Timber Lagging Degredation

[mmdbos]

I have a project where standard yellow pine timber lagging was used for a temporary soldier pile and lagging wall. Lagging is untreated. The owner would like the lagging removed as they are worried about voids developing long-term. It has always been my understanding that timber lagging will lose its strength over time, but not its volume, so leaving the lagging in place is not a big deal as long as it is not asked to support any loads. I cannot find a reference for this to convince the owners engineer. Can anyone confirm this and does anyone have a reference that I could provide?

Thanks for your help.

 

[PSlem]

I'm at home, but I think it is in the Spencer, White and Prentis Underpinning book.

 

[hokie66]

Pine floats. Dirt doesn't. So the decayed material will occupy less space than the wood. I'm with the owner.

Don't you have termites where you are? Wood left in the ground attracts them.

 

[born2drill]

There was a good discussion on this very same topic a while back.

http://www.eng-tips.com/viewthread.cfm?qid=109779

 

[DRC1]

Untreated timber lagging has been used for a very long time. It is generally left in the gound with little resulting settlement. Although it will decay, it does not change volume, as once the excvation is backfilled, thher is little net pressure. Any compression is minor. There has been several theads on this issue. I have temporarily relocated my office, so I do not have access to texts. In addition to the Spencer White book, I believe Ratay address the issue in his temporary structures book.

 

[PEinc]

See attached excerpt from FHWA-RD-75-130.

The only place I see lagging removed is the upper 2 to 4 feet below finished grade.

www.PeirceEngineering.com

 

[InDepth]

I think PEInc hit on a few key words.

The real question the owner should be asking is: "To what depth does lagging need to be removed in order to meet a specified settlment criteria?"

Removal of lagging in the upper 2-4 feet will minimize any settlement immediately adjacent.

Next time, though, I would specify preservative treatment.

 

[dirtydude]

DRC1 said it best, the wood will decay, but matter never disappears. Some other references that are helpful are the stake tests performed by the National Forest Service. They give some actual data regarding amount of treatment versus wood life in the ground.

I don't have a name of the reference, but you can google 'National Forest Service stake tests' and it should come up.

Lastly, regarding 'pine floating', it would be awfuly hard for it to float after overcoming the shear force of concrete poured against it as well as the insitu soil it was retaining when that concrete was poured.

 

[mmdbos]

Thanks for all of your input. It is nice to see that many of you have the same experience that I have.

 

[hokie66]

dirtydude,
My statement that pine floats was just comparing the SG of pine to the soil which is the decayed product. The decayed product is the same matter, but it occupies less volume.

 

[PEinc]

I have exposed old, untreated lagging on a number of new projects. The lagging is still there but has lost its strength. There are no voids created as the wood degrades.
Except for the upper few feet of the soldier beams and lagging, a temporary sheeting wall with untreated lagging is usually abandoned in place. On permanent soldier beam walls with CIP or precast concrete facings, untreated lagging is also used and left in place. However, some owners, such as PADOT, require some minimal amount of treatment for lagging that will remain in place. Often this lagging is un-dried, hardwoods, treated only to refusal. If a permanent soldier beam wall will have exposed lagging as its finished face (which I do not recommend), then the lagging should be kiln dried, properly treated softwood.

http://www.PeirceEngineering.com

 

[wedlmic]

All things left to themselves will become simple. Even treated timber lagging. It is common practice to use untreated mixed hardwood timber lagging in temporary excavation support systems. It is also common practice to remove the top three feet (minimum) of the soldier beams/piles and lagging. SWP would strap the top three feet and place it behind the rear flange - this allowed you to burn off the top three feet of soldier beam/pile and pull the lagging out of the ground as a unit. The requirment is mostly due to future interference with utilities etc., settlement was not considered an issue. If the specs did not call for treated lagging and did not require complete removal then standard practice would be just to remove the top three feet. Otherwise Cost-Plus - ouch.

Wedlmic