Ground Improvement Techniques By Dr. K H S M Sampath Need for ground improvement • Rapid urbanization and industrial growth driving demand for land . In order to meet this demand, land reclamation & utilization of unsuitable or environmentally affected lands have been taken up • Where poor ground conditions make traditional forms of construction expensive, it may be economically viable to attempt to improve the engineering properties of the ground before building on it • To improve load bearing capacity and shear strength Need for ground improvement • Mechanical properties are not adequate • Swelling and shrinkage • Collapsible soils • Soft soils • Organic soils and peaty soils • Sands and gravelly deposits. • Foundations on dumps and sanitary landfills • Handling dredged materials • Handling hazardous materials in contact with soils • Use of old mine pits Need for ground improvement Effect of shrinkage Collapsible soil Effect of swelling Effect of liquefaction Need for ground improvement Ground improvement techniques help • to reduce permeability • to reduce compressibility • to increase shear strength • to increase bearing capacity Full or partial replacement of soil • One of oldest and simplest methods is simply to remove and replace the soil • Soils that will have to be replaced include contaminated soils or organic soils • Method is usually practical only above the groundwater table • Most common admixture is Portland Cement. • When mixed with soil, forms soil-cement which is comparable to a weak concrete. • Other admixtures include lime and asphalt. • Objective is to provide artificial cementation, thus increasing strength and reducing both compressibility and hydraulic conductivity. • Used to reduce expansion potential of clays. • Used in surface mixing applications. Mechanical Compaction • This method utilizes mechanical compactors and rollers which reduce voids • This method has very low depth of influence and hence can be used only for shallow soils • In the event of weak soil being present up to certain depth, the same is replaced with a layer of good soil • Sheepsfoot roller and vibratory rollers are used for cohesive and cohesionless soils respectively • Need to achieve a specified degree of compaction considering the field dry density and maximum dry density • Compaction may be done in different layers to achieve the required fill height Vibro Compaction • Vibro compaction densifies clean, cohesion less granular soils by means of a down hole vibrator which is typically suspended from a crane and lowered vertically into the soil under its own weight. • Penetration is usually aided by water jets integrated into the vibrator assembly. After reaching the bottom of the treatment zone, the soils are densified in lifts as the probe is extracted. • During vibro compaction, clean sand backfill is typically added at the ground surface to compensate for the reduction in soil volume resulting from the densification process. • The vibratory energy reduces the inter-granular forces between the soil particles, allowing them to move into a denser configuration, typically achieving a relative density of 70 to 85 percent. The treated soils have increased density, friction angle and stiffness. Vibro Compaction Dynamic Compaction – Impact method • • • • Uses a special crane to lift 2-50 tons to heights of 7m-35m and then drop these weights onto the ground. Cost effective method of densifying loose sands and silty soils up to 15 to 30 feet deep. Good densification occurs up to a depth of 5 m-10 m. There are two main methods: • Rapid Impact compaction: In this method, the soil is compacted under the pounding action of heavy hammer • Dropping of heavy weight: In this method, steel or concrete weight around 500-600kN is dropped from a height of 40-50m with the help of a crane Dynamic Compaction – Impact method Micro-blasting • The micro-blasting technology is used for the improvement of subsoil under civil and hydro engineering structures. • It employs high-energy explosions to modify the surrounding soil. • The energy generated by the explosion of 1 kg of TNT = the energy of 5 tons of tamper falling free from a height of 100 m. • Applications: harbour areas, reclaimed islands, breakwaters, road embankments and airfield pavements, nuclear power plants, dams, etc. • Blasting is most effective in loose sands that contain less than 20% silt and less than 5% clay. • Although blasting is quite economical, it is limited by several considerations, as it produces strong vibrations that may damage near by structures or produce significant ground movements. Loose soil before blasting Underwater explosive compaction explosive compaction with surface charges explosive compaction with hidden charges Densified soil after blasting Grouting & Injection • Grouting is normally done to fill the cracks present in soil or rock strata • It proves effective in following situations • When foundation is to be below ground water table • When site is located in an area where direct access is restricted • When design of foundation is restricted by no. of boundaries & contact zones • Excavation is not possible besides the proposed structure • The grout used may be a combination of cement slurry with different admixture • Bentonite slurry may be commonly used grout, it is basically a highly plastic clay which has advantages and disadvantages • Due to its highly plastic nature, it can enter into even its smallest present crevice • It compacts quickly & forms a monolithic structure • It is costly and hence used in special situations like to protect the sides of tunnels from collapsing Grouting Methods Permeation grouting • Permeation grouting, also known as cement grouting or pressure grouting, fills cracks or voids in soil and rock • Permeates coarse, granular soils with flowable particulate grouts to create a cemented