SOIL IMPROVEMENT

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Soil improvement in its broadest sense is the alteration of any property of a soil to improve its engineering performance. This may be either a temporary process to permit the construction of a facility or may be a permanent measure to improve the performance of the completed facility. The result of an application of a technique may be increased strength, reduced compressibility, reduced permeability, or improved ground water condition.

Need for Stabilization of Earth Roads

An earth road is one whose foundation and wearing surface is composed of solely of the natural soil present originally on the site. Soils can be classified into two categories – cohesion less and cohesive soils. It has been observed that regions that are predominantly clayey do not usually have sandy materials. Clays must be considered as very important and often determining soil component since it has two objectionable qualities that make it the most troublesome of the materials to be dealt with. It swells when subjected to wetting, and shrinks with drying.

Clays and silts are low-grade construction materials, which find use in impervious elements such as cores (dams), cut-offs, they are poorly drained, and they shrink and swell. Also, clays when wet lose all strength; they are highly compressible, producing undesirable settlement as sub-grades of highways. Sands, though, having good drainage properties are also not suitable, as they lack cohesion and spread laterally under vertical loads. Thus, either of the two types alone cannot take the traffic independently. Therefore, combination of the two in certain specific proportions and thorough compaction with or without the use of additives may result in a stable sub-grade. A stabilized material may be considered as a combination of binder-soil and aggregates preferably obtained at or near the site of stabilization, and compacted so that it will remain in its compacted state without detrimental change in shape or volume under the force of traffic and exposure of weather. Several materials have been used as soil stabilizing agents. Of these, the best stabilizer will be the one involving minimum cost and at the same time providing durable effect. The technique is mainly applied in Road construction soil, and is termed as Mechanical Stabilization or Granular Stabilization. The process of mechanical stabilization is used both for base-courses as well as surface-courses. A good mechanically stable base or surfacing usually consists of a mixture of coarse aggregates (gravel, crushed rock, slag, etc.), fine aggregates (natural or crushed stone, sand, etc.), silt and clay, correctly proportioned and fully compacted. The use of correctly proportioned materials is of particular importance in the construction of low-cost roads. The principle of grading soils may be applied to the improvement of sub-grade soils of low bearing

capacity, by adding to them materials having particle sizes that are lacking, e.g. sand can added to clay sub-grades and vice versa.

Techniques of Soil Improvement

The various techniques of soil improvement are:-

1 Surface Compaction

2 Drainage Methods

3 Vibration Methods

4 Precompression and consolidation

5 Grouting and Injection

6 Chemical Stabilization

7 Soil Reinforcement

8 Geotextiles and Geomembranes

9 Other Methods

These techniques are briefly described as follows:

1. Surface Compaction

One of the oldest methods of soil densification is compaction. Construction of a new road, a runway, an embankment or any soft or loose site needs a compacted base for laying the structure. If the depth to be densified is less the surface compaction

alone can solve the problem. The usual surface compaction devices are rollers, tampers and rammers. All conventional rollers like smooth wheel, rubber-tyred, sheep foot, vibratory and grid rollers can be used.

2. Drainage Methods

Ground water is one of the most difficult problems in excavation work. The presence of water increases the pore water pressure and decreases the shear strength. Further heavy inflow of water to the excavations is liable to cause erosion or collapse of the sides of open excavations. Certain methods are available to control the ground water and ensure a safe and economical construction scheme.

Common drainage methods are Well-point Systems, Deep- well Drainage, Vacuum Dewatering system, Dewatering by Electro-osmosis etc

3. Vibration Methods

Vibration methods can be effectively used for rapid densification of saturated noncohesive soils. Vibrations and shock waves in loose deposits of such materials cause liquefaction followed by densification accompanying the dissipation of excess pore water pressures. Some of the mostly adopted vibration methods are blasting, Vibrating probe, Vibratory rollers, Vibro-displacement Compaction Piles, Vibrofloatation, Heavy Tamping etc.

4. Pre-compression and Consolidation

This method aims to consolidate the soil before construction. Various techniques adopted are Preloading and Surcharge Fills, Vertical Drains, Dynamic Consolidation, Electro osmotic Consolidation etc.

