Variation of Shear Modulus During Primary Consolidation Chris and Dayakar by pimpdaddymust


									             Variation of Shear Modulus During Primary
                               Chris Jaeger1 and Dayakar Penumadu 2

                                  Civil & Environmental Engineering

            A major portion of the present research program was experimental and included resonant column
testing of Kaolin clay obtained from Akrochem Corp., OH (with commercial name SC #25) with various
strain amplitude and confining pressures. The Shear Modulus and the Damping ratio of soil over a wide
range of shear strains can be obtained using the combined resonant column torsional-shear. The Resonant
column-torsional shear apparatus can cover a wider shear strain range of 10-4 % to 6%. The test
equipment was originally built by Soil Dynamics Instruments. This has the capability to test both solid as
well as hollow cylindrical specimens. For the present research, solid cylindrical specimens having 71 mm
diameter and 193 mm height were used. The dynamic loading for the resonant column testing is applied
using a Hardin oscillator. The loading is applied to the top of the specimen while the bottom of the
specimen is rigidly fixed.
            Confining pressures up to 700 kPa can be applied inside the cell. The cell and the pore pressure
are measured using two (Model AB 4 type) pressure. Anisotropic stress conditions can be applied using a
double acting belloframe that is mounted on a frame attached to the top of the chamber lid. By applying
pressure to the top or the bottom of the belloframe, downward or upward force on clay specimen can be
applied. The change in length of the specimen is measured by a LVDT, located on a yoke just above the
bearing connection between the piston shaft and the rotating shaft. The apparatus has special provisions
for volume change measurements and for flushing and back pressure saturation of the specimen. The top
and the bottom platens have porous plastic plugs attached to them, which uniformly distribute or collect
the pore fluid. The bottom as well as the top platen has two drain lines each. One of these lines from the
top and the bottom platen are connected to the flush lines. The other two lines are connected to a pore
pressure transducer which in turn is connected to the back pressure tank (used during saturation) and
burette (used during consolidation). The volume change measurements are made during the consolidation
stage by monitoring the change in height of the pore fluid in the burette.

    Class of 1999, Oral Presentation
    Associate Professor, Civil & Environmental Engineering

        A Hardin type electro-magnetic oscillator is used to apply torsional vibrations to the specimen
that vary sinusoidally with time. The oscillator is a single-degree of freedom system in which a spool
shaped spring couples a central mass and the specimen cap attachment to a hollow cylindrical mass with
large rotational inertia. The oscillator has four magnets attached to it, which are alternatively excited to
get the desired torsional vibrations. The large inertia mass of the oscillator essentially provides a fixed
reaction such that the forcing torque produces vibration on the central mass and specimen cap. A
sinusoidal wave generator is used to power the coils of the electro-magnet. The output of the wave
generator is amplified so that it has enough power to induce vibrations of the soil specimen corresponding
to shear strains ranging from 10-4 % to 10-2 %. The response of the specimen to the induced vibrations is
measured using a piezo-electric accelerometer mounted on the top platen at a known distance from the
axis of rotation. The electric charge coming from the piezo-electric transducer is converted to DC voltage
by a charge amplifier and is read using a voltmeter.
        The excitation frequency is adjusted to produce first resonance of the system. The system
includes the specimen, top platen and the Hardin oscillator. At resonance, the acceleration and the torque
are ninety degrees out of phase with each other. The resonance condition is determined by plotting input
sinusoidal signal (corresponding to the applied torque) on the X-axis and the accelerometer output on the
Y-axis of an oscilloscope (Tektronix 2212 60 MHz digital oscilloscope) to produce an ellipse (from
Lissajous figures).
The shear modulus values are determined for kaolinite clay consolidated isotropically at 40 psi and 70 psi
as shown below. Resonant frequency is directly proportional to shear modulus. The variation of void
ratio with time during consolidation is also presented below. Currently empirical relationships are being
developed that relate the variation of shear modulus with void ratio during primary consolidation.

                                             Resonant Freq vs Void Ratio

           Res. Freq. (Hz)

                             170                                                      40 psi
                             160                                                      70 psi
                               0.65   0.75       0.85     0.95    1.05     1.15
                                                 Void Ratio, e

                             Void Ratio vs Time

Void Ratio, e

                                                            40 psi
                                                            70 psi
                       0   500                1000   1500
                                 Time (min)


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