Geoscience Reference
In-Depth Information
10.17 Project reliability
•
Reliability is based on the type of project and structure.
•
Lowest value of strength is not used in design unless only limited samples.
•
Design values are references to a normal distribution as this is what is applied to
steel and concrete design, and many codes apply this normality concept also to
soil and rock. As commented above non normality of soils and rock applies.
•
Ultimate conditions (strength criteria) and serviceability (deformation criteria)
requires a different acceptance criterion. The literature is generally silent on this
issue and a suggested criteria is provided in the table.
Table 10.17
Ground conditions acceptance based on type of project.
Type of
Typical design values
Comment
project
Ultimate
Serviceability
Structure
1%
5%
5% for a normal distribution is likely to be 10% to 30% for a
lognormal distribution.
Road
5%
10%
10% for a normal distribution is likely to be 30% to 50% for a
lognormal distribution: 20% is typically close to the median
value.
•
Correct Distribution needs to be applied, ie non normal.
•
At interfaces such as embankments next to a bridge structure then tighter controls
would be required. This would be 1% to 5% serviceability for major to minor
roads, respectively.
•
If the above is translated into a physical criteria, then this in terms of absolute
conditions, eg if 10% design is used then no more than 1m in 10m of road length
would be above a criteria of say 50mm acceptable movement.
10.18 Road reliability values
•
The desired road reliability level is based on the type of road.
•
A normal distribution is assumed, and comments on the non normality of soil and
rocks apply.
Table 10.18
Typical road reliability levels.
Road class
Traffic
Project reliability (typical)
Highway
Lane AADT
>
2000
90-97.5% (95%)
Lane AADT
2000 (rural)
85-95% (90%)
≤
Main roads
Lane AADT
500
85-95% (90%)
>
Local roads
Lane AADT
500
80-90% (85%)
≤
•
These values do vary between road authorities.
Search WWH ::
Custom Search