5. Grouting and Injection

Grouting is a process whereby’ stabilizers, either in the form of suspension or solution are injected into subsurface soil or rock for one or more of the following applications: -Control of ground water during construction

-Void filling to prevent excessive settlement

-Strengthening adjacent foundation soils to protect them against damage during excavation, Pile driving, etc.

-Soil Strengthening to reduce lateral support requirements

-Stabilization of loose sands against Liquefaction

-Foundation Underpinning

-Reduction of machine foundation vibrations

Grouting is done by Suspension Grouts which include grouting with Soil, Soil-cement Mixes, Cement, Lime, Displacement Grouting and by Solution Grouts using "one shot" or "two shot" systems.

6. Chemical Stabilization

Chemical Stabilization has been widely used in the form of lime, cement, fly ash and the combination of the above is widely used in soil stabilization. Chemical Stabilizations reduce permeability of the soils, improve shear strength, increase bearing capacity, decrease settlement and expedite construction. Chemical Stabilization is used for surface soils more successfully.

Mixtures of soils and chemicals are mixed either mechanically in place or by batch process. Some of the chemicals used are Lime, Cement, and Fly Ash etc.

7. Soil Reinforcement

Soil Reinforcement is in the form of a weak soil reinforced by high-strength thin horizontal membranes. A large variety of materials such as rubber, aluminum and thermoplastics have been used successfully.

8. Geotextiles and Geomembranes

Geotextiles are porous fabrics manufactured from synthetic materials, which are primarily petroleum products and others, such as polyester, polyethylene, polypropylene and polyvinyl chloride, nylon, fibreglass and various mixtures of these. Geotextiles are used as separators, filters, Drains, reinforcement, geomembranes etc.

9. Other Methods

Other methods include Thermal methods, Moisture barriers, Prewetting, addition or removal of soils, etc.

EARTHQUAKE GEOTECHNICAL ENGINEERING AND ITS PRACTICES-IV-GROUND IMPROVEMENT FOR LIQUEFACTION HAZARD MITIGATION

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Ground Improvement for Liquefaction Hazard Mitigation Ground Improvement in IS Code        “In poor and weak subsoils, the design of conventional shallow foundation for structures and equipment may present problems with respect to both sizing of foundations as well as control of foundation settlements.

Traditionally, pile foundations have been employed often at enormous costs. A more viable alternative in certain solutions, developed over the recent years, is to improve the subsoil itself to an extent such that the subsoil improvement would have resultant settlements within acceptable limits. The techniques for ground improvement has developed rapidly and has found large scale application in industrial projects.”

IS 13094 : 1992 (Reaffirmed 1997)          Ground improvement is indicated if

Net loading intensity of the foundation exceeds the allowable bearing pressure as per IS 6403:1981

Resultant settlement or differential settlement (per IS 8009 Part 1 or 2) exceeds acceptable limits for the structure

The subsoil is prone to liquefaction in seismic event

Types of Ground Improvement by Function

1. Excavation, fill placement, groundwater table lowering2. Densification through vibration or compaction3. Drainage through dissipation of excess pore water pressure4. Resistant through inclusions5.   Stiffening through cement or chemical addition

Densification through vibration and compactionVibrating probe/vibroflotation

         Vibrations of probe cause grain structure to collapse densifying soil; raised and lowered in grid pattern

Most Suitable Soil Type

Saturated or dry clean sand

Max effective treatment depth

20 m, ineffective in upper 3-4 m.

Special materials required

None

Special equipment required

Vibratory pile driver or vibroflot equipment

Properties of treated material

Can obtain up to Dr = 80%

Special advantages and limitations

+ Rapid, simple, cheaper than VR stone columns, compaction piles – less effective than methods that employ compaction as well as vibration, difficult to penetrate stiff overlayers, may be

ineffective for layered systems

Relative Cost Moderate

Vibro-compaction/replacement stone/sand columns

Steel casing is driven in to the soil, gravel or sand is filled from the top and tamped with a drop hammer as the steel casing is successfully withdrawn, displacing the soil

Most Suitable Soil Type

Cohesionless soil with less than 20% fines

Max effective treatment depth

30 m

Special materials required

Granular Backfill

Special equipment required

Vibrofolt equipment, steel casing, hopper for backfill

Properties of treated material

Can obtain high relative density

Special advantages and limitations

+ Rapid, useful for a wide range of soil types– May require a large volume of backfill, noisy

