The viable to attempt to improve the engineering

The increasing infrastructure growth in urban and metropolitan areas has resulted in a dramatic
rise in land prices and lack of suitable sites for development. These factors have forced the building
industry to look for cheaper land for construction. As a result, construction is now carried out on
sites which, due to poor ground conditions, would not previously have been considered economic
to develop. These types of land include low-lying areas of marine and estuarine Quaternary
deposits 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 the
soil properties in these sites.
One of the techniques extensively used in soft soils is vibro-replacement, which consists of
replacing some of the soft soil with crushed rock or gravel to form an array of stone columns
beneath the foundation. Although the use of conventional stone columns in soft soil deposits was
found to benefit foundations in many respects they are generally suited to soils with an undrained
cohesion (cu) above 5–10kPa. Below this strength, the lateral support provided by surrounding
soil may be insufficient to prevent column failure through excessive radial expansion. In the last
twenty years, this limitation has prompted investigation into geosynthetic column encasement to
provide the required lateral support to columns installed in extremely soft soils.1.2. GROUND IMPROVEMENT
Ground improvement is the modification of the ground properties to achieve required
ground conditions for the particular use of the ground. 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. This can be done by reducing the
pore 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 force
includes 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 through
trenches and boreholes, and in fine grained soils it is achieved by applying external loads or
electrical forces.3. Physical and chemical modification: Includes mixing o the soil with adhesives, these
adhesives may include industrial by products or waste4. Modification by inclusion of confinement: Reinforcement by fibers, fabrics imparts
tensile strength to a constructed soil mass. In-situ reinforcement is achieved by nails and
anchors. Stable earth retaining structure can also be formed by confining soil with concrete,
Steel, or fabric element.Improvement of Cohesive soils
Cohesive soils such as soft clay have large void ratio and high water content. Ground improvement
is required to reduce the void ratio and the water content to increase its strength so that the bearing
capacity is increased and the compressibility is decreased. The methods which are commonly used
in the practice are
1. Pre-compression
2. Sand drains
3. Wick drains
4. Stone columnsImprovement of Cohesion-less soils
Cohesion-less soils when in loose condition with N-value than 10, have a low shear strength hence
the bearing capacity is quite low. If a deposit of loose sand exists at the site of construction, the
ground improvement can be achieved by inducing strong vibrations in the ground to densify the
soil, thus the relative density of the soil is increased which increases the N-value and the bearing
capacity.
The methods used to densify the cohesion-less soils are
1. Vibroflotation
2. Terra probe
3. Dynamic Impaction
4. Compaction by blasts
5. Compaction piles1.2.1. Granular columns
Granular piles also known as stone columns was adopted in European countries in the early
1960s, are usually installed by vibration techniques, which involves drilling a large diameter holes
into the soft clay and filling them with gravel to form vertical stone columns, formation of stone
columns using a vibroflot is quite suitable technique for improvement of cohesive soils. Stone
columns act as flexible vertical reinforcing elements that increase the bearing capacity and reduce
the settlement of the soil mass.
The columns being flexible in nature bulge outward when the load is applied on them, these derive
their 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. Stone
columns in compressive loads fail in different modes, such as bulging. A long stone column having
its length greater than 4 times its diameter (critical length) fails by bulging irrespective of whether
it is end bearing or floating. Granular columns accelerate the consolidation process of clay as they
function 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 measures
taken to prevent the bulging of the stone columns are
a. Encasing the stone column with geo-grid
b. Using pervious concrete
c. Skirting stone column with concrete
d. 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 ANALYSIS
The finite element method is a numerical technique for solving physical problems governed
by a differential equation or an energy theorem. The finite element method provides an
approximate 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 finite
element mesh.
The finite element mesh is represented by a system of algebraic equations to be solved for
unknowns 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 to
represent the physical problem. FEM has two characteristics that distinguish it from other
numerical 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 unknown
quantity or quantities.The method originated from the need to solve complex elasticity and structural analysis problems
in civil and aeronautical engineering. Its development can be traced back to the work by
A.Herinkoff and R.Courant. In China, in the later 1950s and early 1960s, based on the
computations of dam constructions, K. Feng proposed a systematic numerical method for
solving partial differential equations. The method was called the finite difference method based on
variation principle, which was another independent invention of finite element method. Although
the approaches used by these pioneers are different, they share one essential characteristic: mesh
discretization 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 solution
Finite element method involves dividing the domain of the problem into a collection of
subdomains with each subdomain represented by a set of element equations to the original
problem, followed by systematically recombining all sets of equations into global system of
equations for the final calculation. The global system of equations has known solution techniques
and 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 be
applied 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 FEM
The 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 equations
can be evaluated individually.01.4. PLAXIS
PLAXIS 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 an
initiative of the Dutch Ministry of Public Works and Water Management. The initial purpose was
to develop an easy-to-use 2D finite element code for the analysis of the river embankments on the
soft 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 and
stability in geotechnical engineering. Incorporates advanced constitutive models for the simulation
of the nonlinear, time dependent and anisotropic behavior of soils and/or rock. Also incorporates
special 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 to
dynamic 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 in
geotechnical engineering and hydrology.Plaxis 3D Tunnel: Enables to perform three-dimensional analysis of deformation and stability in
tunnel projects.Plaxis 3D Foundations: Finite element package for three-dimensional deformation analysis of
foundation structures.111.5. NEED FOR PRESENT STUDY
The rapid growth in the population and urbanization has led to use of soils that are
unsuitable for construction, such soils include clay deposits, peat soils, recent sills, marine clays
etc. These soils have low bearing capacity, high compressibility etc. hence these soils need to be
improved by using suitable ground improvement technique. The main objective of the ground
improvement is to improve the characteristics of soil, usually includes increasing shear strength
and 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 the
surrounding soil mass against bulging, so the stone columns cannot be used effectively in the soft
clays (10-20KPa), silts and organic soils.
This leads to the study of innovative technique such as providing encasement to the stone column
with geotextile. The geosynthetic encasement will resist the lateral bulging of stone column and
cause increase in load carrying capacity because of its high stiffness. As the increase in stiffness
leads 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 giving
prescribed deformation. The model is three dimensional finite element, which is capable
to determine the mode of failure of a stone column under geometry/soil/encasement, as
general, punching and bulging form.? To perform a parametric study to establish the effect of governing parameters believed to
effect the performance of single geosynthetic encased stone column in layered soil.? To determine the load settlement, lateral bulging behavior of single geosynthetic encased
stone column by varying the various parameters.? To validate these models with available experimental and numerical data in the literature.