wood lagging

[shorebob]

As a shoring designer, I'm running into some clients and consultants who won't allow pressure treated (.4 pcf per AWPA guidelines) 3x12 DFir wood lagging to remain in the ground, citing potential voids and resulting soil subsidence. This is particularly problematic when the lagging is used as a backwall form. Any experience out there pro or con this ? (To DRC1 & PEinc, I've read your postings of Feb. 3, 2004. Thanks.)

 

[PEinc]

In over 30 years of designing and building temporary and permanent earth retention walls, I have rarely seen treated wood used for lagging unless it is permanent, exposed lagging. Sometimes treated lagging is used when someone is worried about termites near a museum or some other "critical" building.

According to FHWA's 1976 Report No. FHWA-RD-75-130, Lateral Support Systems and Underpinning, Vol. III, Construction Methods, Page 275 & 276, Horizontal Wood Sheeting, "The material used for sheeting is commonly untreated wood. Occasionally, because of concern over future deterioration, specifications require treated wood, concrete planking, or steel sections. The issue of wood rotting is presently (1976!)controversial. Many contractors have found that even with deterioration the fabric of the wood remains intact, thus preventing earth from filling the space occupied by wood."

On Page 47, "There is a divergence of opinion among practitioners as to whether or not wood can be left in place permanently above the ground water table. Some claim that deterioration of the wood leads to lateral movement of soil and therefore ground settlement. Others point to many examples of wood remaining strong enough to provide the necessary resistance to prevent closing the space occupied by wood. Given these diverse opinions, one has no alternative other than to be conservative when adjoining structures must be protected. Therefore, the viable options are to remove lagging that would be permanently above the ground water level or to treat with chemicals to prevent future deterioration."

According to Alan Macbab's Earth Retention Systems Handbook, 2002, Page 188, "Lagging can be utilized in either temporary or permanent applications.....Permanent applications occur when lagging is the final exposed fascia for retaining walls constructed utilizing soldier piles.....When its use is permanent, it almost always is treated. Some municipalities require that temporary lagging be treated, but these are in the minority."

In Winterkorn and Fang's 1975 Foundation Engineering Handbook, Chapter 22, Underpinning, Page 630, it says, "All pit work is sheeted with 2" x 8" untreated horizontal timbers as excavation proceeds."

Schnabel Foundation Company's Specifications for a Contractor designed Tiedback Retaining Wall using Soldier beams, Lagging, Tiebacks and Concrete Facing, Section 2.20 states, "Lagging shall be untreated, non-stress graded rough cut lumber, No. 3 or better...."

I've never seen anyone not allow treated wood to remain in the ground. In over thirty years and hundreds of walls in many states, I have never removed lagging except for the upper 2 or 3 feet near finished grade. Treated or untreated, lagging is never removed and can not be removed from a soldier beam and lagging wall with a permanent concrete facing poured against the lagging.

 

[PEinc]

P.S. In what Forum did you see our 2/3/04 posts on lagging?

 

[GeoPaveTraffic]

I agree with PEinc. While I don't quite have 30 years, I've never seen temporary lagging removed more that 2 or 3 feet from the surface.

 

[shorebob]

To PEinc. Thanks very much for your comprehensive reply. This mirrors my experience that lagging need not be removed in light of no records of deleterious effects. I'd be curious in what region of the country you've seen untreated lagging used. Here in SoCal it's the standard. To answer your question, I'd read your posting in the Earth Retention Engineering Forum, responding to "what are criteria for using soldier piles" (Thread 255-86011), para.9

To GeoPaveTraffic. Thanks for your corroboration.

 

[PEinc]

I've designed and built lagging walls throughout the Mid-Atlantic and New England states, California, and Hawaii. My former employer, Schnabel Foundation Company, designed and built walls nationwide. Most used untreated lagging.

 

[DRC1]

I would like to echo what PEinc said. We will take out lagging to a depth of about 5 feet or so with out difficulty, but below that it gets tricky. If your goal is to reduce ground movements, then don't take out the lagging.What ever small movement you might have due to rot you have signifcantly more risk removing the lagging if the soil comes in on you.

 

[PSlem]

I worked with Spencer, White, & Prentis. I believe White is the one credited with exposing 50 year old untreated lagging and stating that while its strength was gone, it still occupied the space. Most of their lagging was tear outs of old NYC buildings. I've put in lagging all over the East and did not use any PT. A lot of jobs specify the lagging shall be placed in the soldier pile flanges up to 3' from the top where it shall be placed behind the soldier piles. This facilitates burning the piles off below grade.

 

[PEinc]

PSlem,

You are correct. Although, lots of people have dug up very old lagging boards. By the way, are you aware that SWP is out of business - again. Too bad.

 

[PSlem]

I also worked for Kerr, Franki, Soletanche...Now when I fill out job history for PE applications I wonder what they think.

 

[PEinc]

You're the "Kiss of Death" to your employers. You run them all out of business! (Just kidding)

 

[PSlem]

I prefer to think it was my leaving that did it.:-}

 

[Schwager]

What is the most common type of wood used in lagging operations?

 

[PEinc]

Along the U. S. East Coast, lagging is usually ungraded, rough cut, mixed hardwoods.

 

[DRC1]

I used to use Southern Yellow Pine but that has gotten expensive so as PEinc said, we pretty much use mixed hardwoods 8-12 full.

 

[GeoPaveTraffic]

In the mid west we generally use 3-inch rough cut oak with soldier pile spacings of 8 to 9 feet.