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Sunday, 19 February 2012

MECHANICAL & CEMENT STABILISATION OF SOIL

Land Stabilisation has been carried out since Roman and Egyptian times with many well documented cases in recent history. The strength and stability of all soils improve on compaction. For certain purposes, the strength gained , even after compaction , may not be adequate . The use of the soil as a construction material can be extended over a much wider field by soil stabilization

Soil Stabilization is the permanent physical and chemical alteration of soils to enhance their physical properties. Stabilization can increase the shear strength of a soil and/or control the shrink-swell properties of a soil, thus improving the load bearing capacity of a sub-grade to support pavements and foundations.

The most stable types of soils are those which contain granular material which derive their strength from friction and clayey materials which do so from cohesion. Thus the properties of granular soils can be improved by mixing them with gravel and sandy material.

Again moisture content plays a very important role in preserving the strength of soil material, even after compaction. Thus capillary moisture provides a useful binding force, and for relatively pervious, granular, soils which drain quickly, use of some hygroscopic additive which would maintain their moisture, is conductive to stability.

On the other hand, for clayey soils, the problem is one of preventing them from getting saturated, which causes them to loose strength. This is partly accomplished by proper drainage, but can be usefully supplemented by adding a water proofing agent to the soil.This means that virtually any soils found on site can be improved for bulk fill applications and to build roads, pavements, embankments, reinforced earth structures, railways, housings and industrial units.

 

 

Types of stabilization:

     Broadly stabilization can be classified under the following categories:

·         Mechanical stabilization

        (Ex. De watering, compaction and so on).

·         Process stabilization

         (Ex. Electrical, thermal so on).

·         Chemical stabilization (Ex. Cement, bitumen, lime and so on).

 

Mechanical stabilization.

Mechanical stabilization is accomplished by mixing or blending soils of two or more gradations to obtain a material meeting the required specification. The soil blending may take place at the construction site, a central plant, or a borrow area. The blended material is then spread and compacted to required densities by conventional means.

 

Chemical stabilization.

Chemical stabilization is achieved by the addition of proper percentages of cement, lime, fly ash, bitumen, or combinations of these materials to the soil. The selection of type and determination of the percentage of chemical to be used is dependent upon the soil classification and the degree of improvement in soil quality desired. Generally, smaller amounts of chemicals are required when it is simply desired to modify soil properties such as gradation, workability, and plasticity. When it is desired to improve the strength and durability significantly, larger quantities of additive are used. After the additive/chemical has been mixed with the soil, spreading and compaction are achieved by conventional means.

                    

Methods of Mechanical stabilization

Removal &Replacement:-

Excavate unsuitable soil and replace compaction fill used when soil is too loose use same soil for fill which has high unit weight which have engineering properties. Removal will be done first soil has excessive organics. It is expensive method because we want to dispose and import the soil. Both is suitable only above ground water table. Earthwork operation is different if soil is wet.

Precompression:-

For improving soil we have to cover them with a temporary surcharge fill. Preloading, surcharging. Suitable for soft clayey and silty soils because static weight of fill cause them consolidate thus improves settlement of strength properties after the properties

attains, surcharge id removed and construction proceeds surcharge fills 3-8 m, settlement 0.3-1 m.

 

Insitu densification:-

Method of densifing shallow soil using heavy vibratory rollers upto 2m. 

 Vibrpcompaction: Two methods Terraprobe and vibroflot. Terraprobe consists consists      vibratory pile hammer attached to steel pipe. Pile is vibrated. A vibroflot contains vibrator and water jet. Depth upto 3- 15m, silt content less than <12-15%.

 

Dynamic compaction:

cost effective method of densifing loosen sandy and silty soil. Primary zone of influence typically extends to depth of 5-10m with lesser improvements below these depths. It is used to treat 5-10m with lesser improvements below these depths .It is used to treat liquefaction prone soil, collapsible soil. It is evaluated by performing STP&CPT tests before and after construction.

 

Blast Densification: -

curious than above one. It consist of drilling a series of boring & using them to place explosive underground. It is effective in clean sands. Because of vibration of safety issues it is only suitable for remote sites.

Insitu Replacement:-

It is intended to provide load bearing members that extend through weak strata. The stone column acts as vertical drain thus helps in accelerate consolidation settlement of mitigate seismic liquefaction problem.

