Date of Award

Spring 1-1-2013

Document Type

Thesis

Degree Name

Master of Science (MS)

First Advisor

Shideh Dashti

Second Advisor

John McCartney

Third Advisor

Youssef Hashash

Abstract

When investigating the seismic response of buried structures in geotechnical experiments, a reliable measure of dynamic earth pressure is necessary. Tactile pressure sensors are flexible, thin sheets containing a matrix of sensels (sensors), allowing them to measure pressure distributions with minimal intrusion and avoid soil arching effects. Each of the sensor's sensels can record pressure at high sampling rates (e.g., up to 20,000 samples/second).

Calibrating tactile pressure sensors for use in dynamic centrifuge modeling with granular materials has been challenging in the past. In particular, they have been unable to capture the full amplitude content of a dynamic signal at high frequencies. This is due to the low sampling rate of older sensor models (leading to signal aliasing) and the sensor's own frequency response. In this thesis, a dynamic calibration procedure is proposed and evaluated to correct for the tactile sensor's frequency response. Further, different static calibration procedures are compared and evaluated in this study.

The dynamic response of tactile pressure sensors was characterized by loading them with sine-waves of different frequencies using a controlled loading machine at the University of Colorado, Boulder. A transfer function was calculated to relate the pressure sensor's signal to the reference load cell recording as a function of frequency. A digital filter was developed based on this transfer function and applied to pressure sensor recordings to recover the original, high frequency signal. The reliability of the proposed dynamic calibration procedure was then tested through a series of blind dynamic tests with the loading machine and dynamic centrifuge tests with water.

Two methods of static calibration were explored and compared in this study: (1) loading of the sensor with a controlled loading machine; and (2) placing the sensor at the bottom of a centrifuge container covered with the test sand and spun to different g-levels. Calibration results from each method were applied to sensor recordings of static lateral earth pressure on a tunnel wall during a centrifuge model test. For the particular tactile sensor tested here, static calibration with the loading machine yielded better comparisons with theoretical estimates of lateral earth pressure.

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