Transmission Tower Foundation behind Soldier Pile Wall 1

[dfranks]

I am working on a soldier pile wall design that will retain a large transmission tower foundation. The foundation is 17 feet deep and 7 feet wide. There is a 4400K-ft moment a the top of the shaft in the direction of my wall, as well as a 62K horizontal force. I have a basic understanding of how the foundation designers determined the depth of their shaft based on passive pressure (using Brom's I think), but I can't come up with a logical way to convert that needed resistance into a load on my soldier pile wall.
The good news is that the cut in front of this tower foundation is only about 6 feet. The bad news is that it is only 7 feet away from the foundation. (obviously interupting the soil wedge that would have been providing the passive pressure the utility folks were expecting).

Anybody got any suggestions on how to take the "tower loads" into account for my wall?

 

[dcarr82775]

I'm not sure I understand your design problem. I assume the transmission tower base is just a big round blob of reinforced concrete, and that you are making a cut of 6' below grade not below the bottom of the foundation.

If the designers used Broms that would imply a constant uniform resistance over the height of the foundation to resist the bending and horizontal forces. Just apply that horizontal force to the solder beams in addition to soil loads. I'm not sure how Brom's method relates to passive 'wedges'.

 

[dfranks]

dcarr82775- Thanks for your input. You assumptions about my sitution seem correct and your comment about Broms using uniform resistance is helpful. I hadn't used Broms in the past and didn't note the niform assumption.

The struggle now is determining how exactly to transmit those loads through a layer of soil and determine their net effect on a specific length of retaining wall. Typically when working with wall design you can think it 2 demensions (i.e. per linear foot type designs), but the loads from this tower are definitely 3 dimentional.

 

[dcarr82775]

I use (3) pier diameters when calculating the resistance so I would spread out the Brom pressure over at least 3*7'=21'.

 

[FixedEarth]

dc;

Is your foundation width and drilled pier diameter both 7 ft? The reason I ask is, the passive wedge factor of 3 is applied only to the drilled pier diameter containing the soldier beam and is independent of foundation width.

Soldier piles are flexible in that the lateral deflection on the ground surface almost always govern. In this case we have only a 6 ft cut but large lateral stresses. So we need to do two things- first let us get the lateral stresses from static earth pressure, surcharges + any GWT if applicable. The surcharge is strip load-so you need width of load, intensity of load and setback. Then apply this to the retained + burried portion of the soldier pile. The strip pressure will look like a question mark but when you add the static, etc, it will resemble like a trapezoid.

Next, draw a FBD of the soldier pile with that trapezoidal loading and then you can determine the required embedment and max. moment.

If you post a diagram with the soils data, loadings and geometry, I can post what I am getting.

 

[dfranks]

Fixed Earth:

Here is a simple drawing of the section. (no soils data yet)

The tower foundation diameter is 7ft. The Soldier Pile pier diameter is open for adjustment. Your comment about the factor of 3 only being applied to the soldier pile pier is understood, but I agree with DCarr8775 that Broms uses a similar factor of 3 that needs to be factored back out as you convert soil resistances into loads for the soldier pile wall.

What is GWT?

Dcarr8775:

When you said that the use of Broms would indicate a uniform load distribution, does that assume that our soil is cohesive? In my study it seems like Broms uses the traditional passive triangle for soils without cohesion.

 

[dcarr82775]

I assume GWT is ground water table.

To me, Brom's resistance is for cohesive soils where the ultimate resistance is taken as 9*cohesion*pier diameter. for better or worse i have always assumed 3 pier diameters is included in the 9.

For cohesionless soils or soils that exhibit a little of both I use a Coulomb(sp?) type of passive pressure soil loading, again with 3 pier diameters.

 

[FixedEarth]

dfranks;

Yes, GWT is Ground Water Table as dcar82775 mentioned.

I used your attached diagram and came up with the attached solution. I converted your shear to a moment and added that to the exisiting Moment. Then I converted your moment and came up with a vertical load (P = M/e). Then I assumed a 10 ft length of the existing foundation and came up with an equivalent strip load pressure of 950 psf. I then conservatively, assumed it as a uniform pressure (ignored the 7 ft distance) and calculated your required soldier pile section.

It Would be nice to hear from others confirm or disagree.

 

[mudman54]

this type of problem is similar to when you have a lateral-loaded drilled shaft with a wrap-around retaining wall abutment; we use LPILE or COM624 and sum up the soil resistance results at the proper elevations. this load in the soil is then spread out at to the 7-feet distance of your retaining wall.

 

[dfranks]

FixedEarth: Perhaps I am missing the obvious, but how did you convert the shear and moment in an drilled caison into a 950psf strip load?

Please be as detailed as you can in your explaination.

Otherwise, the program you use looks pretty slick.

I appreciate everyone's input on this.

 

[FixedEarth]

dfranks;

I didn't save the quick calcs from last week. However, this is what I did:

I took your shear load & multiplied by the moment arm and added that to your larger moment. Then I used P = M/e Then I took that P as a line load and the 7.25 ft as a setback and put that into Lateral Stress Software.(you can do it by hand or by spreadsheet, and it is a good excercise.) Then the lateral stress profile of this line load looks like a question mark. I just extended the lagest bulp of the question mark and made it constant with depth (conservative). Then multiplied that by 3 to convert it to a unifrom vertical surcharge (normally, lateral stress is 1/3 rd of the vertical stress).

Attached is the line load formulas for quick hand computation. Sorry swamped with work and need to catch up.

So as a recap get your P and then use the attached line load equation, and multiply by 3, your highest lateral stress and it should be close to 950 psf.

One other thing- you need to estimate your embedment depth. I used 1.3H as embedment. H being your cut height.

By the way the Canadian Foundation Engineering Manual covers these stresses rather well.

 

[RFreund]

FixedEarth:

I'm slightly confused on using P=M/e. Are you assuming that e=7' (existing footing width). Then this P acts at the bottom of the existing footing and therefore the lateral pressure resulting from this load would act below the bottom of the existing footing (thus not effecting the retaining wall). However I would think that the moment of the existing footing would cause a lateral pressure from lateral bearing which would act along the depth of the existing footing which in turn would put a lateral pressure on the retaining wall. Is this what you are accounting for by applying the uniform load resulting from P=M/e of the existing?

Thanks

EIT