mass. Process • Depending on the conditions, Portland cement or microfine cement grout is injected under pressure at strategic locations through single ‘port’ or multiple ‘port’ pipes. • The grout particle size and void size must be matched properly to allow the cement grout to permeate. The grouted mass has an increased strength, stiffness, and reduced permeability. Grouting Methods Compensation or fracture grouting • Injection of a cement slurry grout into the soil creating and filling fractures that then lift the overlying soil and structures. Process • A pipe with rubber sleeves covering ports is inserted into a pre-drilled hole beneath a structure and grout injected under pressure at strategic locations through the ports. • Once the hydrofracture pressure of the soil is exceeded, fractures open in the soil and are immediately expanded by the influx of grout. This results in the controlled heave of the overlying soils and structures. Grouting Methods • Compaction grouting • When low-slump compaction grout is injected into granular soils, grout bulbs are formed that displace and densify the surrounding loose soils. • Used to repair structures that have excessive settlement Process • An injection pipe is inserted, typically to maximum treatment depth, and the grout then injected as the pipe is slowly removed in lifts, creating a column of overlapping grout bulbs. • The expansion of the grout bulbs displaces surrounding soils, and the grouting increases the density, friction angle, and stiffness of surrounding granular soils. Grouting Methods • Jet grouting • Uses a special pipe with horizontal jets that inject grout into the ground at high pressures. • Commonly used for ground water control projects. • Suitable for stabilizing soils before excavation or tunneling. Process • Jet grouting creates in situ columns of soilcrete (grouted soil), using a grouting monitor attached to the end of a drill stem. • The jet grout monitor is advanced to the maximum treatment depth. Then high velocity jets (cement grout with optional water and air) are initiated from ports in the monitor. • The jets erode and mix the in-situ soil with grout as the drill stem and monitor are rotated and raised. Pre-loading • Simply place a surcharge fill on top of the soil that requires consolidation • Weight of the surcharge fill is decided based on the load from the structure • Once sufficient consolidation has taken place, the fill can be removed, and construction takes place • Surcharge fills are typically 10-25 feet thick and generally produces settlement of 1 to 3 feet. • Most effective in clay soil Graph: Time vs. Settlement Pre-loading - Application • Can be used to densify sanitary landfills • Used to facilitate consolidation settlement • Pre-load is applied in the form of an imposed earth-fill left for a long period over an area to be compacted • When soil is being laid on the garbage and kept for long period, the leach-ate present in it is squeezed out, as a result, there is reduction in volume of garbage and more space is available Pre-loading • Advantages • Requires only conventional earthmoving equipment • Any grading contractor can perform the work • Long track record of success • Disadvantages • Surcharge fill must extend horizontally at least 10 m beyond the perimeter of the planned construction, which may not be possible at confined sites • Transport of large quantities of soil required • Surcharge must remain in place for months or years, thus delaying construction Vertical Drains • Vertical drains are installed with pre-loading to accelerate the drainage of impervious soils and thus speed up consolidation • These drains provide a shorter path for the water to flow through to get away from the soil • Time to drain clay layers can be reduced from years to a couple of months • Sometimes vertical drains are filled with sandy soils which are known to be the most cohesionless soils and therefore drainage through sand layers is very easy and convenient. Known as Sand Drains. Vertical Drains Vertical Drains - Advantages • Minimum disturbance to soil layer during installation. • High water discharge capacity. • High tensile strength prevents the collapse of flow path. • Fast and easy installation up to 40 m. Wick Drains • Also known as Prefabricated Vertical Drains (PVD) are prefabricated geotextile filterwrapped plastic strips with molded channels. • These act as drainage paths to take pore water out of soft compressible soil, so it consolidates faster, often from decades to months. • Installed by being pushed or vibrated into the ground. • Most are about 100 mm wide and 5 mm thick. Compaction Piles • Compaction piles are used to compact loose granular soils, thus increasing their bearing capacity, preventing liquefaction, reducing settlement • Sand compaction piles : • It consists of driving a hollow steel pipe with closed bottom & fitted with collapsible doors • Sand is filled in the hollow steel pile • Similar to Tremie method , the pipe is slightly lifted • Due to the load the bottom plate opens out and sand backfills the void created during driving of pipe • The refilled sand prevents the surrounding soils from collapsing Stone Columns • The method of installation is similar to that of compaction piles • The size of stones used is 6-40 mm • The spacing of stone columns is kept between 1-3m • It is particularly applied to soft soils • It is not suitable for highly organic soils Stone Columns Wet soil mixing • Wet soil mixing, also known as the deep mixing method, improves the characteristics of weak soils by mechanically mixing them with cementitious binder slurry. Process • A powerful drill advances a contra mixing tool as binder slurry is pumped through the connecting drill string, mixing the soil to the target depth. Additional mixing of the soil is completed as the tool is withdrawn