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OCR for page 98
99 DESIGN FOR LATERAL LOAD design of rock-socketed shafts. O-cell testing almost always demonstrates that base resistance provides a significant por- Methods for analysis of rock sockets under lateral loading are tion of total axial resistance under service load conditions. readily available to foundation designers, but currently are Data from Crapps and Schmertmann in Figure 23, chapter subject to uncertainties regarding their reliability. The survey three, show direct evidence of significant base load transfer shows that all states currently use the p-y method of analysis. when appropriate construction and inspection methods are Criteria for p-y curves in rock have been proposed and these applied to base conditions. Furthermore, O-cell and STN are the most widely used at the current time. However, the testing often result in higher values of allowable side resis- proposed criteria were described as "interim" when they first tance than would be calculated using the recommended appeared, because of the insufficient field load test data avail- prediction equations, which are intended to be conservative. able for validation. Research aimed at improving p-y curve criteria for rock has been described. The proposed methods Lateral load testing of rock sockets can be conducted also require additional verification by comparisons with field using O-cell and STN methods. The STN method may be load testing. A major feature of p-y methods of analysis is that particularly applicable for design of shafts subject to they provide structural analysis of the reinforced-concrete dynamic lateral loading, such as impact and seismic. Lateral shaft that incorporates the nonlinear momentEI relationship. O-cell testing has been demonstrated successfully, although This feature provides a useful interface between geotechnical research is suggested to develop procedures to relate lateral and structural design. O-cell test results to p-y curve criteria and to parameters used in other analytical methods. Conventional static lateral load Analysis methods based on elastic continuum theory have testing is still the most common method and is a proven been developed for lateral loading. The Carter and Kulhawy approach to verifying performance and studying load trans- method (1992) requires a minimal number of parameters and fer behavior. Lateral load testing on instrumented shafts is is easy to implement by hand or spreadsheet, but is applica- the only reliable method for validating p-y curves for design. ble only over the range of elastic response. The Zhang et al. method (2000) provides the complete nonlinear response, but requires more input parameters and relies on a finite-difference CONSTRUCTABILITY AND INSPECTION computer solution. These methods may be useful in the Pre- liminary Foundation Design phase (Figure 2, chapter one), Issues of constructability and inspection are related directly for making first-order assessments of trial designs to satisfy to rock-socket design and performance. Load testing, espe- service limit state criteria for lateral displacements. They are cially with O-cell methods, has helped to identify the effects most applicable when the ground profile can be idealized as of various construction methods on rock-socket perfor- consisting of one or two homogeneous layers; for example, mance. For example, the perception that construction of rock soil over rock. sockets is best facilitated by using full-depth casing and tak- ing measures to permit a "dry" pour has been shown to have detrimental effects on side and base resistances. Use of water LOAD TESTING or slurry, when subjected to appropriate quality control, pro- vides better performance by eliminating inward seepage, A positive development for drilled shaft design has been the trapping of cuttings behind casing, and potential for smear- introduction of several innovative field load testing methods. ing as the casing is removed. The Osterberg Cell (O-cell) and Statnamic (STN) tests can be conducted in less time, at lower cost, and with less equip- Tools available for incorporating constructability into ment than conventional axial load testing methods. This has rock-socket design through specifications, plans, and inspec- given transportation agencies the option of incorporating tion procedures are identified in several publications, includ- load testing into the design process on individual projects ing the FHWA Drilled Shaft Manual and the Participants and developing databases of shaft performance in specific Manual for the National Highway Institute Inspectors Certi- geologic environments. The experience of the Kansas De- fication Course. Several state agencies have developed partment of Transportation is described as a model example model drilled shaft specifications that incorporate proven for incorporating O-cell testing into a comprehensive pro- constructability practices (see for example, Washington State gram that has resulted in more efficient use and design of DOT Geotechnical Design Manual 2005). Recent develop- rock-socketed shafts. Many of the states surveyed have taken ments in concrete mix design, such as self-consolidating advantage of O-cell and STN testing and this has resulted in concrete, are expected to provide improved constructability. a significant increase in load test data. It is suggested that a Inspection tools such as the shaft inspection device used database of load test results be developed, analyzed, main- by Florida and North Carolina have direct implications for tained, and made available to the wider research community. design. By providing a means to verify base conditions un- der water or slurry construction, designers are better prepared The survey shows that states using the O-cell for axial to include base resistance in socket design. Construction of load testing are less likely to neglect base resistance for "technique" or "method" shafts and contractor constructability