Timber Lagging - Long Term Performance

[Derteeberd]

Can anyone direct me to a reference regarding the long term perfromance of timber lagging when subjected to a wet environment?

I am looking at designing a soldier pile wall that would use timber lagging so the retained soil would be free draining. We would like to install a cathc basin in front of the wall to catch surface water and route seepage water out from the wall. Then construct a new concrete wall in front and hopefully utilized reduced earth pressures from the soldier pile wall with tiebacks.

Any recommendations would be appreciated.

 

[Focht3]

Timber - even pressure treated materials - don't seem to last more than 10 to 15 years (in my area.)

I'm not a big fan of this approach, partly because the damn things are ugly and soil fines are easily washed between the boards in wet environments. They aren't much good as permanent walls higher than about 10 feet in wet clay environments. And you would need to install a proper filter and drain behind the wall if you are retaining clays.

The soldier piles have to be painted regularly, and rust along the web and backside of the flanges. Critters often burrow into the space behind them - which renders filter fabrics useless.

They are cheap when compared to concrete walls, and can have a small installation footprint. But I'd look hard at a MSE wall as my first choice.

 

[Derteeberd]

My Problem is that this wall is 30 feet high and we don't have enough room behind the wall to excavate due to trees etc.

We planned to install in front of the soldier pile wall a crushed rock drain and catch basin, then a new concrete wall designed for low earth pressure and tied to the soldier pile wall.

Soil nailing is not realling an option due to the condition of the material behind the wall and the anticipated groundwater.

It sounds like sheeting is probably the best answer with tiebacks and burn some holes into it for drainage into the drain rock.

Do you agree?

 

[Focht3]

What is the soil profile? Installation limitations?

 

[PEinc]

Focht3 is correct about the lagging. I've seen lagging in service after 7 years and it isn't pretty. I've also seen buried lagging uncovered after 15 years and after 60 years (around old subways). It is still intact (no voids created below ground) but the strength is gone. I've also built permanent soldier beam walls with treated timber lagging and precast concrete lagging. Both lagging types have their advantages and disadvantages.

Have you considered just building a tiedback, soldier beam and lagging wall with an attached reinforced concrete facing (either cast in place or precast)? These walls are very economical for high walls. The soldier beams, lagging, and tiebacks would support the earth loads before the concrete facing is constructed. The soldier beams, tiebacks, and concrete facing would support the earth loads after the facing is constructed. You build only one wall. You don't build two walls (sheeting wall and conventional concrete retaining wall) and hope that each holds its share of the load.

For a permanent tiedbsack wall, the soldier beams are usually coated with coal tar epoxy. They can be either driven or drilled in place. The lagging is usually untreated, mixed hardwoods. The tieback anchors usually have an additional level of corrosion protection. Miradrain or EnkaDrain chimney drains are usually attached to the timber lagging under or behind the attached permanent concrete facing. There are usually weep holes from the chimney drains, through the facing, at the bottom of the facing.

You would need an easement for both temporary and permanent tieback anchors if they cross property lines. Temporary easements are easier to get than permanent, especially if there are utilities behind the wall. Tiebacks should pass under the utilities, not over them in case future utility work is required.

Check out examples of permanent tiedback walls at

http://www.schnabel.com

 

[Derteeberd]

I understand what you are talking about but my greatest concern is how I can construct a 30' high catch basin behind the exposed facing wall to route surface water away from the wall (no way to route on surface without causing other problems). That is why I was looking at Soldier Pile with timber lagging then using a concrete wall in front of that (about 5 feet) that is tied back to the Soldier pile wall.

 

[Focht3]

As I understand your concern, you are worried about dealing with surface water behind the wall (i.e. at the top of the wall.) Although I have to admit that the bits and pieces you have provided are a bit confusing. Could you provide a description of the geometry of your problem and the site constraints? We also need to understand the subsurface conditions in order to really help you.

 

[PEinc]

Can't you just build a shallow catch basin at the top of the wall between 2 of the soldier beams and then run a drain pipe into and down through the concrete facing to an outlet at the bottom? You might need to set those two soldier beams back a little bit to accomodate the "downspout."

 

[Derteeberd]

That is a good idea, thanks.

I just got off the phone with a contractor discussing the soil conditions, site access, etc. My thoughts about the use of lagging and running sands has been confirmed by them that it would be very difficult to construct.

Access to the top of the wall is limited so secant piles or drilled shafts are not easily constructed either.

Sheet pile might be the only option and cutting holes for drainage with a concrete facing attached to the sheet pile. Then we have to address how far off the wall the facing might need to be offset due to two rows of walers etc and how to make that connection.

 

[MSEMan]

I think there is a better way. There is a really neat (patented) system on the market that incorporates tied back H piles with precast concrete facings. Drainage is provided by single size stone placed between the facings and the lagging. the steel soldier piles have shear studs welded onto them to secure the facing.

