Lec 12 Highway Engineering - Soil Stabilisation

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Lecture 12 129

Highway Eng. Soil Stabilisation 14 15

Dr. Firas Asad

In this lecture;

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A- Soil Stabilisation

B- Mechanical Stabilisation

C- Chemical Stabilisation

Soil Stabilisation

The information listed in this lecture is mainly taken from the Iraqi General

Specifications for Roads and Bridge (SCRB, 2007, R6), Traffic and Highway

Engineering (Garber and Hoel, 2009) and Highways (OFlaherty, 2007).

A- Soil Stabilisation

Why should we stabilize soils? Soil stabilization is any treatment (including,

technically, compaction) applied to a soil to improve its strength and reduce its

vulnerability to water. Soil is regarded as stable

The continued demand for aggregates nowadays has resulted in shortages of stone

and gravel in many countries. As a result, greater attention is now being paid to the

use of locally-available substitute materials such as stabilized soils to meet road

construction needs.

if the treated soil is able to

withstand the stresses imposed on it by traffic under all weather conditions without

deformation. This definition is applicable for in situ soils, embankment soils and

base/subbase soils.

In practice, the main methods by which soils are stabilized for road purposes are:

(1) mechanical or granular stabilization; (2) cement stabilization; (3) lime and lime-

pozzolan stabilization; and (4) bituminous stabilization. The Figure below is a general

guideline to selecting the proper method of soil stabilisation.


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B- Mechanical Stabilisation

Mechanical stabilization involves improving the gradation of a raw soil by admixing a

coarse and/or fine material (usually 10 to 50 per cent) with the aim of achieving a

dense homogeneous mass when compacted. Mechanical stabilization is also known

as granular stabilization and soilaggregate stabilization.

C-

Chemical Stabilisation

Chemical stabilization is the blending of the natural soil with chemical agents.

Several blending agents have been used to obtain different effects, as shown in the

Table. The most commonly used agents are Portland cement, asphalt binders, and

lime.


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C-1 Cement Stabilisation (Cement-stabilised soils)

Cement stabilization of soils usually involves the addition of 5 to 14 percent Portland

cement by volume of the compacted mixture to the soil being stabilized. This type of


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stabilization is used mainly to obtain the required engineering properties of soils

that are to be used as base course materials. Generally, all types of soil can be

stabilized with cement; especially well-graded granular materials.

The procedure for stabilizing soils with cement involves:

Pulverizing (crushing) the soil

Mixing the required quantity of cement with the pulverized soil

Compacting the soil cement mixture

Curing the compacted layer

Internationally, cement stabilization is most commonly used in subgrade capping

and/or subbase layers in major road pavements, and in subbase and/or base

courses of secondary-type roads. It is never used in surface courses because, as well

as having poor resistance to abrasion, it must be protected from moisture entry into

the cracks that will certainly form in the cement-treated material.


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Factors which ensure that cement stabilization is very widely used are:

(1) cement is available in most countries at a relatively low price;

(2) the use of cement involves less care and control than many other stabilizers;

(3) more technical information is available on cement-treated soil mixtures

(4) most soils (except those with high organic matter or soluble sulphate contents)

can be stabilized with cement.

A soils gradation and plasticity are the most important factors affecting its

suitability for cement stabilization. Cement-treated soil mixes exhibit similar

moisturedensity relationships as raw soils, i.e. for a given compactive effort there is

one moisture content that will give a maximum dry density. Compaction should be

completed as soon as possible after the addition of cement to a soil.

Generally, enough quantity of cement should be used and with the right amount of

water and proper compaction and curing.

C-2 Asphalt Stabilisation (Bitumen stabilized subgrade or Subbase soils)

The mechanisms involved in the stabilization of a soil with a bituminous material

(usually hot bitumen, cutback bitumen, or bitumen emulsion)

Asphalt stabilization is carried out to achieve one or both of the following:

Waterproofing of natural materials

Binding of natural materials

Waterproofingthe natural material through asphalt stabilization aids in maintaining

the water content at a required level by providing a membrane that impedes the

penetration of water, thereby reducing the effect of any surface water that may

enter the soil when it is used as a base course. In addition, surface water is

prevented from seeping into the subgrade, which protects the subgrade from failing

due to increase in moisture content.


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Binding

Several types of soil can be stabilized with asphalt, although it is generally required

that less than 25 percent of the material passes the No. 200 sieve.

improves the durability characteristics of the natural soil by providing an

adhesive characteristic, whereby the soil particles adhere to each other, increasing

cohesion.

The mixing

The material then is spread evenly in layers of uniform thickness, usually not greater

than 6 in. and not less than 2 in. Each layer is properly compacted until the required

density is obtained using a sheeps foot roller or a pneumatic-tired roller.

of the soil and bituminous materials also can be done in a central or

movable plant (plant mixing) or at the roadside (road mixing). In plant mixing, the

desired amounts of water and bituminous material are automatically fed into the

mixing hoppers, whereas in road mixing, the water and bituminous material are

measured and applied separately using a pressure distributor.

C-3 Lime Stabilisation (Lime stabilized subgrade or Subbase soils)

Lime stabilization is one of the oldest processes of improving the engineering

properties of soils and can be used for stabilizing both base and subbase materials.

In general, the oxides and hydroxides of calcium and magnesium are considered as

lime, but the materials most commonly used for lime stabilization are calcium

hydroxide Ca(OH)2and dolomite Ca(OH)2+ MgO.

Clayey materials are most suitable for lime stabilization, but these materials should

have PI values less than 10 for the lime stabilization to be most effective.

When lime is added to fine-grained soil, cation exchange takes place, with the

calcium and magnesium in the lime replacing the sodium and potassium in the soil.

The tendency to swell as a result of an increase in moisture content is therefore

immediately reduced. The PI value of the soil is also reduced. Pozzolanic reaction


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may also occur in some clays, resulting in the formation of cementing agents that

increase the strength of the soil. When silica or alumina is present in the soil, a

significant increase in strength may be observed over a long period of time. An

additional effect is that lime causes flocculation of the fine particles, thereby

increasing the effective grain size of the soil.

The percentage of lime used for any project depends on the type of soil being

stabilized. The determination of the quantity of lime is usually based on an analysis

of the effect that different lime percentages have on the reduction of plasticity and

the increase in strength on the soil. The PI is most commonly used for testing the

effect on plasticity, whereas the unconfined compression test, the Hveem

Stabilometer test, or the California bearing-ratio (CBR) test can be used to test for

the effect on strength. However, most fine-grained soil can be effectively stabilized

with 3 to 10 percent lime, based on the dry weight of the soil.


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Lec 12 Highway Engineering - Soil Stabilisation