Geoscience Reference
In-Depth Information
•
The stress of the underlying layer also needs to be considered.
•
Factor of safety needs to be applied and is the same for piles and shallow
foundations.
Table 22.10
Ultimate bearing capacity with failure in flexure or punching.
Failure mode
Strength range
Design ultimate strength
Flexure
Flexural strength
∼
5% to 25% UCS
Use 10% UCS
Punching
Tensile strength
50% flexural strength
Use 5% UCS
∼
22.11 Factors of safety for design of deep foundations
•
The factor of safety depends on:
- Type and importance of structure.
- Spatial variability of the soil.
- Thoroughness of the subsurface program.
- Type and number of soil tests performed.
- Availability of on site or nearby full - scale load test results.
- Anticipated level of construction inspection and quality control.
-
Probability of the design loads actually occurring during the life of the
structure.
Table 22.11
Typical factors of safety for design of deep foundations for downward loads (Coduto, 1994).
Classification of
Design life
Acceptable probability
Design factors of safety, F.S.
structure
of failure
Good
Normal
Poor
Very poor
control
control
control
control
10
−
5
Monumental
>
100 yrs
2.3
3.0
3.5
4.0
10
−
4
Permanent
25-100 yrs
2.0
2.5
2.8
3.4
10
−
3
Temporary
<
25 yrs
1.4
2.0
2.3
2.8
•
Monumental Structures are large bridges or extraordinary buildings.
•
Permanent structures are ordinary rail and highway bridges and most large
buildings.
•
Temporary structures are temporary industrial or mining facilities.
22.12 Control factors
•
The control factors referenced in the above table are dependent on the reliability of
data derived from subsurface conditions, load tests and construction inspections.
•
Examples of good and very poor control are:
-
Bored piles constructed with down the hole inspection for clean out and
confirmation of founding layers - good control.
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