Verification of geotechnical reliability using load tests and integrity tests - Risk and Reliability in Geotechnical Engineering - page 542

0.010

Prior inverted gamma distribution
k = 6.3, v = 906.3
Updated inverted gamma distribution
k = 8.3, v = 1056.3

0.008

0.006

0.004

0.002

0.000

0 100 200 300 400 500 600

Mean thickness of toe debris (mm)

Figure 14.8 A comparison between updated and prior distributions of mean toe debris thickness.

14.6.4 updating reliability of piles based on interface coring tests
Having obtained the updated distributions of the occurrence probability and thickness
of toe debris based on the tests, the reliability of the piles can be updated. The follow-
ing load statistics are adopted: λ QD = 1.08, λ QL = 1.15, γ D = 1.25, γ L = 1.75, COV QD = 0.13,
COV QL = 0.18, and Q D / Q L = 2.0. These statistics are similar to those in Section 14.5.2.
When the soil shaft resistance, the rock shaft resistance, and the end bearing are all consid-
ered, the bias factor and COV of the pile capacity are assumed as λ R = 1.15 and COV R = 0.17
following McVay et al. (1998). The test piles in the calibration exercise are assumed to have
been constructed properly. If excessive toe debris exists, then λ R will decrease due to the
reduction in the pile capacity.
Ideally, the effect of toe debris on the behavior of axially loaded, large-diameter bored
piles founded on rock should be studied by systematic field tests. Such tests will be costly,
and numerical modeling is often resorted to instead. Zhang et al. (2006a) reported the
behavior of large-diameter bored piles with different lengths and toe debris of varying thick-
ness using a nonlinear finite-element program. The relationships between the applied load
and the pile-head settlement for piles of three lengths (25, 50, and 75 m) and three diameters
(1.2, 1.5, and 2.0 m) were obtained. Based on these relationships, the pile capacity reduction
factors defined by Equation 14.27 can be obtained. Figure 14.9 shows the reduction factors
plotted against the toe debris thickness. As expected, the reduction factor decreases with
the toe debris thickness. The toe debris has a significant effect on the capacity of the 25 m
long piles, but a minor effect on the 75 m long piles. Obviously, when a pile is very long, the

Table 14.7 Summary of updated items in three cases

PDF of occurrence probability
of toe debris, f (p d )

PDF of toe debris
thickness, f (t) Case Assumptions

Case 1 No toe debris detected Updated Not updated

Case 2 Toe debris detected

but not repaired

Updated Updated

Case 3 Toe debris detected

and repaired

p d = 0 , x = 0 Not relevant