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