An investigation into the load-settlement behavior of two drilled shafts, founded
in shale, is presented. The motivation for this research is to advance the understanding
on how drilled shafts react under loading in stiff clays and shales. The objectives of the
study are to measure the strengths within the subsurface material at the test site, estimate
the unit side shear and unit end bearing of the shale-shaft interaction by running two axial
load tests, and compare the results to the current design methods that are used to predict
the axial capacity of drilled shafts.
A comprehensive field investigation, performed by Fugro Consultants, provided
strength profiles of the subsurface material at the test site. Through the cooperation of
the Texas Department of Transportation (TxDOT), the Association of Drilled Shaft
Contractors, and McKinney Drilling Company, two drilled shafts were installed at a
highway construction site in Austin, Texas. The load tests were performed by Loadtest,
Inc.; using the patented Osterberg-Cell™ loading technique to axially displace the shafts.
Ensoft, Inc. installed strain gauges at multiple levels within the shafts, making it possible
to analyze the shaft mobilization during loading.
Ultimate end bearing values of about 100- and 120-ksf were measured for Test
Shafts #1 and #2, respectively. The current methods for estimating unit end bearing,
developed by TxDOT and the Federal Highway Administration, provide fairly accurate
predictions when compared to the measured information. The ultimate side resistance
obtained near the O-Cell™ in each test was about 20-ksf, however, the measured ultimate
side resistance steadily decreased nearing the tip of the shaft. For the zones where the
side resistance was believed to be fully mobilized, the TxDOT design method accurately
predicts the side resistance. A limited amount of information is currently available for
load tests performed in soils with TCP values harder than 2-in per 100 blows. Additional
load test information should allow for a stronger correlation between TCP tests and unit
resistances for very hard clay-shales, as well as, allowing for further evaluation of the
shale-shaft interaction near the shaft tip. The results presented herein demonstrate the
effectiveness of the current design methods for drilled shafts and the non-uniformity of
side resistance within one- to two-diameters of the shaft tip. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/21935 |
| Date | 05 November 2013 |
| Creators | Burkett, Terry Bryce |
| Source Sets | University of Texas |
| Language | en_US |
| Detected Language | English |
| Format | application/pdf |
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