SUBMITTED BY :-
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Mechanical properties of soil are not adequate. Swelling and shrinkage Collapsible soils Soft soils Organic soils and peaty soil Sands and gravelly deposits, karst deposits with sinkhole formations Foundations on dumps and sanitary landfills Handling hazardous materials in contact with soil. Use of old mine pits
Ground Improvement refers to a technique that improves the engineering properties of the soil mass treated. Usually, the properties that are modified are shear strength, stiffness and permeability. Ground improvement has developed into a sophisticated tool to support foundations for a wide variety of structures. Properly applied, i.e. after giving due consideration to the nature of the ground being improved and the type and sensitivity of the structures being built, ground improvement often reduces direct costs and saves time.
GROUND IMPROVEMEN T TECHNIQUES
MECHANICAL MODIFICATION
HYDRAULIC MODIFICATION
PHYSICAL & CHEMICAL MODIFICATION
MODIFICATION BY INCLUSIONS & CONFINEMENT
The choice of a method of ground improvement for a particular object will depend on the following factors. Type ype and degree of improvement required ● T ● T Type ype of soil , geological structure, seepage conditions ● Cost Availability vailability of equipment and materials and the quality of work ● A 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 oxicity or corrosivity of any chemical additives . ● T ● Reliability of method of analysis and design. ● Feasibility of construction control and performance measurements
NOTE : If soil is moist, freezing is applicable to all type of soil.
● Increase shear strength. ● Reduce compressibility. ● Increase Bearing Capacity. ● Reduce Settlement. ● Reduce permeability. ● Reduce liquefaction potential. ● Control of swelling and shrinking. ● Prolong durability.
Soil density is increased by the application of mechanical force, including compaction of surface layers by static vibratory such as compact roller and plate vibrators. TYPES OF COMPACTIO N
SHALLOW SURFACE COMPACTIO N
DEEP SURFACE COMPACTIO N
SHALLOW SURFACE COMPACTION Achieved by static pressure and or dynamic pressure caused by rollers , impact or vibration.
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Smooth steel rollers Pneumatic tired rollers Sheepfoot rollers Grid rollers
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Tamper or rammers Plate compacters Vibrating rollers Impact rollers
S.NO.
EQUIPMENTS
MOST SUITABLE SOIL
1
Smooth wheel roller
Well graded sand, gravel mixtures, asphalt
2
Rubber tired rollers
Coarse grained soils with fines
3
Sheepfoot rollers
Clayey soil
4
Grid rollers
Weathered rock well graded coarse soils
5
Vibrating plate
Course grained soils 4 to 8% fines
6
Tamper and rammers
All type
7
Impact roller
Wide range of moist and saturated soil
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2-3 smooth metal rollers useful in compacting base layers and paving mixtures also used to provide a smooth finished grade generally are self-propelled, i.e. has an engine compaction is provided by the weight of the machine
TAMPER : • Tampers are devised that compact soil by delivering a succession of light, vertical blows • They are held in place and operated by hand • Tampers are powered pneumatically (compressed air) or by gasoline engines • They are limited in scope and compacting ability - good for small jobs and tight spaces • Layers are limited to 6" or 150mm with a tamper
● PRECOMPRESSION : Pre loaded by means of a surcharge on the surface in an array of Boreholes, causing a ground to consolidate .
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Precompression is a process in which a soil mass to be used as f o u n d a t i o n bed is preloaded to improve its properties and then design loads are applied. It is normally used in improving properties of cohesive soils.
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The preloading results s e t t l em e n t t o s o i l before construction process starts. Preloading is applied with a mass of earth fill which is left above area for a long period. The period is determined by the adequate settlement or desired settlement
Deep compaction Techniques: EXPLOSION:
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Explosives are detonated on the surface in an array of boreholes causing a loose soil structure to collapse.
The explosive charges generally contain 30% gelatin dynamite of special types having ammonite and 60% dynamite. Normally the charges place at 2/3 of the thickness of required stratum to be densified for founding building structure.
The charges should be placed at (3-8) m interval and for a particular location 3-5 blasts are usually generated to have required compaction. Now we will know the mechanism of compaction. This method of compaction is suitable for cohesionless soil of fully saturated condition. Shock waves generated by explosion results l i q u e f a c t i o n to sand resulting a densification of the surrounding depositions. But in case of partial saturated soil capillary action obstruct the densification tendency by preventing soil particles to come close. So this method is not useful for partial saturated soils.
Now we have to know the zone of influence of explosion densification. Up to 25m depth is considered effective in blast operation. Uppermost soil up to 1 m depth is displaced in random manner leading improper densification. This random densified zone is compacted by usual compaction method (roller).
