Environmental Engineering Reference
In-Depth Information
3.5
3.0
Factor of safety commonly
used for design verified by
conventional load tests
2.5
2.0
1.5
All tests positive
Number of positive tests out of 4 tests
Number of positive tests out of 3 tests
Number of positive tests out of 2 tests
Number of positive tests out of 1 test
1.0
0.5
0.0
0
1
2
3
4
5
Number of positive tests
Figure 14.4
Factor of safety for single driven piles that are required to achieve a target reliability index of
2.0 after verification by proof load tests.
The updating effects of the majority of the proof tests conducted to twice the design load
and the tests in which the piles have a mean measured capacity of twice the design load
are different.
Table 14.4
compares the updated statistics of the two types of tests. For the
proof tests that are not conducted to failure, the updated mean bearing capacity ratio and
β values si
g
niicantly increase with the number of tests. For the tests that are conducted to
failure at
x
= 1. 0 the updated mean value will approach 1.0 and the updated COV value
will approach the assumed within-site COV value of 0.20 as expected when the number of
tests is sufficiently large. This is because test outcomes carry a larger weight than that of the
prior information as described in
Equations 14.13
and
14.14
.
14.5.3 accuracy effect of design methods
The example in the previous section focuses on a single design method. One may ask if the
observations from the example would apply to other design methods.
Table 14.5
presents
Table 14.4
Comparison of the updating effects of proof tests that pass at
x
T
=
1.0 and
tests in which the test piles fail at a mean bearing capacity ratio of
x
T
=
1.0
Tests that pass at x
T
=
1.0
Tests that fail at xT
T
=
1.0
Number of
tests
Mean
COV
Mean
COV
β
β
0
1.30
0.50
1.49
1.30
0.50
1.49
1
1.59
0.41
2.23
1.06
0.27
1.89
2
1.68
0.39
2.45
1.04
0.24
1.98
3
1.74
0.38
2.58
1.04
0.23
2.02
4
1.77
0.38
2.67
1.03
0.22
2.04
5
1.80
0.37
2.74
1.03
0.22
2.05
10
1.89
0.36
2.94
1.02
0.21
2.09
Note: Mean and COV of the bearing capacity ratio
x
.
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