Timber Lagging - Short term loading

[rlewistx]

I had a thread going in the Earthwork/grading forum about soldier pile shoring. I guess it was the wrong forum. It was also getting too long.

I have a question about the design load for lagging. I have a temporary soldier pile shoring condition and I was wondering what pressure to use for the wood lagging design. There is a peak soil stress for the soldier pile design that occurs theoretically at one point. What soil pressure should be used for the wood lagging, some percentage of this peak value?

Thanks for your help.

 

[PEinc]

Timber lagging for temporary soldier beam and lagging walls is usually not designed. However, if the wall is along a railroad, the railroad will make you design the lagging "their way" and you will have thick boards (if they even let you use lagging).

For most temporary walls, 3 inch nominal thickness, untreated, ungraded, mixed hardwoods (along the east coast of the US) are used. 3 inch boards have been used successfully on very deep excavations (> 60 feet) even where by design you would need thicker boards. 3 inch boards work for most temporary walls as long as the soldier beam spacing is not too large (about < 10 feet) and the retained soils are not too softand as long as there is not a large adjacent surcharge.

There are several methods for designing lagging boards which try to account for reduced earth pressures due to arching.

Refer to FHWA-RD-75-130, Lateral Support Systems and Underpinning, Volume III.

Refer to Handbook of Temporary Structures in Construction by Robert T. Ratay.

 

[krd]

To account for arching, the design load on lagging can be approximated by using a triangular distribution: maximum soil pressure at the soldier pile reducing to zero at midspan of the lagging then increasing to maximum at the next soldier pile. Analyse as simple beam.
If your calcs indicate 4&quot; wood lagging is required then you will have to use good judgement based on geotech experience, either your own or someone else's, to stay with 3&quot; wood. Or, if you don't feel confident in this situation, sometimes it's cheaper to buy the wood than the dirt engineer.

 

[Focht3]

A few details for those who didn't participate in the other thread: ( thread256-67233 )

The trench will be in northeast Texas in a (primarily) clay stratigraphy. The trench will be for an equipment pit inside an existing structure. And it will be 20 feet deep and at least 100 feet long.

I've already commented on this, and have pretty firm feelings - so I'll refrain from commenting until after [blue]rlewistx[/blue] has gotten responses from a number of you -





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

 

[jheidt2543]

I read the thread noted by Focht3 and it leaves me wondering:

If most of the floor slab is going to be removed and if the building foundations are some 50' away from the proposed trench, why not open cut the trench? You should at least check the cost of the tieback or soldier pile system against an open cut solution. Yes, it is a lot of dirt to move, but it could also save a lot of money.

I have been involved in a number of similar foundations in operating facilities, from the contractor's point of view. If there were no building foundations to support and there was no ground water to contend with, we almost always open cut for the machine bases inside a building. It does make for a big hole in the ground and requires a place to store the excavated material for use as backfill, but it is a lot cheaper. Just a thought!

 

[PEinc]

If you open cut to build the trench walls, you probably would be building > 20 feet high, cantilevered, cast-in-place concrete retaining walls. These walls would be pretty big. It sounds like someone wants to minimize the excavation, backfill, and plant disturbance.

 

[jheidt2543]

PEinc,

By open cut I mean, a trench approximately 20' wide at the bottom and 40' wide at the top (size depends on the soil's angle of repose), no earth retention system at all. My thought was to at least look at the cost of open cut vs. a temporary earth retention system. There are costs and advantages for each and if cost is the major concern, then the open cut method may well be the way to go. Of course, if the Owner's operations are interrupted, then that is one of the disadvantages. However, I have done this in operating industrial buildings many times, all it takes is some room and a good plan.

 

[Focht3]

Open cut would have to be 2*(20'*1.5) + trench width - at least 60 feet wide, not counting the trench width. But [blue]jheidt2543[/blue] has a point: if the floor will be removed, open trench construction makes some sense. It's worth looking at, anyway.