 

 

 

                                 Modern equipments used for stabilization

 

 

 

 

 

 

 

 

CHEMICAL STABILIZATION

Grouting:-

Injection of special liquid or slurry material into ground for improving the soil. Two types: cementitious, chemical grout. Cement grout made of Portland cement that hydrates after injection forming a solid mass. Chemical grout includes wide range of chemicals that satisfy once they are injected into ground

Four principal methods:

Intrusion grouting: filling joints in rock by injecting grout through pipe cementitious grout is suitable. Used to prepare foundation for dams.

Permeation grouting: injection of thin grouts into soil to permeate into voids. Chemical grout is used.

Compaction grouting: used a stiff grout that is injected into ground under high pressure through a pipe. Grout is too thick. used to repair structures that have excessive settlement.

Jet grouting: uses a special pipe equipped with horizontal jets that inject grout into soil at high pressure.

Stabilization using admixtures: Portland cement is used as admixture. Cement +lime+ asphalt  thus increasing strength of reducing both compressibility and hydraulic conductivity.

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:                                                               

·         pulverization;

·         cement content;

·         moisture content;

·         mixing;

·         compaction;

·         finishing;

·         curing          .

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

 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.

 

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.

 

Insitu deep mixing:-

Uses rotating mixer shafts, paddles or jet that penetrates into ground while injecting of mixing Portland cement. It includes deep cement mixing, deep jet mixing. The treated soil has greater strength, reduced compressibility than original soil.

Surface mixing: upper soil gets ripped, applying the admixture mixing with special equipment and compacting. Once mixture has cured it forms very hard and durable soil.

It forms a layer called sub base in highways and airports. It is no more than 200 mm.

Reinforcement:-tensile reinforcement members improve the soil stability of load carrying capacity. Used in construction of compacted fill slopes of earth retaining structures.

 

 

                               Equipments used for Cement stabilization

 

 

Transverse single-shaft mixer processingsoil-cement in place.

 

     Multiple-transverseshaft mixer mixing soil, cement, and water in one pass.

Bulk portland cement being transferred pneumatically from a bulk transport truck to a job truck

 

 

Mechanical cement spreader attached to ajob dump truck spreading cement in regulated quantities

 

  Equipments for lime stabilization

 

           

                                Spreading of lime slurry

 

 

                                          Portabatch lime slaker

 

 

 

                             Equipments used for Bituminous stabilization

                    

 

      

                     Rotary mixer on primary road project

 

                         Distributor applying asphalt.

 

Uses of Stabilization.

Pavement design is based on the premise that minimum specified structural quality will be achieved for each layer of material in the pavement system. Each layer must resist shearing, avoid excessive deflections that cause fatigue cracking within the layer or in overlying layers, and prevent excessive permanent deformation through densification.

As the quality of a soil layer is increased, the ability of that layer to distribute the load over a greater area is generally increased so that a reduction in the required thickness of the soil and surface layers may be permitted.

 

a. Quality improvement.

The most common improvements achieved through stabilization include better soil gradation, reduction of plasticity index or swelling potential, and increases in durability and strength. In wet weather, stabilization may also be used to provide a working platform for construction operations. These types of soil quality improvement are referred to as soil modification.

 

b. Thickness reduction.

The strength and stiffness of a soil layer can be improved through the use of additives to permit a reduction in design thickness of the stabilized material compared withan unstabilized or unbound material.

 

 

Improvements:

Following are the improvements observed after soil stabilization.

1.      Increase of shear strength and bearing capacity

2.      Reduction of susceptibility to shrinkage and swelling

3.      Improved resistance to weather and traffic

4.      Reduction of water content to improve strength, workability and compaction

 

Advantages of soil stabilization :

1 Saves money :

 We can usually save significant sums of money by land stabilisation compared to the traditional "dig and dump" method. Dig and dump incurs the cost of vehicle movement, landfill tax and buying aggregates.

SavingsbyDesign:                           
            Soils treated with lime, cement and other binders can be designed to be stronger than conventional granular sub base. Using this type of material in a pavement or foundation means that the strength is considerably enhanced. This strength can be used to reduce the thickness of the foundation or the thickness of the subsequent layers. Concrete or blacktop can be laid directly onto stabilised soil. Savings in granular sub base, concrete and bituminous materials are all possible

2 Environmental effect       

Imagine removing 50 lorry loads of soil and bringing in 50 loads of imported material.