The following influence(R):
expression
gives
an
idea
about
radius
Where M=mass of explosive charges (kg) C=constant (considered 0.04 in case of 60% dynamite)
of
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HEAVY TAMPING:
A large mass is dropped in to the ground surface, causing compaction and possibly long term consolidation.
Soil densification by dynamic compaction (DC), also called "heavy tamping" is a well-known compaction method. The method was "rediscovered" by Menard, who transformed the crude tamping method into a rational compaction procedure. Soil is compacted by repeated, systematic application of high energy using a heavy weight (pounder). The imparted energy is transmitted from the ground surface to the deeper soil layers by propagating shear and compression waves types, which force the soil particles into a denser state. In order to assure effective transfer of the applied energy, a 1 to 2 m thick stiff layer usually covers the ground surface. Pounders can be square or circular in shape and made of steel or concrete. Their weights normally range from 5 to 25 tons and drop heights of up to 25 m have been used. Heavier weights and larger drop heights have been used for compaction of deep soil deposits, but are not very common.
VIBRATION PILES:
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Densification is achieved by a vibratory probe or piles.
A loose soil or non-homogeneous granular fill can be compacted to depth by the penetration of vibrating probes or vibroflots. The main purpose of Vibrocompaction is to increase the density of the insitu soils by vibration.
Hydraulic Modification: ● Free pore water is forced out of soil via drains or wells. ● Course grained soils; it is achieved by lowering the ground water level through pumping from boreholes, or trenches. ● In fine grained soils the long term application of external loads (preloading) or electrical forces (electrometic stabilization) Techniques involved in hydraulic modification are: • • •
Sand drains Dewatering by electro-osmosis Well point drainage
SAND DRAINS Sand drains is a process of radial consolidation which increase rate of drainage in the rate of drainage in the embankment by driving a casing into the embankment and making vertical bore holes. These holes is back filled with suitable grade of sand.
Process of construction of drains The driven casing is withdrawn after the sand has been filled. A sand blanket is placed over the top of the sand drains to connect all the sand drains. To accelerate the drainage, a surcharge load is placed on the sand blanket. The surcharge is usually in the form of dumped soil. Mechanism of consolidation The pore water pressure is increased by the applied surcharge load in the embankment. The drainage occur in the vertical and horizontal directions. The horizontal drainage occure because of sand drains. The sand drains accelerate the the process of dissipation of excess pore water created by the surcharge. Spacing of drains The drains are generally laid either in a square pattern or a triangular pattern. The spacing (s) of the drains is kept smaller than the thickness of the embankment (2H) in order to reduce the length of the radial drainage path.
Zone of influence
The zone of influence of each drain in a triangle pattern is hexagonal in plan, which can be approximated by an equivalent circle of radius R, where R = 0.525 S. In case of a square pattern, the radius of circle of influence R is equal to 0.554 S. The radius of the sand drain is represented by r w
Dewatering by Electro-Osmosis : Electro-Osmosis is a method of drainage of cohesive soils in which a direct current is used. When a direct current Is passed through a saturated soil between a positive electrode (anode) and a negative electrode (cathode), pore water migrates to the cathode. The cathode is a well point which collects the water drained from the soil. The water collected is discharged, as in a conventional well point system.
The phenomenon of electro-osmosis can be explained with the help of the electrical double layer. Cations are formed in pore water when the dissolved minerals go into solution. These cations move towards the negatively charged surface of clay minerals to satisfy the electrical charge. As the water molecules acts as dipoles, the cations also attract the negative end of dipoles. When the cations move to the cathode, they take with them the attracted water molecules. In fact, the entire outer part of the diffuse double layer which is loosely adsorbed to the soil particles gets sheared along a plane.
Anodes are in the form of steel rods located near the toe of the slope of the excavation. Cathodes are in the form of perforated pipes, resembling well points, installed in the soil mass about 4 to 5m away from the slope of the cut. The electrodes are so arranged that the natural direction of flow of water is reversed and is directed away from excavation. This arrangement is required to prevent sloughing of the slopes. In many cases, mere reversing of the direction of flow helps in increasing the stability of the slope even if there is no significant decrease in the water content of the soil. The system requires about 20 to 30 amperes of electricity per well at a voltage of 40 to 180. The consumption of energy is between 0.5 to 10 kWh/m3 of soil drained. Because of specialised equipment and high electricity consumption, drainage by electro-osmosis is expensive compared with other methods. The drain water in a cohesive soil of low permeability (k=1x10 -5 to 1x10-8m/sec) Electro-osmosis also helps in increasing the shear strength of the cohesive soil.
