Timber Retaining Wall with Deadman 2

[RFreund]

We have a new project we are considering taking on which is the replacement of an existing failed timber wall which has already partially been rebuilt (Stop Work due to no engineered drwgs). The wall is about 8' tall at its tallest point. I believe the new construction consists of 6x6 timber facing with timber deadmen. The problem is I'm not very familiar with the design of timber walls of this nature. It seems similar to sheet piling with tiebacks and they may have used a timber pile behind the wall every so often. I've attached a sketch showing what I believe the wall construction will resemble and my assumption of what the analysis should look like. I would appreciate any comments on the sketch as well as any design guides or manuals for walls similar to this.

couple concerns/questions:
Is this type of wall construction acceptable?

How do the deadman anchors' resisting force propagate through the wall (the joints are staggered)? Shear x-fer thru nails is my assumption, but is there a way to calc this or is there a recommended spacing of the anchors?

Thanks again (lots of questions from me this week)?


EIT

 

[SixDegrees]

The sketch looks fine. My only comment is that the deadman approach does not seem to be compatible with a wall face that has no bending stiffness. There needs to be horizontal equilibrium of forces and moment equilibrium. And the wall face needs to be able to transmit shear and bending these forces.

Maybe a better option if you have access to the back of the wall would be to lay in a High Density Polyethylene (HDPE) grid, you'd only need one about 6 feet long, every 2 feet, and you could just attach these between the timber crib boards and you'd have a very nice Mechanically Stabilized Earth (MSE)Wall.

 

[RFreund]

I still have yet to go to the site and see what has actually been constructed, but I do like that idea.
Thanks.

EIT

 

[FixedEarth]

RF;

8 ft retained height is not tall enough to warrant a tieback. The design is very similar to that of soldier beam and lagging.

 

[PEinc]

Are you allowed to move the face of the new wall out about 2 feet in front of the existing timber wall? If so, it could be cheaper to install coated or galvanized soldier beams and stack either pressure treated timber lagging or precast concrete lagging and then backfill the space between the lagging and the old wall with crushed stone with a geotextile over the face of the old wood wall. There would be no need to remove and dispose of the old wall. There would be little or no excavation. There would be a small amount of easy crushed stone backfill.

www.PeirceEngineering.com

 

[irawanfirmansyah]

I suggest to use timber facing wall and nails of steel rebar, instead of timber deadmen.

 

[RFreund]

irawanfirmansyah -

are you saying that the steel rebar is driven down (vertically) through the timber that is extending back (perpendicular to the wall) into the earth? Is there a way to calculate the resistance that this rebar provides?

EIT

 

[irawanfirmansyah]

RFreund,
Yes,steel rebar go trough the timber facing wall, extending into the ground, making an angle of 10 - 20 deg to horizontal plane.
If the ground is not too hard, you can use backhoe to push rebars into the ground.
Frictional resistance of nails can be calculated as driven piles/bored piles, depending on method of nail installation, whether you drill the ground or you push the nails into the ground;
- in clay/silt : fs = alpha x Cu
alpha = 0.7 - 0.8 for pushed nails
0.5 for drilled nails.
- in sand : fs = sigma vert x Ks x tan phi
or 2 N SPT (unit kPa)

 

[PEinc]

It's not as simple as driving a rebar into the ground. You need to make a connection between all of the timbers and the rebars or soil nails. Without knowing exactly how theoriginal and newer walls were built, you can't be sure if your redesign is appropriate. I would consider the soldier beam wall in front or I would apply reinforced shotcrete to the timber wall and then anchor it back. That way, I'm not counting on a failed or improperly rebuilt timber wall.

http://www.PeirceEngineering.com

 

[RFreund]

I was out at the site and they have part of the wall constructed similar to my sketch except there is no deadman at the end of the tieback only a 1/2" rebar. The other part of the wall uses a soldier pile that is installed behind the wall. The spacing however is about 12' and I don't believe either of these will work. The wall is only about 5.5' tall. I was thinking about installing more of the soldier piles. Or possibly the soil nails idea.

I have not design many or any soldier pile walls and only run the calc a handful of times. Normally I would use the California T&S Manual to find embedment required where they use an effective width = 0.08*phi < 3 . However they do not reduce the passive pressure, instead they increase the embedment by %30 percent or so. Is this design method suitable for permanent structures?
Or should I use an effective width = 3*w and reduce Kp by 1.5 or 2? (may end up being similar designs)
Or is it necessary to use a more rigorous design method, any suggestions?

EIT

 

[PEinc]

For passive resistance of soldier beams, I would just use 3 x the soldier beam flange width (or concrete drill hole diameter) and then increase the calculated required embedment by at least 20%.

http://www.PeirceEngineering.com

 

[irawanfirmansyah]

RFreund,
If you have LPile, you can easily calculate the required embedment and wall inner forces (See the thread by jalthi "Tangent/Secant/Drilled Shaft Wall Modelling in LPile", in this forum).
8' wall is relatively a short wall, and it should be designed as cantilever wall