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Foundation of Bedrock 2
- Thread starter akvargas
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My worry as geotechnical engineering (with our COV values of 30%) progresses to "codes" - that if you are wholly and totally within the code and something goes wrong . . . who would be at fault? The code committee? We all know it is the engineer - the lawyers would say - your judgment knew that the code was deficient or wrong and what did you do?
bigH said:
Could someone blame you if you can show you followed rigorously the law? If something goes wrong then it is the unfavourable fate, tough luck, Murphy's law, you name it. Of course you have the obligation to be updated on the state of the art and nobody will object if, by sound technical reasoning, you'll use a more conservative approach. A more conservative approach means higher construction costs so you should always justify that.
What parameter the 30% COV refers to?
See Tables 8.1 and 8.2 in Baecher and Christian's "Reliability and Statistics in Geotechnical Engineering (Wiley, 2003)
As Examples:
1. CBR: 17 - 58%
2. Angle of Friction in clays - 12 - 56%
3. Cohesion (undrained sands - 25 - 30%
4. Consolidation Coefficient - 25 - 100%
5. Liquid limit - 2 - 48%
6. Moisture Content - 6 - 63%
7. Permeability - 200 - 300 %
8. Undrained Shear Strength Clay Triaxial - 5 - 20%
9. Su (index Su) 10 - 35%
10. OCR - 7 - 30%
11. Ratio of Su/p' 5 - 15% (a good one)
As Examples:
1. CBR: 17 - 58%
2. Angle of Friction in clays - 12 - 56%
3. Cohesion (undrained sands - 25 - 30%
4. Consolidation Coefficient - 25 - 100%
5. Liquid limit - 2 - 48%
6. Moisture Content - 6 - 63%
7. Permeability - 200 - 300 %
8. Undrained Shear Strength Clay Triaxial - 5 - 20%
9. Su (index Su) 10 - 35%
10. OCR - 7 - 30%
11. Ratio of Su/p' 5 - 15% (a good one)
Ah great BigH, we are talking the same language here.
The Eurocode 7 has developed a way to use cautious representative values of the geotechnical parameters, the so called 'characteristic values'.
These are a function of the size of foundation, the size of the dataset and the variability or standard deviation or COV.
The EC7 tells you which is the appropriate point-estiamte value for the whole distribution; with a 30% COV and a small dataset and small foundations it may become pretty low and cautious. You use that, you're pretty safe, dude.
If anything goes wrong then something was wrong in sampling, or correlation, or testing, or in adopting an initial value which wasn't representative.
Engineering judgment keeps being king.
The Eurocode 7 has developed a way to use cautious representative values of the geotechnical parameters, the so called 'characteristic values'.
These are a function of the size of foundation, the size of the dataset and the variability or standard deviation or COV.
The EC7 tells you which is the appropriate point-estiamte value for the whole distribution; with a 30% COV and a small dataset and small foundations it may become pretty low and cautious. You use that, you're pretty safe, dude.
If anything goes wrong then something was wrong in sampling, or correlation, or testing, or in adopting an initial value which wasn't representative.
Engineering judgment keeps being king.
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