PILES AND PILING FORMULAS 109thickness of the consolidating soil layers penetrated by the piles, and their
undrained shear strength, respectively. Such forces as Qgdcould only be
approached for the case of piles driven to rock through heavily surcharged,
highly compressible subsoils.
Design of rock socketsis conventionally based on
(4.13)whereQdallowable design load on rock socket, psi (MPa)
dssocket diameter, ft (m)
Lssocket length, ft (m)
fRallowable concrete-rock bond stress, psi (MPa)
qaallowable bearing pressure on rock, tons/ft^2 (MPa)
Load-distribution measurements show, however, that much less of the load
goes to the base than is indicated by Eq. (4.6). This behavior is demonstrated
by the data in Table 4.1, where Ls/dsis the ratio of the shaft length to shaft
diameter and Er/Epis the ratio of rock modulus to shaft modulus. The finite-
element solution summarized in Table 4.1 probably reflects a realistic trend if
the average socket-wall shearing resistance does not exceed the ultimate fR
value; that is, slip along the socket side-wall does not occur.
A simplified design approach, taking into account approximately the compati-
bility of the socket and base resistance, is applied as follows:
1.Proportion the rock socket for design load Qdwith Eq. (4.6) on the assump-
tion that the end-bearing stress is less than qa[sayqa/4, which is equivalent to
assuming that the base load
2.CalculateQbRQd, where Ris the base-load ratio interpreted from Table 4.1.
3.IfRQddoes not equal the assumed Qb,repeat the procedure with a new qa
value until an approximate convergence is achieved and
The final design should be checked against the established settlement tolerance
of the drilled shaft.
Following the recommendations of Rosenberg and Journeaux, a more realis-
tic solution by the previous method is obtained if fRuis substituted for fR. Ideally,
fRushould be determined from load tests. If this parameter is selected from data
that are not site specific, a safety factor of at least 1.5 should be applied to fRuin
recognition of the uncertainties associated with the UCstrength correlations.*
FOUNDATION-STABILITY ANALYSIS
The maximum load that can be sustained by shallow foundation elements at
incipient failure (bearing capacity) is a function of the cohesion and friction
angle of bearing soils as well as the width Band shape of the foundation.
qqa.Qb( /4)ds^2 qa/4].Qd
dsLsfR
4ds^2 qa*Rosenberg, P. and Journeaux, N. L., “Friction and End-Bearing Tests on Bedrock for High-
Capacity Socket Design,” Canadian Geotechnical Journal, 13(3).