Relative Cost Moderate

Dynamic Densification (heavy tamping)•         A heavy weight is dropped in a grid pattern, for several passes

Most Suitable Soil Type

Cohesionless soil, waste fills, partly saturated soils, soils with fines

Max effective treatment depth

30 m, less at the surface, degree of improvement usually decreases with depth

Special materials required

None

Special equipment required

Tamper and crane

Properties of treated material

Good improvement and reasonable uniformity

Special advantages and limitations

+ Rapid, simple, may be suitable for soils with fines– lack of uniformity with depth, not possible near existing structures, may granular backfill surface layer

Relative Cost low

          Other methods

Displacement piles: densification by displacement of pile volume, usually precast concrete or timber piles

Compaction grouting: densification by displacement of grout volume

Stiffening through cement or chemical additionPermeation or penetrating grouting: High permeability grout is injected into the ground at numerous points, results in solidified soil mass

Most Suitable Soil Type

Saturated medium to coarse sand

Max effective treatment depth

> 30m

Special materials required

Grout

Special equipment required

Mixers, tanks, pumps, hoses, monitoring equipment

Properties of treated material

Impervious, high strength where completely mixed

Special advantages and limitations

+ Produces a hard, stiff  mass of soil, useful for existing structures as it causes little or no settlement or disturbance, low noise– Area of permeation can vary,

can be blocked by pockets of soil with fines, difficult to determine the improved area, requires curing time

Relative Cost

Least expensive of grout systems, but moderately expensive compared to vibro methods

Earthquake resistant design of geotechnical structuresGeotechnical structures like,          Retaining wall/Sheet pile        Slope        Shallow foundations        Deep foundationsMust be designed to withstand the earthquake loadingSeismic Design of Retaining WallMononobe-Okabe (1926, 1929) Method

Seismic Slope StabilityWedge Method of Analysis by Terzaghi (1950)

Seismic Bearing Capacity of Shallow Foundations Seismic Bearing Capacity of Shallow Strip Footings

Guideline as per Indian Code•         According to IS 1893, isolated RCC footing without tie beams, or

unreinforced strip foundation shall not be permitted in soft soils•         Shallow foundation elements should be tied together so that they move

uniformly, bridge over areas of local settlements, resist soil movements which ultimately reduces the level of shear forces induced in the elements resting on the foundation

•         Buried utilities, such as sewage and water pipes, should have ductile connections to the structure to accommodate the large movements and settlements that can occur under seismic loading

Ground Improvement Technique

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ground improvement

Essential prerequisite for High Speed corridor is to have control on the degradation of track geometry so as to keep various tolerances well within the specified limits. Degradation of track geometry is a function of Track Design, Axle-Load, Speed, and Sub-Grade characteristics. Improvement of sub-grade in poor ground areas is recognized as one of the most significant factor.

The considerations involved and the methodologies to be adopted for sub-grade improvement are described here. The engineering process to overcome the problems presented by poor ground areas is discussed along with the examination of the various options available, outlining their various advantages and limitations.

Interface Between Track, Sub-grade and Ground

Degradation of track geometry is a function of Track Design, Axle-Load, Speed, Vehicle and Sub-Grade characteristics.

For the track carrying mixed traffic, design has two differing requirements; light weight passenger train at high speeds and heavily loaded freight train at lower speed. This leads to the requirement of sub-grade, which can provide the necessary surface and alignment required for high-speed service, at the same time withstand heavy axle load without resulting in rapid deterioration or requiring frequent maintenance. Trade-off for cant and cant deficiency between high-speed passenger train and stability of slow speed heavy freight trains also needs to be considered.

Another issue related to high speed is whether to have conventional coaching stock with increased super elevation or have tilting train for higher speed. Techno-economical solutions have to be sought to enable safe running of train at higher speeds on conventional railway track without expensive alignment work rather than to design for too much differential speed on the same track.

The track system comprising of Rails, Sleepers, Ballast and Sub-ballast is normally separated from the sub-grade by a layer of geo-textile separator. Track sub-surface layers (ballast, sub-ballast and sub-grade) provide the required support to track structure.

