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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
