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6. Soil Stabilisation

6.1 Introduction

Stabilisation:

Term may be used to describe physical or chemical treatment used to maintain or improve the stability of natural materials occurring in, or utilised in, engineering structures.
Term more commonly applied to processes in road engineering.
In roadworks, stabilisation includes any physical or chemical treatment intended to improve or maintain the load-carrying capacity and resistance to moisture of in-situ or locally available soils or fragmented rock materials.
Compaction is a method of stabilisation, but the term usually implies a change in constitution resulting from incorporation of some material which will modify or counteract deficiencies in the engineering properties of the soil.

6.2 Applications of Stabilisation

The most common uses of stabilisation in roadworks are:

to improve local granular soils or crushed rock materials to provide satisfactory pavement materials,
to improve subgrade soils to enable pavement thickness to be reduced,
to improve paving materials by providing increased resistance to the effects of moisture, and
to provide a 'working platform' over a weak subgrade (to facilitate construction).

6.3 Stabilisation Methods

Three major factors must be considered in selecting the method of stabilisation to be used - soil type, use and cost.

6.3.1 Soil Type

Stabilisation Method	General Range of Additive	Soil Type
Stabilisation Method	General Range of Additive	A3	A-2-4 to	A-2-7	A4	A5	A6	A7
Mechanical	10 to 50%	Variable	Good	_____	_____	Fair	_____	Difficult
Cement	1 to 3%	Variable	Variable	Good	_____	Fair	_____	Difficult
Hydrated Lime	1 to 4%	Poor	Poor	Good	_____	_____	_____	Fair
Bitumen	1 to 4%	Fair	Good	Good	Fair	Difficult	.	.
Bitumen Emulsion	2 to 6%	Variable	Good	Good	Fair	_____	Difficult	.
Tar	2 to 4%	Variable	Good	Good	Fair	_____	Difficult	.

(A2 = Silty or clayey gravel and sand; A3 = Fine sand; A4 and A5 = Silty soils; A6 and A7 = Clayey soils)

6.3.2 Proposed Use

Material to be used in upper paving courses must be stabilised to A1 quality.
Material for lower courses may be stabilised to produce quality less then A1 material.

6.3.3 Relative Cost

Stabilisation must be economically viable.
Cost of stabilisation must be compared to cost of importing suitable materials not requiring stabilisation.
Costs of stabilising are usually dominated by cost of stabiliser, and cost of mixing.

6.4 Types of Stabilisation

6.4.1 Mechanical Stabilisation

Mechanical stabilisation is the process of mixing two or more soils to obtain a desired particle size distribution and/or reduce plasticity. Examples would be the addition of a sandy material to a clayey material, or the addition of a fine material (usually of low plasticity) to a coarse material which is deficient in fines.

Theoretically any soils can be mixed to improve quality. In practice, however, the process is best reserved for those materials having low plasticity indices or which are non-plastic. Thorough mixing of materials is important and it is very difficult to adequately mix plastic materials (e.g. heavy clays).

The optimum proportions for mixing are determined from laboratory tests.

6.4.2 Portland Cement Stabilisation

Cement treatment takes two forms:

cement modification - which uses up to about 3 percent cement (by mass) and aims to reduce plasticity without producing a rigid material; and
cement stabilisation - which uses higher percentages of cement and produces a stiff, semi-rigid pavement material.

The design of cement-soil-water mixtures is based on selecting the minimum cement content required to provide sufficient strength and durability to enable the material to function as a satisfactory layer in the pavement structure. The amount of cement required is determined by laboratory testing, usually using the unconfined compressive strength test.

Construction practices significantly effect the subsequent performance of cement stabilised materials, and each of the following aspects must be closely controlled:

pulverisation;
cement content;
moisture content;
mixing;
compaction;
finishing; and
curing.

Rapid compaction after mixing is possibly most important as cement hydrates relatively quickly.

6.4.3 Hydrated Lime Stabilisation

When hydrated lime (or quick lime) is aded to a soil in the presence of moisture a series of reactions is set in motion. The actual physical and chemical processes which occurs are quite complex. The reaction between lime and soil can be considered in three major, overlapping stages:

agglomeration of fine clay particles, though base exchange;
weak cementing action, due to calcium carbonate formation; and
slow, long-term cementing action.

The reaction of lime with soil depends on the type of clay minerals present in the soil. For the reaction to be effective the soil must contain kaolinite or montmorillonite minerals. If the clay minerals are illite or chlorite, a pozzolan must be added to produce the desired effects. The normal pozzolan which is used is fly ash.

Small amounts of lime (1% to 3%) may reduce the soil plasticity and this process is referred to as lime modification. The more normal process is the addition of 3% to 6% lime, although there is now a school of thought which suggests the process is more effective with fairly high lime contents (in the order of 10%).

Mix design is based on the selection of the lime content necessary to provide required strength and durability. Lime contents may be determined by strength tests (e.g. CBR), or by Atterberg Limits, or by pH values.

Construction processes are similar to those used for cement stabilisation. Adequate pulverisation of the soil to be stabilised is very important, and this may be facilitated by partially pulverising, adding portion of the lime, repulverising and then adding the balance of the lime.

6.4.4. Bituminous Stabilisation

The addition of a bituminous material to soil or crushed rock material is intended to either provide a cohesive binder for non-plastic materials, or to waterproof a cohesive material.

The type of binder best suited to a particular application depends on cost, soil type, climate, and availability of mixing equipment. The most appropriate binder from a technical perspective is determined by laboratory testing.

Bituminous binders which have been used for stabilisation work include bitumen, cut-back bitumen, bitumen emulsion and tars.

6.5 Trends in Stabilisation

Early stabilisation works were carried out using agricultural machinery and road graders. Major improvements to stabilisation techniques were achieved with the development of pulverising machines, and the introduction of single and multi pass stabilising equipment.

Plant mix methods are now being increasingly used instead of on-the-ground stabilisation methods, as improved quality control can be obtained and production is less affected by weather conditions. However road-mix methods are still the most economical for most jobs in Australian rural areas because of the relatively short job lengths undertaken and the cost of shifting mixing plants.

The depletion of sources of natural materials inevitably results in increasing use of stabilisation methods. The need for more durable roads for modern traffic loadings means that stabilisation of subgrades and inferior paving materials will receive increasing consideration.

LINKS TO SITES ON STABILISATION

The Australian Stabilisation Industry Association has a very useful Web site. The 'Austab Guidelines' and 'Reference Materials' sections have a lot of good technical papers on various aspects of stabilisation.

Page last modified 25 June 2002.