One 30 tones load of lime can eliminate these 100 vehicle movements. Less cost, less congestion and no furious neighbors. An environmental solution with many benefits.

 

3 Saves waste :

There is no need to import new material when the soil on site can be used after a simple treatment process             . 
             Even Type 1 sub base is not required as the same strength and properties can be achieved using the soils on site. Costly and time-consuming importation of new material and generation of large quantities of waste is therefore eliminated.

4. Saves landfill taxes

             Soil stabilisation uses the soils available on your site. These are improved to give the properties required for construction.

             This can vary from a simple process to enable use in landscaping or embankments right through to use in sub base. All the available soils can be used, so tipping is virtually eliminated. No need for any more tipping charges, just stabilise the soils on site and use them.

 

Case study:                 

 Construction with Portland Cement.

 

a.      General construction steps.

In soil-cement construction the objective is to thoroughly mix a pulverized soil material and cement in correct proportions with sufficient moisture to permit maximum compaction. Construction methods are simple and follow a definite procedure:

 

(1) Initial preparation

(a) Shape the area to crown and grade.

(b) If necessary, scarify, pulverize, and prewet the soil.

(c) Reshape to crown and grade.

(2) Processing

(a) Spread portland cement and mix.

(b) Apply water and mix.

(c) Compact.

(d) Finish.

(e) Cure.

 

b. Mixing equipment.

Soil, cement, and water can be mixed in place using traveling mixing machines or mixed in a central mixing plant. The types of mixing equipment are

(1) Traveling mixing machines.

      (a) Multiple-shaft mixer

      (b) Windrow-type pugmill

(2) Central mixing plants.

      (a) Continuous-flow-type pugmil

      (b) Batch-type pugmill

      (c) Rotary-drum mixers

 

Whatever type of mixing equipment is used, the general principles and objectives are the same. Some soil materials cannot be sufficiently pulverized and mixed in central mixing plants because of their high silt and clay content and plasticity. Almost all types of soil materials, from granular to fine grained, can be adequately pulverized and mixed with transverse-shaft mixers.

The exception is material containing large amounts of highly plastic clays. These clays may require more mixing effort to obtain pulverization. Revolving-blade central mixing plants and traveling pugmills can be used for nonplastic to slightly plastic granular soils. For coarse, nonplastic granular materials, a rotary-drum mixer can provide a suitable mix; however, if the material includes a small amount of slightly plastic fines, mixing may not be adequate.

Equipment for handling and spreading cement.

There are a number of methods of handling cement. On mixed-in-place construction using traveling mixing machines, bulk cement is spread on the area to be processed in required amounts by mechanical bulk cement spreaders. Bag cement is sometimes used on small jobs.

Cement spreaders for mixed-in-place construction are of two general types: those that spread cement over the soil material in a blanket  and those that deposit cement on top of a partially flattened or slightly trenched windrow of soil material . Cement meters on continuous-flow central mixing plants are of three types: the belt with strikeoff, screw, or vane. Cement for batch-type pug mill mixers and rotary drum-mixers is batch weighed.

 

Construction.

Construction with soil cement involves two steps-preparation and processing. Variations in these steps, dictated by the type of mixing equipment used, are discussed in this chapter. Regardless of the equipment and methods used, it is essential to have an adequately compacted, thorough mixture of pulverized soil material and other proper amounts of cement and moisture. The completed soil-cement must be adequately cured.

Preparation.

Before construction starts, crown and grade should be checked and any fine grading should be completed. Since there is little displacement of material during processing, grade at the start of construction will determine final grade to a major extent. If borrow material is to be used, the subgrade should be compacted and shaped to proper crown and grade before the borrow is placed.

Any soft subgrade areas should be corrected. To avoid later costly delays, all equipment should be carefully checked to ensure it is in good operating condition and meets construction requirements of the job. Guide stakes should be set to control the width and guide the operators during construction. Arrangements should be made to receive, handle, and spread the cementand water efficiently.

