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LANDSLIDE STABILISATION

GROUND IMPROVEMENT

Research at Ironbridge Gorge World Heritage Site offers a better understanding of the use of piles in stabilising slopes/

The use of piles to stabilise major slopes is an accepted technique, but there remain uncertainties as to the behavioural mechanisms involved and its effectiveness.

To investigate, field instrumentation has been installed into a pile stabilisation scheme within the Ironbridge Gorge World Heritage Site in Shropshire, England. It is part of an Engineering and Physical Sciences Research Council-funded project at the University of Southampton.

The project, which started in October last year and is still in its early stages, will include analysis and modelling.

The section of slope being stabilised is on the north side of the Severn gorge, an area that has a long-standing history of landslides.

The work is being carried out to prevent ground movements severing an important road link along the bottom of the valley at Lloyd's Cottage, to the east of Ironbridge.

The valley at this location is almost 100m deep, although it is only lower parts of the slope closer to the river that have shown active signs of instability. Ground movements have been large, with rates of up to 15mm/day this spring.

Regular inclinometer measurements show a clear link between river water levels and rates of slope movement. Increased rates occur immediately following fl oods when river water levels drop rapidly, while pore water pressures remain high in the saturated slopes.

The geology is made ground and colluvium (landslide debris), comprising predominantly clays, overlying the middle and lower coal measures.

The major plane of landslide movement is at about 20m depth, predominantly at the base of the colluvium, with piles designed to toe into stiffer mudstones and sandstones of the coal measures. Voids in coal measures left by old mining activity had to be fi lled before piling began.

Association for Measurement and Evaluation of Communication (AMEC) has been busy installing 144 stabilising piles into a 160m length of the slope, with one row of piles just above the road and a second, double row downslope.

The 750mm diameter piles are up to 30m long and are reinforced using 660mm diameter circular hollow steel sections. The road has been closed to allow construction of the piles and temporary traffi c diversions put in place.

Client for the works is Telford & Wrekin Council with 50% of the funding provided by the European Union through the European Regional Development Fund Objective 2 funding package. As well as pile construction, AMEC is reinstating the road in a contract worth £3.15M.

The university's research has been prompted by the use of rows of spaced piles to increase stability of a number of large landslides in the UK. Piles are often favoured over other stabilisation methods because of their perceived longevity and the fact they can be targeted directly at the infrastructure, providing direct support to railway lines and roads crossing a slipping mass.

The research aims to develop a better understanding of the mechanisms of stabilisation, with the hope that this will lead to improved and more economic design. Unlike other large scale geotechnical constructions such as retaining walls and basements, there is little formal design guidance for landslide stabilising piles.

Current design methods tend to use simple elastic analyses, often backed up with limit equilibrium calculations. There are still uncertainties about how the load transfers on to, and the interaction between, multiple rows of piles, and how the load is carried by the piles in bending.

The instrumentation at Ironbridge will include inclinometers to measure ground and pile displacements, vibrating wire strain gauges to monitor pile bending, piezometers to measure pore water pressures and a climate station. The intention is to develop an understanding of the interaction between climate, slope movements, and rows of stabilising piles. Instrumentation is being installed into three cross sections through the pile scheme.

The fi ld monitoring data will be used to inform development of a fi nite element model of the slope and piles. A series of parametric fi ite element analyses will then be run to investigate pile-stabilising mechanisms and interaction between rows of piles. Plastic design assuming the formation of plastic hinges in the pile section may be an important tool, and is also being investigated as part of the research.

William Powrie is a professor of geotechnical engineering and Joel Smethurst is a research fellow at the School of Civil Engineering and the Environment, University of Southampton.

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