earthworks improvementusing quicklime

Introduction

Unsuitable earthworks materials can be improved withquicklime to enhance geotechnical properties. Thisresults in:

A reduction in moisture contentImproved bearing capacityReduced plasticityLess shrinkage or swelling

This allows weak materials to:

Be handled more easily by earthmoving plantCompacted to a high standardSupport construction trafficBe treated with other binders for even greater strength

The objective of soil ‘Improvement’ with quicklime is toachieve an immediate reaction, which significantlystrengthens the soil due to the removal of moisture anda chemical change in clays. It is not intended to induceany long-term strength gain, which occurs when soils are‘Stabilised’. However where clay soils are present evenmodest additions of lime may cause some long-termstrength improvement. Soil stabilisation will be coveredin another BLA data sheet.

This data sheet considers the use of quicklime to improvecohesive, granular and chalk materials that areunacceptable due to plasticity or moisture content. Theimproved material will be deemed acceptable underclasses 1, 2, 3 and 4 of the Specification for HighwayWorks (SHW)(ref 1). The advice given here is applicableto any situation where a naturally won fill materialcomplying with the above classes would have been used.

History

Lime treatment of soils was introduced into aDepartment of Transport Guidance Note in 1976 andinto the full D Tp Specification ten years later. LimeImprovement has been carried out in large quantities inthe UK for many years and has now been included in aHighways Agency draft advice note (ref 2). Typicalapplications include roads, warehouses, service areas, carparks and leisure developments (ref 1 & 3). Specificallythis technique has also been used on the Channel TunnelRail Link through Kent, Manchester Airport secondrunway, Rockingham Motor Speedway at Corby and theM60 Motorway.

The nature of Improvement

The addition of quicklime to cohesive, granular and chalkmaterials causes a reduction in moisture content aswater is used in the hydration process. This can beenough to produce a material with acceptableengineering properties.With cohesive soils there is a simultaneousimprovement in the condition of the clay particles dueto a chemical change brought about by the calcium ions.This chemical change is known as ‘Modification’ and isadditional to the drying process.This reaction essentiallytransforms the material into a different soil withenhanced geotechnical properties. Compactioncharacteristics, MCV(moisture condition value) results,plastic limit and bearing capacity are all changedbeneficially in this reaction.

These reactions occur immediately the quicklime isdispersed into the soil. Stiff clays will lose plasticity andbecome more friable due to the change in soil properties.This need only be enough to achieve the requiredspecification for the fill material, which is usually when95% passes a 28mm sieve. This will enable therequirements of classes 1,2,3 & 4 to be met but furthertreatment will be necessary to meet other specificationsof higher strength.

Preliminary site study and investigation

As far as possible the site investigation should identify the different groups of materials that exist on the site and the classes they fall into. It will then be possible todetermine the sources of unacceptable soils that require improvement before they can be used.As the reaction between different soils and quicklime will vary it is importantto obtain representative samples of all site soils for laboratory testing. Generally grading should comply with the specification for the acceptable material and it will be themoisture content, MCV and plastic limit that are corrected by using quicklime.

Soil samples will be mixed with varying percentages of quicklime and allowed to mature in a sealed container. The duration of curing should reflect the proposed siteoperation between mixing and compaction and 2 hours is a good average. MCV and moisture content tests results will be taken on these samples and the MCV rangeestablished for acceptable limits. The lower range will establish the higher moisture content point where strength loss is unacceptable and the upper range where thematerial is too dry for acceptable compaction. This is usually in the range of 8 - 12.

The MCV test is only applicable to fine grained soils and may not be appropriate for granular material or chalk. In this case the moisture content should be tested foracceptability. Mixtures of granular and cohesive soils may use either or both tests depending on the make up of the soil. Moisture content ranges are usually OptimumMoisture Content to OMC-2%. On wet materials a matrix of results may be useful to determine the addition of quicklime required to cater for all the conditions that maybe encountered on site.

Specific tests that are required when adding quicklime to cohesive material are total sulfate, total sulfur and organic content. Sulfates may induce swelling in the improvedmaterial and organics can affect the soil / quicklime reaction. Large percentages of organics such as peat are not recommended for stabilisation and further guidance shouldbe sought if this is to be contemplated.

Construction

The site investigation results will give an indication of the quantities and location of the unacceptable material, the moisture content, plasticity, organic content andsulfate content. Further tests on the current moisture content may be required if conditions have changed since the original site investigation was done. It can then bedetermined whether treatment at source will be required to improve excavation, handling and placement at the point of use.

