In continental Europe, the mass stabilisation of soft non-organic soils with lime and cement columns has been carried on for more than 20 years in order to reduce settlement and improve the stability of embankments and slopes.
The stabilisation of organic soils such as clay and peat is a relatively new technology which has developed rapidly in recent years, thanks mainly to new machinery produced by Finnish manufacturer Allu. This trend towards stabilisation is expected to continue as the method is highly cost effective as well as reducing amounts of spoil and requirements for imported stone, together with the associated needs for haulage.
Stabilisation is a ground improvement method for soft soil layers. Its aim is to increase the firmness and reduce the compressibility of the layers by mixing an appropriate amount of a binder agent with the soil. Commonly used binders include lime, cement, or a mixture of both.
Normal deep stabilisation involves creating reinforced zones which are vertical, columnar and rectangular, whereas the aim of mass stabilisation is to mix the binder agent as uniformly as possible throughout the volume of the treated soil layer. The soil can be stabilised either by forming columns of stabilised soil, or by stabilising the entire soil volume. With existing equipment columns can be stabilised to about 25 metres, and mass stabilisation can be used to depths of about five metres. The two methods may be combined.
The stabilisation process brings about significant changes to the chemical and physical properties of peat and clay. The pH value will quickly rise to 11 or 12 and the curing will start. Depending on the binder, some reactions will take place in a few weeks but others may take months. Using only cement, most of the strength develops during the first month after treatment but with lime, gypsum, slag or ash the strength will continue to develop after the first month.
Mechanical interactions are also important. Cement requires very thorough mixing since it does not diffuse as does lime. While cement increases the strength and brittleness of the stabilised soil, lime produces increased ductility. However, a too stiffly stabilised material is not necessarily the best solution since it will behave like a pile. The idea is to produce a stabilised soil that mechanically interacts with the surrounding material, so that loads are carried by both.
Because the local subsoil forms part of the construction, careful site investigation is essential in order to choose the correct binder and application process.
Stabilisation can also be used in combination with other ground improvement methods requiring deeper treatment. The top portion of the soil layer can be stabilised throughout by mass stabilisation while deeper soil layers are reinforced by column stabilisation.
The main purposes of soil stabilisation are:
(1) To increase the strength of the soft soil in order to:
- increase the stability of an embankment
- increase the bearing capacity
- reduce the active loads on retaining walls
- prevent liquefaction
(2) To improve the deformation properties of the soft soil in order to (static loads)
- reduce settlement
- reduce the time for settlements
- reduce the horizontal displacements
(3) To increase dynamic stiffness of the soft soil in order to:
- reduce the vibrations to the surroundings
- improve the dynamic performance
(4) To remediate contaminated ground by:
- creating an environmental barrier (solidification)
- stabilisation of the contaminated ground
- creating a geohydrological barrier
Mass stabilisation involving the use of moderate amounts of a binder agent (< 100 kg/m3) usually increases the shear strength of soil layers to at least 50-100 kPa or even higher. Mass stabilisation is especially suited as a ground improvement method for structures that require reinforcement of large soil volumes as advantageously as possible, but where absolute non-settlement of the ground is not expected, and the soil is not subject to excessive single point loads. As a binder agent, two-component binder mixes are widely used but three-component binders are more versatile and can be more effective for many cases. The most important components are lime, cements, blast furnace slag and gypsum. Industrial by-products such as high quality fly ashes can be exploited for certain cases, especially in the stabilisation of peat. Applications for soil stabilisation include highway and ground construction, foundations for industrial buildings, yards and bridges, sports ground and storage sites, protection of adjacent structures, reduction of earth pressure, stabilisation of very soft soils for tunnel boring, foundations for landfill areas, underwater protection layers, cable/pipe trenching, river, lake and embankment slopes. Other applications include the handling and isolation of contaminated soils and the neutralisation of toxic waste. There are two ways to seek tenders for stabilisation projects. The bid is based on the definition of the working procedure, or on a bearing capacity and other important features. In the first case the contractor’s only responsibility is to work according to the instructions. On the second case the contractor has to have good knowledge and experience of stabilisation work. He decides how to do the job and takes responsibility that all the requirements are fulfilled. While economics and the need to reduce waste continue to bring about increased use of the stabilisation technique, another key driver has been the introduction of machinery to carry out the work. Thorough mixing at all levels is crucial, and is being achieved by Finnish company Allu. The company’s screener crusher processing attachment for screening, crushing and mixing various materials is already widely used in Ireland. Applications include processing glass and green wastes, crushing and screening excavated material so it can be backfilled as pipeline bedding, and processing gypsum and concrete wastes. Allu’s AS38H windrow turner can mix even high density materials (2,300 kg/m3) at up to 4 m3/hour. The company’s EPM System together with Allu SM, Allu VP and Allu OilQuick represents a complete solution for soil stabilisation in pipeline construction. Mounted on a 35 tonne excavator with quick release which includes automatic hydraulic couplings, the system comprises a fast-fill silo for placing the binder agent and a rotating mixer head which is lowered into the soil. These proven machines have now been joined by the Allu PF, a track mounted pressure feeder which continuously feeds binding agent via a hose to the mixer head. This greatly speeds the processing of large areas as the excavator operator has remote control of the PF unit.