Lateral earth pressure - short term and long term analysis 1

[tpk72]

I want to make sure I have a good understanding of the term effective stress, and how it impacts short and long term analysis (lets say a sliding analysis).

If you are calculating lateral earth pressures, your vertical stresses will be the same for long term or short term analysis, correct? The only item that changes is the c and phi values that impact your lateral earth pressures when you apply your Ko, Ka, or Kp factor. In other words, the vertical stress below the water table will be the effective unit weight *H, regardless of if you are doing a short term or long term analysis. It only impacts the K value.

Is that correct?

 

[oldestguy]

Don't forget the effect of that water at its density times H

 

[tpk72]

Right oldestguy - I should have included that in my discussion. Thanks!

 

[Doctormo]

Anything under the water table would use buoyant weights for downward forces (saturated weight minus weight of water) regardless of analysis. Lateral earth pressure forces would also use buoyant weights below the water table but the lateral force from a water pressure differential would have to be added. I assume you are looking at short term being the phi = 0 condition and long term as c = 0 or some sort of phi + c relationship.

The analysis can be a little more complicated than your example. It is rare that an analysis is for a completely static water table for all time. Water tables can vary and pore pressures in soil can take a long time to equalize, and so on. Long term strength properties can be difficult to define in many soils and they can change when the moisture content changes. Generally, it is best to be safe when water is involved and make conservative design assumptions regardless of soil test results in my opinion.

 

[BigH]

Doctormo gave a good discussion - but i don't see how the unit weight of the soil affects the K coefficient - they are independent of each other -

the K coefficient is dependent on strength parameters and geometry. The Ko value can be affected, too, by overconsolidation pressures - such as induced high lateral stresses due to compaction.

 

[Doctormo]

BigH - Sorry if I said something that was not clear, the K coefficient is independent of soil weight and is a function of the geometry and strength parameters used as you stated.

However, it is not always as simple as taking some test results, calculating a K, then applying that to the weight of the soil and height of wall. The math is that simple but the conditions have to be evaluated properly for water effects, tension cracks, etc. which is a little more complicated.

 

[Okiryu]

What can be a good literature reference for retention structures and earth pressures?

 

[BigH]

Doctormo - I am not in disagreement with your second comment. To evaluate the overall stability (or lateral pressures) the points you make about water effectys, tension cracks, wall adhesion factors, whether Rankine or logrithmic spiral surface etc. are definitely to be considered but the unit weight of the soil, again, does not affect the actual value of k. k is affected by the strength parameters and for ka and kp, the amount of movement that can occur - again, not related to unit weight. It affects the pressures and the other parameters need to be addressed. It seemed to me that the OP suggests that the unit weight affects the k value.

 

[fattdad]

I'm not sure I agree that total stress analyses (short term loading/failure) would be based on buoyant unit weight.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Doctormo]

BigH - I agree although I can think of some ways of calculating Ka other than equation solutions (ie: trial wedge) that may come out slightly different with different weight soil zones of the same strength. Not totally sure and don't have time to test my theory.

f-d - I agree also. Cohesion is not normal force dependent thus not weight dependent. However, the K part of the equation would be normal force dependent as the calculation is not normally based on c. The original question was about effective stress analysis so we will get back to how to use cohesion in an earth pressure analysis and so on...