The increasing infrastructure growth in urban and metropolitan areas has resulted in a dramaticrise in land prices and lack of suitable sites for development. These factors have forced the buildingindustry to look for cheaper land for construction. As a result, construction is now carried out onsites which, due to poor ground conditions, would not previously have been considered economicto develop. These types of land include low-lying areas of marine and estuarine Quaternarydeposits which are characterized by very poor geotechnical properties due to their low strength.Various ground improvement techniques have been employed in order to artificially improve thesoil properties in these sites.
One of the techniques extensively used in soft soils is vibro-replacement, which consists ofreplacing some of the soft soil with crushed rock or gravel to form an array of stone columnsbeneath the foundation. Although the use of conventional stone columns in soft soil deposits wasfound to benefit foundations in many respects they are generally suited to soils with an undrainedcohesion (cu) above 5–10kPa. Below this strength, the lateral support provided by surroundingsoil may be insufficient to prevent column failure through excessive radial expansion. In the lasttwenty years, this limitation has prompted investigation into geosynthetic column encasement toprovide the required lateral support to columns installed in extremely soft soils.
1.2. GROUND IMPROVEMENTGround improvement is the modification of the ground properties to achieve requiredground conditions for the particular use of the ground. Where poor ground conditions maketraditional forms of construction expensive, it may be economically viable to attempt to improvethe engineering properties of the ground before building on it.
This can be done by reducing thepore water pressure, by reducing the volume of voids in the soil, or by adding stronger materials.The four groups of ground improvement techniques are:1. Mechanical modification: Soil density is improved by application of mechanical forceincludes compaction by compact rollers and plate vibrators.
2. Hydraulic modification: The free pore water is forced out of the soil via drains or wells,in coarse grained soils it is achieved by lowering the ground water table by pumping throughtrenches and boreholes, and in fine grained soils it is achieved by applying external loads orelectrical forces.3. Physical and chemical modification: Includes mixing o the soil with adhesives, theseadhesives may include industrial by products or waste4. Modification by inclusion of confinement: Reinforcement by fibers, fabrics impartstensile strength to a constructed soil mass. In-situ reinforcement is achieved by nails andanchors. Stable earth retaining structure can also be formed by confining soil with concrete,Steel, or fabric element.Improvement of Cohesive soilsCohesive soils such as soft clay have large void ratio and high water content.
Ground improvementis required to reduce the void ratio and the water content to increase its strength so that the bearingcapacity is increased and the compressibility is decreased. The methods which are commonly usedin the practice are1. Pre-compression2. Sand drains3. Wick drains4. Stone columnsImprovement of Cohesion-less soilsCohesion-less soils when in loose condition with N-value than 10, have a low shear strength hencethe bearing capacity is quite low. If a deposit of loose sand exists at the site of construction, theground improvement can be achieved by inducing strong vibrations in the ground to densify thesoil, thus the relative density of the soil is increased which increases the N-value and the bearingcapacity.The methods used to densify the cohesion-less soils are1.
Vibroflotation2. Terra probe3. Dynamic Impaction4. Compaction by blasts5. Compaction piles1.2.1.
Granular columnsGranular piles also known as stone columns was adopted in European countries in the early1960s, are usually installed by vibration techniques, which involves drilling a large diameter holesinto the soft clay and filling them with gravel to form vertical stone columns, formation of stonecolumns using a vibroflot is quite suitable technique for improvement of cohesive soils. Stonecolumns act as flexible vertical reinforcing elements that increase the bearing capacity and reducethe settlement of the soil mass.The columns being flexible in nature bulge outward when the load is applied on them, these derivetheir axial capacity on account of the passive lateral resistance offered by clay against their bulging,hence granular columns cannot be used effectively in very soft clays, organic soils and silts. Stonecolumns in compressive loads fail in different modes, such as bulging. A long stone column havingits length greater than 4 times its diameter (critical length) fails by bulging irrespective of whetherit is end bearing or floating. Granular columns accelerate the consolidation process of clay as theyfunction as vertical drainage paths for dissipation of excess pore water pressure.
Usually the columns fail by bulging when they are subjected to load. The preventive measurestaken to prevent the bulging of the stone columns area. Encasing the stone column with geo-gridb. Using pervious concretec. Skirting stone column with concreted. Applying circumferential nail1.
2.2. Encased stone column:One common approach for treatment of soft clay soils is the installation of stone columns.The load capacity of the stone columns depends very much on the shear strength of the surroundingsoft clay soil. The stone columns help in both reduction of settlements and accelerated preconsolidation of clay soil deposit. However, in case of extremely soft clay soils, the stone columnformation may be difficult due to lateral spread of stones.
The contamination of stone aggregateby the ingression of soft clay soil may inhibit the drainage function of stone columns. Thegeosynthetic encasement of stone columns is an ideal solution for enhancing the performance ofstone columns in such conditions. The geosynthetic encasement helps in easy formation of thestone column and improves the strength and stiffness of the columns. By providing reinforcement,the length of the stone column can be increase.Advantages of encased stone column:1. Impart lateral confinement2. Increases load carrying capacity and stiffness by many fold3.
Stresses are transferred to the deeper strata4. Higher lengths of stone columns are possible5. Lateral squeezing of stones is prevented6. Higher degree of compaction can be achieved7. Prevents the clogging of stone columns8.