Well point drainage The wellpoint system is one of the most versatile of pre-drainage methods which can pump a few gallons per minute in fine sandy silts or many thousands of gallons per minute in coarse sands and gravels.
A wellpoint system consists of a number of wellpoints spaced along a trench or around an excavation site, all connected to a common header, which is attached to one or more wellpoint pumps. Wellpoint systems are most suitable in shallow aquifers where the water level needs to be lowered no more than 15 or 20 feet. Due to the vacuum limitation of the pump, excavations that are deeper will require multiple stages of wellpoint systems. When designing a wellpoint system, it is necessary to give first consideration to the physical conditions of the site to be dewatered. Things to consider include: • • • • • • •
The physical layout Adjacent areas Soil conditions Permeability The amount of water to be pumped Depth to imperviousness Stratification
Physical and chemical modification: Stabilization by physical mixing adhesives with surface layers or columns of soil. Adhesive includes natural soils industrial byproducts or waste. Materials or cementations or other chemicals which react with each other and/or the ground.
When adhesives are injected via boreholes under pressure into voids within the ground or between it and a structure the process is called grouting. Soil stabilization by heating and by freezing the ground is considered thermal methods of modifications.
The physical properties of soils can often economically be improved by the use of admixtures. Some of the more widely used admixtures include lime, Portland cement and asphalt.
Soil-lime Stabilization Lime stabilization improves the strength, stiffness and durability of fine grained materials. In addition, lime is sometimes used to improve the properties of the fine grained fraction of granular soils. Lime has been used as a stabilizer for soils in the base courses of pavement systems, under concrete foundations, on embankment slopes and canal linings. Quicklime is the most commonly used lime; the followings are the advantages of quicklime over hydrated lime • higher available free lime content per unit mass • denser than hydrated lime (less storage space is required) and less dust • generates heat which accelerate strength gain and large reduction in moisture content according to the reaction equation below
CaO+H2O
Ca(OH)2 + HEAT
Soil-Cement Stabilization: Soil-cement is the reaction product of an intimate mixture of pulverized soil and measured amounts of Portland cement and water, compacted to high density. As the cement hydrates, the mixture becomes a hard, durable structural material. Hardened soil-cement has the capacity to bridge over local weak points in a sub grade. When properly made, it does not soften when exposed to wetting and drying, or freezing and thawing cycles.
Bituminous Soil Stabilization: Bituminous materials such as asphalts, tars, and pitches are used in various consistencies to improve the engineering properties of soils. Mixed with cohesive soils, bituminous materials improve the bearing capacity and soil strength at low moisture content. The purpose of incorporating bitumen into such soils is to water proof them as a means to maintain a low moisture content.
Fly –Ash Fly ash is a byproduct of coal fired electric power generation facilities; it has little cementitious properties compared to lime and cement. Most of the fly ashes belong to secondary binders; these binders cannot produce the desired effect on their own. However, in the presence of a small amount of activator, it can react chemically to form cementitious compound that contributes to improved strength of soft soil. Fly ashes are readily available, cheaper and environmental friendly.
Blast Furnace Slags These are the by-product in pig iron production. The chemical compositions are similar to that of cement. It is however, not cementitious compound by itself, but it possesses latent hydraulic properties which upon addition of lime or alkaline material the hydraulic properties can develop.
GROUTING Among other technique of stabilization techniques, the grouting is one of the most expensive methods where some kind of stabilizing agent inserted into the soil mass under pressure. The pressure forces the agent into the soil voids in a limited space around the injection tube. The agent reacts with the soil and /or itself to form a stable mass. The most common grout is an admixture of cement and water, with or without sand.
The principle of grouting is to introduce a substance into rock fissures or into soil by pumping fluid (called grout) down a small diameter tube in the required location.
VARIOUS FUNCTIONS INVOLVED IN SOIL & ROCK GROUTING
The three basic functions involved in soil and rock grouting are as follows: • Permeation or penetration In this situation the grout flows freely with minimal effect into the soil voids or rock seams. • Compaction or controlled displacement In this case the grout remains more or less intact as a mass and exerts pressure on the soil or rock. • Hydraulic fracturing or uncontrolled displacement In this condition the grout rapidly penetrates into a fractured zone which is created when the grouting pressure is greater than the tensile strength of the soil or rock being grouted.
It has a large number of applications such as: • • • • •
Control of water problems by filling cracks and pores. Prevention of sand densification beneath adjacent structures due to pile driving. Underpinning using compaction (displacement) grouting. Reducing vibrations by stiffening the soil. Reducing settlements by filling voids and cementing the soil structure more firmly.