The number of cement and water trucks required depends on length of haul, condition of haul roads, and anticipated rate of production. For maximum production, an adequate cement and water supply is essential. The limits of the different materials and their corresponding cement requirements should be established by the project engineer. Prewetting by adding moisture before cement is applied often saves time during actual processing.

Silty and clayey soils may require extra effort to pulverize them, particularly if they are too dry or too wet. Soils that are difficult to pulverize when dry and brittle can be broken down readily if water is added and allowed to soak in; whereas, sticky soils can be pulverized more easily when they have been dried out a little. Most specifications require that the soil material be pulverized sufficiently so that at the time of compaction 100 percent of the soil cement mixture will pass a l-inch sieve and a minimum of 80 percent will pass a No. 4 sieve, exclusive of any gravel or stone. Gravel or stone should be no more than 2-inches maximum size.

The final pulverization test should be made at the conclusion of mixing operations. When borrow material is specified, it should be distributed on an accurately graded, well-compacted roadway in an even layer or uniform windrow, depending on the type of mixing equipment to be used. It should be placed by weight or volume as required by the specifications.

 

 

(2) Processing.

For maximum efficiency and to meet specification time limits, a day’s work should be broken down into several adjacent sections rather than one or two long sections. This procedure will result in maximum daily production and will prevent a long stretch of construction from being rained out in case of a sudden severe rainstorm.

 

(a)   Handling and spreading cement.

Bulk cement is normally trucked to the jobsite in bulk transport trucks or shipped to the nearest railroad siding in enclosed hopper cars. Compressed air or vibrators are used to loosen the cement in the hopper cars during unloading. Transfer to cement trucks is done pneumatically or by a screen or belt conveyor. The trucks are usually enclosed or fitted with canvas covers.

 

The cement is weighed in truckloads on portable platform scales or at a nearby scale. Soil materials that contain excessive amounts of moisture will not mix readily with cement. Sandy soils can be mixed with a moisture content at optimum or slightly above; whereas, clayey soils should have a moisture content below optimum when cement is spread. Cement should not be applied onto puddles of water. If the soil material is excessively wet, it should be aerated to dry it before cement is applied. Handling and spreading procedures for different types of equipment are presented below.

1. Mechanical cement spread, mixed-inplace construction.

A mechanical cement spreader is attached to the dump truck. As the truck moves forward, cement flows through the spreader, which regulates the quantity of cement placed on the prepared soil. To obtain a uniform cement spread, the spreader should be operated at a constant, slow speed and with a constant level of cement in the hopper. A true line at the pavement edge should be maintained with a string line.

 

The mechanical spreader must have adequate traction to produce a uniform cement spread. Traction can be aided by wetting and rolling the soil material before spreading the cement. When operating in loose sands or gravel, slippage can be overcome by the use of cleats on the spreader wheels or by other modifications; sometimes, the spreader is mounted on a tractor or high lift. The mechanical cement spreader can also be attached directly behind a bulk cement truck.

 

Cement is then moved pneumatically from the truck through an air separator cyclone that dissipates the  air pressure, and falls into the hopper of the spreader.Forward speed must be slow and even. Sometimes a motor grader or loader pulls the truck to maintain this slow, even forward speed. Pipe cement spreaders attached to cement transport trucks have been used in some areas with variable results. Improvements in this type of equipment are being made.

 

 

2. Bagged-cement spread, mixed-in-place construction.

When bags of cement are used on small jobs, a simple but exact method for properly placing the bags is necessary. The bags should be spaced at approximately equal transverse and longitudinal intervals that will ensure the proper percentage of cement. Positions can be spotted by flags or markers fastened to chains at proper intervals to mark the transverse and longitudinal rows. When the bags are opened, the cement should be dumped so that it forms fairly uniform transverse windrows across the area being processed. A spiketooth harrow, a nail drag, or a length of chain-link fence can be used to spread the cement evenly. The drag should make at least two round trips over the area to spread the cement uniformly.

 

3. Cement application, central-mixingplant construction.

When a continuous-flow central mixing plant is used, the cement is usually metered onto the soil aggregate and the two materials are carried to the pugmill mixer on the main feeder belt. Variations in moisture and in gradation of the soil aggregate will result in variations in the amount of material being fed onto the feeder belt. A high bulkhead placed in front of the soil hopper will help to obtain a more uniform flow through the soil material feeder.

