Borough Council, English Partnerships, Halcrow Group, Mowlem Remediation, Mowlem Piling and Foundations, Envirotreat Technologies


Ground improvement solutions for the M60 Orbital motorway widening scheme, South Manchester

Integration of the geotechnical contractor into the design team proved successful in solving settlement problems on the M60 motorway widening.

The 7.5km long, £106M widening of the M60 orbital motorway (Manchester's outer ring road), around south Manchester will provide three and four dual lanes between junctions 5 and 8. Work started in July 2003 and is expected to finish this spring.

The road runs through the Mersey Valley, creating physical constraints that means widening has to be asymmetrical, switching from one side of the road to the other.

Around junction 7 and between junctions 6 and 7, the motorway runs on a 10m high embankment which includes a road bridge, a rail bridge and one over the river. Bridge abutments and piers are supported on piles penetrating through the soft alluvium and glacial deposits into the Triassic bedrock.

The widened motorway is carried on embankment, with new bridges built alongside existing ones and new abutments and piers built on piles down to bedrock to minimise settlement and reduce impact on existing foundations.

At tender stage, the Amec-Alfred McAlpine joint venture recognised the signifiant geotechnical risk associated with building bridge approach embankments on a 5m thick layer of soft compressible alluvium.

The poor ground, coupled with programme restraints, meant there was little time to allow the new widened embankments to be surcharged and left to consolidate. So alternative ground improvement methods were sought to speed up construction and reduce long-term settlements.

At tender stage the conceptual design was to strengthen the foundation for the embankment using ground improvement to protect bridge piles from lateral soil pressures from the new embankments by transferring loads to the underlying competent glacial deposits.

Pennine Vibropiling was brought in to develop a ground improvement scheme with the JV and consultant Halcrow. Detailed design was subject to the Highways Agency new procedures for geotechnical certification - Managing geotechnical risk (HD22/02).

The design process incorporated staged investigation to satisfy third party concerns and sought to integrate construction issues by JV review at each stage.

Embankment loads were transmitted to competent glacial sands and gravels using a load transfer platform on vibro concrete columns (vccs), substantially removing lateral loads on the bridge piles and ensuring settlement criteria for the abutments were met. The vccs were installed in 10m wide zones behind the widened bridge abutments.

Staged construction was assisted by band drains and/or vibro stone columns, allowing the ground (through accelerated drainage and consolidation) to gain sufficient strength during construction to support the full height embankments.

To protect the vccs from lateral loading due to lateral displacement of the soft alluvium the ground was further stiffened by a 10m wide zone of vibro stone columns (vscs) with geogrid extended over the first few rows of vscs from the adjacent vcc zones.

Using the dry bottom feed method, in all about 1300 vccs were installed to depths ranging from 7.3m to 10.5m and about 8000 vscs to depths ranging from 6.3 m to 13m - depths depended upon working platform level.

Using the two methods together smoothed out the settlement profile behind piled abutments, ensuring good vehicle ride quality and mitigating future maintenance issues.

Once ground improvement had been carried out, a testing and monitoring scheme was used to verify its performance. And while widening is still under way, the main areas where the ground improvement was carried out is now being trafficked and is performing satisfactorily.

The scheme was let under a design and build, ECC Option C, target cost contract. The non-adversarial basis of this type of contract has proved very successful. It has engendered an open approach by all parties and has made partnering, joint problem solving and collaborative working more than aspirations.

The integration of Pennine Vibropiling into the design team was a direct result of the type of contract used.

(A full report of this project was featured in Ground Engineering's June 2004 issue. )

Colin Serridge, principal geotechnical engineer, Pennine Vibropiling

Project team

Highways Agency Amec-Alfred McAlpine JV Halcrow Pennine Vibropiling

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