Active Pressure Combined with Water Pressure 1

[structurebeton]

I have to design a concrete pit, with water table at ground level. I am a little confused at what is the actual design pressure I have to use:

Option A: Water pressure 62.4 psf and I ignore the soil weight?

Option B: Water Pressure 62.4 psf + Active effective pressure which I think is (115 pcf x ka=38 psf with ka=0.33) = 100.35 psf total equivalent fluid pressure?

Any opinions on this will be helpful. Option B seems overkill, but apparently that is what should be done...?

Thanks.

 

[structurebeton]

In my previous post I forgot to deduct the buoyancy from the active soil pressure, therefore it would be:

Option B: 62.4 psf + (115-62.4)x0.333 = 80 psf of equivalent fluid pressure

Can someone tell me if I am not doing this calculation correctly? Or if my assumptions are wrong?

Thanks.

 

[hokie66]

Now you have it. The second post is correct. But please avoid cross-posting in future.

 

[hokie66]

I have read this OP closer, and your answer is correct for active pressure. But for a pit, you probably have more like at rest pressure, and probably not triangular loading. Suggest you seek geotechnical advice.

 

[fattdad]

I would have to agree you take a close look at whether active or at-rest earth pressure is applicable. It is not likely to make a huge difference as it would only relate to the bouyant weight of the soil. Active earth pressure would be about 17 pcf and at rest would be about 26 pcf. Add to that your water pressure. So, it'd be about a 9 pcf difference.

There are two reasons why you may want to use at-rest earth pressure: The pit may be so small that you won't get sufficient deflection at the top to fully mobilize the active state, or you may want to limit movement at the top to minimize the potenital for tension cracks on the concrete. If it were my job, I'd use at-rest earth pressure.

f-d

p.s., Hokie66, what's cross-posting?

¡papá gordo ain’t no madre flaca!

 

[hokie66]

fattdad,

The site monitors frown on posting a question in more than one forum. He posted this one in the "earth retention engineering" forum.

 

[structurebeton]

Yes, I wasn't sure in which forum people would pay attention to my question, which in fact applies to both.

ST

 

[oneintheeye]

so fattdad for submerged pits/tanks etc. you would recomend at rest pressure? we always use active. Bit worried now. No geotech has mentioned this before.

 

[ashjun]

herewegothen
Your solution is correct. You would use at rest pressure only under situations when (in addition) tie-back or struts are used to retain the earth

 

[civilperson]

No, the active pressure requires that movement away from the soil is possible by the wall. A pit is surrounded by the soil and can not move as required, use at rest pressure for the pit.

 

[hokie66]

herewegothen,

In my opinion, civilperson is correct and ashjun is incorrect. In 30 years of design experience, I have never used or had advice to use active pressure for a pit like the one structurebeton described. In the other forum, he said that the pit was 8' square, so very rigid.

 

[PEinc]

The pit is small (short wall lengths). Even with using the highest earth pressure, the total amount of resteel will be relatively insignificant. The true earth pressure is debatable considering arching, compaction, and unspecified construction sequence. Design the walls to span horizontally from corner to corner. Use at-rest pressure and be done with it.

 

[BigH]

to PEinc. and civilperson.

 

[structurebeton]

Honestly, I have never seen a geotechnical report recommending "at rest" pressure for wall design. In fact the worst I have seen is a movement allowed design pressure of 40 psf and a rigid design pressure of 60 psf (for walls that are not allowed to move because of proximity of building).

However, I thought the requirement was more to design a "stiffer" wall so deflection is minimized, not the other way around. At rest pressure can be considerable...

 

[oneintheeye]

accepted that the pit in question is very small but at what point do you switch to active pressure then? I have never seen any design guide etc use passive pressure for submerged tank/wall design. Although highways authority do require it, resulting in massive thick walls.

 

[BigH]

BTW, how deep is your pit? It makes a theoretical difference of about 0.5/0.33 - but if you are compacting behind the wall to any extent, even if you use active, you will have to bump it up for residual horizontal stresses induced due to compaction.

Also, from Terzaghi, Peck and Mesri (article 45.4.2) ". . . if a retaining wall is proportional to withstand active earth pressure with a suitably conservative margin of safety with respect to sliding, overturning, and bearing capacity, the actual displacements of the wall will be less than those corresponding to the active state, and the pressures will exceed the active values. . . . Nevertheless, if earth-pressure measuremetns were to be made on such a conservatively designed wall, the pressures would exceed the active values. This apparent contradiction has led to confusion concerning the appropriate basis for design. Logically, design to resist acive earth pressures is appropriate with respect ot the external stability of the wall. On the other hand, the structural design of the components of the wall, such as the stem or base slab of a cantilever wall or the reinforcement in a reinforced-soil wall, must take account of the larger pressures tha the wall must resist at displacements smaller than those corresponding to the active state." It goes on further to talk about the several factors that are not always separable. Suggest any interested colleague read this section from TP&M.
Don't see why your wall thickness increases considerably - all you have to do is throw in a bit more steel as dgilette suggested. Your pit is so small - and it is assumed that you will build the walls - all four of them first, that at the least, the at-rest pressures act. You should be worrying about how much you are going to compact the soil behind.

 

[oneintheeye]

with respect depending on how big your wall/tank is (assume high), there is only so much steel you can throw in before it becomes uneconomical not to increase size.

I would also comment on your compaction allowance by saying if you've designed your wall for earth pressure, surcharge loading plus water table. Is it then resonable to assume on top of that at the same time you apply a compaction load?

Another point i would consider is that if the pressure is 'temporarily' not enough to cause wall to deflect enough to mobilise active pressure, the higher passive pressure will cause deflection thereby in itself mobilising active state.

Despite above not openly disagreing with you, just playing devils advocate on this one and looking at it from other angle.

I suppose we are going a little off the OP question on a 8" pit now, so apologies.

Oh and thanks for the quote BigH, very interesting.

 

[hokie66]

herewegothen,

Just to make sure you are clear, at rest pressure is between active and passive. Active requires deflection to reduce the pressure, passive requires deflection in the other direction to increase the pressure, and at rest is at rest.

 

[oneintheeye]

sorry, yes meant at rest not passive. Typing when working on other things, sorry.

 

[DRC1]

For a free wall it really doesn't matter if it is designed for active or at rest for overturning unless movement effects are an issue. For a pit the wall will need to be constrained at the top so the lid will fit and movement will be constrined by adjacent walls. Thus the walls should be designed for at rsst. Big H's point that wall movements may not be enough to devlop the reduced pressure so walls should be strucurally designed for at rest. For a 8 foot wall, I don't think you will find a significant differance.