The chance of loss of cement due to wind can be minimized by the use of a small plow attachment that will form a furrow for the cement in the soil aggregate.

 

After the cement is added, a second plow attachment a little farther up on the main feeder belt closes the furrow and covers the cement. A cover on the main feeder belt will also minimize cement loss due to wind. One of three types of cement meters-belt, screw, or vane-can be used to proportion the cement on a volumetric basis.

Each requires a 450- to 750-pound capacity surge tank or hopper between the cement silo and the cement feeder. This tank maintains a constant head of cement for the feeder, thus providing a more uniform cement discharge. Compressed air of 2- to 4-pounds per square inch pressure should be used to prevent arching of cement in the silo and the surge tank.

Portable vibrators attached to the surge tank can be used instead of air jets. A positive system should be included to stop the plant automatically if the cement flow suddenly stops. The correct proportion of cement, soil material, and water entering the mixing chamber must be determined by calibrating the plant before mixing and placing operations begin.

 

 

 

Mixing and application of water.

Procedures for applying water and mixing depend on the type of mixing machine used. A thorough mixture of pulverized soil material, cement, and water must be obtained. Uniformity of the mix is easily checked by digging trenches or a series of holes at regular intervals for the full depth of treatment and inspecting the color of the exposed soil-cement mixture.

Uniform color and texture from top to bottom indicate a satisfactory mix; a streaked appearance indicates insufficient mixing. Proper width and depth of mixing are also important.

Following are methods of applying water and mixing for the different types of mixing machines.

 

1. Windrow-type traveling mixing machine.

Windrow-type traveling mixing machines will pulverize friable soil materials. Other soils,however, may need preliminary pulverizing to meet specification requirements. This is usually done before the soil is placed in windrows for processing. The prepared soil material is bladed into windrows and a proportion pulled along to make them uniform in cross section.

When borrow materials are used, a windrow spreader can be used to proportion the material. Nonuniform windrows cause variations in cement content, moisture content, and pavement thickness. The number and size of windrows needed depend on the width and depth of treatment and on the capacity of the mixing machine. Cement is spread on top of the partially flattened or slightly trenched, prepared windrow.

The mixing machine then picks up the soil material and cement and dry-mixes them with the first few paddles in the mixing drum. At that point water is added through spray nozzles and the remaining paddles complete the mixing. A strikeoff attached to the mixing machine spreads the mixed soil-cement. If a motor grader is used to spread the mixture and a tamping roller is used for compaction, the mixture should first be loosened

 

to ready it for compaction. If two windrows have been made, the mixing machine progresses 350 to 500 feet along one windrow and then is backed up to process the other windrow for 700 to 1,000 feet. The cement spreading operation is kept just ahead of the mixing operation. Water is supplied by tank trucks. A water tank installed on the mixer will permit continuous operation while the tank trucks are being switched.

As soon as the first windrow is mixed and spread on one section of the roadway, it is compacted. At the same time a second windrow is being mixed and spread. It in turn is then compacted. Finishing of the entire roadway is completed in one operation. Water requirements are based on the quantity of soil material and cement per unit length of windrow.

                     

Multishaft traveling mixing machine.

Since most multi-shaft traveling mixing machines have a high-speed pulverizing rotor, preliminary pulverization is usually unnecessary. The only preparation required is to shaping the soil material approximate crown and grade. If an old roadbed is extremely hard and dense, prewetting and scarification will facilitate processing. Processing is done in lanes 350 to 500 feet long and as wide as the mixing machine.

Cement is spread on the soil material in front of the mixing machine. Cement spreading should be completed in the first working lane and under way in the second lane before mixing operations are begun. This ensures a fullwidth cement spread without a gap between lanes and keeps spreading equipment out of the way of mixing equipment. See figures 4-12 and 4-13 for an illustration of the construction sequence.

 

Single-shaft traveling mixing machine.

Soil-cement construction with single-shaft traveling mixers differs from the preceding examples in that more than one mixing pass is required. The basic principles and objectives are the same, however.Shaping, scarifying, and pulverizing the roadway are the first steps of preparation, as described previously in this chapter. Since most single-shaft

 

traveling mixers were not designed to scarify, the soil material may need to be loosened with a scarifier. Prewetting the soil material is common practice.

