Above Ground Vertical Tank Foundation on Weak Grade. 3

[ABU312]

We are designing tank (Dia 20m -30m) foundations for a Petroleum product storage facility. the soil investigation revealed that the sub-grade is made of Pit(Organic matter) to a depth of 19-23m below existing ground level. we initially proposed two options thus: Piling or soil improvement by Vibro-flotation technique.
The client refused the two option base on cost consideration and suggested soil replacement to about 4m. our geotechnical expert advised that the pressure bulb analysis shows that the pressure at the bottom of replaced soil is still more than the bearing pressure of the pit.
The client insist that it has work for them elsewhere. However, they are willing to carryout gradual loading while observing for failure. and are also willing to manage the significant settlement expected. This we believe is not right.
I will appreciate any ideas.

Thanks.
Abu

 

[Ron]

Run like hell from this one. Do not put a vertical tank on poor soil conditions....leaning tower of Pisa.

Unless your client is a licensed professional engineer with the capability to accept responsibility for all design decisions, they should not be dictating your engineering judgment. Yes, it might have worked in another location. That could very well be anecdotal and they got lucky. The long term settlement of this tank could be tremendous.

If you continue with this, make sure your professional liability insurance premiums are paid.

 

[bimr]

Have you done a thorough analysis of the site with borings so that you are assured that the entire site has this pit material?

Regarding:

"The client insist that it has work for them elsewhere."

Have the client demonstrate where this has been accomplished by supplying project plans from one of these projects. You can review the documents and inspect the work.

Regarding:

"However, they are willing to carryout gradual loading while observing for failure. and are also willing to manage the significant settlement expected."

There is too much risk to the project schedule with this proposal. This gradual loading concept will not be acceptable because of the consequences of failure.

What about the risk to the project schedule if the foundation fails. You will probably lose 1-2 years in the project schedule and this delay and cost will unacceptable to the Project Manager.

 

[BigH]

Pit? Did you mean peat?? I am assuming "peat" - don't forget that when you replace peat (at a unit weight of about 70 pcf with sand or other better material (in your given replacement method), it weighs about 125 pcf - so it is just not the loading of the tank but also the loading due to the increased weight of the replaced material.

As peat "normally" consolidates fairly quickly, would he go for a modified replacement method (using fill and reinforcing it with grids - and much larger than the tank footprint) and then then the use of preloading an even larger area to consolidate the peat (and don't forget any secondary consolidation effects)?

You have indicated that this is in a petroleum products facility - that implies that there are other existing tanks. What did they do about founding these tanks?

I think that your use of piles is probably the most expedient and positive option. And, if he doesn't want to follow that option, then suggest he find another firm that will give him what he wants; and that his medical plan will cover a mental breakdown when his tank fails . . . . (yes, this is facetious).

 

[ABU312]

I appreciate all for your contributions.
I wish i it easy to convince my company to "walk away" from this job...

@BIgH,
Yes I mean Peat, and it is green field. it is actually the first facility that is going to be constructed in a new industrial layout. so no useful experience or reference there.

@bimr,
The client rep had always referred to his unwritten experience and believes that we are being too theoretical.

However, I am putting up a write up on why we would not agree with his engineering judgement and you all have given me some more guides.

Thanks
Abu

 

[BigH]

Why did they buy the site??? With all that peat and thinking that piles aren't necessary - very serious error, I would presume . . . .

 

[MJCronin]

I sense the presence of MBAs....

Remember, loud hysterical client demands uttered verbally during meetings are later remembered only as vague suggestions in a courtroom.....