Generally grout can be used if the permeability of the deposit is greater than 10 -5 m/s. one of the principal precautions with grouting is that the injection pressure should not be sufficient to lift the ground surface. In using compaction grouting where a very stiff displacement volume is injected into the ground under high pressure, however, lifting of the ground surface as a grout lens forms is of minor consequence.
Modification by inclusions and confinement: Reinforcement by fibers, strips bars meshes and fabrics imparts tensile strength to a constructed soil mass. In-situ reinforcement is achieved by nails and anchors. Stable earth retaining stucture can also be formed by confining soil with concrete, Steel, or fabric elements
Vibroflotation is a ground improvement technique used at considerable depth that by using a powered electrically or hydraulically probe, it strengthens the soil. The vibroflotation will compact the soil making it suitable to support design loads. It involves the introduction of granular soil to form interlocking columns with surrounding soil. The technique is used to improve bearing capacity and reduce the possibility of differential settlements that might be allowed for the proposed loads .
The vibroflot is inserted into the ground and typically can be used to improve soil up to depths of 150 feet. Vibroflotation utilizes water and the mechanical vibrations of the vibroflot to move the particles into a denser state. Typical radial distances affected range from 5 to 15 feet .
The vibroflot is suspended from a crane and seats on the surface of the ground that is to be improved. To penetrate the material, the bottom jet is activated and the vibration begins. The water saturates the material to create a “quick sand” condition (i.e. temporarily liquefying the material), which allows the vibroflot to sink to the desired depth of improvement. At that point, the bottom jet is stopped and the water is transferred to the upper jet. This is done to create a saturated environment surrounding the vibroflot, thereby enhancing the compaction of the material. The vibroflot remains at the desired depth of improvement until the material reaches adequate density. The density of the soil is measured by using the power input (via the electric current or hydraulic pressure) as an index. As the material densifies, the vibroflot requires more power to continue vibrating at which point an ammeter or pressure gauge displays a peak in required power. Once this point is reached, the vibroflot is raised one lift (generally ranging from 1 to 3 feet) and compaction ensues until the peak amperage or hydraulic pressure is reached once again. The peak power requirement can be correlated to the density of the soil, so an accurate measurement of the in situ density can be recorded
STEPS OF VIBRO-FLOTATION
ADVANTAGE OF VIBROFLOTATION •
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When the process is done properly, it will reduce the possibility of differential settlements that will improve the foundation condition of the proposed structure. It is the fastest and easiest way to improve soil when bottom layers of soil will not provide good load bearing capacity. It is a great technology to improve harbor bottoms It can be done around existing structures without damaging them . It does not harm the environment No excavations are needed, reducing the hazards, contamination of soils and hauling material out from the site No need to manage table water issues, neither the permits required to manage water discharge and dewatering issues. When vibroflotation is performed at a site, it will reduce the possibility of liquefaction during an earthquake
Vibro Stone Columns are designed to improve the load bearing capacity of insitu soils and fills and to reduce differential settlements of non-homogeneous and compressible soils, allowing the use of shallow footings and thinner base slabs.
Stone Columns are formed by inserting a vibrating probe to incorporate granular aggregate into the ground via the resulting void. This is followed by the re-compaction of granular aggregate. Both Top and Bottom feed techniques are available, depending on the stability of the insitu soils and water level. The Stone Columns are typically installed under uniformly loaded structures, such a building slabs and embankments, on a regular grid spacing. A load transfer platform can then be designed to spread the load from the structure to the improved ground. This technology is well suited for the improvement of soft soils such as silty sand, silts, clays and non homogeneous fills. Due to their lack of lateral confinement organic soils, peat and very soft clays are not suitable for this method, and other ground improvement methods need to be considered.
METHODS OF STONE COLUMN
WET/TOP FEED METHOD
DRY/BOTTO M FEED NETHOD
WET / TOP FEED METHOD
STEP 1 : Penetration of probe STEP 2 : Backfilling of aggregate and compacting STEP 3 : Consolidation of granular fill and finishing the column
DRY/BOTTOM FEED METHOD
STEP 1 : Penetration of probe STEP 2 : Installation of aggregate through separate duct along the vibro probe. STEP 3 : Consolidation of granular fill and finishing the column
ADVANTAGE OF STONE COLUMN
Technically and potentially economical alternative for deep compaction. Alternative for dynamic compaction, deep blasting etc. Quicker than preloading the site. It increases the bearing capacity, reduces the settlement, liquefaction potential.
