Lime columns are a tried and tested method of improving clays and silts, particularly in Sweden, although use in the UK to date is limited. The joint venture between UK's Stent Foundations and Swedish contractor Hercules Grundloggning hopes to revitalise interest in the use of the method in the UK.
The move draws on Stent's experience in the UK and Hercules' expertise in the use of dry lime and cement powder for improving soft clay and silt.
One of the reasons for this apparent lack of interest in lime columns is that use is limited to certain soil types, says Stent's Tom Schofield. 'The method is largely restricted to soft estuarine clay, silt and organic rich soil, materials that are not as common in the UK as they are in Holland and Scandinavia, where lime columns are more widely used.'
But lime stabilisation is not unheard of in the UK. Since 1987, Loughborough University's Geotechnical Engineering Research Group has been carrying out extensive research into lime stabilisation on clay soils. London Underground has used the technique to stabilise the failing cutting slopes and embankments throughout its network and has funded some of Loughborough's research, particularly in the application of lime columns, piles and nails.
Stent has also carried out research into the lime pile treatment of clay slopes. The work involves field and laboratory studies on a London Clay embankment, investigating a range of different mixes of lime, cement and additives for stabil- isation.
Lime (used in combination with cement), when mixed insitu with soft clay, increases the soil's stiffness, permeability, erosion resistance and improves its volume stability against swelling and shrinkage.
There are a number of different mechanisms at work when lime is added. Initially, a dry lime powder will react with any moisture in the clay and hydrate. This results in an increase in effective stress, as the moisture content falls.
As hydration of the lime continues, it expands and its calcium ions bond with clay minerals, increasing the soil's plastic limit, lowering its liquid limit and forming a calcium silicate gel. There is an optimum amount of added lime that results in a maximum increase in the soil's plastic limit. Any more lime added past this 'lime fixation point' is used in gel formation. The gel coats and binds other clay particles, increasing the stiffness of the clay. And along with these processes, the pore water pressure of the reworked clay reduces, causing consolidation and a further increase in the soil's shear strength.
The lime and cement mix is varied according to site require- ments, calculated from strength measurements of samples from test columns. In general, a greater cement content offers higher strength, and less lime added reduces the amount of moisture removed from the soil. 'The aim is to achieve a balance bet- ween the two,' says Schofield.
Lime col- umns are formed using a hollow stem auger system. Once the required depth of column is reached, rotation is reversed, the auger is withdrawn and the lime and cement mix is injected by dry compressed air through the hollow stem. The retraction rate is slower to ensure the mix is fully incorporated with the reworked soil. It is possible to carry out mixing to 25m, although more commonly columns are formed between 10m and 15m, with rates of up to 50m/hour achievable.
The whole operation is computer controlled, which Schofield says ensures the columns are formed with an even distribution of lime and cement.
The method can be used to accelerate the settlement of wet, plastic clay. Typically, 600mm diameter columns are used, placed between 1m and 1.5m apart in a grid across the site. 'The columns effectively increase the permeability of the ground by an order of up to 1,000, and consolidation therefore tends to be rapid, up to twice the rate of conventional wick drains,' says Schofield. 'Each column can perform as well as three wick drains and offers around 20% cost saving on other more conventional methods.'
Lime columns are especially useful for embankment stabilisation. For temporary works, such as road cuttings, steeper cuts can be made if lime columns are used to stabilise the ground, reducing land take. And lime columns can be used to replace sheet piles as support for temporary excavations.
The method is used extensively in Sweden for foundations of rail and road pavements. 'It is the most common foundation type for these structures in Sweden,' says Schofield.
One of Hercules' current con- tracts is for a large road contract between rebo and Arboga on the E18/E20, 160km west of Stock- holm. Some 155,000, 600mm diameter columns (785,000m) and 1,000, 500mm diameter columns (9,000m) were installed in just 12 months for the main contractor NCC and the Swedish Road Authority.
Schofield adds that the method can also be considered for ground improvement below raft found- ations for low-rise buildings, with loads of between 50kN/m2 and 75kN/m2. And the method is also effective for treating soils with high organic content. 'In 95% of cases we can stabilise peat, although this is subject to an on-site trial using a mixture of additives.'
For foundation design, the soil and lime columns are treated as a composite material with more and more of the applied load being carried by the columns with depth and the surrounding soil offering lateral support. 'Design relies on the fact that the settlement of the soil must equal the settlement of the columns and is a factor of the strength, length and size of the columns,' explains Schofield. The original designs are drawn up by Stent and then checked by Hercules, as the Swedish company has more experience of working to the accepted standards laid down by the Swedish Geotechnical Institute.
Schofield believes that demand for the technique will continue to grow as people realise the technical and economic benefits, helped partly he says by the increasing popularity of lime piles in the early 1990s.