Timber Lagging - Short term loading
Timber Lagging - Short term loading
(OP)
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.
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.





RE: Timber Lagging - Short term loading
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.
RE: Timber Lagging - Short term loading
If your calcs indicate 4" 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" wood. Or, if you don't feel confident in this situation, sometimes it's cheaper to buy the wood than the dirt engineer.
RE: Timber Lagging - Short term loading
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 rlewistx has gotten responses from a number of you -
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Timber Lagging - Short term loading
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!
RE: Timber Lagging - Short term loading
RE: Timber Lagging - Short term loading
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.
RE: Timber Lagging - Short term loading
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Timber Lagging - Short term loading
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.
RE: Timber Lagging - Short term loading
RE: Timber Lagging - Short term loading
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 VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Timber Lagging - Short term loading
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
RE: Timber Lagging - Short term loading
rlewistx, one or two walls?
RE: Timber Lagging - Short term loading
Thanks.
RE: Timber Lagging - Short term loading
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.
RE: Timber Lagging - Short term loading