Longevity of Timber Lagging
Longevity of Timber Lagging
(OP)
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?
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?





RE: Longevity of Timber Lagging
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.
RE: Longevity of Timber Lagging
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
RE: Longevity of Timber Lagging
http://www.eng-tips.com/viewthread.cfm?qid=109779
It seems this issue has been controversial for about 40 years.
RE: Longevity of Timber Lagging
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.
RE: Longevity of Timber Lagging
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!
RE: Longevity of Timber Lagging
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
RE: Longevity of Timber Lagging
Has anyone ever come across a study that either supports or refutes the hypothesis that ground movements will result from the deterioration of wood?
RE: Longevity of Timber Lagging
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.