Verification of geotechnical reliability using load tests and integrity tests - Risk and Reliability in Geotechnical Engineering - page 540

(a) 40

35

30

25

20

15

10

5

0

>300

<100 100-200 200-300

ickness of toe debris (mm)

(b) 0.015

Data

Inverted gamma distribution
Mean = 145, COV = 0.40

0.012

0.009

0.006

0.003

0.000

0 100 200 300 400 500 600

Mean thickness of toe debris (mm)

Figure 14.6 (a) Histogram of thickness of toe debris; (b) probability density of mean thickness of toe debris.

To obtain the information on toe debris thickness, as-built information of 263 bored piles
constructed at five different sites in Hong Kong in 2000 is adopted for analysis. The data
were reported by the Hong Kong Construction Association (HKCA 2003) and the study
was conducted in response to recent bored pile cases where confirmatory interface coring
revealed pile toe irregularities. The results are summarized in Figure 14.6a . About 70% of
toe debris found is less than 200-mm thick. The thickness of toe debris in 20 piles is larger
than 300 mm. Based on these data, a probability density of the mean thickness is plotted
in Figure 14.6b . Through the Kolmogorov-Smirnov goodness-of-fit test at a significance
level of 5%, an inverted gamma distribution as shown in Equation 14.23 with parameters
κ = 6.3 and ν = 906.3 is confirmed to fit the frequency diagram of the mean thickness. The
corresponding mean and standard deviation of the inverted beta distribution are 145 and
58 mm, respectively.

14.6.3 updating the priors based on interface coring tests
After some coring tests have been conducted, information on the occurrence probability and
thickness of toe debris can be obtained. In a field study, 67 bored piles 2.5 m in diameter