Precast vs. Timber lagging

[mdstveng]

New Here. In my relatively short experience, I have seen mostly precast panels with CIP facing and studs welded to the front flange used in my state, Maryland. In reading through some threads, I noted that PEinc stated precast panels aren't the way to go unless certain conditions are present (7 April, 2006). I wanted to ask for input regarding the two alternatives, as I do not want to be limited in my designs, or my suggestions to clients.

 

[PEinc]

I have used precast panels for low, cantilevered wall where the panels could be stacked from the bottom and then be backfilled. This works OK for low walls where the ground behind the wall can be safely sloped. These walls can be economical.

I have also designed and built permanent tiedback walls with temporary, but left-in-place, timber lagging. After the wall was constructed down to subgrade, we came back and studded on precast lagging to the face of the soldier beams. This was early 1980's technology. Now outdated. The walls are structurally OK but are not the best looking walls.

I've seen many cut situation, tiedback walls with precast lagging that were built by others, looked terrible when finished, and were dangerousand difficult to build because the contractor had to excavate a very high, very steep, unsupported slope so that the panels could be stacked from the bottom up. A terrible method. Not economical.

 

[mdstveng]

In my experience, precast lagging panels have been used for top-down construction, then once the excavation is completed, studs are welded to the front flanges, and a CIP wall is placed, and later finished with and inch or two of aesthetic concrete.

Do you consider a timber lagging system with a CIP wall that encases the front flange a more appropriate top-down construction design?

 

[PEinc]

Yes. You can not easily or safely install precast lagging from the top down. Lagging should never be slid down as the site gets excavated. Therefore, you dig about 5 feet and install lagging. Dig another 5 feet and install another 5 feet of lagging BENEATH the first 5 feet of lagging, always placing the lowest board first.

Once a wall is lagged and tiedback, the deflections have already occurred. Then the finished CIP, shotctete, or precast facing can be attached to the soldier beams.

A CIP facing does not "encase" the front flange of the soldier beam. The facing is poured or sprayed against the tiedback wall and is attached usually by welded shear studs. Also, don't forget, the wall also needs some drainage system such as geocomposite chimney drains.

A CIP wall can use a form liner for aesthetics. A shorctete wall can be sculpted. A precast facing can have aesthetic details cast in. I have never seen or even read about a separate 2" thick aesthetic coating being used.

Precast panels are heavy. It is hard to make sure there is full, tight earth contact behind precast lagging, especially if you try to slide them down while adding additional panels at the top of the wall. Also, tieback anchors may interfere with sliding down the precast panels (unless you use a through-beam tieback connection detail). If you try to build a wall with through-beam tieback connections and precast lagging tucked behind the soldier beam flanges, the soldier beams may become very large which will then require large diameter drill holes which adds significant cost. In addition, the soldier beams have to be positioned perfectly to assure proper bearing of the precast against the flanges.

A CIP or shotcrete facing hides, or compensates for, the inevitable, as-built, wall discrepencies.

If (as you describe above) you are studding on a CIP facing, why would you also need to use precast lagging?????? Use wood lagging, usually untreated.

 

[mdstveng]

I follow regarding the precast vs. timber. Maryland DOT apparently likes the front flange to be encased. I have details illustrating this, if you would like.

Can you tell me where I might be able to find some details illustrating the configurations you mentioned?

I appreciate your input, as I would like to make sure that my clients get the most out of their money.

 

[PEinc]

FHWA has Geotechnical Engineering Circular No. 2 and No. 4. They are design manuals for anchored walls.

If you have the Maryland DOT details, I would like to see them. Please give me a web site to check or send them to me. Google "Peirce Engineering, Inc."

I have never seen anyone encase the front flange of a soldier beam. What is the purpose? Corrosion protection of the front flange? If so, why worry about the front flange rotting but not worry about the soldier beam web rotting too?

"Apparently likes" doesn't necessarily mean "requires." Check Maryland DOT's requirements. Just because they suggest or show a bad detail, it doesn't mean you have to use it. You are ultimately responsible for the design.

 

[mdstveng]

I will send them to you. Maryland's DOT is quite parochial and often will turn suggestions into requirements. They also have not published soldier pile wall standards.

Do the FHWA circulars have sample design details or just descriptions that can be interpreted.

