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