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OCR for page 99
100 reviews before publication of the final design and bidding proposed by several researchers. These design methods are phases are additional tools for incorporating constructability. limited because roughness is not a commonly measured pa- rameter in the field. Construction techniques are constantly under development and innovative methods that can lead to RESEARCH NEEDED TO ADVANCE improved quality should be encouraged and, where appro- STATE OF PRACTICE priate, developed further through research. Information gathered for this study suggests that develop- ment of improved practices for design and construction Consider developing a manual or design circular fo- of rock-socketed drilled shafts might be achieved through cused specifically on drilled shafts in rock. the following research or wider dissemination of existing Research is needed for axial load tests on instrumented information. shafts for the purpose of evaluating prediction equa- tions for base resistance in rock; O-cell and STN tests are ideal for this purpose. Site Characterization Identify efficient and inexpensive field roughness mea- surement methods that can be incorporated into design Studies are needed to better define the best methods for equations; correlate roughness parameters to rock type, determination of rock mass deformation modulus specif- drilling tools, groundwater conditions, etc. ically for use in rock-socket design. In situ methods, Investigate the potential of base grouting as a quality including borehole jack and PMT, may yield different assurance tool for rock-socketed shafts. results and both could be compared with the most up-to- date correlations with RMR and GSI. A survey of contractors could be conducted to identify Design for Lateral Loading the rock mass information most useful for evaluating construction in rock; avoiding overemphasis on "weak- Methods developed for analysis of deep foundations in soil, est" materials. especially the p-y curve method, are the methods of choice Application of geophysical methods to rock-socket for laterally loaded rock sockets. The principal limitation lies design requires further research and development. in the lack of proven p-y curve criteria for rock and IGM. Guidelines are needed for matching appropriate meth- This problem could be addressed by first conducting a com- ods to site conditions. Case histories of successes and prehensive analysis of all existing load test results to evaluate limitations could be published and distributed. proposed models, followed by research involving additional Research is needed relating rock drillability to rock mass field load testing against which p-y curve criteria can be eval- characteristics; correlations to RMR or GSI warrant uated and calibrated. investigation. Relationships between the reliability of rock and IGM Conduct research to collect and analyze all existing engineering properties and resistance factors used in lateral load test results, with the goal of establishing load and resistance factor design could be investigated uniform criteria for p-y curve development. and quantified sufficiently to support the resistance val- Transportation agencies could undertake research in- ues recommended in AASHTO specifications; this volving lateral load tests on properly instrumented topic could be the subject of ongoing research. rock-socketed shafts, designed specifically for testing and calibration of p-y criteria for rock and IGM. Design for Axial Loading Structural Design Sufficient analytical tools exist for the reliable design of Structural issues of concern to foundation designers, as iden- sockets under axial loading. However, much of this informa- tified by the survey, included uncertainty regarding appar- tion is widely scattered in the literature. The FHWA Drilled ently high shearing forces in shafts analyzed using p-y curve Shaft Manual and the 2006 Interim AASHTO LRFD Bridge analyses and questions pertaining to moment capacity of Design Specifications include some available methods, but short, rigid sockets. These issues can best be addressed by are not concise and clear in the presentation, and include rigorous analytical methods in conjunction with load tests on some out-of-date methods. Numerous equations are presented carefully instrumented shafts in rock. A structural issue that in the literature for estimating base resistance of drilled shafts has yet to be investigated as it pertains to deep foundations is in rock. Surprisingly, very little data are available by which the effect of confining stress on the strength and stiffness of proposed methods can be evaluated. Studies are needed in- reinforced concrete. It may be that concrete strength could be volving field axial load testing in which rock mass properties significantly increased under confining stresses typically en- are well-documented and design equations for base resis- countered over the subsurface depths of many bridge foun- tance can be evaluated. Equations for incorporating rough- dations. More economical structural designs may be possible ness as a design parameter for unit side resistance have been if this issue is investigated and applied in practice. Permanent

OCR for page 99
101 steel casing contributes to the structural capacity of drilled Management of Load Test Data shafts. Design methods that account explicitly for the steel casing are lacking in current design codes. Large amounts of data from load tests on rock-socketed shafts, conducted for the purpose of research or for specific Consider fundamental research with the goal of quantify- transportation projects, have been acquired, especially since ing the effects of geologic confining stress on reinforced- the development of new testing methods. These data can concrete shear, moment, and compression behavior. be used most effectively if they are made available from a Incorporate the results into structural design of drilled single source and organized in a systematic manner. shafts. Conduct research and development of methods that ac- Investigate placing those into a national database of count for permanent steel casing in the structural design load test results for rock-socketed drilled shafts, for use of drilled shafts. by transportation agencies and researchers.