Lime & Cement Stabilisation

Lime and/or cement stabilisation is often used to improve the properties of site won materials, to enable their use in a pavement and other like areas, such as dam foundations and building pad sites. Lime stabilisation of clay material reduces entrance cracking, whilst increasing the hardness of the material by up to ten times. The use of cement as a binder, after lime, can further increase the strength and durability towards that of concrete. Various binder blends, away from lime and cement, such as slag or fly ash, are commonly utilised for further benefit dependant on site conditions and requirements.

Water Truck

Especially during the drier months, water must be added to ensure optimum moisture content is maintained for compaction. Depending on soil conditions and moisture content water can be added before and or after spreading any binder or directly into the mixing chamber by linking the water truck to the mixer where appropriate. Not only is water vital to ensure optimum moisture content at compaction, water initiates the necessary chemical reactions with most binders.

Compaction

Compaction commences after mixing. Typically stabilised materials are compacted to 95% standard, however higher compaction standards are achievable. Insitu mixing up to 400mm in a single layer requires compaction equipment large enough to achieve density throughout a layer this thick. Typically large self propelled vibratory padfoot rollers are used initially for deep compaction followed by a similar smooth drum to complete compaction of the full layer.

Final Trimming

It is normal to commence trimming the pavement before the completion of the compaction operation, ensuring good bonding of any corrected shape before it is finished.

General Benefits of Soil Stabilisation

Saturated, wet sites can be treated to provide a working platform within a day for project continuation during wet periods/seasons.

Stabilisation recycles existing pavement by pulverising the existing pavement to 25mm down. Lime and or cement or other binders are then mixed with water as necessary. No imported materials and increased production rates means cost savings.

Strength gains often over CBR 15% or 5 times the previous strength are the result of the realignment of particles and adjustment of moisture content allowing compaction at optimum moisture content.

Reduce Plasticity Index (PI) in cohesive materials. For example a material with PI 20 will typically stabilise to PI < 10, say 8.

Considerations for Stabilisation

By seeking our advice early during the development stages of a project we can ensure savings are maximised by optimising the use of stabilisation in designs to reduce double handling and import and export of materials. We have qualified engineers and project managers offering sound advice with years of experience. In order to assess a site accurately in terms of stabilisation, ideally the following information is considered:

Geotechnical data including site conditions, material type and depth, sub-grade and existing pavement material.
Construction conditions and loading.
Geometric site layout proposed and existing.
Proposed minimum area to be treated.
Specification requirements typically in terms of density, CBR strength or binder content if provided.
Proposed award and commencement dates.

Using Lime

Note that there are many variations of lime available but only quicklime is considered suitable for lime stabilisation in the pavement construction industry and general field construction activities. Quicklime is calcium oxide (CaO) supplied commercially in a dry powder form. Agriculture Lime is a calcium carbonate (CaC03) and not suitable for pavement construction. Hydrated Lime is calcium hydroxide (Ca(OH)2) often used in the laboratory for lime saturation testing, not generally used on site for pavement construction.

Hydrated lime (calcium hydroxide), is produced by reacting water with quicklime (calcium oxide). CaO + H2O => Ca(OH)2. When calculated using the atomic weights, this converts practically to 5t Quicklime + 3t Water => 7t Hydrated lime + 1t Water Evaporation.

The pozzolanic reaction between lime with water and the silica and alumina in clay results in an ionic exchange, which permanently realigns the clay particles forming friable conglomerates. The new alignment of the particles provides less ability for the clay to absorb water around the particles. This makes the clay more waterproof, less expansive and therefore reduces the plasticity and linear shrinkage. The PI is often more than ½ and the shrinkage is often 10% of what it was. Practically this results in improved permeability less shrinkage cracking providing less chance of piping failure and seepage.

In a lime saturated environment (typically 3% to 4% quicklime), the clay-alumina and clay-silica become available to react with the free calcium to form calcium aluminates or silicates. The pozzolanic reaction is illustrated by the following equations:

Ca2+ + OH- + Available Clay Aluminium Calcium Aluminate Hydrate (CAH)
Ca2+ + OH- + Available Clay Silica Calcium Silicate Hydrate (CSH)

General Stabilisation for Lining Systems and Cohesive Expansion Material

Reduce or eliminate the need for imported clay liner by stabilising insitu materials.
Reduce permeability.
Reduce Linear Shrinkage rate up to 10%.
Environmental benefits of reduced geotextile, borrow pit clay and quarry import.
Additional environmental benefits from reducing extra excavation and disposal by modification to suitable material.
Improved structural stability through realignment of soil particles by ionic exchange between clay and lime.
Increased Strength and durability.
Reduced dispersion means reduced dispersion piping failure and increased erosion protection.
Pulverisation to 40mm down of clay, extremely weathered limestone, mudstone and siltstone provides smaller diameter conglomerates and homogenous material throughout the stabilised layer eliminating lenses, streaks, rock fissures and faults providing reduced seepage.

Lime & Cement Stabilisation