Applying water at this stage of   construction saves time during actual processing operations because most of the required water will already have been added to the soil material. In very granular materials, prewetting prevents cement from sifting to the bottom of the mix by causing it to adhere more readily to the sand and gravel particles. Mixing the soil material and cement is easier if the moisture content of the raw material is two or three percentage points below optimum.

However, very sandy materials can be mixed even if the moisture content is one or two percentage points above optimum. Moisture should be applied uniformly during prewetting. By mixing it into the soil material, evaporation losses are reduced. Because of the hazard of night rains, some prefer to do the prewetting in the early morning.

After scarifying and prewetting, the loose, moist soil material is shaped to crown and grade. Cement is spread by a mechanical cement spreader or from bags. Occasionally, the prewet soil material becomes compacted by cementspreading equipment. In such cases, mixing can be hastened by loosening the material again after cement is spread, usually with the scarifier on a motor grader. The scarifier teeth should be set so that the cement will flow between them and not be carried forward or displaced by the scarifier frame. The mixer picks up the soil material and cement and mixes them in place.

Water, supplied by a tank truck, is usually applied to the mixture by a spray bar mounted in the mixing chamber, or it can be applied ahead of the mixer by water pressure distributors. The soil material and cement must be sufficiently blended when water contacts the mixture to prevent the formation of cement balls. The number of mixing passes depends on the type of soil material and its moisture content and on the forward speed of the mixer.

 

Central mixing plant.

Central mixing plants are often used for projects involving borrow materials. The basic principles of thorough mixing, adequate cement content, proper moisture content, and adequate compaction apply. Friable granular borrow materials are generally used because of their low cement requirements and ease in handling and mixing. Pugmill-type mixers, either continuous flow or batch, or rotary-drum mixers are used for this work. Generally the twin-shaft continuous-flow pugmill is used on highway projects. Facilities for efficiently storing, handling, and proportioning materials must be provided at the plant. Quantities of soil material, cement, and water can be proportioned by volume for weight.

Mixing is continued until a uniform mixture of soil material, cement, and water is obtained. To reduce evaporation losses during hot, windy conditions and to protect against sudden showers, haul trucks should be equipped with protective covers.

To prevent excessive haul time, not more than 60 minutes should elapse between the start of moistmixing and the start of compaction. Haul time is usually limited to 30 minutes. The mixed soilcement should be placed on the subgrade without segregation in a quantity that will produce a compacted base of uniform density conforming to the specified grade and cross section.

The mixture should be spread to full roadway width either by one full-width spreader or by two or more spreaders operating in staggered positions across the roadway. Less preferable is the use of one piece of spreading equipment operating one lane at a time in two or more lanes. No lane should be spread so far ahead of the adjoining lane that a time lapse of more than 30 minutes occurs between the time of placing material in adjoining lanes at any location. The subgrade should be damp when the soil-cement is placed. Bituminous pavers have been used for spreading soil-cement although modification may be necessary to increase volume capacity before they can be used.

Compaction equipment should follow immediately behind the spreader. When second lane to serve as a depth guide when placing the mix in the second lane. Water spray equipment should be available to keep the joint areas damp. The amount of water needed to bring the soil-cement mixture to required moisture content in continuous-flow-type

 

mixing plants is based on the amount of soil material and cement coming into the mixing chamber per unit of time. The amount of water required in batch-type central mixing plants is similarly calculated, using the weights of soil material and cement for each batch compacting the first lane, a narrow compacted ridge should be left adjacent to the

 

Compaction.

The principles governing compaction of soil-cement are the same as those for compacting the same soil materials without cement treatment. The soil-cement mixture at optimum moisture should be compacted to maximum density and finished immediately. Moisture loss by evaporation during compaction, indicated by a greying of the surface, should be replaced with light applications of water. Tamping rollers are generally used for initial compaction except for the more granular soils.

Self-propelled and vibratory models are also used. To obtain adequate compaction, it is sometimes necessary to operate the rollers with ballast to give greater unit pressure. The general rule is to use the greatest contact pressure that will not exceed the bearing capacity of the soil-cement mixture and that will still “walk out” in a reasonable number of passes.

