Verification of geotechnical reliability using load tests and integrity tests - Risk and Reliability in Geotechnical Engineering - page 522

Verification of geotechnical reliability
using load tests and integrity tests
Limin Zhang
14.1 IntroDuCtIon
Pile tests are an important means to cope with uncertainties in the design and construction
of pile foundations and avoid the use of excessively conservative designs. Both quantitative
tests, such as static or dynamic load tests, and qualitative integrity tests, such as coring
examination and cross-hole sonic logging, are routinely employed. Pile tests serve several
purposes. Some of their functions include verification of design parameters (this is especially
important if the geotechnical data are uncertain), establishment of the effects of construc-
tion methods on foundation capacities, meeting regulatory requirements, and provision of
data for the improvement of design methodologies in use and for research purposes (Zhang
2004). In addition to these functions, savings may be derived from pile tests. With pile tests,
lower factors of safety (FOSs) or higher-strength parameters may be used. Hence, there are
cost savings even when no changes in the design are made after the tests. For example, the
U.S. Army Corps of Engineers (USACE) (1993), the American Association of State Highway
and Transportation Officials (AASHTO) (1997), and the Geotechnical Engineering Office
(2006) recommend the use of different FOSs depending on whether load tests are carried
out or not to verify the design. The FOSs recommended by the USACE are shown in Table
14.1 . The FOS under the usual load combination may be reduced from 3.0 for designs based
on theoretical or empirical predictions to 2.0 for designs based on the same predictions that
are verified by a sufficient number of proof load tests.
Engineers have used proof pile tests in the allowable stress design (ASD) as follows. The
piles are sized using a design method suitable for the soil conditions and the construction
procedure for the project considered. In designing the piles, an FOS of 2.0 is often used when
proof load tests are scheduled. Preliminary piles or early test piles are then constructed,
necessary design modifications made based on construction control, and proof tests on the
test piles or some working piles conducted. If the test piles do not reach a prescribed failure
criterion at the maximum test load (i.e., twice the design load), then the design is validated;
otherwise, the design load for the piles must be reduced or additional piles or pile lengths
must be installed.
Integrity tests are also performed on selected piles to detect defects such as toe debris,
short piles, voids, honeycombing, cracks, cavities, and necking. If the detected defect sizes
are larger than the tolerable defect sizes, the defects need to be repaired or the defective
piles need to be replaced. Whether actions are taken or not after the tests, the information
obtained from these tests will reduce the uncertainties involved in the design variables, such
as occurrence probability of defect and defect size. As the distributions of these variables
affecting the reliability of the pile foundations are updated using the additional information
from the tests, the reliability of the pile foundations can be updated.
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