Molasses tank 1

[jo2cal]

I am designing a foundation of molasses tank (1500 kg/m^3) 12.0m dia by 14.0m height.The allowable soil bearing capacity of 150kN/m^2.I tried the ring beam option but ,the soil alone cannot support the weight of the product (206Kn/m^2).Do I have to build the foundation made of thick concrete pad to distribute the load? I am looking for answers to minimise the cost of the foundation. Thanks for comments.

 

[fattdad]

Who developed the "allowable bearing capacity"? It seems that there is some mis-communication in that the geotechnical engineer needs to give you direct recommendations on whether the ground can support the loads required by the project (i.e., the areal loading of the product). The "foundation" design really relates to the piece of concrete that's holding up the tank wall and has nothing to do with the support of the product load.

Then again, if the geotech feels this needs to be pile supported, that's another matter entirely!

f-d

¡papá gordo ain’t no madre flaca!

 

[Zambo]

f-d isn't this what jo2cal is saying. He has tried the annular foundation option which will support the tank walls, but the allowable bearing capacity of the soil does not support the weight of the product i.e 150KN/m2 against 206KN/m2.

Now he wants to know whether he can spread the load over a wider area using a pad foundation under the full area of the tank and extending for the required dimension beyond the tank wall.

The pad (or raft) foundation is one option. Another would be driven concrete piles.

 

[fattdad]

If possible, provide in this forum results from the subsurface exploration - soil type and N-values. It would be interesting to see whether there is any justification to complicate a ground supported tank foundation, by limiting the "bearing capacity" to something less than the tank loading.

f-d

¡papá gordo ain’t no madre flaca!

 

[VoyageofDiscovery]

You have to design this foundation for the 150 KPa. It is not a matter of minimizing the cost, it will be what it needs to be if it is spread footing design.

Piles is another option but it will be more expensive as there is more labour and design.

HTH

VOD

 

[GeoPaveTraffic]

I see three options:

1. Use piles as VOD indicates - On the plus side the foundation should perform well, however as indicated it is likely to be relatively expensive.

2. Use spread footings at 150kg/m^3 - On the plus side the foundation is likely to perform well (assuming the allowable bearing pressure was correctly determined), however, the foundation will be much larger than the tank and the thickness of the foundation will need to be quite large to create a stiff foundation to spread the load. Also, likely to be relatively expensive.

3. Look at the soil conditions and the loading (I think this is what Fattdad was trying to get at). Can the tank be stage loaded and the soil allowed to consolidate to gain strength? Was the allowable bearing pressure determined on the basis of allowable settlement instead of bearing failure? A 12m diameter tank is not an extreamly common design element, talk with the geotechnical engineer, look at the way the tank will be used and how the soil and the structure will interact. I would be surprised if there is not another option besides 1) and 2).

 

[fattdad]

Further comment: Usually geotechnical engineers assign bearing pressure based on total anticipated settlement, typically limited to 1 in or less. For a ground-supported tank, this is not reasonable. You will likey realize several times this amount - and that's not bad.

If you have a geotechnical engineer on board that has no experience with the performance of ground-supported tanks this is the real problem. If the "foundation" design is based on the performance of the ring-wall and the "bearing capacity" based on typical considerations for spread footings you will not get to the answer that you need. Rather, you need to look at the response of the ground to the areal load from the product.

Let's consider "bearing capacity". If you were to run the numbers on the rational bearing capacity formula, you would find that the bearing capacity for a 40-ft diameter foundation would be MUCH greater than the bearing capacity of a 2 ft wide "ring-wall" foundation. This has to do with the lenght of the shearing surface. If you consider that the shearing surface for the 40 ft diameter tank extends to depths in excess of 40 ft (perhaps 60 ft), you have to even wonder whether the subsurface exploration has these data. Heck for all we know there is bedrock at your site below the depth of 25 ft!

There is no way that this group can even comment on the adequacy of the geotechnical recommendations with out you providing some sense of the subsurface conditions at the site. Similarily, we can't really comment on whether you "should" make a larger diameter mat to spread out your product load or use a deep foundation. Too much speculation!

I would expect that the geotechnical engineer has just given you a capacity based on 1 in of settlement and you in turn need to assess to what extent you are willing to let the center of the tank deform more than 1 in.

Good luck.

f-d

¡papá gordo ain’t no madre flaca!