ko or ka values?
ko or ka values?
(OP)
I understand we use ko (presure at rest) values if the wall is rigid.
I have a solder piles (king piles) propped at both ends. As the piles is not stiff, the soil behind will move and hence ka value is adopted. But later pile will not deflect. In this case, should I used ko? If the pile is not strong enough, plastic hinge will formed and allows lateral movement and the soil lateral pressure goes back to ka?
Question is why most text book and even software like WALLAP ueses ka in the design of retaining wall structures?
I have a solder piles (king piles) propped at both ends. As the piles is not stiff, the soil behind will move and hence ka value is adopted. But later pile will not deflect. In this case, should I used ko? If the pile is not strong enough, plastic hinge will formed and allows lateral movement and the soil lateral pressure goes back to ka?
Question is why most text book and even software like WALLAP ueses ka in the design of retaining wall structures?





RE: ko or ka values?
RE: ko or ka values?
Ka values develop when the wall rotates slightly. If rotation is undesirable use Ko values Ko vales will give you amore substantial wall with little or no movement. If you design on the basis of Ka, be sure the wall face is battered so that when it does rotate, it does not look like it is leaninig. I suspect software companies use Ka to give more economical wall designs.
RE: ko or ka values?
RE: ko or ka values?
nowdays I just look at how much control I have during construction. If technicians will be able to test compaction and we can specify select backfill, then I use Ka. However, If I have no observation and testing input, then I resort to Ko. If you use the equivalent fluid pressure method it comes down to using either 35 psf/ft or 55 psf/ft. You may pick any value in the middle ground and be able to sleep at night.
RE: ko or ka values?
A couple of comments. First, if the wall is supported at both ends then neither Ka or Ko should be used. Instead as DRC1 indicated an emperical pressure envelope should be used. If it is not, the top strut could be overloaded and fail.
Second, twice you have said "...and even computer programs takes ka values and hence reduce the prop force...". What does this have to do with ANYTHING. Please don't say that just because it is in a computer program that it must be right! If you don't know why the computer program is using a value AND agree with that value then you should get rid of the computer program and stop trying to design something that you don't understand.
RE: ko or ka values?
I go on about this because I do not follow what it is you are trying to do but am some what concerned it may have some stability issues during construction.
RE: ko or ka values?
Am I correct to say:
1. Ko to ka occurs if the soldier pile deflects at ko and reduces when lateral pressure becomes ka?
2. When the piles deflect at ka, over a period of time, will the lateral soil coefficient ka becomes ko if pile does not yield or deflect?
In both cases, will the governing factor be ko instead of ka as taught by the soil mechanics text books? So will ko be the correct lateral pressure coefficient to be used in the design?
RE: ko or ka values?
To answer direct questions:
1. If there is sufficent rotation (varries for soil types, but lateral wall movements on the order of .5% of the wall height) the soil pressure will reduce to active. Note that passive pressures typically need approximately 10 times more movement to achieve full passive pressure. Note also we are talking wall rotation allowing mobilzation of the retained soil mass, not just localized deflection.
2. In theroy no, once the wall has deflected, the soil mass has dropped and there is no strength gain. In reality, the area may be disturbed,Groundwater patterns change,surcharges are added or taken away, so I might expect some increase over time depending on the situation.
To analyze your situation is some what complicated. I am understanding your situation that your cassions have a slab at the top and are drilled in below final grade. This analysis is somewhat complicated because it is a mixed system. The cassions will not be flexable.I am not sure how the shotcrete facing will transfer loads to the cassions. If you are incorperating the cassions into the final strucure, then I can see why you are using them. If they are soley for earth retention, they seem to be overkill.
The important question is will the cassions be deep enough to achieve fixity below grade and at what piont. If not you can use a stadard Broms method to analyze as a soldier pile. You will get Ka conditions if the cassion toe tries to kick into the excavation, but since this sytem is so stiff, I might be inclined to use Ko or at least very conservative Ka values. Due the large diameter, I would use less than 3d for the width of the passive zone. If the pile does devlop fixity, It can be anlyzed using Dr. Blum's equivilent beam (see Pile Buck)with the same cautions as Brom's method.
I would like to say that this is a very unique system.This is not a stardard case and should not be compared to text book solutions. Depending on site and soil conditions, other methods, such as subgrade moduls reation, very well may be more appropriate. I would STRONGLY urge you to have the design either done or reviewed by a firm experienced in complicated earth retention
Good Luck