Assessing the value of information to design site investigation and construction quality assurance programs - Risk and Reliability in Geotechnical Engineering - page 510

2.00

Marine/
Alluvial

Target reliability= 99.4%

1.75

1.50

1.25

Marine

Alluvial

1.00
0.00

0.05 0.10
Axial capacity coeceint of variation (d)

0.15 0.20 0.25

Figure 13.35 Relationship between additional partial safety margin required to achieve a target reliabilty and
coefficient of variation due to spatial variability in axial pile capacity in different offshore fields.

with alluvial and marine deposits has the greatest variability and smallest horizontal correla-
tion distance ( Figure 13.34).
It is common in offshore practice to not have soil borings exactly at the location of struc-
tures because locations and configurations for facilities often change during the design due
to other considerations (e.g., reservoir engineering) and because new facilities are added in
the future. The consequence of not having a site-specific boring to design a pile foundation
is increased uncertainty due to spatial variability in pile capacity. If a target reliability is
desired, then this reliability can be achieved when there is additional uncertainty by increas-
ing the factor of safety with an additional partial safety factor or decreasing the resistance
factor with an additional partial resistance factor (e.g., Gilbert et al. 1999a, b). Figure 13.35
shows how this additional partial safety factor increases with an increasing coefficient of
variation in the axial capacity due to spatial variability.
The value of information from drilling a new soil boring quantifies the expected benefit
of this boring in reducing uncertainty in the foundation design. This value of information
is shown in Figure 13.36, represented as an expected savings in pile length, versus the dis-
tance to an existing boring. The greatest value from a new soil boring is in the most variable
geologic setting (i.e., “Marine/Alluvial” in Figure 13.36). The value is smallest when the

25

Marine/Alluvial

20

Alluvial

15

10

Marine

5

0

0 500 1000

Distance from existing boring (m)

1500 2000

Figure 13.36 Value of information for new boring versus distance from existing boring in designing axially
loaded pile in different offshore fields.