Soil stabilisation using lime
Benefits of soil stabilisation using lime
Soil stabilisation using lime is beneficial as it:
- Improves resource efficiency by improving the quality of on-site materials and reducing the need for imported materials
- Demonstrates good practice by minimising costs, carbon and resource demands
- Reduces construction traffic by avoiding transport of excavated and imported materials
- Reduces project costs by using on-site materials as a primary resource
- Is established practice with a proven history of successful use across the UK and internationally
- Is delivered by a competent supply chain able to meet the varied demands of differing projects
- Is covered by standards and specifications – including key civil engineering and foundation applications across the infrastructure spectrum
Introduction to soil stabilisation using lime
Soil stabilisation with lime can be achieved by applying to soil a controlled dose of quicklime, hydrated lime, or liquid lime (calcium hydroxide in water – also called milk of lime).
Quicklime is commonly used to reduce the moisture content of soils through the hydration process, which both chemically absorbs water and through the heat generated by the hydration reaction causes evaporation of moisture. The modified soil can then be more easily further modified by other binders if required.
Clay soils are permanently modified by the addition of lime to a more granular material that can be readily compacted.
Different treatments are available to suit virtually every soil type and can produce a range of different strengths according to use.
A significant advantage of improving soils with lime products is the use of in situ materials. This eliminates the need to remove material off site and saves the cost of haulage and disposal. Treating materials with lime on site often has a positive impact on the construction programme as it is usually considerably quicker to treat soils on site rather than to excavate, remove and import suitable replacement materials.
The mechanisms of soil stabilisation with lime
Improvement using quicklime
Once quicklime is mixed with the moisture bearing soil an exothermic (heat-producing) reaction takes place.
CaO + H2O = Ca(OH)2 + 1,140 kJ/kg CaO
quicklime water hydrated lime heat produced
In a homogeneous mixture, the quicklime reacts with the moisture present in the soil. This exothermic reaction generates significant amounts of heat energy which will dry the soil (temperatures can reach in excess of 100ºC) as well as chemically binding 32% of its own weight of water as hydroxide.
Modification and stabilisation only occur with clay soils. When quicklime, hydrated lime or liquid lime is added to a clay soil, the clay platelets go through an ion exchange process, which introduces calcium into the clay surface and causes a change in the way the clay platelets align, as shown in the pictures below. This gives an increase in soil strength and will normally occur quite rapidly (usually within two hours of mixing but can take up to a day depending on site conditions).
Aligned clay particles surrounded by water meaning the particles can slide easily – resulting in low strength clay soil.
Clay particles following lime modification – showing a reduced water layer and realignment – which increases the strength of the clay soil.
The silica and alumina contents of clay soils will react with the calcium present in the quicklime, hydrated lime or liquid lime to form calcium silicate hydrates or calcium aluminate hydrates. These reactions are similar to those that occur in hardening cement; they are long term and strength gain can continue for many years.
The treatment process
The process is simple and can range from basic plough and disc harrow through to purpose built lime spreaders and powerful rotavators. Soil stabilisation using lime can be carried out on a wide range of project sites.
Lime products are introduced into the soils and mixed thoroughly. The most common process is to treat the soils in horizontal layers, in situ.
It is also possible to excavate the untreated soil into a stockpile and then feed the material into a mixing processor where the lime is added. The mixed material can then be placed where it is required ready for the next stage.
The mixed material of lime and soil is allowed to mellow, where stabilisation reactions continue to occur and the mixture changes in characteristics, becoming stiffer.
Where required, other binders may be introduced as a further treatment to increase the ultimate strength and stiffness.
Whichever method of mixing has been used, the next stage is the compaction of the material to produce a competent surface with the designed loadbearing capacity. Further construction layers are subsequently installed onto the stabilised layers.
Britpave provide guidance on soil stabilisation (available here).
The Safe Handling of Lime document (available here) gives some general guidelines on the handling requirements for lime products.
However, please refer to the supplier's Safety Data Sheets for the complete safety information referring to an individual product being considered.
Information on soil stabilisation using lime is available from:
BLA have produced a leaflet that explaining the benefits of soil treatment using lime – available here.
Britpave publications provide advice and guidance on soil stabilisation – available here.
A recent study on the performance of a lime treated German embankment placed in 1979 is available here.