Date of Award
2025
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Civil and Environmental Engineering
Department
Civil and Environmental Engineering
First Advisor
Christopher Baxter
Abstract
This body of work consists of three manuscripts. The first manuscript focuses on the evaluation of constant normal stiffness tests on sand using a newly developed active control system. The second manuscript is related to prediction of axial capacity of drilled and grouted micropiles for offshore foundations. The third manuscript corresponds to evaluation of quality control measures for cyclic resistance of sand using cyclic direct simple shear equipment.
Small diameter drilled piles (micropiles) are being considered for use as an economical foundation for floating offshore wind structures. On land, micropiles are typically load tested to confirm design capacity; this is not feasible for offshore piles. Because of this limitation, there is a need to better understand the frictional behavior of small-diameter piles (such as grouted micropiles) to improve predictions of capacity. During the axial loading of micropiles, dilation or contraction of the shear band leads to changes in normal stress around the micropiles. Knowing this change in normal stress will lead to better predictions of micropile shaft resistance.
Typically, to model the frictional resistance around piles, interface shear tests are performed in the laboratory under constant normal load (CNL) conditions. This condition does not account for the changes in normal stress due to dilation/contraction at the interface during shear. An improvement would be the widespread adoption of the constant normal stiffness test, and the objective of the first manuscript is to present the results of a newly developed, active control system for performing constant normal stiffness tests on sand. This is accomplished by a feedback loop incorporated into a modified direct shear machine in which the ratio of the change of normal stress (Δσ’n) to the change of normal displacement (Δy) is checked and adjusted to maintain constant normal stiffness during shear. The ability of the control system to maintain constant normal stiffness during shear was found to be excellent, and test results on both dilative and contractive samples demonstrated consistent trends of shear strength and changes in both shear band thickness and normal effective stress.
The objective of the second manuscript is to assess whether constant normal stiffness (CNS) testing in the laboratory can more accurately model the axial capacity of small-diameter grouted micropiles. A suite of active control, monotonic constant normal stiffness (CNS) tests at various relative densities and soil spring stiffnesses were performed to obtain shear band dilation or contraction (y) for samples of Monterey sand at consolidation stresses corresponding to depths typical of drilled grouted micropiles. The results suggest that there is a unique relationship between the amount of shear band dilation and stress-corrected relative density and that an increase in soil spring stiffness suppresses dilation. Most importantly, a conceptual framework is presented that may improve our understanding of the static capacity of small diameter piles that includes non-linear normal stiffness changes around the pile. This framework allows for site specific information (e.g. small strain shear modulus from a seismic cone) to be incorporated into a targeted laboratory testing program.
The objective of the third manuscript is to evaluate quality control measures used for cyclic direct simple shear (CDSS) testing. A cyclic direct simple shear (CDSS) test is an element laboratory test that is being increasingly used in geotechnical engineering to evaluate the cyclic behavior and liquefaction susceptibility of clean sand. Despite the proliferation of commercial CDSS systems, there is still considerable uncertainty in the results of CDSS tests reported in the literature. Variabilities in testing equipment, control systems, sample preparation methods, failure criteria, testing procedures, and quality control measures make it difficult to obtain unique and soil-dependent test results. Given this variability, comparing results across studies requires careful application of quality control measures to ensure that the data are valid and meaningful. A suite of active control constant volume CDSS tests were conducted on samples of Monterey and Ottawa sand with different stress histories and relative densities. Various quality assurance/quality control (QA/QC) measures published in the literature were applied and evaluated. The results of tests that passed the QA/QC checks compared well with published results despite differences in equipment and some testing procedures. Improved communication and transparency regarding QA/QC procedures in CDSS testing is recommended to ensure comparability of results across different studies and ensure that CDSS test results are reproducible.
Appendix A presents an initial study pre-dating the first manuscript that focused on the development of a passive control constant normal stiffness (CNS) test in the laboratory to more accurately simulate boundary conditions near the pile-soil interface under monotonic and cyclic loading.
Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.
Recommended Citation
Babaee, Amir, "MONOTONIC CONSTANT NORMAL STIFFNESS TESTS FOR IMPROVING THE SKIN FRICTION RESISTANCE PREDICTION OF DRILLED PILES IN SAND" (2025). Open Access Dissertations. Paper 4438.
https://digitalcommons.uri.edu/oa_diss/4438