Sequence of construction is as follows:

Augur pile holes, sink H sections and place concrete.

Excavate top down placing the lagging behind the back flange of the H section.

Install soil/rock anchors as excavation proceeds.

Pour a small concrete strip footing at the base.

Build up precst concrete facings, pour reinforced concrete connections. Place drainage stone between the facings and the lagging as the work proceeds.

Its a really super system. If you need more info contact me at DPMcK02@aol.com.

 

[PEinc]

MSEMan,
That type of wall is not patented. However, some types of precast panels and their connections are patented. I know that both Schnabel Foundation Co. and Terra Tech, Inc. have patents on their precast panels. However, the wall you described is essentially the same as what I suggested. Cast in place walls use geocomposite chimney drains while precast panel walls use stone fill between the lagged wall and back of panels.

Derteeberd,
Running sands can be a problem for lagging installation. Are the sands running because they are loose, dry, and poorly graded or because they are below a water table? If groundwater is a problem, the owner will have a problem after the wall is built. A single wall will not be a very efficient cut-off. If the sands are just loose (not water borne), then you may have to close the soldier beam spacing to about 6 or 7 feet so that lagging can be installed with a little more control. Cast in place concrete facings are usually attached to the soldier beams with welded, headed shear studs. Precast facings usually have some type of welded, threaded connection to accomodated threadbar connections between the soldier beam and inserts in the back of the concrete panels. I personally do not like the precast panel systems. I have concerns about the long term corrosion of the connections (usually galvanized). I've designed and built both types of walls. I prefer cast in place facings.

Have you thought about accepting design build proposals from tiedback retaining wall contractors? You provide the soils information, the wall location, the general length and height, possibly any special loads the wall may be subject to, requirements for the catch basins, etc. The contractor will price the most appropriate and economical wall they can build. They are responsible for the type of tieback anchor and making sure it tests to the required capacity. They also pick the type of soldier beam, drilled or driven, and whether or not tieback wales are required (usually not).

 

[Focht3]

Hey, guys - let's take a deep breath and ask a few fundamental questions before we get this wall designed and the drawings sealed! We still don't know what the site conditions are, other than Derteeberd has a possible surface water problem, "running sands", limited installation footprint - and the wall will be 30 feet high! Am I the only one still concerned that Derteeberd hasn't told us anything about the site location and the subsurface profile?

Derteeberd: What are the soil conditions? Do you have recent geotechnical data? Please describe the subsurface conditions.

 

[HarleyDD]

A simple solution here is a plain tied back sheet piled wall. Assuming no major obstacles to soil penetration. The height is not a problem. Weep holes are only necessary if you have/need to drain the retained soils. Otherwise simply include the water head in the design and choose a sheet pile for the capacity. For a permanent seal the ready made V at the locks can simply be welded with a sealing weld (not structural) for a permanent water tight finish. The piles can be coated with a colored epoxy coating for a good finish. And nothing to build in front of the wall.

 

[Focht3]

There are lots of possible solutions, but...

We still don't know what the subsurface conditions are!

 

[ARIFSIDDIQUI]

IN GENERAL IT IS FOUND THAT CLASS 1 TMBER LAGGING CAN LAST TILL 6 TO 7 YEARS UNDR WET CONDITIONS BUT IT'S PERFOMRMANCE GETS DETERIORATED BECUASE OF CORROSIVE ATMOSPHERE FURTHER IT WILL DEPEND ON THE SALT CONTENT OF THE WATER AS WELL.

BUT A STUDY SUGGESTS LEAVING THE WEEP HOLES THROUGH THE LAGGING AND PUTTING THE DRAINGE PILE THROUGH THE HOLES WILL EXTEND THE LIFE CONSIDERABLE BUT I DONOT THINK FOR ANY STRUCTURE YOU REQUIRE MORE THAN 1-2 YEARS OF SPAN FOR SUPPROTING THE TEPORARY TIMBER LAGGING.

IF THE TIMBER LAGGING REQUIRES FOR MUCH MORE THAN THIS THAN YOU CAN PUT SOME ADDITIONAL COATING ON THIS WITH WEEP HOLES PROVISION. SOLDIER PILE WRAPPED IN VERY THIN MS SHEETS CAN REALLY SERVE YOUR PURPOSE FOR LAST LONG BUT YOU NEED TO CHECK THE ECONOMICAL CONSIDERATIONS OUT OF IT.

ARIF SIDDIQUI (DELHI METRO PROJECT - NEW DELHI)

ARIF ALI SIDDIQUI
STRUCTURAL DESIGN ENGINEER
IMCC 8 JANTAR MANTAR MARG
NEW DELHI INDIA

 

[jimbeck]

I have used preserved wood timber lagging on several projects. Design for the at rest pressure, aned all should be well.