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Rock-Socketed Shafts for Highway Structure Foundations (2006)

Chapter: Chapter Six - Conclusions

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Page 96
Suggested Citation:"Chapter Six - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2006. Rock-Socketed Shafts for Highway Structure Foundations. Washington, DC: The National Academies Press. doi: 10.17226/13975.
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Page 96
Page 97
Suggested Citation:"Chapter Six - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2006. Rock-Socketed Shafts for Highway Structure Foundations. Washington, DC: The National Academies Press. doi: 10.17226/13975.
×
Page 97
Page 98
Suggested Citation:"Chapter Six - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2006. Rock-Socketed Shafts for Highway Structure Foundations. Washington, DC: The National Academies Press. doi: 10.17226/13975.
×
Page 98
Page 99
Suggested Citation:"Chapter Six - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2006. Rock-Socketed Shafts for Highway Structure Foundations. Washington, DC: The National Academies Press. doi: 10.17226/13975.
×
Page 99
Page 100
Suggested Citation:"Chapter Six - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2006. Rock-Socketed Shafts for Highway Structure Foundations. Washington, DC: The National Academies Press. doi: 10.17226/13975.
×
Page 100

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97 This synthesis identified technologies and practices available to transportation agencies for utilization of rock-socketed drilled shafts as reliable and cost-effective structure foundations. All thirty-two of the state transportation agencies responding to the survey are currently using rock-socketed shafts, some quite ex- tensively (more than 20 projects per year). The single Canadian agency responding to the survey has not used drilled shafts extensively to date. Use of rock-socketed drilled shafts for transportation structures has increased significantly over the past 25 years and technologies applied to design, construction, and testing have advanced considerably. The design process for structural foundations by state transportation agencies is outlined in chapter one. Responsi- bilities typically are separated into geotechnical and struc- tural categories. Site characterization, geomaterial property evaluation, and design issues related to geotechnical capac- ity or load-deformation analysis are normally addressed as geotechnical issues, whereas structural modeling and rein- forced-concrete design are normally carried out by structural engineers. Design for lateral loading requires significant input and analysis by both geotechnical and structural personnel. The overall process of design and construction (i.e., engi- neering) is shown to consist of highly interrelated factors, requiring an integrated approach to drilled shaft foundations. Figure 3 in chapter one illustrates the process in the form of a flowchart. Adequate site characterization is needed to obtain the basic information required for both geotechnical analysis and construction. Constructability issues are best addressed during the design process, when decisions such as whether to include side resistance, base resistance, or both must be made on the basis of anticipated subsurface condi- tions, construction methods, load testing, inspection meth- ods, and experience. SITE AND GEOMATERIAL CHARACTERIZATION The most valuable and reliable information for rock-socket design is obtained by drilling and taking core samples of the rock at the location of each structural foundation. Careful logging of rock core, photographic records, and proper han- dling of core to obtain samples for laboratory testing provide the basic information that will be used for rock mass classi- fication, evaluation of engineering properties of intact rock and rock mass, and baseline information needed to assess constructability. Drilling also provides the means to conduct in situ tests. Every transportation agency that responded to the survey currently relies on rock coring as the primary source of design and construction information for rock-socketed shafts. Geophysical methods can provide additional valuable in- formation when applied appropriately by competent users. NCHRP Synthesis 357: Use of Geophysics for Transporta- tion Projects (Sirles) identifies the major geophysical methods that are applicable to geotechnical investigations and found that overall use of geophysics by transportation agencies is ex- panding. Seismic refraction for establishing depth to bedrock is the most common use of geophysics for drilled shafts in rock. However, of 33 responding agencies, only 8 (24%) re- ported using geophysics, including 7 that use seismic refrac- tion and 1 that uses electrical resistivity. These data suggest that geophysical methods are not used widely for investiga- tions related specifically to foundations in rock. Survey results from the Sirles study show that agency experience is mixed, with both successful and unsuccessful cases being cited. Factors associated with successful cases (for depth to bedrock) are: sufficient number of borings to validate and correlate the seismic results, interpretation by qualified geo- physicists, and clear understanding of the capabilities and limitations of the technology. Geophysical methods that show potential for rock site in- vestigations include electrical resistivity tomographic profil- ing and borehole televiewers. Multi-array resistivity methods have shown the ability to provide accurate images of subsur- face profiles in karstic terrains when used in conjunction with borings. Borehole televiewers, both acoustic and opti- cal, may have limited applicability to rock foundations. They are primarily useful for providing detailed information on structural discontinuities. For large or critical rock-socket projects, where the orientation and condition of discontinu- ities in situ is a critical concern, these devices can supplement information obtained from more conventional core logging. Other potentially useful methods are downhole seismic and crosshole seismic. A case described by LaFronz et al. in 2004, “Geologic Characterization of Bridge Foundations, Colorado River Bridge, Hoover Dam Bypass Project,” showed good correlation between rock mass modulus from downhole seismic testing and rock mass modulus from correlations to CHAPTER SIX CONCLUSIONS

