Conc. Wall on Soldier Pile W/ Tiebacks

[rlewistx]

I am working on designing a wall for a 20 ft. deep trench in an industrial building. The building is up and the slab in poured and now they need special equipment that needs a long deep trench. I was thinking about using soldier piles with tiebacks and wood lagging to shore the trench. Then put a concrete face in front of the soldier piles. I did a lot of this design work back in 1985 after getting out of college. Back then I had a Sr. engineer who guided me through the design. I remember quite a bit of it but I don't remember how to tie the concrete face into the tieback system so that when the wood lagging rots the concrete face holds back the soil.

Another question regards setting the piles in place. I need to visit the site but my gut feeling that a pile driver might not work. I was thinking initially of drilling holes, setting the pile and backfilling with very lean concrete. Then when the trench is excavated the lean concrete can be broken away to allow the wood lagging to be inserted.

I would appreciate any insight into solving this problem. Maybe my soldier pile solution isn't the best?

Thanks much!

Rich

 

[Taro]

Your shoring system sounds reasonable. Consider welding headed studs to the exterior flange of the steel soldier pile. The studs will hold the concrete in place and the piles will be efficient composite members after the concrete is in place.

 

[RDK]

Can you have cross bracing in the trench?

If so then the easiest solution is to brace the trench wall across the trench to the other side. Then there is no need for tiebacks. You could drive or casy in place solider piles, brace the tops and install sheet piling spanning between the piles longitudinal to the trench and then excavate between them.

What soil conditions do you have?
How wide is the trench?
How much headroom is available?

There are some low headroom drilling rigs available. You could try talking to some pile contractors out there to get a feeling for what equipment that have available. There is no use designing a system that no contractor around has the equipment to build.


Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[Focht3]

Use a drilled pier soldier pile wall instead of driving piles, installing lagging, etc, etc. It's stiffer and stronger, and will move less before you install the tie-backs. [blue]RDK[/blue] is right - bracing will be cheaper. But you may need the space -

And remember to put in a thick, heavily reinforced floor in the bottom of the trench. It will act as a brace between the sides of the trench, reducing your tie-back requirements -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[rlewistx]

Thanks for all your responses. I really appreciate them very much.

I only know a little from a preliminary phone call. I am trying to learn what I can before I meet with the contractor so that I can talk intelligently.

It is my understanding I can't brace to the other wall. The equipment in the trench will be in the way.

Is my idea of backfilling the hole with lean concrete after the soldier piles in inserted a good idea or not? If not, what else would I do? Whatever is backfilled needs to be cut away to install the lagging.

If I weld studs to the face of the pile and then encase it in concrete how to I encase the whaler and the tieback. Do I make the wall very thick and encase them in the concrete face wall?

Appreciate all your help and insight.

 

[DRC1]

I did something like this several years ago. We drilled in sand to the bottom of the pile, inserted the pile and poured lean concrete to the excvation line. After the concrete got an inital set, the balance of the hole was filled with sand. The can was removed and once all the piles were complete the excavation was completed using tucked lagging and soil annchors. Rather than using a wale, which gets complicated with one sided forming, we fabricted sleeves in the piles with pipe. This allowed us to lock the anchors off to each pile and eliminate the wale. The lagging was tucked behind the flanges. Once the excavation was complete, studs were welded on the piles and a waterproof membrane was applied to the face of the lagging to act as dampproofing. The concrete sub installed his bars and placed a one sided form. The form can secured to the piles by welding connectors to the piles or by pouring a strip footing and bracing the form with rakers to the footing. Some things to consider are :
1.) The anchors are permenant so they should be double corrosion protected and considerations made for long term creep.
2.) I assume the bottom of the trench is above the water table
3.) I assume there will be no utility interference for the anchors and that there is sufficent overhead and workroom
4.) the cost of the piles will be considerably more than conventionally installed piles, but you are saving the cost of additional excvation to access and form the backwall, the cost of forming and the cost of backfilling.
5.) You may want to look at using soilnailing to support the trench. This would also be an option that could work.
Good Luck

 

[Focht3]

Where is your site? (city & state) What are the soil and groundwater conditions? Your "handle" ([blue]rlewistx[/blue]) suggests that you are in Texas, as am I. I assume that your site is in Texas, and has a predominantly clay profile. The following comments are primarily based on my geotechnical engineering experience in Houston, Sam Antonio, Corpus Christi, Dallas/Ft. Worth, Austin, and parts between; I have been involved in the design of retaining walls, basements and bulkheads in most of those areas. Most of the sites had clay soils with water tables within the exposed wall height. All of the sites relevant to this specific post were south and east of the Balcones Escarpment.

