Active Pressure Combined with Water Pressure
Active Pressure Combined with Water Pressure
(OP)
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.
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.





RE: Active Pressure Combined with Water Pressure
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.
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
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!
RE: Active Pressure Combined with Water Pressure
The site monitors frown on posting a question in more than one forum. He posted this one in the "earth retention engineering" forum.
RE: Active Pressure Combined with Water Pressure
ST
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
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
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
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.
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
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...
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
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.
RE: Active Pressure Combined with Water Pressure
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.
RE: Active Pressure Combined with Water Pressure
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.
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
Interesting comments. I happen to agree with everybody suggesting that you use the at-rest pressures and also happen to disagree with the notion that active earth pressures are appropriate.
Here's my perspective, which is likely to rehash some of what's been stated: Active earth pressure for "retaining walls" requires about 1 inch of outboard movement at the top of a 10 ft tall wall. I visualize a concrete pit as something that is cylindrical or tall-box-like in shape. As such you either have compressional "hoop" stress or corner stresses that will hinder any rotational movement at the top of the pit. If you design for active earth pressures and the pit experiences at-rest earth pressures, you will end up putting more stress on the steel reinforcement and run the risk of developing concrete cracks that fully penetrate the structure. That may lead to leaking or other distress (unlikely it would lead to failure; however). Considering that the difference between the submerged active and at-rest pressures is seemingly trivial (i.e., less than 10 pcf), you should plan on no movement at the top of the wall.
I once worked with a structural/geotechnical engineer that designed cantilevered retaining walls for at-rest pressures just so that there would be no movement. Belts and suspenders, eh?
f-d
¡papá gordo ain't no madre flaca!
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
f-d
¡papá gordo ain't no madre flaca!
RE: Active Pressure Combined with Water Pressure
I am surprised at the simple conclusion you have drawn in your post. Please look at some of the research papers which cite active earth pressure actually measured to be lower than developed from the Rankine theory.
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
I am sure there are occasions where measured earth pressure is less than assumed, maybe usually, but we normally design walls based on assumptions and recommendations, not measured pressure.
RE: Active Pressure Combined with Water Pressure
Measuring exsiting earth pressures gives us a better insight into the failure mechinsm of soil. However, or knowledge and confidence has not increased sufficently so that we may replace the exsiting methods with more agressive values.
RE: Active Pressure Combined with Water Pressure
It must be very difficult to reliably measure long-term earth pressure.
RE: Active Pressure Combined with Water Pressure
My theory is that the type failure you described is often caused by volume growth of the backfill rather than any fundamental change in the pressure characteristics. This is especially true with cohesive backfill. When the clay cracks during dry seasons, the cracks attract debris, and the backfill gradually grows laterally with each cycle.
RE: Active Pressure Combined with Water Pressure
You could well be right, the pressure due to swelling reactive clay would be considerable.