Soil Stabilisation has been used in engineering projects in the UK since the 1960's. No other process allows such a rapid improvement in site condition and foundation strength. The addition of quicklime to any soil or slurry will result in instantaneous drying. When added to a cohesive soil the following changes also occur.
Once treated with lime even the most plastic soil will break down into fine particles. This makes it suitable for the addition of cement, fly ash and ground granulated blast furnace slag that will impart significant strength and make the material suitable for subbase applications. Once the binders have been added and the correct moisture content is achieved the treated soil is compacted to a high standard to promote further strength gain and long-term durability.
In-situ Treatment of Large Sites
In-situ Larger Sites.
Soil Stabilisation is essentially a mixing process that can be carried out in a number of ways. Normally it is an in-situ operation where the binders are mixed into the ground in a layer by a powerful rotovators and then compacted with a roller. The layer is nominally 300mm but any number of layers can be used.
In-situ Treatment of Small Sites
In-situ Smaller Sites.
For restricted access sites, tractor-mounted rotovators are available but these still have a very powerful mixing action. Binders can be applied by spreading them on the ground before mixing or they can be applied during the rotovation process. The latter method reduces any potential dust problems.
Central Mixing Plant Treatment of Sites
Ex-situ or Central Mixing Plant.
For less cohesive materials and sites without in-situ mixing availability it is possible to mix the soil and binders at a central mixing plant. This involves hauling the soil to the plant for treatment and returning it to the point of deposition. This is usually slower than the in-situ method.
The chemistry and mechanisms by which soil is treated and stabilised by binders is described below
The mechanisms of Lime Treatment of Soils
1. Improvement (quicklime addition only)
Once quicklime is mixed with the moisture bearing soil an exothermic (heat producing) reaction takes place.
CaO + H2O = Ca(OH)2 + 1,140 kJ per kg of 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 it’s own weight of water as hydroxide to form hydrated lime.
2. Modification (quicklime or hydrated lime / liquid lime addition)
The next steps, Modification and Stabilisation only occur with clay soils. When quicklime or hydrated 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).
Figure 1: Clay particles aligned and surrounded by water, allowing them to slide easily, which results in a low strength clay soil.
Figure 2: Clay particles after modification with lime, showing reduced water layer and realignment, which significantly increases the clay strength.
3. Stabilisation (quicklime or hydrated lime / liquid lime addition)
The silica and alumina contents of the clay soil will react with the calcium present in the lime to form calcium silicate hydrates or calcium aluminate hydrates. This reaction is slow to proceed and is similar to the reactions that occur when cement cures. The strength gain can continue for over 10 years.
Once treated with lime even the most plastic soil will breakdown into fine particles. This makes it suitable for the addition of cement, fly ash and ground granulated blast furnace slag that will impart significant strength and make the material suitable for subbase applications. Once the binders have been added and the correct moisture content is achieved the treated soil is compacted to a high standard to promote further strength gain and long-term durability.