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Evaluation of torsional load transfer for drilled shaft foundations : final report : SPR 304-701.
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2016-05-01
[PDF-12.28 MB]
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Edition:Final report
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Abstract:Despite the prevalence of the use of drilled shafts for the support of traffic signal and signs along
state highways, relatively little is known about the torsional load transfer between the structure and soil providing
its support. A review of literature indicated that just three full-scale torsional loading test series have been
conducted, and these did not report the observation of the torsional load transfer. To help address this gap in
knowledge, two instrumented test shafts, which were designed to support signal pole type SM3 based on ODOT
Standard Drawing TM653, were constructed to evaluate the torsional capacity and load transfer of these shafts at
full-scale at the Oregon State University (OSU) Geotechnical Engineering Field Research Site (GEFRS).
Two shafts were constructed: one shaft designated as the torsion test drilled shaft with production base (TDS)
was constructed using the dry method, whereas another shaft designated as the torsional drilled shaft with
frictionless base (TDSFB) was constructed by placing bentonite chips the bottom of the cavity to create near-zero
base shear condition. Monotonic, quasi-static and cyclic loading tests were performed using two hydraulic
actuators and a displacement couple. The imposed rotation and corresponding torque was monitored using string-potentiometers
and load cells, respectively. Embedded strain gages were installed on both test shafts over five
depths to measure shear strains and reveal the load transfer of the drilled shafts in torsion. The torsional load
transfer is back-calculated from the instrumentation data and described in detail.
Existing design procedures for predicting torsional capacity of drilled shafts were investigated. The CDOT
Design Method and the Florida District 7 Method, both of which can treat layered cohesive and cohesionless soils,
were selected to estimate the torsional capacities of the test shafts and compared with the test results. However,
these design methods appeared to over- and under-predict the torsional capacity, respectively, indicating the need
for the development of improved methods for assessing torsional capacity.
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