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

 

[PEinc]

jheidt2543 is correct that an open cut is usually more economical than a sheeted cut. But, I would think that the excavated trench width at top would be a lot wider than what even Focht3 has indicated. The top of excavated trench width for a pair of cantilevered concrete retaining walls would be Focht3's 60 feet plus the finished, inside, trench width plus two wall stem thicknesses plus two footing heel lengths plus forming room behind both wall footings. The total width of excavated trench at original grade could be approximately (2x30'slopes) + (2x3'stems) + (2x5'heels) + (2x2'forming work areas) + finished trench width = 80' plus finished trench width. This is an awful lot of disturbance inside a plant and we haven't even talked about adding work area for the contractor.

If the plant owner can live with permanent, underground, anchor easements, permanent soil nail walls or tiedback walls could be the better way to go.

 

[jheidt2543]

I've re-read the the original post for the third time, the heat must be getting to me. I don't see where PEinc's getting two retaining walls are required. However, I do agree that the hole could end up 60' wide, if the soil requires that the trench be sloped at 1:1.

 

[Focht3]

The soil will have to be sloped at 1.5:1 (H:V) as a minimum; it may have to be more like 2:1. But I gave it the benefit of the doubt.

Given the symmetry of the problem, two separate walls probably aren't needed - so 60' plus the full width of the structure is closer to the minimum size. But that could easily rise as a result of lots of things we can't anticipate. It will be large, regardless.



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

 

[rlewistx]

Thank you all for your input. I really appreciate it very much.

To end the speculation let me explain it a little further. The trench needs to be about 120' long by about 30 feet wide at the base to construct a foundation for a 2,000 ton press. At the narrow end it is 5 feet from an existing press that must remain in operation. This end must be shored. At the other end it is also near equipment and mist be shored. Along the length the will excavate out enough to slope the cut and not require shoring. So I have probably 2 lines of shoring about 30-40 feet long.

Thanks for all you help!

Rich

 

[PEinc]

The original post said, &quot;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.&quot; Equipment trenches usually have two long sides. I can't remember seeing a one-sided, permanent, equipment trench with the opposite side sloped. Possible, but not likely. I could be wrong.

rlewistx, one or two walls?

 

[rlewistx]

The more I became involved in the project the more it changed. At first I was assuming I needed a long trench excavated and concrete side walls. It was my understanding the equipment would sit in the trench. Then I found out the equipment had a special concrete foundation and I needed to design shoring to hold back soil so the foundation could be built and then backfilled. There were too many jogs in the wall to use the shoring as the foundation wall. As some people commented, the more we talked the more it looked like over excavating the sides and bench cutting the slope would be best, except in 2 areas, at the ends of the long trench where over excavation could not be done. We finally settled on one, possible 2 small area that required shoring and the remainder of the hole will be over excavated and benched. My how it evolved with time!

Thanks.

 

[PEinc]

Remember, sheeting is defined as a flexible support system. Flexible is a relative term. Flexible means deflection which means settlement. The existing presses or foundations in front of your proposed sheeting will surcharge the sheeting. Usually, a structure very close to the excavation gets underpinned. If not, the sheeting should be as stiff as possible. Tiebacks should be installed at or slightly below the bottom of the existing foundations to minimize wall movements prior to tieback installation.

When, as a last resort, I have sheeted in front of foundations (instead of underpinning), I have backfilled the soldier beam drill holes with structural concrete instead of lean mix concrete or flowable fill. I also closed the soldier beam spacing from about 8' c.c. to about 6' c.c. This closer spacing along with the structural concrete helped stiffen up the support wall. I also would use a heavier soldier beam than usual. Doing this will give a wall similar to that proposed by Focht3 in your other thread.

Make sure you have accurate elevations of the adjcent structures before, during, and after the trench construction.

 

[rlewistx]

Thanks for the helpful advice. I am told that the existing press right next to this one has an identical foundation which means the bottom of the foundations match. Now that I think of it though this was verbal and not written down anywhere. I will confirm it to make sure but my assumption during design was that the foundation of the adjacent press did not influence earth pressure on this wall. I did include a surcharge load for the existing 8&quot; concrete slab and 200 psf live load.