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BIN THERE

TESTING

What relevance has agricultural research to geotechnics? Paul Wheeler visits the National Soil Resources Institute at Silsoe College to find out.

After 20 years studying, working in and writing about the geotechnical industry, I feel I have a reasonable grasp of the type of apparatus you are likely to encounter inside most commercial and research geotechnical laboratories. So it was with some curiosity that I visited the laboratory of the National Soil Resources Institute at Silsoe College, near Bedford.

The lab, in fact, boasts three facilities that have very obvious geotechnical and geoenvironmental applications, but as far as I'm aware, you won't find any of them within a university soil mechanics departments.

Most striking is the giant soil bin, apparently one of only two in the world.This is a 20m long, 1.7m wide and 1m deep test pit in which a soil profile can be built up in 50mm layers, at controlled density, grain size and moisture content.

Such has been the success of the soil bin, that Silsoe is currently investing £1.25M constructing two more within a new purpose built laboratory.These, says Dr James Brighton who is project managing the expansion, will be 40m long; one is 5m wide and 0.75m deep and the other 2.5m wide and 2.5m deep.They should be fully commissioned by next summer.

Applications of the existing soil bin have extended beyond obvious farming related issues such as studying how different tyre pressures on tractors compact and rut the ground to different depths and the effect this has on the soil as a growing medium. Recent projects have investigated runway design, the performance of geogrids, traction of off-road vehicles, the performance of earthmoving equipment and even cable installation and retrieval.

The second largest bit of kit in the lab is a giant soil erosion rig. This comprises a rainfall simulator sitting above a 5t soil table that can be orientated in three dimensions (up, sideways and diagonally across) at slopes up to 30infinity.

Researchers can build up an instrumented 300mm deep soil bed, orientate the test bed to the required slope and then turn on the rain simulator.This can be used to study surface erosion, gulley formation, sediment transfer and also pollution migration and the effectiveness of buffer strips - essentially any situation where there is interaction between soil and rainfall.

Silsoe also boasts a glass-sided tank 4m long, 1.5m wide and 1m deep, in which soil can again be prepared to uniform conditions, instrumented and then submerged with water. This has proven particularly useful for developing methods of installing seabed cables using ploughs and high speed jetting.

Silsoe College, part of Cranfield University, was set up as the National College of Agricultural Engineering in the early 1960s. It started life as a teaching institute, but by the late 1960s people were beginning to do research based degrees.

A pioneer in the field was Professor Dick Goodwin, who completed his doctorate in 1970 and has been the centre's driving force over the last three decades.

His early work started looking at the application of soil mechanics and dynamics from an agricultural perspective. Mostly this was about erosion and drainage and improving soil as a growing medium, but Goodwin's work also investigated the dynamics of ploughing, compaction and traction.

The college built a new facility in 1984 which included the fullyinstrumented soil bin, unique in the western world. Run a tractor tyre over the bin, or pull a plough blade through it, and you can determine the stresses and strains in the soil at full scale, but under a laboratory controlled environment.

The availability of this facility meant Silsoe's research interests have broadened significantly to include, for example, the performance of geogrids, and applications ranging from runway performance to tyre design for off road vehicles.

As Professor Goodwin says: 'In terms of calculating and estimating forces, what we do is based on the same underlying physics as soil mechanics. 'Calculating the stresses in the ground during ploughing for example, is essentially reverse bearing capacity at failure - but turned on its side.'

Over the years, Silsoe has diversified from its pure agricultural base - to anywhere where a soil dynamics perspective can add value. Goodwin feels Silsoe offers a good basis in science that could be applied to different areas, but 'we're not really competing with civil engineers', he adds. 'Many of the projects we look at are 'too light' [in the sense of low stress] for mainstream soil mechanics' But when the new lab, complete with the enlarged soil bins, opens next year, Goodwin muses 'we're happy to talk to anybody-'

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