UNIT.1Ground improvement is the primary application of many geotechnical construction techniques, permitting construction on soils by changing their characteristics.Need for 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 are often used to improve sub soil properties in terms of their bearing capacity, shear strength, settlement characteristics, drainage, etc. These techniques have a wide range of applicability from coarse grained soils to fine grained soils. Depending upon the loading conditions and nature of soil, a suitable technique which is also economical needs to be adopted. Ground improvement techniques continue to make considerable progress, both quantitatively and qualitatively, as a result of not only technology developments but also of an increasing awareness of the environmental and economic advantages of modern ground improvement methods. The selection of the correct ground improvement technique at an early stage indesign can have an important effect on foundation choice and can often lead to more economical solutions when compared to traditional approaches. The expansion in the use of ground improvement techniques is further assisted by the increasing need to develop marginal land. All ground improvement techniques seek to improve those soil characteristics that match the desired results of a project, such as an increase in density and shear strength to aid problems of stability, the reduction of soil compressibility, influencing permeability to reduce and control ground water flow or to increase the rate of consolidation, or to improve soil homogeneity. These methods are used when replacement of the in-situ soils is impractical because of physical limitations, environmental concerns, or is too costly. 0bjectives of ground improvement:Ground improvement methodologies have the primary functions to: * Increase bearing capacity, shear, or frictional strength, * Increase density, * Control deformations, * Accelerate consolidation, * Decrease imposed loads, * Provide/increase lateral stability, * Form seepage cutoffs or fill voids, * Increase resistance to liquefaction, and * Transfer embankment and/or ERS loads to more competent layers. According to Ground Improvement Methods, There are three strategies available to accomplish the above functions representing different approaches. *Increase shear strength, density, and/or decrease compressibility of foundation soil, *Use lightweight fills to significantly reduce the applied load on the foundation soil, and *Transfer the load to a more competent (deeper) foundation soil.

METHODS OF GROUND IMPROVEMENT:The techniques are divided into three categories:1. Compaction techniques that typically are used to compact or densify soil in situ.2. Reinforcementtechniques that typically construct a reinforcing element within the soil mass without necessarily changing the soil properties. The performance of the soil mass is improved by the inclusion of the reinforcing elements.3. Fixation techniques that fix or bind the soil particles together thereby increasing the soils strength and decreasing its compressibility and permeability.* Compaction: -dynamic compaction, -vibro compaction * Structural Reinforcement: -stone columns, -vibro concrete columns, -soil nailing, -micro piles* fixation: - Permeation grouting - Jet grouting - Soil mixing(dry soil mixing, wet soil mixing)Dynamic compaction (DC), also known as dynamic deep compaction, was advanced in the mid-1960s by Luis Menard, although there are reports of the procedure being performed over 1000 years ago. The process involves dropping a heavy weight on the surface of the ground to compact soils to depths as great as 40 ft or 12.5m. The method is used to reduce foundation settlements, reduce seismic subsidence and liquefaction potential, permit construction on fills, densify garbage dumps, improve mine spoils, and reduce settlements in collapsible soils.Vibro compaction (VC), also known as Vibroflotation was developed in the 1930s in Europe. The process involves the use of a down-hole vibrator (vibroflot), which is lowered into the ground to compact the soils at depth.The method is used to increase bearing capacity, reduce foundation settlements, reduce seismic subsidence and liquefaction potential, and permit construction on loose granular fills.Stone columns refer to columns of compacted, gravel size stone particles constructed vertically in the ground to improve the performance of soft or loose soils. The stone can be compacted with impact methods, such as with a falling weight or an impact compactor orwith a vibroflot, the more common method. The method is used to increase bearing capacity (up to 5 to 10 ksf or 240 to 480 kPa), reduce foundation settlements, improve slope stability, reduce seismic subsidence, reduce lateral spreading and liquefaction potential, permit construction on loose/soft fills, and precollapse sinkholes prior to construction in karst regions.Vibro concrete columns (VCCs) involve constructing concrete columns in situ using a bottom feed vibroflot. The method will densify granular soils and transfer loads through soft cohesive and organic soils. The method is used to reduce foundation settlements, to increase bearing capacity, to increase slope stability, and as an alternative to piling.Soil nailing is an in situ technique for reinforcing, stabilizing, and retaining excavations and deep cuts through the introduction of relatively small, closely spaced inclusions (usually steel bars) into a soil mass, the face of which is then locally stabilized. The technique has been used for four decades in Europe and more recently in the United States. A zone of reinforced ground results that functions as a soil retention system. Soil nailing is used for temporary or permanent excavation support/retaining walls, stabilization of tunnel portals, stabilization of slopes, and repairing retaining walls.Permeation grouting is the injection of a grout into a highly permeable, granular soil to saturate and cement the particles together. The process is generally used to create a structural, load carrying mass, a stabilized soil zone for tunneling, and water cutoff barrier.Jet grouting was conceived in the mid-1970s and introduced in the United States in the 1980s. The technique hydraulically mixes soil with grout to create in situ geometries of soilcrete. Jet grouting offers an alternative to conventional grouting, chemical grouting, slurry trenching, underpinning, or the use of compressed air or freezing in tunneling. A common application is underpinning and excavation support of an existingstructure prior to performing an adjacent excavation for a new, deeper structure. Super jet grouting is a modification to the system allowing creation of large diameters (11 to 16 ft, or 3.4 to 4.9 m) and is efficient in creating excavation bottom seals and treatment of specific soil strata at depth.Soil mixing mechanically mixes soil with a binder to create in situ geometries of cemented soil. Mixing with a cement slurry was originally developed for environmental applications; however, advancements have reduced the costs to where the process is used for many general civil works, such as in situ walls, excavation support, port development on soft sites, tunneling support, and foundation support. Mixing withdry lime and cement was developed in the Scandinavian countries to treat very wet and soft marine clays.Dry soil mixing is a low-vibration, quiet, clean form of ground treatment technique that is often used in very soft and wet soil conditions and has the advantage of producing very little spoil. The high speed rotating mixing tool is advanced to the maximum depth, disturbing the soil on the way down. The dry binder is then pumped with air through the hollow stem as the tool is rotated on extraction. It is very effective in soft clays and peats. Soils with moisture content, greater than 60% are most economically treated. This process uses cementacious binders to create bond among soil particles and thus increases the shear strength and reduces the compressibility of weak soils. The most commonly used binding agents are cement, lime, gypsum, or slag. Generally, the improvement in shear strength and compressibility increases with the binder dosage.Wet soil mixing is a similar technique except that a slurry binder is used making it more applicable with dryer soils (moisture contents less than 60%). The grout slurry is pumped through the hollow stem to the trailing edge of the mixing blades both during penetration and extraction. Depending on the in situ soils, the volume of grout slurry necessary varies from 20 to 40% of the soil volume. The technique produces asimilar amount of spoil (20 to 40%) which is essentially excess mixed soil which, after setting up, can often be used as structural fill. The grout slurry can be composed of Portland cement, fly ash, and ground granulated blast furnace slag.

