Development and validation of the downhole freestanding shear device for measurement of the dynamic properties of clay.

摘要

The Downhole Freestanding Shear Device {lcub}DFSD{rcub} is designed to automatically carve a column of soil 4-inches in diameter and up to 16-inches long, at the bottom of a cased borehole, while maintaining the original field stress level throughout the specimen creation process. The device is capable of measuring the local stress-strain response of the freestanding soil column under cyclic torsional shear loading.; The development of the device has been completed and its performance has been validated in an elaborate laboratory testing program. DFSD laboratory tests have been conducted on two cohesive soils reconstituted in specially-designed “smart” consolidometers. The tests were designed to simulate field tests, and therefore, all the phases of a DFSD test have been evaluated. Measurements from DFSD tests were compared with those from independent laboratory tests conducted on identically-prepared samples at the University of Texas, Austin and the University of California, Los Angeles. The results clearly indicated a successful performance of the DFSD. Shear modulus and damping ratio were obtained at strains as small as 0.0005% and up to 1%. The results indicate that the specimen creation process has a minimal effect on dynamic soil properties. The DFSD-measured absolute and normalized modulus degradation curves and the damping ratio curves were in good agreement with the curves predicted by the independent tests. Moreover, the DFSD has been shown to correctly capture the effect of soil plasticity, stress level, and stress history on dynamic soil properties.; Equipment for DFSD field transportation and deployment has also been developed. A customized self-sufficient trailer was designed to transport the device to the test site and then to serve as a control station. The trailer houses a power generator, an air-compressor, and the DFSD umbilical and other accessories. The DFSD and its field transportation and deployment equipment have been implemented in field tests conducted at the Richmond Field Station. The results from these tests have also indicated a successful DFSD performance. Dynamic soil properties were obtained over a wide strain range with the same quality as in the laboratory tests. DFSD field shear modulus measurements compared well with the measurements made by cross-hole tests conducted at the site. (Abstract shortened by UMI.)