ko or ka values? 1

[ozziz]

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?

 

[UcfSE]

Most retaining walls will deflect enough to allow the active earth pressure wedge to form, reducing the design load for the wall. This saves money for the wall construction.

 

[DRC1]

If you have a soldierpile that is proped top and bottom, you would design the first stage, that is with the topped proped and excavting to the second prop with active & pasive pressures.Once the 2nd prop is in, you would design on the basis of emperically derived pressure envelopes. Note that in order to get the second prop in, you will probably need to have sufficent toe below the excavation to hold the pile until the strut is in. For information on the pressure envelopes, see a text such as Bowels Fondation design or Pile Buck Steel Sheet Pile Design Manual
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.

 

[ozziz]

I am refering to the deflection of the soldier pile around bottom of mid-span assuming it is quite rigidly fixed at both ends. We can take ko if the pile is rigid but what happens if the pile deflects near bottom of the mid-span? The value will change to ka. To be conservative one will take ko values but most structural design text books and even computer programs takes ka values and hence reduce the prop force by 20 to 30%.

 

[fndn]

If at some point of the wall, at rest pressures will be seen, then use Ko. On your latter question, if retaining walls are not restrained then Ka applies, however we also have to consider other factors. For example in a basement wall for residences, you may use Ka and majority of the time see no problems, mainly due to the low height of the wall.

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.

 

[GeoPaveTraffic]

ozziz,

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.

 

[DRC1]

I am somewhat confused as to what you are looking for. You say the soldier pile is rigidly fixed at both ends. How are you achieving the fixity at the top and the bottom? Although a substantial connection can be made to the top wale, this is usually not done and the wale/strut connection is more of a pin connection. At the bottom, fixity can be achieved by driving the pile deep enough, however the point of fixity usually occurs a few feet below grade. At any rate, in order to install a brace, it must be possible to excavate safely at least 2 feet below the level of the brace, which generally makes a bottom strut unecessary. An exception to this is a cofferdam that may be excvated in the wetand a concrete floor is tremied in which acts as a strut. To do this, at least 2 levels of upper bracing are required.
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.

 

[ozziz]

Maybe I miss out some salient points. See, I am designing a 6m deep underpass using 2.5 ft dia bored piles at 4.5 ft spacing. In between the bored piles, we have arched shotcrete wall. This 2.5 ft dia bored piles are essentially are like soldier piles - propped at the top by a 2.5 ft thick cover slab and piled deep at the base. We do not have any waling or ground anchor tie back. Depending on the pile stiffness, the pile will deflect and trigger soil movement from ko to ka.

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?

 

[DRC1]

This is much clearer.
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