The sub-grade provides a stable platform for the ballasted track structure. The track system distributes the loads from the rolling stock to a safe level such that these stresses do not produce undue strains in the sub-grade that would cause non-recoverable deformations and progressive degradation of the track geometry, affecting the safety and ride quality.

The design of the ballasted track system is influenced by the characteristics of the sub-grade, in particular the resilience modulus of the sub-grade soils. Resilience modulus has significant influence on ability to maintain track geometry. Condition deterioration at locations where sub-grade changes from geo-technical to structural element is a chronic problem. Track deterioration in these areas could be abnormally high and may require 8 – 10 times more maintenance. These areas require transition structures.

Improvement in sub-grade results in the reduction in the rate of track geometry degradation and measurable lower maintenance cost.

Common problems due to poor sub-grade

Poor sub-grade may result into:

Massive shear failure – attributable to the low shear strength of the sub-grade material

Progressive shear failure or general sub-grade failure due to the stresses imposed by the axle loads progressively squeeze the overstressed sub-grade clays to the side.

Attrition or local sub-grade failure where the repeated loading on the sub-grade, especially in the presence of water reduces the sub-grade to slurry which can “pump” to the surface.

Sub-grade settlement that can be caused by consolidation, moisture content changes or progressive deformation due to repeated traffic stresses.

Slope stability of embankments and cuts also need to be assessed and the possibility of massive shear failure has also to be precluded. For most projects the chosen sub-grade material predominantly consists of well-compacted residual soil fill material that offers a high shear strength and modulus of resilience, precluding the possible occurrence of progressive shear failure.

Higher axle load can impose higher stresses on sub-grade, which consequently gives rise to accelerated track deterioration Settlement of the sub-grade can occur independent of extent of axle-load in the case of compressible sub-soils and this can cause degradation of the rail track particularly if the settlements are not uniform.

Poor sub-base conditions can result in excessive and uneven track degradation. Uneven track degradation results in costly maintenance and may even adversely affect the track safety. Further, the non-uniform nature of the soft soils will result in differential settlements, which will lead to rail track degradation over time.

Ground Improvement Options

Improvement of the sub-grade is integral with and dependent on the improvement of the underlying natural ground formation. Ground treatment is required at poor ground areas, as the naturally occurring sub-soils may be unable to support the embankment and rail system without exceeding the requirements of the client’s design brief.

Various methods of ground treatment for soft ground can be broadly categorized into the structural (rigid) and the geotechnical solutions based on various considerations, which included the height of fill, thickness and compressibility of the soil as well as time and cost. Following methods of ground treatment can be adopted for various poor ground conditions:

Vibratory surface compaction and Deep vibro-compaction Removal and replacement of soft cohesive deposits of limited

thickness

Preloading of existing soft/loose fill Preloading with vertical drains. Dynamic Replacement. Stone Column Piled Embankments in areas having soft soil to large depths Viaduct for high embankments on ground having very deep soft soils

with organic deposits.

Vibratory surface and Deep Vibro-compaction

Surface vibratory compaction is used for densification of loose cohesionless soils using vibratory roller.

Deep vibro-compaction can be done for the loose sandy deposits having less than 15% of fines for depths up to 10 m. Compaction is carried out by inserting the probe up to the design depth of improvement and allowing the soil around the probe to get compacted for certain time interval. Then the probe is raised by about 0.5m to compact the soil around the vibrator and the process is repeated.

Removal and Replacement

For localised areas with soft soils of limited depth and thickness, removal of unsuitable material and replacement with suitable fill may be carried out. These unsuitable materials were encountered in valleys and low-lying areas and may be replaced with well-compacted suitable fill. Excavation and replacement could be carried out up to 5m to 6m.

The removal and replacement may be required to be carried out even in cutting areas where the naturally occurring soils were found to be of a low shear strength and high moisture content. Subsurface drainage may have to be introduced in most of these areas.

