1. Chapter 3: Application of Reliability Based Design to everyday geotechnical problems

2. What did we do ?
Why did we do it ?
What did we find ?
Practical RBD

3. What did we do ? What did we do ?
Chose 7 simple structures (Orr, 2005)
One soil type (sand, f’k = 32o, gk = 20kN/m3)
Found Eurocode-compliant solution
Determined b (FORM and Monte Carlo)
Calculated FoS (Mean & characteristic values)
Repeated for range of parameters & CoV
What did we do ?

4. Footings
Single pile
Retaining walls 4 3 5
What did we do ? 1 2 6

5. What did we do ? Soil Type Non-cohesive sand:
f’k = 32o, log-normal, CoV=0.1
gk = 20kN/m3, normal, CoV=0.05
rf,g = 0.2
What did we do ?

6. What did we do ? Loading: Gk fixed value, = mean
Qk log-normal CoV = 0.25
Wk Gumbel CoV = 0.50
What did we do ?

7. Why did we do it ? Why did we do it ?
Ease of application of RBD methods
Check reliability of Eurocode compliant designs
Variation in reliability index with:
variation in parameters f’ and g
variation in CoV
Compare with working stress design
Why did we do it ?

8. What did we find ? What did we find ? b Reliability Ind.
b values 3.2 – 3.7 (< 3.8)
b FORM ~ b M/C
b reasonably constant for all types of structure
Global FoS
FORM
M/C
Mean
Char
0. Strip vertical load
3.49
3.45
5.18
2.50
1. Square vertical load
3.51
3.46
4.86
2.40
2. Square inclined load
3.69
3.58
6.58
2.60
3. Pile
3.36
3.35
2.76
1.73
4. Gravity wall
3.33
3.3
6.88
3.12
5. Cantilever wall
3.40
3.39
2.34
1.63
6. Anchored wall
3.24
3.23
1.43
1.25
What did we find ?

9. What did we find ? What did we find ?
FoS determined using mean values too high
FoS using charac-teristic values about right
b Reliability Ind.
Global FoS
FORM
M/C
Mean
Char
0. Strip vertical load
3.49
3.45
5.18
2.50
1. Square vertical load
3.51
3.46
4.86
2.40
2. Square inclined load
3.69
3.58
6.58
2.60
3. Pile
3.36
3.35
2.76
1.73
4. Gravity wall
3.33
3.3
6.88
3.12
5. Cantilever wall
3.40
3.39
2.34
1.63
6. Anchored wall
3.24
3.23
1.43
1.25
What did we find ?

10. What did we find ? FORM gives similar results to Monte Carlo
b values bit low but generally OK
b values consistent for all problem types
FoS highly variable – poor measure
What did we find ?

11. What did we find ? Characteristic values: FORM design point values:
QHk = 638 kN
QVk = kN
QHk = 400 kN
GVk = kN +
310 kN
QVk = kN
What did we find ?

12. What did we find ? FORM gives similar results to Monte Carlo
b values bit low but generally OK
b values consistent for all problem types
FoS highly variable – poor measure
DA B* will give different eccentricity to DA1
What did we find ?

13. Effect of varying f’ and g
What did we find ?

14. Effect of varying CoV
What did we find ?

15. What did we find ? FORM gives similar results to Monte Carlo
b values bit low but generally OK
b values consistent for all problem types
FoS highly variable – poor measure
DA B* will give different eccentricity to DA1
b values consistent for range of material properties
b values below target for high CoV of material properties
Designing for normal FoS may be unreliable
What did we find ?