Friable silty and clayey sandy soils will compact satisfactorily using rollers with unit pressures of 75 to 125 pounds per square inch. Clayey sands, lean clays, and silts that have low plasticity can be compacted with 100- to 200-pounds per square inch rollers. Medium to heavy clays and gravelly soils required greater unit pressure, i.e., 150 to 300 pounds per square inch. Compacted thickness up to 8 or 9 inches can be compacted in one lift.

Greater thicknesses can be compacted with equipment designed for deeper lifts. When tamping rollers are used for initial compaction, the mixed material must be in a loose condition at the start of compaction so that the feet will pack the bottom material and gradually walk out on each succeeding pass. If penetration is not being obtained, the

 

scarifier on a motor grader or a traveling mixer can be used to loosen the mix during start of compaction, thus allowing the feet to penetrate. Vibratory-steel-wheeled rollers and grid and segmented rollers can be used to satisfactorily compact soil-cement made of granular soil materials. Vibratory-plate compactors are used on nonplastic granular materials.

Pneumatic-tired rollers can be used to compact coarse sand and gravel soilcement mixtures with very little plasticity and very sandy mixtures with little or no binder material, such as dune, beach, or blow sand. Some permit rapid inflation and deflation of the tires while compacting to increase their versatility. Pneumatic-tired rollers pulled by track-type tractors equipped with street plates can be used to compact cohesionless sand mixtures. The weight and vibration of the tractor aid in compaction. Heavy three-wheeled steel rollers can be used to compact coarse granular materials containing little or no binder material.

Gravelly soils that contain up to about 20 percent passing the No. 200 sieve and have low plasticity are best suited for compaction with these rollers. Tandem-steelwheeled rollers are often used during final rolling to press down or set rock particles and to smooth out ridges. There are two general types of road cross section: trench and featheredge. Both can be built satisfactorily with soil-cement.

In trench-type construction, the shoulder material gives lateral support to the soil-cement mixture during compaction. In the featheredge type of construction, the edges are compacted first to provide some edge stability while the remaining portion is being compacted. The edge slope should not be steeper than 2:1 to facilitate shaping and compacting. Shoulder material is placed after the soil-cement has been finished.

Occasionally, during compaction and finishing, a localized area may yield under thecompaction equipment. This may be due to one or more causes: the soil-cement mix is much wetter than optimum moisture; the subsoil may be wet and unstable; or the roller may be too heavy for the soil. If the soil-cement mix is too damp, it should be aerated with a cultivator, traveling mixer, or motor grader. After it has dried to near optimum moisture, it can be compacted. For best results, compaction should start immediately after

 

 

the soil material, cement, and water have been mixed. Required densities are then obtained more readily; there is less water evaporation; and daily production is increased.

 

Finishing.

There are several acceptable methods for finishing soil-cement. The exact procedure depends on equipment, job conditions, and soil characteristics. Regardless of method, the fundamental requirements of adequate compaction, close to optimum moisture, and removal of all surface compaction planes must be met to produce a high quality surface. The surface should be smooth, dense, and free of ruts, ridges, or cracks.

When shaping is done during finishing, all smooth surfaces, such as tire imprints and blade marks, should be lightly scratched with a weeder, nail drag, coil spring, or spiketooth harrow to remove cleavage or compaction planes from the surface. Scratching should be done on all soil-cement mixtures except those containing appreciable quantities of gravel.

The surface should be kept damp during finishing operations. Steel-wheeled rollers can be used to smooth out ridges left by the initial pneumatic-tired rolling. Steel-wheeled rollers are particularly advantageous when rock is present in the surface. A broom drag can sometimes be used advantageously to pull binder material in and around pieces of gravel that have been set by the steel-wheeled roller. Instead of using a steel roller, surfaces can be shaved with the motor grader and then rerolled with a pneumatic-tired roller to seal the surface.

Shaving consists of lightly cutting off any small ridges left by the finishing equipment. Only a very thin depth is cut and all material removed is bladed to the edge of the road and wasted. The final operation usually consists of a light application of water and rolling with a pneumatic-tired roller to seal the surface. The finished soil-cement is then cured.

 

 

 

Curing.

