tank bottom in submerged tank - rising water level

[oneintheeye]

this may be a mouth full but here goes.

Your designing a tank, smallish size. Using plastic soil conditions.

The tank imposes a surcharge on the soil.

For the structural design you would tank the weight of walls spread over the base area which say produces a pressure of 15 kn/m2 (again just pulling figures out of air).
Therefore the pressure is applied over slab area and forces calculated.

Now if the water table rises above base and provides uplift of 10kn/m2.
Now is the pressure used to calculate forces 10 + 15 or 15 as the water buoyancy takes some of the pressure off soil.
Or two load cases. 1) wall weight over slab 2) force due to uplift of water.

For a continuous base and wall things will get more detailed.

So how would people approach? I'm having doubts i do it the most economical and no one seems too convinced here.

 

[JStephen]

What size and kind of tank is it?

Normally, I would not assume that the weight of walls was spread over the bottom unless it was a very small tank or intentionally had the bottom stiffened to accomplish that.

With water around the tank, the downward force on the top of the foundation would be the same. You would need to check for uplift of the tank and check for overturning stability including the uplift effect. Also most tank bottoms are not designed to resist uplift forces.

 

[oneintheeye]

Its concrete and lets assume the tank is sufficiently small to be able to take plastic soil behaviour over slab area. Slab is flat and is suitable to take these forces.

I dont get what you mean by tank bottoms are not designed to resist uplift. The tank itself would resist uplift (i.e. not move) but the external water pressure will put forces on the bottom and walls.

 

[JoeTank]

If the water pressure exceeds the weight of the tank bottom, then you have a net uplift on the bottom slab. All this happens at the same time that external water pressure is pushing inward on tyhe tank walls.

Joe Tank

 

[oneintheeye]

yes i realise that. What i'm asking is do you add this uplift to the effective bearing pressure by the weight of the walls or not?

 

[JoeTank]

If you are assuming that the tank weight is spread out over the tank base area, then there is a water uplift pressure for which you will design the slab (after deducting for the slab dead load, of course). A flood condition around the tank will add directly to this uplift design load. Yes, I would include the water pressure uplift to the slab design.

Joe Tank

 

[oneintheeye]

would you not have a condition then though where the weight of the walls is taken up by the buoyancy of the water (or by the soil) meaning you only ever have one of these forces (soil or water) acting on tank base.

 

[Ussuri]

OK, having a think about this.

Pressure on the base is due to the weight of the walls.

A rising water table will reduce the weight of the tank.

Once the water table is high enough and the tank has floated you have a boat, subject to pure hydrostatic pressure.

To my mind as the water table rises your 15kN/m^2 reduces (eventually to zero when the tanks becomes neutrally buoyant) but the uplift pressure (hydrostatic) increases to a maximum of 10 kN/m^2 assuming this is depth of water required to displace the weight of the tank and make it float.

So everywhere in between is going to be a summation between the two cases.

As the weight of the base is not included when calculating the pressure (15kN/m^2) and the water table has rose the depth of the base you would have the 15kN/m^2 + the uplift component from the depth of base?

Maybe do a plot of one against the other?

I dont know if this is correct but it seems to be they way my mind thought about it.

 

[miecz]

The total pressure (soil+hydrostatic water) acting on the underside of the tank is due to the weight of the empty tank, 15 kN/m[²]. If the water table rises to 15 kN/m[²], then the tank is buoyant and the soil bearing pressure is zero. If the water table rises beyound that, then the tank will rise to the elevation that will result in a hydrostatic pressure of 15 kN/m[²];soil bearing pressure is still zero. If the water table provides a pressure of 10 kN/m[²], then the soil bearing pressure is 5 kN/m[²].

 

[JStephen]

"I dont get what you mean by tank bottoms are not designed to resist uplift."

On larger flat-bottom tanks, the bottom is a flexible thin steel plate or concrete slab that is supported by the subgrade. It acts as a membrane to retain the fluid, and does not act like a blind flange retaining the pressure.

For a cylindrical tank with flat bottom, partially submerged, the hydrostatic force upward on the bottom is exactly the same as the hydrostatic force downward on the part of the foundation directly under the tank. So you should never have a net increased load from surrounding water. You would have an uplift or partial uplift that would need to be accounted for.

 

[BigH]

JoeTank - a flooding condition - which is transient in nature would not necessarily lead to uplift conditions. If the ground is granular and free draining - you might be right. If, however, the ground is cohesive and the actual groundwater table is at a depth below ground level - the cohesive nature is relatively impermeable and the flooding condition would not cause, in the transiet short term, the ground water to rise - therefore the flood condition would actually be a surcharge.
Question is why would the groundwater rise? Why would you let the bottom of your tank (and some height above the base) be continually under water? - assuming that your tank is one 'above' ground and not a buried tank.

 

[oneintheeye]

people seem to be confirming my view that the water will take the load of the tank and so reduce the pressure from the soil.

BigH, the tank in my theoreical situation is buried. I would think that ground water can 'rise' due to seasonal variations in railfall etc.

Jstephen I have already stated that the tank is suitably small for the bottom to act structurally, not be a thin 'membrane' connected to a thicker edge and wall detail. A 'retaining wall analogy' is what you are referring to.

 

[BigH]

In your theoretical situation, it would be nice to have stated that it was a buried tank at the beginning. Sure, you can have some seasonal rise in the groundwater levels - hopefully, you will have records of the groundwater level fluctuations. If your tank is, in fact, buried, it really offers very little "surcharge" on the soil below. Say your tank is founded at 5 m depth and is 3 m square on the bottom and 3 m high. Depending on the liquid, and lets assume that the liquid specific gravity is about 1, you will actually be unloading the soil pressures beneath the tank. This is not a surcharge. (see buoyancy raft foundations). If the groundwater rises - and your tank is empty, it could float for sure. If your tank is full of the liquid indicated, the weight of the liquid would balance the weight of the buoyancy effect - for the most part. Clearly, high water level and empty tank is the design condition. I would suggest, if possible, to put in drainage below the tank and tie the drainage into a sewer or take to a "lower" level by gravity drainage - then you will obviate the high water level.

 

[oneintheeye]

BigH you seem to be misunderstanding the question. Yes the buried tank will in all probability result in a net decrease in soil pressure at the underside of the base, i.e similar to buoyancy raft you describe. I am taking bout forces imposed on the base for the structural design of the element not what its doing to the soil.

 

[himat42]

herewegothen

Please see "Design Analysis of Beams, Circular Plates and Cylindrical Tanks on Elastic Foundations" By: E.S. Melerski
Taylor & Francis Group Lindon, UK, 2006
This book has CD RAM to solve your problems/concerns

 

[kslee1000]

Per your hypothetical condition, the design force on the tank (internal empty)slab subject to raising water table is:

Design Force = Total Weight - Uplift Force + Uplift Force