wood shoring box leading to long term settlement? 3

[SRO]

Greetings,
I work on a fair amount of projects that require wood shoring boxes for pipe trenches, or removal of contaminated soil. I typically specify eastern white pine or him-fir for the 3x10 t&g sheathing and 8x8 wales/struts. Once it has served its purpose we backfill the pit/trench with excavatable flowable fill. Most of every project I've seen, read about, or designed, abandons the shoring leaving it in the ground.

How concerned should I, and every other engineer, contractor, and owner who does these shoring boxes be about degradation of the lumber leading to voids in the soil creating settlement overtime?

Thanks

 

[oldestguy]

An old guy here. I've never seen any settlement problem on former trenches as related to shoring left there. This might be relate to building on a place that once had trees and roots al left in place. Never any problem there either as related to them as to settlement.

 

[SRO]

Thanks oldestguy. I've never had any problems from shoring boxes causing settlement per se, but I was starting to get a little paranoid. Did I read your post correctly in that you have never come across problems from buried stumps creating settlement problems under houses?

 

[SlideRuleEra]

SRO - Whether there is a potential problem, or not, depends on factors such as:

1) Soil temperature, which also influences termite activity.

2) Water table elevation and fluctuations, which contributes to wood deterioration.

For example, in the parts of the south-east USA year round "warm" soil (frost depth = zero inches), high and variable water table, and plenty of termites, buried wood is effectively "gone" in just a few years.

http://www.SlideRuleEra.net

 

[oldestguy]

SRO: As to my experience, I think the buried natural wood, such as tree roots and stumps, even though rotted still occupies sufficient volume so no loss of that volume occurs, Take animals burrowing, that doesn't affect the surface in most cases. I call it "bridging". I have seen problems where the stump was removed and the space wasn't compacted well, resulting in settlement of the foundations there. The volume of the wood in trench bracing is pretty small and bridging is in effect. The way the soil is replaced in more likely to result in problems.

 

[SRO]

SlideRuleEra - First off, thank you for all your advice you have given out over the years on this forum. You have aided me more that you will ever know. That said, I have no doubt that the lumber will eventually deteriorate, I just wanted to get some feedback from other engineers, such as yourself, as to what they've seen for surface settlement over time.

oldestguy - what about fines eroding and filling voids left behind from the decayed material?

 

[oldestguy]

All I know is I have never run into a problems of settlement, etc. as related to decaying wood from natural things such as tree toots or stumps, or trenches with wood lagging left. Take the case of my one son who is college educated, but unable to keep a teaching job due to being too fussy, etc. wanting things perfect.. Is a construction inspector and tester. Has developed a philosophy that construction tests requiring high compaction of trench back fills as not needed. He regularly accepts 90 or somewhat lower percent compaction instead of higher numbers because it works and the roads over the filled trench do not settle.

Edit: Go back and look at some old jobs. Probably no problems.

 

[SlideRuleEra]

SRO - Thank you. In the southeast, above the water table, we can get surface settlement as large, bulky pieces of wood (like buried stumps) decay. The surface depression develops slowly, starting in, say, about 5 years then deepening for another 5 years or so. This can play havoc under a road, slab or foundation, but is usually just filled in if it's in an open area.

For smaller (and thinner) pieces of wood, like concrete forms abandoned in place, not a problem.

I suppose flowable fill would, more or less, retain it's shape and adjacent soil would react as the 3x10s decay. IMHO a deep, continuous wall of decayed 3x10s is too much volume to ignore (In our location). Doubt if decaying 8x8s embedded in flowable fill would cause any problem for a long, long time.

http://www.SlideRuleEra.net

 

[r13]

I've never heard of long term settlement occur naturally, without loading change on top of the trench. But, not as it won't happen, just lack of studies and reports. However, in practice, we always require the contractor to retrieve all temporary excavation supports and move out of the project site. And the tree trumps and degradable materials encountered during excavation to be removed and backfilled. The removal of temporary supports, and underground debris are cost items covered by demobilization, and cleaning and grubbing under earth works.

 

[oldestguy]

Talk about fussy. In about 1965 for a Sears store in a large new shopping center I was asked by their Engineer in Chicago to send him a box of soil from footing grade for his approval. I had OK'd the soil as good for some certain bearing capacity. When it arrived he called me and complained that there were some roots in the sample, hair sized. Took some lengthy phone calls to clear it up as being OK. That partly explains why they no longer exist.

 

[cvg]

i have observed significant settlement potholes and chimneys develop from tree roots and poorly compacted material in arid desert conditions. and in dams/levees or other water retaining earthworks, roots and stumps (and wood shoring) are absolutely not allowed to remain. I have seen sudden dam failure occur because of tree roots. don't even get me started on burrowing animals

 

[oldestguy]

OG comment. My experience is in mid-west.

 

[MTNClimber]

I haven’t seen any major structural issues from buried stumps or wood but I did know someone that had to re-level their brick walkway every couple of years until he gave in and removed the rotting stumps a foot below the walkway. This was in New England.

As for wood shoring, I haven’t seen or heard of any issues. It’s pretty common to leave wood lagging in place on shoring projects. Although I would be very concerned about leaving wood in a dam.

 

[PEinc]

Wood lagging wales, and braces do not deteriorate sufficiently to cause voids. The wood maintains it shape but does lose strength. This has been proven many times when excavations are made next to very old subway systems and building that needed sheeting when originally constructed. A settlement problem can arise when there was improper backfill where there were voids around buried trash and other debris. In this case, soil runs into the original voids which causes lost ground and settlements.

http://www.PeirceEngineering.com

 

[cvg]

termites

 

[SRO]

Thank you everyone for the feedback.

 

[PEinc]

I have a couple structural engineers who insist on treated timber shoring when underpinning historic buildings and museums that have a lot of old wood structure. Their reason is termites, as cvg mentioned.

http://www.PeirceEngineering.com