the Geological Strength Index (GSI). This approach warrants further consideration. The laboratory test most widely applied to foundation design is uniaxial compression of intact rock. Properties obtained are uniaxial compressive strength (qu) and elastic modulus of intact rock (ER). Poisson’s ratio may also be determined. Uniaxial compressive strength is used directly in the most widely applied design methods for evaluating unit side resistance, unit base resistance, and limiting pres- sure under lateral loading. Modulus of intact rock is not used directly, but rather with other rock mass characteris- tics to evaluate rock mass deformation modulus (EM). Other laboratory tests applicable to rock-socket design in- clude the splitting tensile strength (used for side resistance in limestone) and the point load strength (an index of com- pressive strength). Direct shear testing is used to assess shear strength of rock mass discontinuities and can be used to test shear strength of rock/concrete interfaces. Slake durability is used to assess potential for rapid degradation and smearing of weak rocks during construction of rock sockets. Rock mass classification systems have useful applications in foundation design. The Rock Mass Rating (RMR) as given by Bieniawski in Engineering Rock Mass Classifications (1989) incorporates the most important rock mass character- istics (including rock quality designation) that control the strength and deformability of a rock mass. The RMR is use- ful as an overall indicator of rock quality and suitability as a founding material, and is the basis for correlations to rock mass strength and modulus. Approximately one-half of the states responding to the survey reported using RMR in con- nection with rock-socket projects. The GSI introduced by Hoek et al. in Support of Underground Excavations in Hard Rock (1995) can be evaluated on the basis of RMR and is also correlated directly with rock mass strength, through the Hoek–Brown strength criterion, and rock mass modulus of deformation. GSI is now being used in geomechanical models for bearing capacity in rock and for evaluation of limiting lateral pressure for shafts in rock under lateral loading. In situ testing in rock is used primarily to obtain rock mass modulus (EM). Pressuremeter (PMT) and borehole jack are the methods being used. Modulus values obtained by PMT are affected by the scale of the test relative to the scale of rock mass features (discontinuity spacing and orientation) and may or may not be representative for the purpose of foundation analysis. The principal use of rock mass modulus is in analyzing axial and lateral load deformation response of rock sockets. There are several published p-y curve criteria for laterally loaded shafts that incorporate modulus as deter- mined by PMT. The issue of whether the modulus values from PMT are the most appropriate requires further research. Table 15 in chapter two summarizes the rock mass properties required for design of rock-socketed shafts. 98 DESIGN FOR AXIAL LOAD Methods for predicting the behavior of rock sockets under axial loading have developed considerably since the 1970s. The literature review showed that axial load transfer is rea- sonably well understood in terms of its basic mechanisms. Effects of interface roughness, socket-length-to-diameter ratio, modulus ratio, and other variables have been studied analytically and experimentally, providing a broad under- standing of the underlying concepts. Although design meth- ods do not incorporate all of the governing parameters explicitly, understanding the underlying mechanics is useful in many ways, including to provide a framework for under- standing the limitations of empirical design methods. Specific methods for predicting side and base capacities must be in a form that matches the level of knowledge of the ground conditions and that is based on commonly measured rock and intermediate geomaterials (IGM) properties. Chap- ter three of this synthesis provides a summary and review of available methods and it is shown that conservative, reliable, first-order estimates can be made for design values of side resistance on the basis of uniaxial strength of intact rock. Geomaterial-specific methods are presented for Florida lime- stone, residual Piedmont soils (cohesionless IGMs), and weak argillaceous rocks (cohesive IGMs). A method based on direct correlation to Texas Cone Penetration Test results illustrates how some agencies use in-house methods. For base capacity, a variety of methods have been pro- posed in the literature. Because several modes of failure are possible depending on structural characteristics of the rock mass, no single equation is applicable to all conditions. Furthermore, few studies have been conducted comparing proposed bearing capacity models with measured base ca- pacities on socketed shafts loaded to failure. A 1998 study by Zhang and Einstein provided a first-order approximation of unit base resistance from uniaxial strength of intact rock, based on a limited database of field load tests. For intact rock, a conservative upper-bound unit base resistance can be taken as 2.5 times the uniaxial compressive strength. Two methods given in the Canadian Foundation Engineering Manual and incorporated into current AASHTO specifications are recommended for horizontally jointed sedimentary rocks. For highly fractured rock masses, a lower-bound estimate of ultimate bearing capacity can be made in terms of RMR or GSI. Analytical methods for predicting axial load-displacement of rock sockets are needed to design shafts to limit settlement and to determine the percentage of load carried by base resis- tance under service load conditions. Methods based on elas- tic and elastoplastic finite-element modeling are available in the form of charts. Although these methods are useful, they are cumbersome. Simple closed-form solutions that are implemented easily on a spreadsheet are presented. Both approaches require knowledge of the rock mass modulus.

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

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

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

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 360: Rock-Socketed Shafts for Highway Structure Foundations explores current practices pertaining to each step of the design process, along with the limitations; identifies emerging and promising technologies; examines the principal challenges in advancing the state of the practice; and investigates future developments and potential improvements in the use and design of rock-socketed shafts.

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