Soldier Pile System Questions....... 1

[Boltzie]

I'm designing a soldier pile retaining wall for an already failed location. I understand the design and have done a few soldier pile designs for excavation, but I believe this case is a little different. I have the following questions:

1. Should I be using Ka or Ko for the active earth pressure? (Am I just adding some safety factor by using Ko?)

2. How does the design/earth pressures differ when a slip plane has already occurred? The state DOT engineer does not want to excavate out the failed material, just drill/place piles and excavate failed material.

3. What is the typical procedure when using sheet piling or soldier piles & lagging at an existing slope failure? (excavate material, place and compact new behind wall, etc.)

4. How does the design differ when the piles/holes are socketed 5' to 10' into the rock strata? (typically, the concrete is placed to the dredge/lower level. Not sure how many new unknowns I'm adding by adding a new boundary condition (fixed) at the base.)

5. How do the applied loads change when the actual embedded depth is much larger than the calculated depth, D? Am I just increasing the safety factor by embedding more? Do I need to recalculate the applied loads when actual 'D' is much deeper than required 'D'?

Thanks in advance.

 

[DRC1]

1. Typically you use Ka unless you are trying to make the system stiff, eg. to protect a nerby foundation. By using Ko instead of Ka, your applied loads will by about 50% greater than if you use Ka which normally will give you an added measure of comfort. However, in this case since you are filling behind the wall, you may want to use Ka if you are using compaction equipment. Compaction equipment can generate significant earth pressures, and you may want to use Ko to account for that.Note that using Ka over Ko will increase your applied loads, it is more typical to place a factor of safety on the Pasive pressure.

2.Assuming this is a granular material to which you are placing another granular fill over, the basic mechanics don't change because of the slip. What has changed is that the material in the failed zone has moved to a much looser stae, thus lower phi and K than borings or tests prior to the slip would have indicated.If the slope was cohesive material, this becomes somewhat more complicated.

3.Because you are filling in, placing the lagging can be done first. You will have to cut small blocks and block to the opposite flange to hold the boards untill the fill is in place. If you need tiebacks, that can be tricky. I have used heavy duty scaffold to support the tie back rigs. Most are small and can be accomidated by such a system. You will need a crane to place the rig and you should backfill a couple of feet above the hole. You will need to tie the scaffold to the piles to coteract lateral forces from the drill.

4. The drilled in rock sockets filled with concrete can be analyzed as a regular rock socket.

5. That depends on how you model the toe. If the toe is in rock, some additional embedment will increase the capacity, but usually the rock sockets have a high factor of safety anyway. At some point the rock socket will be cable of supporting a load that would yield the pile. at this point additional embedment serves little purpose. Soldier piles toed in soil designed based on free earth, ie. embedment is sufficent to provde lateral restraint only, can increase the factor of safety by increaseing the length. The longer lenthincreases the fixity of the soldier pile until it can be modeled as a fixed earth system - that is the embeddment is suficent to provide lateral and rotational support.After the fixed earth condition is achieved, the increase in length may allow for some variation in soil properties, but the benift of increasing the length beyond that point is marginal.

 

[GeoPaveTraffic]

To what DRC1 said I would add the following:

Depending on the size of the failed slope and the type the loads could be higher than Ko. This is particulary true if the soil is clay or shale.

As for the extra embedment depth, the only down side to extra embedment, is that the pile can become truely fixed. This can lead to a higher moment near the top of rock.

 

[PEinc]

If you have a slope failure, you could have driving forces much higher than conventional retaining wall forces determined from either Ka or Ko. You may have a landslide condition and need to do a slope stability analysis with the proposed wall and tiebacks inserted to provide the required slope global safety factor. You need good soil borings and test data. have any slope indicators been installed that could show the actual failure plane location?

You could analyze the existing failed slope and tweak the soil valuse until you get a safety factor of 1.0 or less and with a scarp location matching what you see in the field. Then you can use those soil values in your proposed analysis.

 

[swine]

Hi DRC1,
With regards to the word fixed. A point of confusion for me is as follows: "Piles may be assumed fixed at 10 feet into bedrock." This is the phrase I read in a soils report a month or two ago, for designing soldier piles.

My first guess is that he means the base of the piles are fixed, a pinned connection more or less. If he meant fixed with regard to soils pressure, then there should be something in the report about soil pressure magnitude and distribution for the fixed condition of the pile, which there isn't.

But my first guess is wrong also, since the fixed/pinned condition refutes a limit put on the allowable passive pressure earlier in the report.

Would anyone care to speculate? The bedrock is 20 or 30 feet down.

 

[GeoPaveTraffic]

Typically, "fixed" in a soils report for a pile or soldier pile wall refers to the point where the bending moment has been disipated and that further embedment of the pile below that point will not result in additional lateral load capacity.

Conversly, if the pile is not installed to that depth, then the amount of lateral load capacity will be lower than otherwise provided.

Where fixity occurs primarily depends on the stiffness of the pile and soil/rock.

 

[DRC1]

I agree with GeoPave Traffic. I would just add in response to the last part of the p[ost I would interpret the report to mean that if you drilled and installed the pile in a 10 ft deep rock socket, the pile would essentially be modeled as a fixed (moment connection) at the top of the rock socket.