Suitability, Feasibility :The choice of a method of ground improvement for a particular object will depend on the following factors. Type and degree of improvement required Type of soil , geological structure, seepage conditions cost Availability of equipment and materials and the quality of work required Construction time available Possible damage to adjacent structures or pollution of ground water resources Durability of material involved ( as related to the expected life of structure for a given environmental and stress conditions) Toxicity or corrosivity of any chemical additives . Reliability of method of analysis and design. Feasibility of construction control and performance measurementsIf soil is moist, freezing is applicable to all type of soil.Dynamic compaction:Applicable soil types: Dynamic compaction is most effective in permeable, granular soils.Cohesive soils tend to absorb the energy and limit the techniques effectiveness.The ground water table should be at least 6 ft below the working surface for the process to be effective.In organic soils, dynamic compaction has been used to construct sand or stone columns by repeatedly filling the crater with sand or stone and driving the column through the organic layer.Vibro compaction:Applicable soil types: The VC process is most effective in free draining granular soils. The typical spacing is based on a 165-horsepower (HP) (124 kW) vibrator. Although most effective below the groundwater table, VC is also effective above.Compaction grouting:Applicable soil types: Compaction grouting is most effective in free draining granular soils and low sensitivity soils. The depth of the groundwater table is not important as long as the soils are free draining.Stone columns:Applicable soil types: Stone columns improve the performance of soils in two ways, densification of surrounding granular soil and reinforcement of the soil with a stiffer,higher shear strength column. The depth of the ground water is generally not critical.Vibro Concrete Columns:Applicable soil types: VCCs are best suited to transfer area loads, such as embankments and tanks, through soft and/or organic layers to an underlying granular layer. The depth of the groundwater table is not critical.Soil NailingApplicable soil types: The procedure requires that the soil temporarily stand in a near vertical face until a row of nails and facing are installed. Therefore, cohesive soil or weathered rock is best suited for this technique. Soil nails are not easily performed in cohesionless granular soils, soft plastic clays, or organics/peatsMicropilesApplicable soil types: Since micropiles can be installed with drilling equipment and can be combined with different grouting techniques to create the bearing element, they can be used in nearly any subsurface soil or rock. Their capacity will depend on the bearing soil or rock.

Emerging trends in ground improvement:

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.

