Dual tube instrumentation was installed at vertical barrier projects in texas to determine the spacing of horizontal cracks that transmit water through expansive clay soils. Site selection was contingent on roadway rehabilitation that included installation of a vertical moisture barrier. To date, six dual tube sites in texas have beeninstrumented and readings taken. Field problems encountered were manufacturing defects with the sensor probe, moisture damage to the scintillation probe, and difficulty in extending readings to required depths due to poor design of the extension rods. The scaler ratemeter is susceptible to damage during normal transport. Data obtained inthe field consisted of depths and count ratios for the source and sensor in the field-installed dual tubes. Density was determined using calibration curves of density versus count ratio supplied by the manufacturer. Plots of the density versus depth for each location were prepared and were studied to determine the crack pattern. Crack spacings of between 4 and 5 in. Were obtained for four sites in climatic zones iii-c and ii-c, while spacings of 7 in. Were obtained for two sites in or near climatic zone iii-b. This paper appears in transportation research record no. 1277, Modern geotechnical methods: instrumentation and vibratory hammers 1990.
Abstract