Considering the weak nature of the soil the spreading of lime is usually best carried out using a towed spreader. Mixing can be carried out by rotovator but the use ofdisc harrows and purpose made ploughs is acceptable providing adequate mixing can be achieved.

The amount of quicklime required to achieve the necessary improvement is determined by testing but 2% addition by weight will substantially improve many soils.

Material can be treated either in situ or after transporting to the point of use. However this is carried out, it is important that the procedure ensures that all the partsof the improved soil are mixed with the correct proportion of quicklime and that untreated material is not included in the finished construction.

UK conditions often determine that the material is transported to the point of use and then treated, but on some larger contracts deep layers are treated and thenmoved to the point of use by scrapers. Either system is equally permissible and further improvement sometimes occurs as treated material is moved and compacted.

Treated material may be stockpiled for use later but the external surfaces should be sealed to prevent deterioration due to the ingress of water.

The procedure described above illustrates the simplicity of this technique. It can quite easily be incorporated into an earthmoving operation and does not delay the rateof construction. In fact due to the superior nature of the improved soil productivity is often higher with less down time due to the weather.

The treatment with quicklime of unsuitableearthworks material to produce general fill

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datatechnicalReferences

1. The Highways Agency. Manual of Contract Documents for Highway Works. Volume 1.Specification for Highway Works. The Stationery Office, Norwich, England.

2. The Highways Agency. Design Manual for Roads and Bridges. Volume 4 Geotechnics and Drainage. Section 1, Earthworks. Treatment of Fill and Capping Material using either Lime or Cement or both. Part X. HAXX/99.

3. Buxton Lime Industries. Stabilisation Project Reports.4. British Lime Association. Model Specification for Earthworks Improvement using Quicklime. BLA

Specification Data Sheet No 1. Quarry Products Association, London.5. Draft prEN227404. Specification for Lime-treated Mixtures for Road Construction and Civil

Engineering – definitions, constituents and laboratory mixture requirements.6. TRANSPORT RESEARCH LABORATORY. Stabilised sub-bases in road foundations: structural

assessment and benefits. TRL Report 248.

Gillingham House, 38-44 Gillingham Street, London SW1V 1HUTel 0207 963 8000 Fax 0207 963 8001

email info@qpa.org

www.britishlime.org

1earthworks

improvement

The British Lime Association is one of the constituentbodies of the Quarry Products Association, the tradeassociation for the aggregate, asphalt and ready-mixedconcrete industries.

Site Testing

The treated material will have to meet the pulverisation,compaction and MCV requirements of the general fillspecification. Site testing to confirm these parametershave been achieved will need to be carried out.

Laboratory tests will have already confirmed theimprovement that can be achieved with quicklime andthe range of MCV values within which the specification isachieved. If these ranges are obtained on site aftertreatment then the required specification will be metsubject to adequate compaction.To achieve these results the spread rate of the quicklimewill need to be checked to confirm it is the same as thatdetermined in the laboratory. If the nature of the soil haschanged between testing and construction then thespread rate may require adjusting to achieve the desiredMCV results. Pulverisation checks should also be carriedout to ensure that adequate mixing has taken place.

As the treated soil should comply with one of the classesgiven in the Specification for Highway Works the methodof compaction will be the one appropriate for that classof material.

For cohesive materials the MCV test is ideal to controlacceptability. In one test it is a measure of moisturecontent, compactability, bearing ratio and shear strength.It is quick to carry out and is simple to use on site.

The MCV test is not so relevant for granular soils andchalk. In these cases the direct measurement of moisturecontent is the preferred control.

Specification

A simple and concise specification for the treatment ofunacceptable earthworks materials is described in BLASpecification Data Sheet No 1 (ref 4).

Benefits

Lime Improvement is a quick and simple operation thatcan be incorporated into any earthmoving contract nomatter how large or small. Because the nature of thesoil is quickly changed to improve the handlingcharacteristics, productivity is often enhanced. Thesegregation of unacceptable material is alsoeliminated. This is simply treated with lime as it isidentified to obtain the soil properties that are required.

The cost of treating unacceptable soils will depend onthe lime addition required and the mixing techniqueemployed. Exact addition rates should be determinedby simple tests as described earlier.

One of the great advantages of improving soils withquicklime is the ability to retain all material on site. Thiseliminates the need to remove material to tip and savesthe cost of tipping and haulage charges. This often hasan impact on the construction programme as it isusually considerably quicker to treat soils on site ratherthan dig, dump and replace.

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