Preloading

For low embankment over soft compressible soil where the poor ground is of limited thickness (short drainage path) or is capable of compressing rapidly under load of excess preload fill due to presence of sand lenses, preloading may be resorted. Preloading of soft soils is based on the consolidation concepts, whereby; pore water is squeezed from the voids until the water content and the volume of the soil are in equilibrium under the loading stresses imposed by the surcharge. This is usually accompanied by gain in shear strength of soil. To a certain extent, the primary consolidation under final loading can be achieved during construction and hence post construction settlement reduces.

Prefabricated Vertical Drains and Pre-loading

However, with increased thickness of the soft clay where the consolidation period is too long for full consolidation of primary settlements, vertical drainage may be incorporated in conjunction with preloading in order to accelerate the settlement. Vertical drains may be proposed in the areas where the thickness of soft soils is limited to less than 10 m and embankment height are low. The anticipated primary and secondary settlements in such areas are limited.

Dynamic replacement

Dynamic replacement may be used for densification of loose cohesionless soils which are up to 5 to 6 m deep and where height of embankment is more than 2.5 m. Dynamic replacement utilizes a heavy pounder, usually lifted by crane to designed height and then dropped onto the soil, in a grid pattern such that the site is adequately covered. Craters formed by the pounder are filled with sand or aggregate and compacted. Due to large vibrations induced by the dropping of the pounder, this method is only suitable at locations away from settlement-sensitive structures.

Stone Columns

Stone columns may be provided in areas where subsoil consists of more than about 5 m thick soft cohesive soil and where stability and stringent considerations cannot be satisfied with conventional removal / replacement of soft material. Stone columns enable the embankment to be constructed to its full height continuously without requiring stage construction.

Piled Embankment and Viaduct

In the areas having low factor of safety against bearing capacity and slope stability; stage construction of the embankment may have to be resorted to, in which waiting period have to be introduced between stages to allow for consolidation and strength gain. When the required construction period extends beyond the limited time frame available, stability berms need to be introduced to reduce the number of construction stages. Moreover, these berms may extend beyond the right of way and require more land to be acquired. In cases of problems of limited time and space constraints it may be necessary to adopt structural solution.

In soft soil areas, embankment height exceeding the pre-consolidation pressure will give rise to excessive settlement. This can be avoided by means of structural solutions such as viaduct or piled embankment. Structural solution is recommended in soft ground conditions with depths exceeding 15 m. Structural solution is also required where settlement

requirement is Zero mm viz Points and crossings / turnout in yards. Where height of embankment is more, cost of pilled embankment may be higher and Viaduct may have to be provided. In both alternatives, the rail system is supported on piles driven through the soft soil and founded within the underlying stiffer material.

The trade off option between viaduct and piled embankment is governed by the embankment height. Economical analysis indicates that viaduct is more feasible for embankment in excess of about 6m, below which piled embankment is favourable.

Transition Structures

Transition structures will be required to be provided at all locations having abrupt change in the sub-grade resilience. Following type of transitions may be required:

At the transition between the vertical drain treatment area, which will undergo residual primary consolidation plus secondary settlement in the long term and the rigid viaduct, transition structure consisting of piled slab followed by an approach slab.

Flexible approach slab as a transition between viaduct and dynamic replacement area.

At all other locations transition structures in form of a mechanical hinge or approach slabs after additional preloading at the interface before construction of the piled embankment to avoid differential settlement between the rigid structures and settling fill.

Overall Cost economy

The type of ground treatment will greatly govern the frequency of maintenance (tamping) and the possession time that is required for maintenance. Significant up front investment may be required to reap long-term savings. General tendency to reduce initial cost (construction cost) of the project is resulting into adoption of methodologies, which gives initial lower cost but may result in higher recurring cost.

An opposite scenario would be the demand for zero total settlement of sub-grade during operation to keep costs of maintenance at lowest possible level, resulting into very high initial cost of construction. System consisting of structural solutions for zero settlement with provision of ballastless track may cost about 2 – 2.5 times that of conventional ballasted track with geotechnical solutions of ground improvement for a given permissible total settlement. If the sub-soil conditions are poor, the life cycle cost of the system can be 3 to 4 times more in case no proper ground treatment is carried out.

Therefore, a trade-off between improvement cost of ground and sub-grade characteristics and maintenance cost arising out of sub-grade deterioration will enable reduction in life cycle cost of maintenance and renewals. However, to harness the true potential of such a trade off, it will be necessary to provide suitable transition structures, which may permit varying methods of ground treatment.

