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]

The pit is buried 10'-6", and will have a slab on top of it.

 

[fattdad]

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.

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!

 

[structurebeton]

Can someone post the equation to calculate "At rest" lateral pressure from the Rankine theory or any other accepted practice?

 

[hokie66]

The coefficient of earth pressure at rest is often taken as 0.5. From Sowers: for loose sand and gravel, k0=0.6; for dense sand and gravel, k0=0.4. Just use this factor instead of active or passive factor in the general equation.

 

[DRC1]

Structurebenton: the EFP of 40 psf is an active pressure and 60 psf is at rest for a granular soil with a unit weight of 120 psf, which is in general, slighly conservative.

 

[muuddfun]

1-sin (phi)

 

[fattdad]

1-sin Phi is the conventional "easy" guess for the coefficient of at-rest earth pressure. There are limitations to this simplified approach for the case of sloping backfill for example. At rest earth pressure can also be evaluated (again with some limitation) by using Ka*1.5. I took this approach in my earth pressure class with Duncan and he agreed (i.e., I got a good grade). You can quickly see that for a phi of 30 degrees, Rankine Ka and 1-sin (phi) are different by this 1.5 factor.

f-d

¡papá gordo ain’t no madre flaca!

 

[ashjun]

Hokie66
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.

 

[ashjun]

sorry for the typo error; it should be "..which cite measured earth pressure to be less than that at rest"

 

[hokie66]

ashjun,

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.

 

[DRC1]

There are many instances in granular soil that vertical or near vertical cuts can be made to fairly significant depths. At that moment in time, there is no lateral pressure generated. The reasons for this effect varry, but are almost always temporal. This is how many excavators get in trouble. Once a wedge does develop, it will generally produce a pressure coefficent of .4 to .6 reducing to .25 to .35 once the wedge has moved sufficently.
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.

 

[apsix]

There are also more than a few cases where undersized retaining walls will happily stand up for 20 years but eventually they tilt or fail, presumably due to consolidation of poorly compacted backfill.
It must be very difficult to reliably measure long-term earth pressure.

 

[hokie66]

apsix,

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.

 

[apsix]

hokie66

You could well be right, the pressure due to swelling reactive clay would be considerable.