CED 310Analysis of Stone Column assisted Raft Foundation

Presented By:Ankit Malhan (2010CE10330)Under the guidance of Dr. G V Ramana

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Table of Content• Objective• Inspiration• Scope of the Project• Literature Review for CPRF• Methodology Used• Settlement analysis of a Unit Cell• What Next?

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Objective• For use in case of High rise buildings, Soft soils, high degrees

of loading many a times we use a foundation called Combined Pile Raft Foundation (CPRF)which is quite common in practice.

• The aim of this project is to design a similar foundation with stone columns, namely Stone Column assisted Raft Foundation.

• Since this being a rather new field of work, quite little research has been done in this exact design concept. A major part of the study is based on research papers on CPRF and Vibro-Stone Columns.

• As a result the obvious goal is to create a 3D model and analyse the bearing capacity and settlement .

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Inspiration• CPRF have been in use for quite some time now. The idea of

combining stone columns to a raft foundation, speaking qualitatively can be quite fascinating.

• Since Stone Columns have much higher Stiffness, Shear Strength and Permeability than soil, we can assume that the pore water movement would be via the columns and hence consolidation can be achieved in a much less time than the traditional CPRF.

• Also high bearing capacities can be expected from Stone Columns

• With the new age techniques installation of stone columns to given design standards is now possible easily.

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Scope of the Project• The scope of the project for now is to analyse the bearing

capacity and to consider settlement and differential settlement for this model.

• Experimental Data is out of the scope of the project. Major part of the analysis would be purely theoretical in nature with examples based on sample data.

• In the later stages based on time constraints the project will deal with comparison of CCRF to CPRF, the pros and cons etc.

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Literature Review for CPRF

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• “CPRF is not a new foundation type, but a design concept”

• Major design challenges includes:• --Bearing Capacity• --Settlement and Differential Settlement(Punching Failure)• --Interaction between raft, soil and piles.

• Criteria for analysis include:• -Number and the orientation of piles.• -Ratio Ac/A, base area of piles vs. total raft area.• -Behavior of piles. 7

Final precautions taken and other methods for differential settlement like hydraulic cushion.

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Methodology• Analysis for a CCRF has been divided into 3 stages:• Stage 1: Complete foundation under loading.

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Stage 2:• Going further down a level we decide on load transfer on the

columns based on their orientation:

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Stage 3:• Analysis of a unit column under defined areal loading:

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Defining Parameters• For the purpose of analysis the basic assumptions taken are:• We have assumed a rectangular raft with cylindrical stone

columns.• A homogenous soil pattern.• A water table at a variable depth comparable to the depth of

the column.* ( so that as an extension later we can define for depth =0 or depth >> depth of column)

• A uniform areal loading on the foundation.

• *: it has been neglected for the purpose of preliminary testing.12

Available Design Models• Preliminary Testing:• -Unit Cell Approach: We define and study a

unit column cell to get preliminary idea of the model.

• In this case the modeling is done in axial symmetry.

• -Homogenization Theorem: Another famous approach in which the soil and column are modeled together as a new soil with improved properties.

• Advantageous only for calculating settlement.

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• For the purpose of final analysis a complex 3D model is built using various numerical models.

• For Unit Cell Analysis:

• We are using polar co-ordinate system.• For the initial calculation we assume that pore water pressure

is not an issue( the water table is further down the depth of the column)

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Settlement analysis of unit cell• Key points of the analysis are:• A 2D settlement will take place here.• Dividing the column into smaller

columns and considering one element. Comparing the change in volume to change in void ratio gives:

• (dz/z)-2(dr/r) = de/(1+e0) • We need another equation to be able

to calculate dz and dr seperately.• Hence, using poisson’s ratio of the

column gives the second equation.• v= (dr/r)/(dz/z) 15

Sample data• For the purpose of preliminary testing sample data that will be

used :• Poisson’s ratio =0.3-0.4• Initial Void Ratio = 1.5-2.7• Coefficient of Compressibility: Based on the effective stress

can be seen from the defined table• Areal Loading = 250KN/m2• Specific Weight of soil and column are : 19 KN/m3 and 25

KN/m3 respectively• Length of column= 30m• Radius of column =1.5m

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Algorithm to be used for discrete analysis of settlement of column• Dividing the column into n= (L/z) no. of discrete smaller

elements.• Going down along the depth each element experiences

increasing levels of radial and axial settlements under incremental stresses.

• Parameters that will change are: n: counter, r= rc-dr, stress, dr, dz. (dr and dz will add all increments from n steps to get total settlement)

• Principal Equation :• (dz/z)*(1-2v) = de/(1+e0) 17

Conclusion• Based on this algorithm we can find more and more accurate

results by increasing number of elements “n” and decreasing depth of each element.

• Furthermore load versus settlement graphs can be made by varying the load.

• Also plugging in for a number of values and comparing can give the significance of each parameter.

• Following this a complex 3 D model can be built moving up the stages.

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