Suitability of Timber Retaining Wall

[ZiggyMan76]

Considering possible structure types for a local sidewalk project; the wall is smallish - about 150 ft long with a max H of just under 7 feet; it takes one right angle turn near the tall end and there is almost no overburden. The (e) embankment is stable and standing at about 1.5:1 (H:V) but property line restrictions dictate a wall rather than another slope. They also severely limit my ability to use any sort of earth reinforcement system.

People seem to like the look and feel of a wood retaining wall. I too appreciate the aesthetics but there are so many concerns: durability, vandalism (e.g. fire resistance), potential for (slow-speed) traffic impact, maintenence, etc.

Would anyone have any thoughts or advice to share on these issues? Can a timber wall be made durable enough to "compete" with other cantilever material types such as concrete, shotcrete or CMU blocks? Can wood treatments or preservatives be used safely where people walk?

Any assistance is appreciated.

 

[SlideRuleEra]

A "permanent" treated lumber retaining wall can be designed and constructed, but cost-effective CCA preservative treatment may no longer be an option for this use. Here are a couple of web sites that offer background info on the wood preservative treatment options that are still available (and considered safe) -

http://www.southernpine.com/pressuretreated.shtml

http://treatedwood.com/

As the information on these websites will show, the material for fasteners used with available treated lumber must also be selected carefully.

In my opinion, treated lumber is not the best choice for retaining walls.

 

[PEinc]

ZiggyMan76,

You need to better describe what you are trying to do with the wall. Is the sidewalk in front of the wall at its base or is the sidewalk behind the top of the wall? Is the wall to hold up an excavated face or is the wall going to retain fill dirt? What do you mean when you say there is almost no overburden? Is there any bedrock present? If so, where is it with respect to the top of the wall? If there is a stable 1.5:1 slope, why do you need a wall?Where is the restricting property line? I assume it is behind the wall. Many answers are needed for us to understand your problem.

 

[ZiggyMan76]

The goal is to construct a sidewalk adjacent to an existing road; the road is narrow (22 ft total for 2 lanes) and was constructed many years ago in a cut section. The new sidewalk will be located just in front of and at the base of the new retaining wall. Most likely the wall will retain an excavated face (e.g. shotcrete or timber lagging bearing on in-situ dirt) but we may be able to obtain a temporary encroachment in order to "slope" the excavation; therefore it could support a wedge of fill between the wall and the in-situ cut face.

Almost no overburden means the slope ends at just about the top of wall (the ground is nealy level behind/above.) The Property Line is located just about at the hinge point (where the cut slope was started and the new wall will top out.)

 

[rockchip]

ZiggyMan76: In my experience, timber walls are mostly constructed as timber cribs, that is they have a 3-D cellular box structure forming pockets within which can be placed fill. The combination of timber facing and "rock-filled boxes" creates a stable mass that can be designed to resist the earth pressure loads. Alternatively, tie-backs or deadman anchors are used to stabilize the facing. Since you already stated you have no room for reinforcing, you are faced with the possibiity of resisting the wall loads with columns installed deep into the ground, like soldier piles. The depth and horizontal spacing would depend on the characteristics of the soil to resist lateral loading. Then you are faced with the prospect of durability of the wall and environmental issues of the wood face treatment. I have seen treated wood crib walls in my area (North Idaho) that have been in place for 100 or more years. Also, by the time you get to designing and installing such a system, it may cost more than a conventional cantilever concrete retaining wall. Other options may be a gravity wall using large precast concrete blocks (Ultrablock, Redi-Rock, Eco-Block, etc.). These are usually available with architectural facing. That's my 2 cents worth!

 

[PSlem]

We've done walls with HP8x36 soldier piles and 6x6 lagging dropped between the flanges. Just drilled a hole and concreted the toe. We tied it back after five feet in height, but you can run the numbers to see what works.