1. 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 preloading 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 stability2. 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.Surchargingis 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 capacity3. 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 stabilization4. GROUND FREEZING:

Ground Freezing Another technique used to stabilize the ground around an excavation and eliminate water inflow is ground freezing. The technique, although somewhat uncommon, is self-explanatory. Water in the ground is frozen by circulating brine at subfreezing temperatures through several pipes placed in the ground. A large refrigeration plant is set up to cool and circulate the brine. The zone of freezing is limited in size and freezing takes a long time to accomplish. This is a relatively costly method of constructiqn and can cause heave of the ground during freezing and subsidenceduring defrosting or melting. This method may in some instances be replaced by other methods such as grouting, slurry methods, and use of earth pressure balance tunnel boring machines. 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 areThermal analysis,Refrigeration system geometry,Thermal properties of soil and rock,freezing rates, Energy requirements, Coolant/ refrigerant distribution system analysis.GROUNDFREEZING APPLICATIONS: Temporary underpinning Temporary support for an excavation Prevention of groundwater flow into excavated area Temporary slope stabilization Temporary containment of toxic/hazardous waste contamination5. VIBRO-REPLACEMENT STONE COLUMNS:

The process of vibroreplacement is essentially the application of the vibrocompaction process to fine-grained in situ soils. The same equipment is used and is described above. The backfill is composed predominantly of crushed rock or stone instead of sand. The major difference in construction methods is that the probe is worked up and down in the hole so that the stone penetrates the soft cohesive soils around the hole. The backfill is not simply used to fill the void left by the probe and densification process as it is with vibrocompaction. During vibroreplacement, thestone backfill actually mixes with the surrounding soils as well as filling the probe hole. This process takes longer than vibrocompaction and uses more stone backfill. 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 methodswhich can be broadly classified in the following manner: Wet top feed method Dry bottom feed method Offshore bottom feed methodVIBRO-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 construction6.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.7.MICRO PILES: Micropiles, also known as minipiles and pin piles, are used in almost any type of ground to transfer structural load to competent bearing strata. Micro piles were originally small diameter (2 to 4 in., or 5 to 10 cm), low-capacity piles. However, advances in drilling equipment have resulted in design load capacities in excess of 300 tons (2.7MN) and diameters in excess of 10 in. (25 cm). Micropiles are often installed in restricted access and limited headroom situations. Micropiles can be used for a wide range of applications; however, the most common applications are underpinning existing foundations or new foundations in limited headroom and tight access locations.Equipment: The micropile shaft is usually driven or drilled into place. Therefore, a drill rig or small pile driving hammer on a base unit is required. The pipe is filled with a cement grout so the appropriate grout mixing and pumping equipment is required. If the bearingelement is to be created with compaction grout or jet grout, the appropriate grouting equipment is also required.Procedure: The micropile shaft is usually either driven or drilled into place. Unless the desired pile capacity can be achieved in end bearing and side friction along the pipe, some type of bearing element must be created (Figure 12.18). If the tip is underlain by rock, this could consist of drilling a rock socket, filling the socket with grout and placing a full length, high-strength threaded bar. If the lower portion of the pipe is surrounded or under lain by soil, compaction grouting or jet grouting can be performed below the bottom of the pipe. Also, the pipe can be filled with grout which is pressurized as the pipe is partially extracted to create a bond zone. The connection of the pipe to the existing orplanned foundation must then be constructed.

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

8.Rammed Aggregate Piers Rammed aggregate piers (RAPs) are a type of stone column as presented in Section 12.3.1. Aggregate columns installed by compacting successive lifts of aggregate material in a compacted in lifts with a beveled tamper to create passive soil pressure conditions both atthe bottom and the sides of the piers. RAPs are generally restricted to cohesive soils in preaugered hold (Figure 12.32). The predrilled holes, which typically have diameters of 24 to 36 in. (0.6 to 1.2 m), can extend up to about 20 ft. As seen in Figure 12.33, aggregate iswhich a predrill hole will stay open. Although constructed differently than store columns or vibro piers (Section 12.3.1) all provide similar improvement to cohesive soils. The vertical tamping used to construct RAPs results in minimal densification in adjacentgranular soils compared to vibratory probe construction.

Rammed Aggregate Piers can be used in some of the following stone column applications that are outlined below:1. Support shallow footings in soft ground.2. Reinforces soils to reduce earthquake-induced settlements, however, does not densify sands against liquefaction.3. Increase drainage and consequently expedite long-term settlement in saturated fine-grained soils.4. Increase global stability and bearing capacity of retaining walls in soft ground.5. Improve stability of slopes if RAPs can be installed to intersect potential shear failure planes.6. Reduce the need for steel reinforcements when RAPs are installed below concrete mat or raft foundations.