A 20 ft deep trench will have damn tall sidewalls. Twenty feet, to be exact.

Keep that proportion in mind as you read my post -

Since the wall will be located within an existing building with concrete floors cast up to/over the top of the wall, movement will be a big issue. Soldier pile and lagging is fine from a stability standpoint, but will move a lot - and you will end up with lots of indications of that movement: CRACKS.

You need a stiff, strong wall system to reduce the occurrence of cracks - and movement at the top of the wall. A drilled shaft wall will do the trick nicely - shafts are typically cast at (pier diameter + 6 inches) on center, but can be constructed to overlap. (Overlapping increases the cost considerably.) Drainage blankets can be inserted between the shafts (with some hand work) and a finished face "gunited" into place. Depending on the subsurface conditions (of course), it is likely you will need two levels of bracing: tiebacks near the top, and the floor slab at the bottom of the trench.

When considering the use of a soldier pile and lagging (SPL) wall, remember that the lagging will rot over time; the "face" wall will have to carry the long term soil pressures. Since the SPL wall should be designed to experience small deflections, the resulting soil pressures will be fairly high. Short-term soil pressures on a cantilever wall in clay soils can be less than the pressure exerted by a fluid with a unit weight of 40 pcf; but the long-term equivalent fluid pressure on a stiff cantilever wall can approach 90 pcf. The SPL wall will have at least two levels of bracing, so the pressure will be closer to a uniform pressure of 0.5*&[ignore]gamma;[/ignore]*H, or around 1,250 psf. (Peck, 1969) That's a pretty big number...and it does not include any water pressure.

I don't think a SPL wall will do well against these loads unless you have two levels of tiebacks (plus the concrete floor) and some very large walers. This SPL wall will be quite thick - and expensive. With greater risks.

Remember that your main issue with this design is deflection...



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[clayR]

rlewistx

A few questions for you...

Is this exc. inside of a building?
How far is the exc. from the building foundations?
What type of foundation?
How far below the foundation will the exc. go?
How wide is the exc.?
Is water a problem?
Overhead restrictions and access problems?

 

[rlewistx]

Thanks again for all your responses. I really appreciate them very much.

To answer some of the questions posted above;
The project is located in NE Texas. There is clay soil.
It is an industrial building.
We can't drive piles because of existing operations. We can drill them.
The contractor does not want to use tiebacks. He has never used them before and is not familiar with them. We are discussing the possibility of using deadmans.
Most of the surrounding slab will be removed and replaced.
The existing building foundations are supposed to be at least 50 feet away from the trench. I haven't been to the site yet.
The contractor wants to temporarily shore the hole so he can get in and build formwork for a big press foundation. The press foundation is about 21'x120' and about 20' tall (below grade).
Subsurface water should not be a problem.
The press foundation wall is designed to support the permanent soil forces so soldier pile wall is only temporary.


Some new questions for thought. If I use a deadman do I still use the triangular shaped active soil pressure? I have a “USS Steel Sheet Piling Design Manual” dated 1969. It illustrates the soil pressure for cantilever walls and walls tied back to deadmans. It uses the traditional triangular shaped soil pressure for active and passive pressures, even with deadman anchors. When I look at the cofferdam section it uses modified soil pressure of the trapezoidal shape. I also have some literature on tieback walls and they use the trapezoidal shaped wall pressure. I don't see the difference between the deadman, cofferdam or tieback. They all have a horizontal force holding back the upper portion of the wall. Why not the same soil pressures?

Does anyone have a recommendation for a tieback contractor in NE Texas that I can give to the contractor I am working with so that he can find out more about using tiebacks.

Anybody have a spreadsheet or mathcad file they have written to solve soldier pile loading?

Thanks again for your help. I really appreciate it very much!

 

[Focht3]

Trapezoidal distribution is correct. Triangular distribution requires a lot of deflection at the top of the wall.

Deadmen will work, too. But the contractor will still have to get the tie rod from the face of the wall to the anchor point. I'd suggest directional drilling rather than trenching - less floor to be removed, less soil to be disturbed. And a faster job.

I realize that a drilled shaft wall may be foreign to you, but it's an excellent option for the problem you have described. The contractor could build the permanent trench in one operation - with no internal bracing at all. The drilling will be the slowest part of the work, and it is common to both retention systems - so the SPL approach doesn't have any significant advantage. While a SPL wall will have fewer piers, the drilled shaft wall doesn't have the hand labor needed to install the lagging. And the drilled shaft approach will have little or no loss of ground behind the wall.

An open work area is a tremendous advantage. Have you discussed it with the contractor to get his thoughts? What has your geotech said about this?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[ccr]

I have done similar systems before and prefer to use 75 to 100 psi Control Density Fill (CDF) for the fill around the piles. This material is easily removed by hand tools but should not be counted on to provide any composite action.