Strength properties of the aggregates are preserved.1.3. FINITE ELEMENT ANALYSISThe finite element method is a numerical technique for solving physical problems governedby a differential equation or an energy theorem. The finite element method provides anapproximate solution. The domain of the physical problem is discretized into the finite elements.
The elements are connected at points called nodes. The assemblage of elements is called finiteelement mesh.The finite element mesh is represented by a system of algebraic equations to be solved forunknowns at nodes. The nodal unknowns may be displacements in structural problems,temperatures in heat transfer problems and velocity or pressure potentials in fluid flow problems.Although the FEM solution is not exact, the solution can be improved by using more elements torepresent the physical problem.
FEM has two characteristics that distinguish it from othernumerical procedures.1. The method utilized an integral formulation to generate a system of algebraic equations.2. The method uses continuous piecewise smooth functions for approximating the unknownquantity or quantities.
The method originated from the need to solve complex elasticity and structural analysis problemsin civil and aeronautical engineering. Its development can be traced back to the work byA.Herinkoff and R.Courant. In China, in the later 1950s and early 1960s, based on thecomputations of dam constructions, K. Feng proposed a systematic numerical method forsolving partial differential equations. The method was called the finite difference method based onvariation principle, which was another independent invention of finite element method.
Althoughthe approaches used by these pioneers are different, they share one essential characteristic: meshdiscretization of a continuous domain into a set of discrete sub-domains, usually called elements.The subdivision of the whole domain has the following advantages:? Accurate representation of complex geometry? Inclusion of dissimilar material properties? Easy representation of total solutionFinite element method involves dividing the domain of the problem into a collection ofsubdomains with each subdomain represented by a set of element equations to the originalproblem, followed by systematically recombining all sets of equations into global system ofequations for the final calculation. The global system of equations has known solution techniquesand can be calculated from the initial values of the original problems to obtain a numerical answer.
Finite Element Analysis has a wide application, the areas where Finite Element Mesh can beapplied are:? Structural analysis: a cantilever, a bridge, an oil platform etc.? Soil mechanics: soil structure interaction, tunnel, dam, retaining wall? Solid mechanics: a gear, an automotive power train etc.? Dynamics: vibrations due to earthquakes.? Thermal analysis: heat radiation of finned surface, thermal stress brake disc? Electrical analysis: piezo actuator, electrical signal propagation.? Bio materials: human organs and tissues. Advantages of FEMThe various advantages which made FEM very popular are its ability to? Model Complex shaped bodies quite easily? Handle several load conditions without difficulty.? Handle different kinds of boundary conditions.? Model bodies composed of several different materials.
? Discretize the bodies with combination of different elements because the element equationscan be evaluated individually.01.4. PLAXISPLAXIS is a finite element program which is developed for the 2D and 3D analysis of deformation,stability and ground water flow in geotechnical engineering.
The development of PLAXIS began in 1897 at Delft University of Technology as aninitiative of the Dutch Ministry of Public Works and Water Management. The initial purpose wasto develop an easy-to-use 2D finite element code for the analysis of the river embankments on thesoft soils of the lowlands of Holland.Plaxis has the following products used for solving the geotechnical problems:Plaxis 2D: Finite element package intended for the two dimensional analysis of deformation andstability in geotechnical engineering. Incorporates advanced constitutive models for the simulationof the nonlinear, time dependent and anisotropic behavior of soils and/or rock. Also incorporatesspecial procedures required to deal with hydrostatic and non-hydrostatic pore pressures in the soil.Plaxis Dynamics: Add on for Plaxis that enables to analyze soils and structures subjected todynamic loads, such as earthquake.
Plax Flow (2D Transient Ground Water Flow): Finite element package intended for the twodimensional and steady-state analyses of saturated and unsaturated groundwater flow problems ingeotechnical engineering and hydrology.Plaxis 3D Tunnel: Enables to perform three-dimensional analysis of deformation and stability intunnel projects.Plaxis 3D Foundations: Finite element package for three-dimensional deformation analysis offoundation structures.111.5. NEED FOR PRESENT STUDYThe rapid growth in the population and urbanization has led to use of soils that areunsuitable for construction, such soils include clay deposits, peat soils, recent sills, marine claysetc.
These soils have low bearing capacity, high compressibility etc. hence these soils need to beimproved by using suitable ground improvement technique. The main objective of the groundimprovement is to improve the characteristics of soil, usually includes increasing shear strengthand decreasing the compressibility.The commonly used soil improvement method for cohesive soils are the stone column,these columns derive their axial capacity from the passive lateral resistance offered by thesurrounding soil mass against bulging, so the stone columns cannot be used effectively in the softclays (10-20KPa), silts and organic soils.This leads to the study of innovative technique such as providing encasement to the stone columnwith geotextile. The geosynthetic encasement will resist the lateral bulging of stone column andcause increase in load carrying capacity because of its high stiffness.
As the increase in stiffnessleads to increase in load carrying capacity.1.6. OBJECTIVE AND SCOPE OF THE STUDY? To develop a numerical model to simulate the cases of single stone column by givingprescribed deformation. The model is three dimensional finite element, which is capableto determine the mode of failure of a stone column under geometry/soil/encasement, asgeneral, punching and bulging form.? To perform a parametric study to establish the effect of governing parameters believed toeffect the performance of single geosynthetic encased stone column in layered soil.? To determine the load settlement, lateral bulging behavior of single geosynthetic encasedstone column by varying the various parameters.? To validate these models with available experimental and numerical data in the literature.