Cedar Lagging in Soldier pile wall

[designrider]

I am designing a canal wall using soldier pile with timber lagging. Timber is required due to constraint of being "restoration of historic properties", otherwise I would use concrete lagging. Additionally, I cannot use ANY wood preservatives to treat the timber. The front of the wall will have variable height water and the back of the wall will retain engineered backfill. Considering the wet/dry cycles and soil exposure, this wall seems doomed to rot even using Cedar or Redwood lagging. Regardless, the client wants to move forward using Cedar. Any suggestions for protecting the lagging from rot? I am proposing using a 12 inch wide drainage rock behind the wall wrapped in geotextile separator fabric. Thanks in advance!

 

[designrider]

Could geocomposite fabric drain or drainage composite materials provide a reliable separation to lengthen lifespan?

 

[oldestguy]

My older "Wood Handbook" from US forest Product Lab has a table of resistance to decay. Left side column shows the best, but even then with species there is high variability.

Note the "2" designation as the best.

Not only the lagging, but also the piles should be decay resistant. Getting resistant piles may be difficult.

 

[oldestguy]

Guess I had better try insert again.

 

[designrider]

Thanks for the decay table oldestguy. The species with high decay resistance identified with the 2 superscript are not readily available in my area, while Western Cedar is. I specified a drainage system using Mirafi G200N geotextile composite fabric abutting the back face of the wall. This is a 10mm thick double-layered filter fabric with voided core. It will extend continuously from top of wall down and fold under the bottom of the lagging to act as a weep system when the canal is dewatered. When the canal is full this voided system will likely fill with water, but it will wash out continuously due to current flow. The most significant benefit is that it will not allow direct soil contact. It is not a fool-proof system, but ideally in will prolong the lagging's lifespan.

 

[oldestguy]

Here is an idea. The house I bought 4 years ago is handicap accessible so the front entry is at grade, no step. Thus the section of front building wall (wood interior) wall above the foundation is below grade about a foot. Wall has brick exterior sitting on brick ledge "supposedly" with a flashing in back. Contractor was no help when asked about flashing says "If they put it in". Not trusting that, I painted the brick below grade with the coating that is supposed to be masonry waterproof and laid a sheet of black plastic against that. Who knows if it helps. I have seen no sign of problems looking and probe testing on inside. No outside efflorescence. House is 14 years old.

So for your wall, in addition to the drainage fabric,and other features, a sheet of plastic, say 5 mil, against the timber also may help. Of course this assumes your rear fill and drainage materials will prevent a build up of excessive hydro pressure behind the wall.

 

[msquared48]

Why don't you use both the structural concrete panel back wall and a non-structural sacrificial cedar plank front?

Mike McCann, PE, SE (WA)

 

[designrider]

msquared48, thanks for the suggestion. I also thought about a timber faux front facing, but to be economical this would only be a 2x or 3x veneer which may be subject to rotting out faster as it has less section area. I would estimate that replacing a faux facing versus replacing structural lagging would be nearly as difficult and thus nearly as costly, especially if it needs to be done more frequently. As is, I am proposing 6x cedar while structurally I only need 4x. This allows a reasonable section loss and it will still be adequate. The client did allow the portion of wall below low water level to use a precast concrete panel for lagging. The upper portion will be the cedar.

 

[PEinc]

I guess concrete lagging that is stained to look like wood and poured with a wood grain architectural finish is no good? I don't like the idea of using wood lagging in a water environment. msquared48's idea is a good one and could probably be done so that the replacement of the wood façade is easy.

http://www.PeirceEngineering.com

 

[designrider]

PEinc, thanks for the input. Per the "standards for restoration of historic properties" it has to be real wood. This is a replacement design, and the existing wall is about 60yrs old and used treated timber lagging. The key word being treated. Now I get to duplicate appearance without the option of using preservatives due to environmental constraints. I proposed a non-toxic treatment, but it was not viable due to the project schedule relative to approval process for a product not on the QPL. I am painted into a corner. So it goes. I will consider re-explore the concrete lagging with cedar facing. In any case, I have done my due diligence to inform the stakeholders that this is not a preferred solution from an engineering perspective.

 

[PEinc]

How about using Ipe, or similar, lumber? This type of lumber is used for boardwalks and other structures where strength and durability are required. The is probably too dense for pressure treatment.
See attached data sheet.

www.PeirceEngineering.com

 

[PEinc]

The is probably too dense for pressure treatment. This hard wood is probably too dense for pressure treatment.

http://www.PeirceEngineering.com

 

[3Fan]

We have specified IPE for pedestrian bridge decking per the fabricators recommendation. "They" claim 20 years of maintenance free usage. Very very dense wood and looks spectacular when new but will haze to grey in a year or so. Pressure washer will make it look new again.

I like the idea of precast lagging with IPE facing. Maybe flange mount the IPE for easy removal/replacement if necessary?

 

[designrider]

I checked the availability of IPE and it only comes in 1x or 5/4x thickness, and I don't like the idea of a laminated or built-up section in this wet environment. I need at least 3x, preferably 4x. I think the cedar is still my best option for a locally available product. I estimate a 20+yr life span if I drain it, provide soil contact separation, and increase design thickness to allow for section loss. The next best solution is concrete lagging with cedar veneer facing. I appreciate all the discussion feedback. Thanks!