Date of Award

2013

Degree Type

Thesis

Degree Name

Master of Science in Ocean Engineering

Department

Ocean Engineering

First Advisor

Christopher Baxter

Abstract

Over the past several years, measuring the shear wave velocity (Vs) of soils has become more common for earthquake site response analyses and for estimating soil type and strength to better assist geotechnical engineers in forming more accurate soil profiles for engineering design. Most of the measurements in the laboratory have been conducted in conjunction with triaxial tests due to the larger sample sizes and availability of equipment in the U.S. It is less common to measure the shear wave velocity in the direct simple shear (DSS) apparatus. This is partly because, up until the past decade, there have been few manufacturers of DSS equipment. In addition, soil samples in the DSS test are typically less than 3 cm high and measuring shear wave velocity over these distances is affected by electrical near-field effects. This is unfortunate considering that the undrained shear strength obtained from the DSS test is preferred because it provides an average value of shear strength compared to other modes of shearing (e.g. compression, extension).

There are two objectives of this thesis. The first objective is to develop a shear wave velocity measurement system for a commercially available DSS apparatus operated at the University of Rhode Island. The second objective is to use this new system to evaluate a possible link between small strain properties (e.g. shear wave velocity and the small strain shear modulus) and large strain properties (e.g. undrained shear strength) of cohesive soils.

A laboratory testing program was performed in which shear waves were generated and measured using piezoceramic bender elements mounted in DSS end caps constructed of brass. Careful grounding and waterproofing of the bender elements significantly reduced electrical noise and near-field effects, enabling clear interpretation of shear wave velocities over a wide range of densities and effective stresses. Three soils were tested in this study: a marine clay from the Gulf of Mexico, a sensitive clay from Maine called Presumpscot clay, and an organic silt from Narragansett Bay, Rhode Island. Shear wave velocity measurements were made at the end of consolidation and prior to undrained shearing of the samples. The ratio of small strain shear modulus (from the shear wave velocity) to undrained shear strength for the three soils was compared to published data of 11 different soils of varying stress histories and plasticity. The agreement with published data was very good, illustrating that the new DSS-Vs system works well and there is a clear link between small and large strain properties of cohesive soils.

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