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The technique

Dry soil mixing's big advantage is that it works in soft and organic clays - soils usually considered inappropriate for ground improvement.

By injection and mixing of a cement and/or lime binder, it turns a soft material into a firm one.

Lime binder is used for pure soft clay, whereas if it is organic the binder is more cement-based.

In Sweden the technique has been used to treat very soft clays to depths of 30m. Applications vary from general ground improvement below roads and pipelines to creating stable blocks of ground outside launch and reception shafts in soft ground tunnelling projects.

A blade between 600-800mm in diameter is rotated into and stirs up the soft ground. Injection pressures are low, typically about 5bar and rarely above 10bar.

Binder volumes are quite modest, about 4-8% of volume. As a result there is very little surface heave and no increase in lateral pressure outside the columns, which allows safe working close to structures and foundations.

Field trials are an essential part of the design. The binder mix and its performance should always be verified on site using pull-out resistance tests, which measure the resistance to pulling out a plug embedded in the column.

From this the post-treatment shear strength can be calculated.

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