SYMPOSIUM ON UNDERGRADUATE RESEARCH EXPERIENCES

Variation of Shear Modulus During Primary Consolidation

Chris Jaeger¹ and Dayakar Penumadu²

Civil and 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.

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.

1. Class of 1999, Oral Presentation
2. Associate Professor, Civil and Environmental Engineering

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