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Unravelling the measurement of Ko

Mark L. Talesnick – Faculty of Civil and Environmental Engineering,
Technion – IIT, Haifa, Israel

Abstract

Knowledge of lateral earth pressures is a required input to geotechnical design and analysis. The determination of lateral earth pressure is often based upon a coefficient of earth pressure at rest, (Ko), for which no rational method for its determination is available.
The submission addresses three issues, which have direct influence on the measurement/determination of lateral earth pressure at rest: Pressure measurement technique, Boundary conditions and Stress history. Testing was carried using two pressure measurement techniques, Null Gage technology and deflecting membrane technology. Tests were performed under three different boundary conditions; free field conditions, confined consolidometer and conditions adjacent to a structural boundary. The tests included multiple load-unload cycles.
Data generated in this study has indicated that soil pressure measurements cannot be reliably performed using deflecting membrane techniques. The small deflections required to produce output signals ruin the actual measurement. Null soil pressure measurement techniques preclude deflection and is therefore inherently more suitable. Boundary conditions must be considered when determining horizontal pressure at rest. Formulations often used in estimation of Ko in cases of unloading do not properly emulate the response of granular soils. The fit to cohesive soils is significantly better.

Short Bio
I am a faculty member in Civil and Environmental Engineering at the Technion – Israel Institute of technology, in Haifa, Northern Israel. I divide my working time between Geomechanics and Engineering for Developing Communities. I like to measure things that are hard to measure.

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