Date of Award
Master of Science in Ocean Engineering
One of the most common tests to evaluate the cyclic strength of soils are constant volume (undrained) cyclic direct simple shear tests. They are easy and fast to perform and best represent in-situ stress conditions during earthquakes. Over the past 15 years the interest in cyclic testing has increased and with this the number of companies developing and manufacturing cyclic direct simple shear devices has also increased. Despite this, the American Society of Testing Materials does not provide any guidelines for cyclic direct simple shear tests, which leads to the issue that testers are left on their own in terms of data quality evaluation. Several recent studies, such as Zehtab et al. (2019), Ulmer et al. (2019) and Konstadinou et al. (in press) have focused on identifying and evaluating factors that can be used to assess sample quality in cyclic direct simple shear test results.
The objective of this thesis was to apply published quality control measures for cyclic direct simple shear (CDSS) to tests performed on a commercially available CDSS apparatus manufactured by the Geocomp Corporation. Tests were performed on samples of Monterey and Ottawa sand, two commonly tested materials in the geotechnical literature. Samples were prepared to a relative density of 40% and samples were consolidated to 100 kPa, 200 kPa, or 300 kPa prior to stress-controlled cyclic shearing. No modifications were made to the CDSS apparatus for this study.
Three quality control measures were focused on: 1) axial strain during cyclic shear (i.e. constant volume conditions); 2) shear strain developed during consolidation; and 3) the effect of consolidation stress on cyclic strength (i.e. Kσ).
For almost all the tests, the axial strain during cyclic shearing met the recommended limits of < 0; 03%. Furthermore, the behavior of each test appeared reasonable (e.g. stress-strain, stress path, etc.) and the relationships between the cyclic stress ratio (CSR) and the number of cycles to failure (N) were internally consistent for each sand.
There was good agreement between the cyclic stress ratio (CSR)/ number of cycles to failure (N) relationship for Ottawa sand with comparable results from Zehtab et al. (2019). However, there was not good agreement between the test results on Monterey sand with the results published by Ulmer (2019).
The measured shear strain during consolidation exceeded recommended values for almost all the tests. This is due to the fact that vertical consolidation stresses apply some load to the horizontal load cell and the software moves the water bath to maintain zero horizontal load during consolidation. It is recommended that modifications to the water bath be made to isolate or detach the horizontal load cell during consolidation.
Finally, CDSS tests at increasing consolidation stresses yielded results that were contrary to established behavior of how the cyclic strength decreases with increasing stress (i.e. the Kσ effect). At this time it is not clear how this effect can be corrected. It is hypothesized that the lack of horizontal stiffness is responsible for the increasing CRR with consolidation stress, and more research is needed to address this issue. It is recommended that the side support arm be replaced with a stiffer frame.
Miescke, Fenia Mika, "QUALITY CONTROL MEASURES FOR CYCLIC SIMPLE SHEAR TESTS ON SAND" (2020). Open Access Master's Theses. Paper 1908.