 

[clayR]

If the existing foundations are at least 50'away,
your options have increased.

I still do not know about overhead restriction?

Without knowing more about what you are looking at, my first choices would be a soil nail wall.

 

[Focht3]

Soil nailing might work; but if the contractor won't consider grouted tie-backs, do you really think he will be interested in soil nailing?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[rlewistx]

Again, thanks for all the information. I really appreciate your insight.

Regard the overhead clearance, I still don't know what it is exactly, but the contractor tells me he can bring a drilling rig in to auger the holes. He will be using a track hoe to dig the pit. I assume there is plenty of clearance.

We have settled on drilled piles with rakers struts. The raker will be down low so initially the pile is a cantilever until the thrust block and the raker are installed. I found some good insight in Bowles "Foundation Analysis and Design" text, 5 ed. On page 793 he gives soil pressure for a braced cut. For Temp. Support Medium Clay the load is triangular in shape with a peak value of 0.3*gamma*H. This is also very close to the peak load of a cantilever pile, just different triangle shape. I'm told the shoring will only be in place for about 6 weeks.

I have another question concerning designing the lagging. What pressure value should the lagging be designed for? Using the peak value seams excessive. Is lagging typically designed for a percentage of the peak load, such as 90% or something like that?

Thanks again.

 

[clayR]

Does anyone have a recommendation for a tieback contractor in NE Texas that I can give to the contractor I am working with so that he can find out more about using tiebacks.

Please provide contact information.

 

[clayR]

Sorry,
I missed your last post, sounds like you got a handle on it.

 

[rlewistx]

contact me by email at

rlewis@lewis-engr.com

Thanks.

 

[PEinc]

Schnabel Foundation Company (

http://www.schnabel.com)

has an office in Houston Texas and installs tiedback walls nationwide. Call Ernie Brandl at 281-531-1103.

Unless you have very high surcharge loads, you should be able to install a single-tier tiedback wall with drilled-in soldier beams, temporary, left-in-place lagging, and a reinforced concrete facing. 20 feet is not very deep.

The lean concrete should be for the entire depth of the drilled hole, not just below subgrade. You can build a wall without tieback wales. Use soldoer beams with sleeves fabricated in them at the proper location or use paired steel channel or wide flange soldier beams where the tieback passes between the paired members and is attached to the paired members with a welded, seat that spans a 4 to 6 inch gap between the paired members.

A soil nailed wall is an option if the proposed trench is not too narrow. You will need at least 20 feet between opposing walls for drill access to the various (approximately 4) nail levels. If the trench is too narrow for soil nails, a single tier of tiebacks could be installed near the top of the soldier beams ( 5 to 7 feet below OG) so that the drill mast and tendon would pass over the opposite wall.

I would think that a drilled shaft wall (with tangent drilled and concreted soldier beams) would be more expensive than a soil nailed wall or a tiedback soldier beam/lagging/concrete facing wall.

Although your trench is 50 feet away from the existing foundations, if you use deadmen and tie rods, you may need to excavate close to the existing foundations because your deadman may need to be located at least 30 feet back from the soldier beam wall. Also, that's a lot of slab to remove and replace.

 

[rlewistx]

PEinc,

Thanks for you response. I really appreciate it very much.

I have a question concerning comment above. You said to backfill the drilled hole with lean concrete the full height. DRC1 above made a good comment about using sand for the upper portion to be excavated and I thought that was a good idea. After reading your post I was wondering if it might not be. Do you think the uncompacted sand might make voids around the pile allowing the soil to move before engaging the pile?

Thanks again.

 

[PEinc]

Do not install loose sand above subgrade in the drill hole. When you excavate to install the lagging, you will lose the sand and have insufficient support (a void) behind the soldier beams. This will put more stress on the soldier beams and will eliminate any arching action between soldier beams. When you stress your tieback anchors, the soldier beams will move backward excessively. You will then bow the lagging boards.

Companies that specialize in tiedback soldier beam walls use low strength concrete or flowable fill for the entire length of the drill hole. (Sometimes, structural concrete is used for the beam toe length when required by specifications.) Lean mix concrete or flowable fill can make it harder to install lagging. Harder but not too hard. Some contractors will attach plywood or styrofoam on the soldier beam flanges for permanent walls so that the lean mix can be more easily removed. You can also consider attaching the lagging to the front flanges of the soldier beams by using welded threaded studs, plates, and nuts. If you do this, buy uniform width lagging boards, not random width. That way, you can weld the studs onto the flanges with a uniform spacing.