Voids Beneath Bottom Plates
Voids Beneath Bottom Plates
(OP)
How do you address voids underneath the bottom plate. Should we cut the plate and refill the void with compacted soil? Is there an acceptance or rejection limit for voids beneath tank floors? Your experience and expert advice is greatly appreciated.





RE: Voids Beneath Bottom Plates
Depends, always depends. The size of the tank and the size of the voids are important. A large tank with significant static pressure will exert a significant bending stress on the floor designed to take no bending stresses. Remember, the API 650 specifically stipulates the full support of the flat bottom. There is a good chance the fillet weld will give in and the floor plate will break. As always, depending on the size, you might dig under the empty tank and use some pressure filling which hardens to similar strength or consistency like the rest of soil. Lift the tank or cut the plate if your calculations show the risk of excessive deformation and weld overstress. Can you afford a leaking tank? if yes, then ignore the leak.
If you need specific help, then you might want to share with us some secrets of your tank like size, content, design standard and conditions, estimated size of voids and some explanation of the origin of those voids that were not supposed be there.
Cheers,
gr2vessels
RE: Voids Beneath Bottom Plates
If the floor fails the 'Fat Guy' test, look into cutting some holes and filling the voids with "Flowable Fill"; it is a slurry of sand, water and a LITTLE cement. Flowable has a compressive strength a little higher than well-compacted soil, but is easy to dig into, and like all cenent mixtures is somewhat alkaline so it is 'friendly' to carbon steel. Any concrete supplier will know what Flowable is, and it is MUCH cheaper than properly compacting dirt.
The fix is pretty quick and fairly cheap; do not take the risk of refilling a tank that has failed the Fat Guy test. You may have the floor rip open at a lap seam and "catastrophically deinventory into the local aquifier". $5-15K repair vs risk of $0.5-20MM environmental damages. Not a risk a prudent engineer would sign off on.
RE: Voids Beneath Bottom Plates
When I walk across a tank roof it flexes as well which is quite alarming....
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Voids Beneath Bottom Plates
a) Is this the right way to address the issue?
b) Referring to API 653 Appendix B, it mentions on local bulges and depressions, should we apply the rules only after the plate has undergone permanent deformation? Appreciate if you can share some lights on this. Thanks