Compression Top Load Reaching Shaft Bottom—Theory Vs. Tests

Crapps, David K.; Schmertmann, John H.

doi:10.1061/40601(256)109

Cited by 8 publications

(3 citation statements)

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“…Loads applied to the shafts are supported by the rock socket through side shear resistance and end-bearing resistance (Horvath et al 1983). Although ''there are significant advantages in designing to include a base [or end-bearing] resistance component'' (Williams and Pells 1981, p. 502), the end-bearing resistance is often ignored in current design practice (Crapps and Schmertmann 2002;Turner 2006). According to Crapps and Schmertmann (2002), the most common reasons cited by designers for neglecting end-bearing resistance in design include settled slurry suspension, reluctance to inspect the bottom, concern for underlying cavities, and unknown or uncertain end-bearing resistance.…”

Section: Introductionmentioning

confidence: 98%

“…Due to the high cost of shaft construction in rock, an overdesign of socket length will lead to increased cost. Crapps and Schmertmann (2002) suggested that accounting for endbearing resistance in design and using appropriate construction and inspection techniques to ensure quality base conditions is a better approach than neglecting end-bearing resistance. They supported their recommendations with field load test results in which load transferred to the base was measured.…”

Section: Introductionmentioning

confidence: 99%

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Prediction of end-bearing capacity of rock-socketed shafts considering rock quality designation (RQD)

Zhang

2010

Can. Geotech. J.

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Existing empirical methods for determining the end-bearing capacity, qmax, use empirical relations between qmax and the unconfined compressive strength of intact rock, sc. As rock-socketed shafts are supported by the rock mass, not just the intact rock, one should consider not only the intact rock properties, but also the influence of discontinuities on rock mass properties when determining qmax. Although semi-empirical and analytical methods have been developed that can consider the effect of discontinuities, they are more complicated than the empirical relations and require information about discontinuities that is often not available or difficult to obtain in engineering practice. In this paper, an empirical relation between qmax and the unconfined compressive strength of rock mass, scm, is developed. The new empirical relation explicitly considers the effect of discontinuities represented by rock quality designation (RQD), which is the parameter normally obtained in engineering practice. The accuracy of the expression for estimating scm based on RQD is verified by comparing its estimation values with those from the existing empirical expressions based on rock mass classification. Two examples are presented to show the application of the newly developed empirical relation between qmax and scm.Résumé : Les méthodes empiriques existantes servant à déterminer la capacité portante en pointe qmax utilisent des relations empiriques entre qmax et la résistance en compression non confinée de roches intactes sc. Puisque les fûts encastrés dans le roc sont supportés par la masse rocheuse et non pas seulement par la roche intacte, il est plus approprié de considérer l'influence des discontinuités des propriétés de la masse rocheuse en plus des propriétés de la roche intacte lors de la détermination de q max . Malgré le fait que des méthodes semi-empiriques et analytiques capables de considérer l'effet des discontinuités aient été développées, ces méthodes sont plus compliquées que les relations empiriques et nécessitent des informations sur les discontinuités qui ne sont généralement pas disponibles ou bien difficiles à obtenir dans la pratique. Dans cet article, une relation empirique entre q max et la résistance en compression non confinée de la masse rocheuse s cm est développée. Cette nouvelle relation empirique considère explicitement l'effet des discontinuités, représenté par la dési-gnation de la qualité de la roche (« RQD »), qui est un paramètre normalement obtenu en pratique. La précision de l'expression permettant d'estimer s cm basée sur la RQD est vérifiée en comparant les valeurs estimées aux valeurs obtenues à l'aide des expressions empiriques existantes basées sur la classification des masses rocheuses. La nouvelle relation entre q max et s cm est inspirée d'une base de données de 43 puits d'essai ayant une valeur de RQD. Deux exemples sont présentés pour démontrer l'application de la nouvelle relation empirique. Ces exemples indiquent que la nouvelle relation entre q max et s cm .Mots-clés : fûts encastr...

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Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

Prediction of end-bearing capacity of rock-socketed shafts considering rock quality designation (RQD)

Zhang

2010

Can. Geotech. J.

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“…In the same paper the author stresses the need for a more economical design, suggesting that load testing should be performed ahead of production shaft installation to guide design rather than merely confirming that the design is safe. Data from Crapps and Schmertmann (2002) from a relatively large number of static tests suggests that base resistance mobilization represents a significant contribution to the overall shaft resistance at downward displacements that correspond to typical serviceability requirements and that end-bearing is generally greater than predicted by numerical calculations using an elastic soil or rock model. Similar cases of overdesign of drilled shafts have been reported by other practitioners.…”

Section: Introductionmentioning

confidence: 99%

Optimization of Drilled Shaft Design Using High Strain Dynamic Monitoring

Sellountou¹,

Duffy²,

Holman³

2016

Geotechnical and Structural Engineering Congress 2016

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High capacity of rock-socketed or end bearing drilled shafts is often not fully exploited due to the highly conservative design that many local codes and local practices around United States dictate. This reduces the cost efficiency of drilled shafts versus other foundation solutions. ADSC is interested in developing a test program procedure that will be economically feasible for engineers and foundation contractors to perform in every project even when load tests are not specified. Static load tests (including conventional top-down and bi-directional load tests) are invaluable but often prohibitive due to high cost and time constraints. Alternatively, high strain dynamic testing of drilled shafts has increased significantly in recent years with well-established testing procedures and analyses. More specifically, ADSC is considering using dynamic load testing to prove the high capacities of drilled shafts in several rock formations around the country and establish limits of extrapolation to larger diameter shafts from tests on smaller diameter shafts. The purpose of this paper is to present the theoretical basis of ADSC's technical endeavor. More specifically the paper will focus into 2 different topics; i) parameters affecting drilled shaft performance and ii) the theory and principals of dynamic testing as applied to rock socketed drilled shafts, as well as, case studies of dynamic tests in rock socketed drilled shafts.

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