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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