 

[PEinc]

These's more information than you will ever want to see. The manuals are available on the FHWA web site. AASHTO and DOT's follow FHWA plus add their own extra special touch. Check out the FHWA anchored wall earth pressure diagrams. They are using non-symmetrical trapezoidal diagrams where the shape is dependent on the tieback locations. Check if you need to use ASD or LRFD design methods.

 

[DRC1]

What we have done in the past is to tie anchors directly to the soldier piles, lag w/ timber, put a membrane up and cast a structural wall against the piles thus the integrity over time of the piles/anchors is not an issue. The form is one sided and either externally braced or tied internally to the soldier piles.

 

[PEinc]

I have never installed a membrane over the wall. Usually, we install a vertical geocomposite chimney drain approximately 20 to 24 inches wide between the soldier beams. Usually, soldier beam walls are not used in areas with enough ground water to warrant a membrane behind the finished face.

 

[PEinc]

mdstveng,

I reviewed the Maryland DOT lagging details you sent me. Terrible! I suggest you call a design/build contractor from the Mid-Atlantic area who probably has designed a wall for Maryland DOT.

Try calling 1. Schnabel Foundation Company in D.C. or 2. Hayward Baker in Odenton, MD or 3. Berkel & Co. in Pasadena, MD or 4. Terra Tech in Berryville, VA. Any of these companies should be able to give you some of their experience with Maryland DOT, if they have any.

If you look at the details, you will see that the DOT has presented crappy details for the lagging and facing and then notes that the designer or contractor has to detail where the tiebacks go and how they will be connected. That's because the DOT hasn't a clue what they are doing.

One of the details notes "Studs shall be installed by Schnabel." Call Harald Ludwig @ Schnabel in D.C. Tell him I sent you.

 

[mdstveng]

PEinc,

Thank you for your help. I did not design the details I sent you, fortunately. Unfortunately, these are typical walls that Maryland's DOT is used to seeing. I would like to provide a better design for them.

I downloaded GEC Circular No. 4. Circular No. 2 is not available on the FHWA website. If you have an electronic copy you could send me I would appreciate it. If not, I'll get it from FHWA.

In any case, I will contact some design/build contractors who have worked in Maryland and see what they have used.

 

[pacobox]

Does anyone know if there are any DOT standards in the Northeast for permanent, or even temporary, Soldier Pile and Lagging walls? or anywhere in the US? i can't find anything on wisconsin's site. they are usually good for some information/ideas.

I am in the philadelphia area and am just trying to find some kind of standards to initiate with.

appreciated.

 

[SlideRuleEra]

pacobox - See Chapter 10, of the "California Trenching & Shoring Manual" at this link

http://www.dot.ca.gov/hq/esc/construction/Manuals/TrenchingandShoring/ch10_soldier_pile.pdf

I see that you have just joined Eng-Tips, welcome. The next time you have a question, just start a new thread rather than placing it at the end of an existing one - you will get more responses.

http://www.SlideRuleEra.net

 

[PEinc]

pacobox,

Be careful if you are trying to design a PERMANENT tiedback wall for a PADOT project. I haven't done one lately, but PADOT had been insisting on redundancy in the design so that, if one tieback failed, the load on that soldier beam would be transfered to and be picked up by other tiebacks. This led to many different and uneconomical wall designs. PADOT also wants LRFD design for temporary and permanent walls. You will have to follow PADOT's Design Manual 4, AASHTO bridge design specifications, and FHWA's Engineering Circulars for anchored walls. Watch out for the non-symmetrical, trapezoidal, earth pressure diagram for tiedback walls.

I'm in the Philadelphia area also. Google "Peirce Engineering, Inc."

 

[pacobox]

hey thanks PEInc. yeah penndot seems to like to be extra safe.

Do you happen to have an electronic copy of Circular 2?

i guess FHWA doesnt have it for download anymore, and i can't seem to find it available for ordering on the NTL site.

 

[PEinc]

Don't have it except in hard copy. Try NTIS.

 

[SlideRuleEra]

Cost $25 to $64, depending on the format

http://www.ntis.gov/search/product.asp?ABBR=PB97173629&starDB=GRAHIST

http://www.SlideRuleEra.net

 

[PEinc]

Thanks, SlideRuleEra. I didn't have the link handy when I responded.

 

[pacobox]

thanks for the help slide rule and PEinc.