The use of time domain reflectometry probes for the moisture monitoring of a drilled shaft retaining wall in expansive clay

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2011-08

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Abstract

Currently there is no consensus on how to account for the lateral earth pressures when designing drilled shaft retaining walls in expansive clay soils. Typically an equivalent fluid pressure is assumed which can range from 40 psf/ft to over 100 psf/ft. The range of assumptions currently in use can cause more than a factor of two difference in the maximum bending moment in the shaft. This range could cause the walls to be over-designed or under-designed. A full-scale test drilled shaft retaining wall was constructed on a site underlain by approximately 50 feet of the expansive Taylor Clay. Analysis of the wall is intended to provide information to be considered in design about the effects of the moisture cycles which cause shrinking and swelling.
In order to monitor the moisture changes within the clay, 20 Time Domain Reflectometry (TDR) probes were installed behind the wall. This thesis discusses the monitoring plan, calibration, installation, and initial results from these probes. The objectives of this thesis is to provide information regarding the site conditions and reasons for using TDR probes for this project and to describe the monitoring plan, calibration, installation, and the field performance of the TDR probes and the moisture values that have been seen on the site to date. Previous studies show that difficulties can be expected when using TDR probes in highly plastic clays. Results from this study are typical of these results seen previously. The initial results show that 4 of the 20 probes are recording reasonable waveforms. However, the waveforms cannot be analyzed using conventional methods. This result was because the waveform reflection that indicates the end of the probe cannot be defined due to attenuation of the signal, which is typical of highly conductive soils. Also, the large amount of scatter in the electrical conductivity values does not allow for the moisture content to be correlated to the electrical conductivity.
In order to use the TDR probes to measure moisture content at the project site, an alternative method needs to be employed to analyze available waveforms. If another method can be successfully employed for the functional probes, the subsequent step would involve recovering the probes that are not functioning properly in order to get a moisture profile along the full cantilevered height of the wall. Direct moisture measurements should also be taken periodically to provide a moisture profile.

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