08-02-2010, 08:29 AM
[attachment=1816]
VARIATION OF DAMPING RATIO AND SHEAR MODULUS WITH PLASICITY INDEX AND CONFINING PRESSURE
INTRODUCTION
Behaviour of soil under cyclic loading is non-linear
Non-linear behaviour commonly characterised by:
Damping coefficient, D
Shear modulus, G
DEFINITION
Damping coefficient
Is defined as the measure of energy dissipation
Shear modulus
Is the ratio of shear stress to shear strain
Plasticity index
Is the difference between liquid limit and plastic
limit
Confining pressure
Is the pressure applied around the surface of a body
EFFECT OF CYCLIC STRAIN ON DYNAMIC PROPERTIES
Shear modulus decreases with increase in shear strain.
Damping increases with increase in shear strain.
HISTORY OF STUDIES
1974 “ Hardin
Studied the effect of confining pressure on dynamic
properties. Found out relations relating shear modulus
reduction to confining pressure and plasticity index.
1993 “Laird
Carried out resonant column tests. Found that increase in confining pressure decreases the shear modulus. Also found that confining pressure affects damping ratio.
2008-Cabalar & Cevik
Found out variation of dynamic properties with
plasticity index and confining pressure.
EXPERIMENTAL STUDY
THEORY
Soil samples with different PI values are collected.
Specimens of required dimensions are prepared and
placed in the apparatus.
Specimens are initially subjected to isotropic
consolidation.
Samples are then loaded harmonically by applying
sinusoidal excitation at one end of specimen.
Test is repeated under different effective stresses and confining pressures.
TESTING APPARATUS PARTS AND THEIR FUNCTIONS
1)rigid pedestal at the fixed base to hold one end of the
specimen with zero vibration.
2)resonant column driver to apply confining pressure
3)electric motor with eight coils and four magnets at the
vibration end to induce sinusoidal excitation.
4)power amplifier and function generator to run the
motor
5)Accelerometer to determine resonance frequency
during the test
6)Bath of silicon oil to prevent migration or air into the specimen
7)Cylindrical membrane to hold the specimen in shape
8)Gas pressurized outer cell which encloses the apparatus
WORKING PROCEDURE
SAMPLE PREPARATION
Soil samples with different values of plasticity index- 47,52,65,71 were taken.
Samples were placed in a split mould with 70mm diameter and 140mm height.
Specimen along with the mould was placed on the pedestal.
Cylindrical membrane was stretched around the specimen.
Vacuum was applied on top to allow the specimen to hold the shape.
Mould was carefully split without disturbing sample.
PROCEDURE
Two stages:
consolidation stage
excitation stage
consolidation stage
confining pressure of 350kPa was applied using
resonant column driver
effective pressure 50kPa was applied
consolidation test was conducted under undrained
conditions
Excitation stage
Sinusoidal excitation was applied on vibration end of
the specimen.
Vibration amplitude of the specimen was noted.
Longitudinal displacement and accelerometer
readings were recorded.
Excitation frequency was increased and the test was
repeated.
On completion of 2 stages:
consolidation of same sample with PI =65 at:
effective pressure = 100,150kPa
confining pressure = 350kPa
excitation under same range of frequencies
Above cycle was repeated for confining pressure of 400 & 450kPa. Then other samples with PI=52,65,71were taken and tested similarly .Results were plotted.
RESULTS
Response of soil to oscillation becomes stiffer as PI increases.
As effective stress increases, G increases and D decreases.
As PI increases, G increases and D decreases regardless of confining pressure
For a given PI, G increases and D decreases as the confining pressure increases.
CONCLUSION
As PI increases, G increases and D decreases.
As confining pressure increases, G increases and D decreases.