Conclusion and recommendations

The methodology to carry out improvement works to ground and sub grade has to be based on requirement of settlement criteria during operations, at the same time to exploit the permitted settlement to use cost effective ground treatment option.

It is recommended to consider Optimization of Life Cycle Cost as one of the requirement during definition and Design development phase. Very prohibitive settlement conditions may lead to significant increase in the Life cycle cost due to very high capital cost, although maintenance and operations cost could be substantially lower.

In case of “Design and Build” contract, a longer maintenance period could be specified to discourage short-term gain by Design & Build Contractor.

Mohan Tiwari, Director (Projects), IRCON International Ltd

LATEST TRENDS IN GROUND IMPROVEMENT TECHNIQUES

The ground can be improved by adapting certain ground improvement techniques. Vibro-compaction increases the density of the soil by using powerful depth vibrators. Vacuum consolidation is used for improving soft soils by using a vacuum pump. Preloading method is used to remove pore water over time. Heating is used to form a crystalline or glass product by electric current. Ground freezing converts pore water to ice to increase their combined strength

and make them impervious. Vibro replacement stone columns improve the bearing capacity of soil whereas Vibro displacement method displaces the soil. Electro osmosis makes water flow through fine grained soils. Electro kinetic stabilization is the application of electro osmosis. Reinforced soil steel is used for retaining structures, sloping walls, dams etc…. seismic loading is suited for construction in seismically active regions. Mechanically stabilized earth structures create a reinforced soil mass. The geo methods like Geosynthesis, Geogrid etc…. are discussed. Soil nailing increases the shear strength of the in-situ soil and restrains its displacement. Micro pile gives the structural support and used for repair/replacement of existing foundations. Grouting is injection of pumpable materials to increase its rigidity. The jet grouting is quite advanced in speed as well as techniques when compared with the general grouting.

GROUND IMPROVEMENT:

Rapid urban and industrial growth demands more land for further development. In order to meet this demand land reclamation and utilization of unsuitable and environmentally affected lands have been taken up. These, hitherto useless lands for construction have been converted to be useful ones by adopting one or more ground improvement techniques. The field of ground improvement techniques has been recognized as an important and rapidly expanding one.

GROUND IMPROVEMENT TECHNIQUES:

1. VIBRO-COMPACTION:

Vibro-compaction, sometimes referred to as Vibrofloation, is the rearrangement of soil particles into a denser configuration by the use of powerful depth vibration. Vibrocompaction is a ground improvement process for densifying loose sands to create stable foundation soils. The principle behind vibrocompaction is simple. The combined action of vibration and water saturation by jetting rearranges loose sand grains into a more compact state.  Vibrocompaction is performed with specially-designed vibrating probes. Both horizontal and vertical modes of vibration have been used in the past. The vibrators used by TerraSystems consist of torpedo-shaped probes 12 to 16 inches in diameter which vibrates at frequencies typically in the range of 30 to 50 Hz. The probe is first inserted into the ground by both jetting and vibration. After the probe reaches the required depth of compaction, granular material, usually sand, is added from the ground surface to fill the void space created by the vibrator. A compacted radial zone of granular material is created

APPLICATIONS:

Reduction of foundation settlements. Reduction of risk of liquefaction due to seismic activity. Permit construction on granular fills.

2. VACCUM CONSOLIDATION:

Vacuum Consolidation is an effective means for improvement of saturated soft soils. The soil site is covered with an airtight membrane and vacuum is created underneath it by using dual venture and vacuum pump. The technology can provide an equivalent pre-loading of about 4.5m high conventional surcharge fill. Vacuum-assisted consolidation preloads the soil by reducing the pore pressure while maintaining a constant total stress.

APPLICATIONS:

Replace standard pre-loading techniques eliminating the risk of failure. Combine with a water pre-loading in scare fill area. The method is used to build large

developments on thick compressible soil. Combine with embankment pre-load using the increased stability

3. PRELOADING:

Preloading has been used for many years without change in the method or application to improve soil properties. Preloading or pre-compression is the process of placing additional vertical stress on a compressible soil to remove pore water over time. The pore water dissipation reduces the total volume causing settlement. Surcharging is an economical method for ground improvement. However, the consolidation of the soils is time dependent, delaying construction projects making it a non-feasible alternative.