Compacted and finished soilcement contains sufficient moisture for adequate cement hydration. A moisture-retaining cover is placed over the soil-cement soon after completion to retain this moisture and permit the cement to hydrate. Most soil-cement is cured with bituminous material, but other materials such as waterproof paper of plastic sheets, wet straw or sand, fog-type water spray, and wet burlap or cotton mats are entirely satisfactory.

The types of bituminous materials most commonly used are RC-250, MC-250, RT-5, and emulsified asphalt SS-1. Rate of application varies from 0.15 to 0.30 US gallons per square yard. At the time of application, the soil-cement surface should be free of all dry, loose and extraneous material. The surface should also be moist when the bituminous materials are applied. In most cases a light application of water is placed immediately ahead of the bituminous application.

 

Construction joints.

After each day’s construction, a transverse vertical construction joint must be formed by cutting back into the completed soil-cement to the proper crown and grade. This is usually done the last thing at night or the first thing the following morning, using the toe of the motor-grader blade or mixer. The joint must be vertical and perpendicular to the centerline.

After the next day’s mixing has been completed at the joint, it must be cleaned of all dry and unmixed material and retrimmed if necessary. Mixed moist material is then bladed into the area and compacted thoroughly. The joint is left slightly high until final rolling when it is trimmed to grade with the motor grader and rerolled. Joint construction requires special attention to make sure the joints are vertical and the material in the joint area is adequately mixed and thoroughly compacted.

When bituminous material is used as a curing agent, it should be applied right up to the joint and sanded to prevent pickup.

 

Multiple-layer construction.

When the specified of soil-cement base course exceeds the depth (usually 8 or 9 inches compacted) that can be compacted in one layer, it must be constructed in multiple layers. No layer should be less than 4 inches thick. The lower layer does not have to be finished to exact crown and grade, nor do surface compaction planes have to be removed since they are too far from the final surface to be harmful. The lower layer can be cured with the moist soil that will subsequently be used to build the top layer-which can be built immediately, the following day, or some time later. With mixed-inplace construction, care must be taken to eliminate any raw-soil seams between the layers.

 

Special construction problems.

 

(1) Rainfall.

Attention to a few simple precautions before processing will greatly reduce the possibility of serious damage from wet weather. For example, any loose or pulverized soil should be crowned so it will shed water, and low places in the grade where water can accumulate should be trenched so the water will drain off freely. As shown by the construction of millions of square yards of soil-cement in all climates, it is unlikely that rainfall during actual construction will be a serious problem to the experienced engineer or contractor.

Usually construction requires the addition of water equivalent to 1 to 1½ inches of rain. If rain falls during cement-spreading operations, spreading should be stopped and the cement already spread should be quickly mixed into the soil mass. A heavy rainfall that occurs after most of  the water has already been added, however, can be serious. Generally, the best procedure is to obtain rapid compaction by using every available piece of equipment so that the section will be compacted and shaped before too much damage results. In such instances it may be necessary to complete final blading later; any material bladed from the surface is wasted. After the mixture has been compacted and finished, rain will not harm it.

 

Wet soils.

Excessively wet material is difficult to mix and pulverize. Experience has shown that cement can be mixed with sandy materials when the moisture content is as high as 2 percent above optimum. For clayey soils, the moisture content should be below optimum for efficient mixing. It may be necessary to dry out the soil material by aeration.

This can be done by using single-shaft traveling mixers with the hood in a raised position, or by cutting out the material with the tip of a motor grader blade and working and aerating with a disc. The maintenance of good crown and surface grade to permit rapid runoff of surface water before soil-cement processing is the best insurance against excessive amounts of wet material.

 

Cold weather.

Soil-cement, like other cement-using products, hardens as the cement hydrates. Since cement hydration practically ceases when temperatures are near or below freezing, soil-cement should not be placed when the temperature is 40 degrees F or below. Moreover, it should be protected to prevent its freezing for a period of 7 days after placement, and until it has hardened, by a suitable covering of hay, straw, or other protective material.

 

Conclusion :

               Stabilised soil is a construction material which is often cheaper than other conventional alternatives. For many purposes it is usually good. Studies on the application of soil stabilisation for roads, buildings, and hydraulic works are being made in many research labs. in the country. As India is  deficient in bituminous material , and resins are yet in developmental stage only, the most promising material for the present is soil-cement. Its use is likely to increase steadily.