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Europe seeks the right mix In Dartford, Kent, the BRE and Keller Ground Engineering have successfully completed the British trial for the Europewide Eurosoilstab project. Bob Essler reports.

GROUND IMPROVEMENT

Eurosoilstab is a three-year research project funded by the European Union under the Brite Euram programme. Set up in 1997 and due for completion in early 2000, it aims to develop the technique of insitu deep soil mixing to treat soft organic clays and peats and to produce a design guide for this method.

The treatment of organic clays and peats has traditionally been extremely difficult, with unpredictable results. It is hoped that the outcome of the project will provide a method whereby these soils can be dealt with successfully.

Applications relate mainly to construction of embankments on soft ground where, in the past, long construction periods have been necessary to allow consolidation of the underlying compressible layers.

In the Netherlands, for example, the traditional method of embankment construction has been to install wick drains, surcharge and wait for one to two years.

The Building Research Establishment and geotechnical contractor Keller Ground Engineering have successfully completed the British trial at Dartford in Kent.

Trials have also been carried out in the Netherlands, Sweden and Finland. Results are due to be published next year following long term monitoring of the test embankments.

Soil mixing involves the rotation of specially designed tools into the ground while injecting a specially designed binder. The binder is either introduced pneumatically using dry materials or injected under pressure in a liquid form. The two methods are thus known as the 'dry' and 'wet' methods.

The dry method has been used extensively in Scandinavia for inorganic clays and the wet method has been mainly used in the UK and Japan.

For Eurosoilstab the UK trial is the only one to use the wet method. This will allow a comparison with the dry method used in the other European trials.

The Dartford trial consisted of three sections, where both column diameter and depth of mixing were varied as well as binder design. One section has been left untreated as a monitoring control.

It is hoped that the design performance predictions of deformation can be compared with the actual field values to validate the design.

BRE is carrying out the site investigation, instrumentation and monitoring and design method tasks while Keller is responsible for the equipment design, process instrumentation design and soil mixing fieldwork.

Binder design was carried out jointly by BRE and Keller, with parallel laboratory trials in Wetherby and Garston.

Extensive instrumentation has been installed by BRE to monitor settlements and changes in ground compression, porewater pressure and earth pressure.

A typical section through the embankment is shown in Figure 1. Rod extensometers have been installed to monitor compression in the columns at different depths and inclino- meters have also been installed to monitor lateral movements.

Boreholes have also been sunk to obtain undisturbed samples for soil property characterisation and for water sampling to assess the environmental impact on the groundwater.

Installation of the columns was completed at the end of 1998 and the embankment was constructed by the end of March 1999. Porewater and earth pressure build-up and deformations were monitored closely as the embankment was built.

This was done in 1m stages because the untreated section could have been susceptible to failure if construction was too rapid.

The embankment will be monitored until the end of the project in February 2000 and it is hoped that further long term monitoring will be possible if the site remains available.

To achieve its aims the project has been split into various tasks designed ultimately to provide a reference procedure for the treatment of soft clays and peats which will become a European standard.

The tasks are:

1 Selection of soils and applications

2 Laboratory and numerical analyses for field tests

3 Development of equipment for field tests

4 Field tests

5 Evaluation of field tests

6 Design guide

7 Development of new binders

8 Development of new equipment

The main output will be a design manual for the treatment of organic clays and peats encompassing design and construction and will be based on joint work by all parties from laboratory test results, numerical analysis and field behaviour.

Bob Essler is associate director of Keller Ground Engineering

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