The soils treated are Organic silt, Varved silts and clays, soft clay, Dredged material The design considerations which should be made are bearing capacity, Slope stability, Degree of consolidation.

APPLICATIONS:

Reduce post-construction Settlement Reduce secondary compression. Densification Improve bearing capacity

4. HEATING:

Heating or vitrifaction breaks the soil particle down to form a crystalline or glass product. It uses electrical current to heat the soil and modify the physical characteristics of the soil. Heating soils permanently alters the properties of the soil. Depending on the soil, temperatures can range between 300 and 1000 degree Celsius. The impact on adjacent structures and utilities should be considered when heating is used. .

APPLICATIONS:

Immobilization of radioactive or contaminated soil Densification and stabilization

5. GROUND FREEZING:

Ground freezing is the use of refrigeration to convert in-situ pore water to ice. The ice then acts as a cement or glue, bonding together adjacent particles of soil or blocks of rock to increase their combined strength and make them impervious. The ground freezing considerations are Thermal analysis, Refrigeration system geometry, Thermal properties of soil and rock, freezing rates, Energy requirements, Coolant/ refrigerant distribution system analysis.

GROUND FREEZING APPLICATIONS:

Temporary underpinning Temporary support for an excavation Prevention of groundwater flow into excavated area Temporary slope stabilization Temporary containment of toxic/hazardous waste contamination

6. VIBRO-REPLACEMENT STONE COLUMNS:

Vibro-Replacement extends the range of soils that can be improved by vibratory techniques to include cohesive soils. Reinforcement of the soil with compacted granular columns or “stone columns” is accomplished by the top-feed method. The important Vibro-replacement stone columns are Ground conditions, Relative density, Degree of saturation, Permeation.

PRINCIPLES OF VIBRO-REPLACEMENT:

The stone columns and intervening soil form and integrated foundation support system having low compressibility and improved load bearing capacity. In cohesive soils, excess pore water pressure is readily dissipated by the stone columns and for this reason, reduced settlements occur at a faster rate than is normally the case with cohesive soils.

There are different types of installation methods which can be broadly classified in the following manner:

Wet top feed method Dry bottom feed method Offshore bottom feed method

 

Summary: Vibro Replacement

Principle Reinforcement Drainage

Applicable soil(s)

Mixed deposits of clay, silt and sand Soft and ultra soft silts (slimes) Soft and ultra soft clays Garbage fills

Effect(s)

Increased shear strength Increased stiffness Reduced liquefaction potential

Common applications Airport taxiways and runways Chemical plants

Storage tanks & silos Pipelines Bridge abutments and approaches Offshore bridge abutments Road and railway embankments

Maximum depth 20-40 m

Land / offshore application Both

VIBRO-REPLACEMENT APPLICATIONS:

Reduction of foundation settlement Improve bearing capacity/reduce footing size requirements Reduction of the risk of liquefaction due to seismic activity Slope stabilization Permit construction on fills Permit shallow footing construction

GROUND TYPE RELATIVE EFFECTIVENESSSANDS EXCELLENTSILTY SANDS EXCELLENTSILTS GOODCLAYS MARGINAL TO GOODMINESPOILS EXCELLENT(DEPENDING ON

GRADATION)DUMPED FILL GOODGARBAGE NOT APPLICABLE

MECHANICALLY STABILIZED EARTH STRUCTURES:

A segmental, precast facing mechanically stabilized earth wall employs metallic (strip or bar mat) or geosynthetic (geogrid or geotextile) reinforcement that is connected to a precast concrete or prefabricated metal facing panel to create a reinforced soil mass.

PRINCIPLES:

The reinforcement is placed in horizontal layers between successive layers of granular soil backfill. Each layer of backfill consists of one or more compacted lifts.

A free draining, non plastic backfill soil is required to ensure adequate performance of the wall system.

For walls reinforced with metallic strips, load is transferred from the backfill soil to the strip reinforcement by shear along the interface.

For walls with ribbed strips, bar mats, or grid reinforcement, load is similarly transferred but an additional component of strength is obtained through the passive resistance on the transverse members of the reinforcement.

