FoS used without consultation 1

[Rakra]

We prepared a geotechnical engineering report and in it we put some general blurb on pile design definitions and methods which the client adopted as the FoS / provided to third parties for use in pile / foundation design. We were not aware and have only just been made aware after questions were raised by the third parties. The statement in the report was this:

Allowable pile capacity is the minimum of the allowable structural capacity of the pile and the allowable geotechnical capacity of the pile including negative skin friction and settlement effects. [highlight #FCE94F]The allowable pile capacity is the ultimate capacity of the pile divided by an appropriate factor of safety, usually not less than 2.0.[/highlight]

The ultimate load capacity of a pile consists of two parts. One part is due to friction called skin friction (fs) or shaft friction or side shear, and the other is due to end bearing at the base or tip of the pile (fb). Both parts are dependent on geotechnical soil parameters derived from field tests such as SPTs. Partial FoS can also be applied to either component to reach the ABC value.

The FoS 2.0 was not recommended or suggested to be used. We had no idea it was used in design. Any thoughts on this?

 

[Smoulder]

Was it your job to provide design parameters? Why don't you use limit state like the piling code? Doesn't look good from here but I don't have the full story.

 

[EireChch]

Sorry, what are you saying? that your report was passed on and the pile contractor took FOS as 2 in their design?

I agree you did not specify what FoS to be used so the ultimate responsibility lies with the contractor??

 

[Smoulder]

@eirechch won't be that simple if there's enough cost for lawyers to get involved.

- if they claimed their full fee they might be deemed to have completed scope of work.

- if scope was to provide design parameters it might be deemed 2.0 was the recommendation.

- if scope was to provide design parameters there might be a question of negligence or error not to provide to the governing code which is limit state.

- if they knew it was build only contact not D&C then the contractor might get to play dumb on design.

Etc etc etc. I'm not a lawyer but I've seen innocuous filler words cause headaches. Copy paste is great until there's a problem.

 

[EireChch]

Smoulder - dont disagree with anything youve said

 

[LittleInch]

Your issue here, from my point of view is those critical last five words "...usually not less than 2.0."

Delete those words and then say something like - The FoS needs to be determined by the pile designer taking into consideration a number of factors such as [then list them]. Say NOTHING about a number of even range of numbers. Or just add after 2.0, but can be up to 5?? depending on local circumstances.

However that horse has bolted here so that to me, especially for a client who isn't expected to know any different, unfortunately looks pretty much like a suggestion to me. Because you don't seem to have said it could be higher than 2, then I can see why an inexperienced client could look at that and say, well 2.0 it is then and run with it.

In reality is 2.0 a bad number in this location?
What are the practical issues here?
Any site survey is only as good as the location where you do it so even if they install piles with a FOS of 2 and yet the pile sinks then just blame ground conditions in the location or poor installation or materials.

What have the third parties been saying about it? That they think 2.0 is too low?
Time to call in the lawyers / insurance company if you're starting to get seriously worried.

All IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EireChch]

I know there is legal worms that can try pin this back on your report but ultimately, if someone does a design are they not the owner of that design and its inputs!!

Piling contractors always get geotech reports with parameters and recommendations and essentially do what they want!! most of the time they know more than us consultants, they own the design and its consequences.

 

[dik]

This could get messy... A factor of safety for soils of 2.0 seems light. Is it based on stress? or settlement? Are you comfortable with a FOS of 2.0?


is bad and the last part, 'usually not less than 2.0' in inappropriate, not specific, and should be removed from future reports. Is there anything that prevents the contractor from using 0.5, for example?

Do you have any provision in the report that requires the contractor to confirm the FOS used? If they used something less than 2 they should have checked with you, but may not be required to.

What factor of safety did they use?

What are the consequences of what they have done?

Just a couple of questions.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

Do you have any disclaimers in your report? Such as test results are representative of the information at that location only and that additional tests may be required?

Any third party limitations? or is the contractor not a third party?

Have you ever asked your company lawyer (if you have one) to review your 'boiler plate' report and provide any comments about possible problems?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[MTNClimber]

Has anything been installed? If not, provide a written communication that clarifies that your report does not recommend a specific factor of safety and that a factor of safety of 2 cannot be used unless a static load test program confirms the geotechnical capacity of the pile meets or exceeds twice the design load.

 

[Rakra]

Thanks everyone for the prompt responses, they've been helpful and highlight the need to be careful in what we report. The design is still underway and the structural engineers have adopted the FoS of 2.0 in their design but are now having to change their design if that is not the case.

Yes, we have a disclaimer in the report about the information being used by third parties and that ground conditions could change between locations.

To add on, we did state in the Conclusion of our report the following:

By way of conclusion, it can be seen that there are several high risk geotechnical conditions present
at this site, some of which are highlighted for further monitoring and/or investigations and are
summarised as follows:

• Presence of potential artesian conditions
• Occurrence of cobbles (>75mm size) and potentially boulder size (>300mm) particles at this
site.
• Highly variable subsurface profile with persistence occurrence of discrete silt containing
soil types occurring in the alternating sand and gravel dominate layers
• High seismic area
• Carryout more analytical lab testing to confirm chloride and sulphate readings

These risks highlight the need for adequate and through planning of foundation installation works
such that associated impacts on construction delivery and costs are minimised. Active supervision
by a suitably experienced geotechnical engineer is strongly recommended during foundation
construction. Given the high unsorted deposition of sediments across the site, where piled
foundations are nominated, pre-drilling at the proposed location is strongly recommended to
inform pile drivability, including preventing pile damage.

7.3 Limitations
The findings and data presented in this report are limited to the borings nominated by the client.
Professional judgment and historical reports have been used to deduce encountered ground
conditions. It should be noted that the borings may not represent potentially unfavourable
subsurface conditions between borings. The commentary presented in this report are applicable
only to this specific site. These data are not to be used or extended to other sites.

 

[LittleInch]

Well you can probably claim the ground is just different, but I think your unfortunate use of numbers in the original report when mentioning FoS is going to give someone sleepless nights.

That sound more like a FoS of about 4 or 5 given those conditions....
Unless the piles go to good solid bedrock I think they are going to be difficult to even get in the ground.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.