The Process

The two main dry powders used are Quicklime (CaO) and Ordinary Portland Cement (OPC), although Ground Granulated Blast Furnace Slag (GGBS) is preferred in soils with a high sulphate content.

Quicklime is predominantly used in clay and silty soils for stabilisation purposes, but is a very useful drying-out agent in sandy and gravelly materials- it reacts aggressively with moisture. Added to clay/silt the CaO reacts with the film of moisture that is present around each particle. This reduction in liquid allows the particles to be compacted together tighter than before, typically achieving 5% air voids or less. In addition to this there is an increase in the CBR and the plastic limit of the material almost immediately. After the initial application pozzolanic (cementing) reactions take place which result in further increases of CBRs, greater frost resistance, improved stability against swell and shrinkage and higher levels of shear and tensile strength.

The addition of OPC to lime treated clay will result in even higher CBRs and provide a non-frost susceptible material. More importantly it is used as a binder when stabilising granular or sandy materials.

In addition Pulverised Fuel Ash (PFA) can be used to provide ‘bulking up in materials that have little, or no natural binding ability of their own. The PFA also has natural cementitious values that also help to improve the strength of the material that has been treated.

The key to stabilisation, however, is the testing of materials before, during and after application to ensure that the specifications for the contract are achieved. With this and a good working practice stabilisation is one of the most economic and environmentally friendly construction practices in use.