Facing panels are typically square, rectangular, hexagonal or cruciform in shape and are up to 4.5m ^2 in area.

MSEW- Mechanically Stabilized Earth Walls, when the face batter is generally steeper than 70 degrees.

RSS- Reinforced Soil Slopes, when the face batter is shallower.

APPLICATIONS:

RSS structures are cost effective alternatives for new construction where the cost of embankment fill, right-of-way, and other consideration may make a steeper slope desirable.

Another use of reinforcement in engineered slopes is to improve compaction at the edges of a slope to decrease the tendency for surface sloughing.

DESIGN:

Current practice consists of determining the geometric reinforcement to prevent internal and external failure using limit equilibrium of analysis.

SOIL NAILING:

The fundamental concept of soil nailing consists of reinforcing the ground by passive inclusions, closely spaced, to create in-situ soil and restrain its displacements. The basic design consists of transferring the resisting tensile forces generated in the inclusions into the ground through the friction mobilized at the interfaces.

APPLICATIONS:

Stabilization of railroad and highway cut slopes Excavation retaining structures in urban areas for high-rise building and underground

facilities Tunnel portals in steep and unstable stratified slopes Construction and retrofitting of bridge abutments with complex boundaries involving

wall support under piled foundations

MICRO PILES:

Micro-piles are small diameter piles (up to 300 mm), with the capability of sustaining high loads (compressive loads of over 5000 KN).The drilling equipment and methods allows micro – piles to be drilled through virtually every ground conditions, natural and artificial, with minimal vibration, disturbances and noise, at any angle below horizontal. The equipment can be further adapted to operate in locations with low headroom and severely restricted access.

APPLICATIONS:

For Structural Support and stability Foundation for new structures

Repair / Replacement of existing foundations Arresting / Prevention of movement Embankment, slope and landslide stabilization Soil strengthening and protection

EXAMPLE:

In India, in some circumstances steel pipes, coated wooden piles are used as cost-effective Options in improving the bearing capacity of foundation or restrict Displacements to tolerable levels and similar uses in stabilization of slopes, strengthening of foundations are common. Sridharan and Murthy (1993) described a Case study in which a ten-storeyed building, originally in a precarious condition due To differential settlement, was restored to safety using micropiles. Galvanized steel Pipes of 100 mm diameter and 10 m long with bottom end closed with shoe, driven at An angle of 60o with the horizontal were used and the friction between the pile and The soil was used as the design basis in evolving the remedial measures. A similar Attempt was made in the present case study in which the bearing capacity of the Existing foundation system of a building was restored to safety using micropiles.

GENERAL GROUTING:

Grouting is the injection of pumpable materials into a soil or rock formation to change the physical characteristics of the formation. Grouting selection considerations are Site specific requirement, Soil type, Soil groutability, Porosity. Grouting can be prevented by Collapse of granular soils, Settlement under adjacent foundations, Utilities damage, Day lighting. Grouting can provide Increased soil strength and rigidity, reduced ground movement, Predictable degree of improvement

DESIGN STEPS:

Identify underground construction problem. Establish objectives of grouting program. Perform special geotechnical study. Develop initial grouting program. Develop performance prediction. Compare with other solutions. Refine design and prepare specifications.

GROUTING TECHNIQUES:

The various injection grouting techniques used by grouting contractors for ground improvement / ground modification can be summarized as follows:

Permeation Compaction Grouting: Claquage Jet Grouting

JET GROUTING:

Jet grouting is a general term used by grouting contractors to describe various construction techniques used for ground modification or ground improvement. Grouting contractors use ultra high-pressure fluids or binders that are injected into the soils at high velocities. These binders break up the soil structure completely and mix the soil particles in-situ to create a homogeneous mass, which in turn solidifies. This ground modification / ground improvement of the soil plays an important role in the fields of foundation stability, particularly in the treatment of load bearing soils under new and existing buildings; in the in-depth impermeabilization of water bearing soils; in tunnel construction; and to mitigate the movement of impacted soils and groundwater.

EXAMPLE:

Teesta Dam – India Cut off / jet grouting and grouting

Upstream and downstream cofferdams. 2 cut-off walls by grouting and jet grouting.