Case studies

Interest in the Swedish method of lime-cement columns is increasing beyond its established geographical base

Swedish geotechnical engineers have long-maintained that ground improvement of very soft organic clays using lime cement columns has the potential for a much wider international application outside of Sweden where the technique was developed.

And while independent, but different deep soil mixing methods (DSM) have developed in parallel in Japan and the USA, there are signs that the Swedish method is spreading.

Undoubtedly this will be helped by fact that Keller recently took full ownership of Gothenburg-based contractor LCM, one of the pioneers of the method. And with Keller embarking on a strategy of transferring the use of its specialist techniques throughout the group globally, it shouldn’t be too long before Keller pushes lime cement columns beyond the established geographical base.

With a domestic market of more than five million linear metres of column a year and over 30 years experience, use of the technique in Sweden is quite routine. You will often spy a couple of operators working quietly away in isolation with a single DSM rig and its attendant self-powered binder supply silo. Typically this is the first sign that a piece of land is earmarked for some future development or construction activity.

DSM is a ground improvement technique in that it increases the stiffness of often very soft clays, improving both the bearing capacity and reducing settlement under load. A dry binder, typically comprising one or more of a mix of cement, lime, gypsum or blast furnace, is injected into the ground, as it is being mixed and churned up in columns by a special mixing tool inserted into the ground by a drilling rig. A chemical reaction between the binder and the soil takes place resulting in a stiffer and drier column of treated ground.

Applications are varied, from reducing settlement below pipelines to mass stabilisation of car parking and delivery areas at out of town retail developments. And in Finland, medium rise structures have been founded directly on DSM columns – although in some countries building regulations don’t yet allow this.

LCM, for example, has installed columns up to 25m in depth for a wide range of applications from supporting embankments to reducing active and increasing passive pressures for retaining structures in weak soils. Columns can be interlocked to provide cellular blocks and in 2001 it developed a tool for mass mixing large volumes of soil in blocks.

At Ku Myren, about 20km south west of Stockholm, this allowed a short link road to be built across soft peaty clay directly upon deep dry-mix stabilised soil. This provided considerable savings over the previous phase of the road’s construction, which relied upon closely spaced driven piles.

In Spring 2004, LCM was working on a new postal distribution centre accessed off this very same road. This latest project involved construction of 20500, 600mm diameter columns, up to 10.5m in depth. The purpose was to treat all hardstanding and road areas; the steel-framed postal depot will be piled.

Column spacing in plan varied from 1.3m to 0.8m, depending upon the loading. Design strength for columns is 150kPa, typical for the technique, and adjacent closely spaced columns alternate in depth between 10.5m and 7.5m.

The design philosophy is that you not creating weak piles, but relying on the interaction between columns and the surrounding untreated soil. Rather like analysis of vibro stone columns, one of the common design methods uses a soft spring/hard spring analogy.

In practice there is a strong reliance on empirical experience in the design and getting the binder right is key. Projects invariably make use of laboratory mixing trials to determine the binder mix and quantities needed to achieve the required strength.

A number of different auger mixing tools are available, each suited to different soil conditions. Binder volumes, rate of rotation and speed of withdrawal are also critical to creating a good quality column and these parameters are all digitally recorded during the process. A typical column takes around five minutes to complete.