to the surface. Columns, typically, are constructed at diameters of 1.0-1.5 m • This process constructs individual mixed columns or rows of overlapping columns with a designed strength and stiffness. Dry soil mixing • Dry soil mixing is a ground improvement technique that improves soft, high moisture clays, peats, and other weak soils, by mechanically mixing them with dry cementitious binder. Process • To construct the columns, a high-speed drill advances a drill rod with radial mixing paddles near the bottom of the drill string into the ground. • During penetration, the tool shears the soils to prepare them for mixing. • After the tool reaches the design depth, the binder is pumped pneumatically through the drill string to the tool, where it is mixed with the soil as the tool is withdrawn. • The dry soil mixing process constructs individual columns, rows of overlapping columns, or 100% mass stabilization, all with a designed strength and stiffness. Ground Anchors Ground anchors transfer tensile loads and consist of an anchor head, a free length and a bond length. Anchors can offer an advantage for basements and large excavations by minimizing horizontal deflections. Common uses • Excavation support • Temporary or permanent slope stabilization • Tie down applications for permanent structures Process A stable borehole is executed to the right diameter and designed length, filled with cement grout and then the anchor (strand or bar with free and bond length) installed. Post grouting can be done after a few hours if necessary. The anchor head is then installed, and stressing completed. Rock Anchors • Rock bolts are tensile units employed to keep rock mass in compression • It is installed as nearly perpendicular to joints as practicable • The ordinary types consist of rods installed in drill holes by driving and wedging, by driving and expanding, or by grouting with mortar or resins • Bolt heads are then attached to rod and twisted against a metal plate to impose the compressive force on mass • Fully grouted rock bolts, provide more permanent bolts than ordinary types • Rock bolts are used in slope stabilization, open excavations, in tunnels, caverns, mines, concrete dam foundations to provide resistance to uplift and sliding Soil Nails Soil nailing uses grouted steel nails to reinforce in situ soils and create a gravity retaining wall for permanent or temporary excavation support. Common uses • • • Stabilise slopes and landslides Support excavations Repair existing retaining walls Process Soil nail walls are generally constructed from the top down. Typically, soil is excavated in 1-2 m deep stages. After each excavation stage, near-horizontal holes are drilled into the exposed face at typically 1-2 m centres. Tension-resisting steel bars are inserted into the holes and grouted in place. A drainage system is installed on the exposed face, followed by the application of reinforced shotcrete wall facing. Precast face panels can also be used. Bearing plates are then fixed to the heads of the soil nails. This installation process is repeated until the design wall depth is reached. The finished soil nails then produce a zone of reinforced slope. Reinforced Earth Technology • The sides to be retained are fixed with sheet piles • The soil on the backside of the sheet piles are removed • A layer of thin metal is laid on ground • Soil is spread on this layer of metal strip and strip is bracketed around the layer of soil • The same process of laying metal strips and soil alternatively is continued till required height is achieved Reinforced Earth Technology Vegetated steep slopes Stabilization with Geo-products • These include use of Geo-textiles, Geomembranes, Geo-grids, Geo-spacers, geo-webs, geo-composites, etc • These in general are porous of polypropylene polyester, nylon or PVC and their variations • The main functions of these are : 1. 2. 3. 4. 5. 6. Separation Fluid transmission Reinforcement Filtration Containment Barrier Electrical methods • Electro-osmosis • This method is employed for cohesive soils (clays) • Metal strips are inserted and a well point system is also employed • The current is passed through metal strips thus becoming the anode and well point system as cathode • Water being charged with anions flows to well point system and is pumped out Thermal Stabilization by Heating Temperature Dielectric Constant Formation of Thermal Gradient in soil Particle Electric Repulsion Flow of pore water Strength of soil Thermal Stabilization by Cooling Temperature Pore water in soil freezes Soil is reinforced Non-vibration sensitive barriers/walls are formed Ice is formed Soil Strength Pre-wetting • The technique is to flood the area prior to construction • As in natural expansive soils, extensive network of fissures and cracks present initially, ponding process is easily facilitated • Because of pre-wetting, the water content will be closer to be attained after construction, hence volume changes will be small subsequently • It is usual to treat the surface with a layer of lime to a depth of 0.3-0.5 m after ponding • This treatment provides a working platform for construction and an impermeable moisture barrier to retard subsequent desiccation of pre-wetted soil Conclusion • Ground improvement is a rapidly developing field as suitable sites for construction are not available these days. • Its applicability has been proven in the recent past for a wide range of structures such as roads, runways, ports, power plants, railways, dams & other infrastructure facilities. • These techniques have been used all over the world for a wide range of soils starting from loose sands, silts, marine clays to weak rocks. • Based on the soil conditions, loading intensity and intended performance, an appropriate ground improvement technique can be designed to attain the desired performance. Thank You.
