Submerged Tank 1

[dik]

I've got a 6' dia tank X 20' long submerged vertically in soil. The x-section of the tank is about 28 ft^2 and the conc base I'm proposing is about 10'X 10'. Assuming the entire system is submerged, is it reasonable to assume that the concrete base has a density of 150-62.4 psf and the included weight of soil is (100 - 28) * (gammasoil-62.4)*20' to resist floatation? or would you neglect the weight of the soil? Assuming no friction along the length of the tank.

Dik

 

[TDAA]

It really is not a matter of friction of the soil vs. the tank. You have designed a base larger that the vault. In order for the base to move up (to any real degree) it will have to move the soil. Therefor, account for the submerged weight of soil to add a resistive force (height * area covered*unit weight).

Also, you mention only the base in the weight of concrete. Use the entire structure. Also, your resistive force due to the concrete does not need to subtract the weight of water. That is giving you the net force (Correct for the soil however). Callc the weight of water displaced, and the weight of you structureand soil over the extended footing. Thoose are the two to compare. Add an appropriate FS.

 

[dik]

Thanks, would you not consider the weight of concrete submerged in water, ie, it displaces its volume? Doesn't a cubic foot of concrete weigh approx 90 pcf under water?

Dik

 

[TDAA]

Yes, that is true, but you are looking at total weight vs. boyant force to determine the factor of safety. That boyant force is the 62.4 pcf you are subtracting off. Using the effective weight would be accounting for the water twice in the calculation.

 

[dik]

Thanks, Dik

 

[oldestguy]

For the soil resistance I usually take the submerged weight of soil within an inverted truncated cone whose bottom edge is at the edge of the footing. Side angle from vertical is taken at 20 degrees, but it depends on what backfill soil you have.

 

[dik]

Thanks oldguy... I treated the systema as a submerged right circular cylinder with a diametre equal to the base that was submerged in water...

Dik

 

[eric1037]

Sorry I didn't see this earlier.

The buoyant unit weight of soil is acutally the saturated unit weight of soil minus the unit weight of water. Since the saturated unit weight is defined as the unit weight of soil when it is saturated (obviously), the saturated unit weight should be slightly higher than the moist unit weight, so your calculations should be a bit conservative.

 

[Hookem]

Why take credit for the soil? If the soil is saturated, it offers no resistance to vertical movement of objects placed in it. Think of an air-filled object being in water and the force trying to push it up to the surface. Now, displace that water with a water-mud slurry. What do you get? Even more force trying to move the object to where nature is trying to equalize forces - at the surface. One may install dry soil, however one cannot state with certainty that the soil will stay dry. Just ask any long term household swim pool owner. When the pool is empty for maintanance and a big rain comes along and there is poor to no soil drainage - the pool floats!!!!!!!!!!!!!

 

[eric1037]

Hookem:

The flaw in your logic is that you assume that saturated soil offers no resistance to vertical movement. Saturated soil does indeed have strength in the vertical direction.

Ever get your foot stuck in the sand on the beach and try to pull it up without pointing your toes?

 

[PEinc]

I agree with eric1037. Hookem's swim pool analogy also does not make sense. The empty pool has no soil above it -neither dry soil nor buoyant soil. Therefore, the swim pool floats like a boat when the ground water table rises.