Slab / Mat Foundation

[TLHS]

In my region, which is reasonably high for seismic forces, it's pretty typical for small, slender tanks to end up on a slab foundation of some sort. It lets you use the full content weight against overturning and you aren't saving much concrete weight to go with a ringwall. Most designs that I've seen of similar installations seem to just finish the concrete flat, set the tank on it, and call it a day.

I've been looking harder at this, and some sources suggest impregnated fibre board, asphalt or a bitumen/sand mix as a topping on the concrete to lower risk of corrosion and/or make up tolerances. I've never seen this done in practice. Does anyone have experience with this installation methodology. Perhaps a detail they'd be willing to share? I'm slightly worried about water migrating under the tank when rain hits the foundation, since sloping away on the foundation area outside of the baseplate feels impractical on a single pour while still keeping the flatness requirements under the wall. However, introducing details that nobody else is using in an area is always going to get pushback, so I'm curious about practice in other areas.

I've done some creative concrete detailing on a larger tank that required a raft foundation for various reasons, but the kind of messing around that was necessary would not be cost effective on a 15ft diameter tank.

 

[HTURKAK]

Just finishing the concrete surface flat is not a good idea.. If the tank experience settlement , the center settlement will be 2.5-3 times of the edge . ( remember for a square raft , the center settlement is four times of the corner ). So the flat surface literally means cone down bottom in long term settlement .. The tank bottom and the surface of the raft should be cone up to compensate settlement and provide a minimum slope..


In past, i have always specified asphalt layer 50 mm or at least bitumen sand layer. and for the periphery ( under the shell ) mastic asphalt or bitumen impregnated board..It is better to provide grooves for leak detection also..

The following sketch is from [ EN 14015-2004] -- Specification for the design and manufacture of site built, vertical, cylindrical, flat-bottomed, above ground, welded, steel tanks .

 

[TLHS]

Thanks for the thoughts.

I'm not worried about differential settlement from the edge of slab to center of slab, it's a double reinforced mat, 2ft thick and it's a less than 15 ft diameter tank. I'll do an beam on elastic foundation analysis as I go forward, but it's going to be rigid enough to hold differential deflection from wall to center of tank to a pretty minimal value. I've also got a fairly firm soil that isn't going to see longer term consolidations.

Regardless, I don't have control over the floor orientation. It's a shop built flat bottomed tank.

If I combine bitumen sand/cold asphalt with grooves for drainage, won't the sand potentially fall in near the edge of the grooves and poorly support the tank. I'm concerned about getting several inches of unsupported floor plate. I've done grooves with vertical vessels, but they don't tend to have a flat bottom plate that bears on the grooved area.

 

[MJCronin]

TLHS ..

What is your proposed scheme for anchor bolts and anchor chairs for this tank in a seismic area ?

What is the aspect ratio for this small tank ?

API-650 has an appendix regarding seismic design of tanks and shell evaluation of "sloshing"

Asphalt impregnated fiberboard and installation issues are discussed here:

https://www.eng-tips.com/viewthread.cfm?qid=359944

https://www.eng-tips.com/viewthread.cfm?qid=84099

https://www.ast-forum.com/ast_forum_tree.asp?master=15884

Fiberboard installation details here:

https://www.ast-forum.com/pictures/datasheet of asphalt impregnated board.pdf

https://www.ast-forum.com/ast_forum_tree.asp?master=12152

Also, more here

https://www.engstack.com/questions/...on-of-atmospheric-storage-tanks-javv7lzrpfzt/

MJCronin
Sr. Process Engineer