Longevity of Timber Lagging

[adh06]

When dealing with city engineers, architects, owner’s reps, and pretty much anyone who is unfamiliar with earth retention we often run into a question about the longevity of the timber lagging used as temporary shoring in a conventional beam and lagging retaining wall. I'm referring specifically to temporary systems where the beams and lagging are ultimately buried and abandoned in place. The aforementioned parties have serious reservations about the system because they believe that in 5 – 10 years the timber lagging will “rot” away, leaving a void that can cause settlement and cracking in adjacent sidewalks and structures.



The usual argument that we defer to is our experience and the track record of this shoring system. Most of downtown San Diego, for example, was built with timber shoring elements of some type over the past 75 years and yet we don’t have differential settlement problems caused by its disintegration. As I’m sure you can imagine this usually doesn’t provide much comfort to the said inquisitors.



Furthermore, the manufacturers and suppliers of the timber are very hesitant to provide any kind of guarantee or information period on the sustainability of their product for this application.



Is anyone aware of any texts, publications, research, etc. on this topic?

 

[concretemasonry]

The lumber used now is commonly very different from the dense, old growth. It is still possible to get some sizes in old growth, but it gets costly.

Also. the treatments processes and chemical have been changed in the last 10 to 15 years as have the concentrations - Health and ecology concerns. The lack of history may be a reason for the reluctance to make real guarnatees.

 

[jdonville]

adh06,

If the timbers end up below the prevailing groundwater level, then I would believe that they would end up lasting indefinitely.

Realistically, though, the timbers do not rot into nothing - the "structure" stays intact even when the strength is no longer there. I would imagine that 3x8 timbers would take a very long time (decades to centuries, depending on the environment) to completely disappear and get replaced by a residual organic soil.

Jeff

 

[adh06]

I guess I should have read back through the old threads:

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

It seems this issue has been controversial for about 40 years.

 

[PEinc]

Nice search, adh06. I was fretting about having to retype all of that info. I forgot it had been addressed before.

The issue really isn't that controversial. On almost all jobs I see and work on, all but the top 3 to 4 feet of lagging is left in place. Almost always it is untreated. Certain owners, usually state highway departments and people who are afraid of termites specify treated wood. In those few cases, the wood is still ungraded, mixed hardwoods which are not kiln dried but are CCA treated to refusal. This provides some token protection at a reasonable cost.

 

[adh06]

I would agree that it's not very controversial among professionals in this field. The controversy arises when people who are unfamiliar with this type of application get involved; and usually they are the ones making the decisions.

My firm will be constructing a CalTrans designed soldier pile wall in the summer that takes conservative design to a new level. For a 7 foot cut height they have specced HP14x117's with 6x12 treated lagging timbers and a 12" CIP concrete facing.

I guess that's one way to address the issue!

 

[jdonville]

adh06,

Wow.

I would try to see if I could bid a less conservative section. It shouldn't be hard - even using FHWA/AASHTO design criteria and guidance.

Jeff

 

[SEshore]

I've read through this tread and the older thread. There are some good points and some good references. I'm not sure that its enough ammunition to convince some of the more stubborn engineers that we've all come across.

Has anyone ever come across a study that either supports or refutes the hypothesis that ground movements will result from the deterioration of wood?

 

[adh06]

I would agree SEshore. Despite the fact that so much of Geotechnical engineering is based on experience and precedence it can still be a difficult sell at times.

I think this would be a fantastic topic for someone to publish a literature review on. Perhaps if the lack of hard research was highlighted in a publication it would spur some post-graduate work.