Rock-Socketed Shafts for Highway Structure Foundations (2006) / Chapter Skim
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Pages 70-95
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From page 70... ...
Illustrative examples demonstrate how load testing can contribute to the economical design of rock sockets. Constructability issues identified by the survey questionnaire are discussed, and practices that can lead to quality construction are identified.
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From page 71... ...
At some combination of rock strength and socket diameter rock augers are no longer cost-effective. One contractor When relatively stiff soil or weak rock cannot be penetrated efficiently with typical soil drilling tools (e.g., open helix augers)
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From page 72... ...
Use of a single parameter, such as uniaxial compressive strength of rock, does not capture all of the variables that determine penetration rates for a given set of conditions. Coring is a widely used method when rock augers are no longer feasible.
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From page 73... ...
that allows circulation of the drilling fluid from the cutting surface up through the bar. FIELD LOAD TESTING The most direct method to determine the performance of fullscale rock-socketed drilled shafts is through field load testing.
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From page 74... ...
If possible, the construction methods anticipated for production shafts should be used to construct test shafts. Axial Load Testing Conventional Axial Load Testing Until the early 1990s the most common procedure for conducting a static axial compression load test on a deep foundation followed the ASTM Standard Method D1143, referred to herein as a conventional axial load test.
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From page 75... ...
Zhan and Yin (2000) describe axial load tests on two shafts for the purpose of confirming design allowable side and base resistance values in moderately weathered volcanic rock for a Hong Kong transit project.
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From page 76... ...
a unit base resistance of 20.8 MPa was reached with no sign of approaching failure. The case presented by Zhan and Yin demonstrates how a set of well-instrumented conventional axial load tests can be used to (1)
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From page 77... ...
, the majority of load tests on drilled shafts are now being done with the O-cell. This is supported by results of this study, in which 17 of 32 states responding to the survey reported using the O-cell for axial load testing of rock-socketed shafts.
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From page 78... ...
These tests illustrate a typical outcome when field load testing is conducted; that is, measured unit side and base resistances exceed presumptive values, sometimes significantly. Load testing results make it possible to achieve more economical designs.
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From page 79... ...
The degree to which this average unit side resistance is valid for design of rock sockets loaded at the head depends on the degree to which side load transfer under O-cell test conditions is similar to conditions under head loading. Detailed knowledge of site stratigraphy is needed to interpret side load transfer.
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From page 80... ...
and loading in compression from the top are the result of differing normal stress conditions at the interface, and that these differences become more significant with increasing rock mass modulus and increasing interface friction angle. These numerical analyses suggest that differences in the response of rock sockets to O-cell test loading and top-down compression loading may warrant consideration in some cases.
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From page 81... ...
Drilled shafts now comprise approximately 70% to 80% of new bridge foundations, and shaft designs are more economical because there is a high level of confidence in capacity predictions, based directly on the load tests. The approach taken by KDOT illustrates how field load testing, in this case with the O-cell, can be incorporated into an overall program leading to increased use and improved design methods for rock-socketed foundations.
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From page 82... ...
Dynamic Impact Testing A dynamic compression load test can be carried out by dropping a heavy weight onto the head of the shaft from various heights. The shaft is instrumented with strain gages and accelerometers to measure the force and impact velocity of the stress wave generated by the dynamic impact.
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From page 83... ...
and Kulhawy and Carter (1992b) proposed a method for interpretation of static axial load tests on rock-socketed shafts.
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From page 84... ...
Carter and Kulhawy (1988) applied the technique described to 25 axial load tests reported in the literature by backcalculating values of the model parameters Er, Eb, c, and (tanφ tanψ)
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From page 85... ...
Methods for conducting lateral load tests on deep foundations include conventional methods, Osterberg load cell, and STN. Conventional Lateral Load Test The conventional method for conducting a lateral load test is given in ASTM D3966 and involves pushing or pulling the head of the test shaft against one or more reaction piles or shafts.
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From page 86... ...
Further research is needed to establish guidelines for proper procedures and to define correct analyses that account for the differences in boundary conditions, load transfer, and soil and rock resistance, compared with a shaft loaded at its head. It is also worth noting that the lateral split socket test may provide a means to measure the in situ rock mass modulus of deformation (EM)
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From page 87... ...
Lateral Load versus Translation Static & Derived Statnamic - Shaft Group (b) FIGURE 75 Results of lateral STN test: (a)
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From page 88... ...
describe a case that seems to contradict some commonly held ideas about casing versus wet hole construction of rock sockets. A drilled shaft installed through 12 m of soil and socketed into rock was constructed using a fulllength casing (to provide downhole visual inspection)
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From page 89... ...
The Washington State DOT uses language in their special provision for rock sockets that reportedly works well and is summarized as follows. For shafts with a specified minimum penetration into the bearing layer and no specified base elevation, the contractor furnishes each reinforcing cage 20% longer than specified in the plans.
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From page 90... ...
; groundwater conditions Competency in rock identification and classification; ability to read and interpret core logs Knowledge of design issues Rock units providing side, base, and lateral resistances Design parameters: shaft locations, socket depths and diameters, reinforcement details Basic understanding of design philosophy for drilled shafts under axial and lateral loading Familiarity with standard specifications, plans, special provisions, shop drawings, and contractor submittals Knowledge of contractor's plan for socket construction Rock excavation tools (augers, coring, hammers, other) and methods (e.g., casing, slurry)
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From page 91... ...
of loose material when base resistance is neglected. When sockets are poured under dry conditions, common inspection methods to verify bottom conditions are either visual inspection or downhole cameras.
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From page 92... ...
Post-grouting of the shaft base is a measure that could be incorporated into design and construction to provide quality base conditions in drilled shafts. It is instructive to observe that most states that have incorporated field load testing of rock sockets into their foundation programs, using a method that allows measurement of base load-displacement, now include both side and base resistances in their design calculations.
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From page 93... ...
Drilled shafts are used extensively for major structures in this region, primarily because it has been recognized that large axial loads can be supported if a shaft is extended to either decomposed or intact rock. Gardner (1987)
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From page 94... ...
Gardner (1987) reviews design methods for axial loading of drilled shafts in Piedmont profiles, including recommendations for design side and base resistances in rock and methods used to determine relative load transfer between side and base.
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From page 95... ...
Load testing is also shown to be a factor in increased use of rock-socketed drilled shafts by transportation agencies. Finally, load testing with the O-cell has been a useful tool for identifying and evaluating poor versus good construction practices.
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