Field investigation and performance-based seismic design of rocking shallow foundations in cohesive soil

• Author / Creator

  Sharma Keshab

• This research investigated the use of rocking shallow foundations in improving the seismic performance of soil-footing-structure systems. A series of lateral loading tests of large-scale rocking foundation systems in natural cohesive soil were conducted and a performance-based seismic design guide was developed with design examples.
  A soil-footing-structure rocking system was fabricated for the present research. Two types of field tests of rocking foundations were performed in a cohesive soil in Edmonton, Canada: firstly by a quick release (snap-back) method and secondly by applying lateral cyclic loading. The loading direction, depth of embedment, and initial static factor of safety were systematically varied. The system consisted of a 1.5 m by 1.0 m concrete footing, steel column, and deck to simulate a prototype bridge system. The footing was intensively instrumented with strain gauges.
  The first part of the field tests characterized the dynamic behaviour of the rocking system using snap-back tests. In total, 27 snap-back tests were conducted, where a variety of initial drift ratios were applied with a maximum value of 8.5%. The damping ratio observed during the oscillations after snap-back release of the shallow foundations ranged from 8 to 30%. Average measured period of the rocking system was elongated by approximately 235% comparing with the period of fixed-base structure. The rocking system on clay exhibited a good recentering ability, which is even better than on sand.
  The second part of the field tests consisted of slow cyclic loadings along the axis of the footing at various drift ratios up to 7%. Twenty-four tests were conducted for foundations with varying initial factors of safety against the bearing failure, loading directions, rotation amplitudes, and embedment. Rocking foundations had the re-centering ability that resulted in less residual rotations and showed non-degrading moment capacity. Rocking foundations were a good energy dissipater. The rocking-induced settlement increased with the cumulative footing rotation and decreased with the factor of safety for vertical bearing capacity. Footing’s mechanical response was quantified from strain gauge readings. The footing remained elastic in tension, which then confirmed that the footing body may perform well in cyclic motion. The transient soil-footing contact areas were estimated with strain gauges and they agreed very well with the measured or calculated contact areas.
  The third part of the field tests characterised the effects of lateral loading obliquity on the performance of rocking foundations. The rocking system was subjected to loading at an angle of 45 with respect to the footing axes. A method of estimating the rocking moment capacity of footing subjected to oblique loading was developed and validated by the present tests. Natural periods, damping ratio, re-centering ratio, settlement, and stiffness degradation during the tests were discussed and compared with the results from previous studies with orthogonal loading.
  Lastly, a performance-based seismic design (PBSD) guide was proposed for the design of rocking shallow foundations. The empirical equations of normalized secant stiffness and damping ratio developed from present field tests were used as input to the PBSD guide. Additionally, rocking-induced settlement and residual drift were checked as performance indicators. An ordinary bridge composed of a column and a nonlinear rocking foundation was considered as an example and then the PBSD was applied to redesign the shallow foundation. Step-by-step design procedure was elaborated with two design examples assumed to be located in British Columbia and California. The design examples showed the feasibility of the PBSD method.

• Subjects / Keywords

  • Performance-based seismic design
  • Cohesive soil
  • Snap-back test
  • Rocking shallow foundation
  • Field test
  • Cyclic loading

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-49tb-8754

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.