Timber lagging in clay 1

[ESSSOE]

HI,

WE are designing a 40 foot high soldier-pile and lagging anchored wall (pile spc 5 feet and 6 feet) in very soft gray varved clay in upstate NY. Can you even use timber lagging to support the clay? At what pile spacing?. Excavation will be open for about two years. We dont want the clay to fail and flow!!. There are some references that support the use of timber lagging and some that dont at various pile spacings. Any experience out there?

Thanks.

 

[PEinc]

The following is copied from my response to your question in another thread:

Are you working for the project owner or for the contractor? What is "very soft clay"? Soldier beams spacings of 5 and 6 feet are very close and therefore probably uneconomical. If the soil really is soft, perhaps you should consider a tiedback steel sheet pile wall. This will give you the strength you need without the concern for timber lagging. Into what type of soil will the tiebacks be anchored?

Because this is a temporary sheeting wall, you probably should not be designing it. You should obtain and provide pertinent information (soil borings, soil properties, wall height, location, special surcharge loadings, easements, etc.) and then issue a performance specification. Let the contractor get design-build proposals from specialty contractors who are very experienced in tiedback wall design AND construction. It will be hard for you to design the wall and tieback anchors if you do not know what type of anchor the contractor intends to use, how he intends to drill and grout the anchors, or the anchor capacity the contractor can obtain in the bond zone soils.

I guarantee that if you design this wall for the owner so that contractors can bid on the wall, the contractor who eventually builds the wall will want to redesign the wall. If so, the owner has wasted a good amount of money on an unused design. If the contractor does not want to at least partially redesign the wall, he probably isn't qualified to build it.

 

[BigH]

PEinc:

 

[jdmm]

PEinc,

I your experience between owners-engineers-contractors is both interesting and disturbing. It seems that implicit in your comments you assume that specialty contractors have access to engineers that have special knowledge in a certain area. In some cases this is true, in others it becomes a bidding war based on perceived factor of safety. There will always be a contractor willing to take more risk. If you are confident in your abilities and have characterized the site then you are the one best capable of assessing the risks.

I take a hard line on alternative designs. When subsititutions are appropriate they are allowed at tender where they are not they are not. Deviations disqualiy the bidders entire bid. Competition based upon a contractors willingness to accept risk should not be encouraged.

 

[PEinc]

"..you assume that specialty contractors have access to engineers that have special knowledge in a certain area." Correct, it's a fact! That's why they are "specialty" contractors. They are the engineers who have developed the various ways to build walls. They are the engineers who, for the most part, have monitored walls in an effort to determine actual pressures, stresses, etc. It was these specialty contractors who authored the initial series of FHWA manuals on tiebacks and anchored walls.

You are correct that a contractor could propose a wall with a lower safety factor or other inferior details compared to other contractors' proposals. However, in almost all cases, the contractor's design gets reviewed and approved by the project engineer prior to construction.

Risk is always a significant part of design and construction. Unfortunately, risk (or aversion of risk) is often the driving force in many anchored wall designs prepared by those engineers who do not specialize in this type of work. Those who are not comfortable with this type of work usually wind up using excessive safety factors. If an engineer is that risk shy, maybe he or she is not experienced enough in that type of design and should not do the design.

 

[GeoPaveTraffic]

PEinc, you have some good points. However, specialty contractors are not always as good as they think they are and OFTEN do not have specific knowledge of the soil conditions where the work is being performed. A 40 foot excavation should be designed by a competent professional who has experience in the geographic area where the work is being performed AND experience with similar types of projects. If the contractors bidding on the work want to submit alternate designs, then the owner can have those designs reviewed and evaluated by the design engineer who has the owner’s interests at the forefront.

As far as the specific question presented by ESSSOE, I'm not familiar with the clays in that part of the country. I have installed timber lagging in medium clays in the St. Louis area with solider piles up to 9 feet on center, although 8 foot is the most common. The soils in that area tend to have enough strength to arch to the soldier piles as the lagging deflects. By the way most of the lagging used is 3-inch rough cut oak.

Hope this helps and good luck.

 

[Focht3]

What are the consequences of the failure of the proposed wall? You can't assess the risks - and select an appropriate factor of safety - until you know the answer to that question.

I'm not so sure that the proposed wall is really a "temporary" structure, since it will remain in place for at least two years. That sounds a lot more like a "permanent" structure in many ways even though it will be removed after only a few years of service. And I wouldn't consider the use of wood for a wall that tall, regardless of whether the soils were sands or clays. Also keep in mind that the earth pressures will end up much closer to at-rest values as a result of wall restraint.

I've seen these kinds of designs done as both "design-bid-build" and "builder turnkey with performance specs". In general, I prefer the turnkey approach for short-term excavations that are integral with the construction activity that requires the "hole" - unless the wall will become a part of the completed structure, of course.

How would I approach this assignment? I think I'd go the turnkey approach, while telling the owner that a healthy evaluation budget will be needed. And I would clearly spell out all the structures at risk (and expected modes of failure) in a letter to the owner, with copies to the bidders, so that the risk remains where it belongs...



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[PEinc]

Focht3,

You are correct that 2 years is relatively long for a temporary wall. However, the simple corrosion protected tiebacks, uncoated soldier beams and wales, and untreated timber lagging should not deteriorate significantly in 2 years - unless there is a highly aggressive environment.

Timber lagging is used every day for braced and tiedback temporary sheeting walls of significant height supporting average, competent soils. Please see project reports with pictures for walls on Schnabel Foundations Company's web site (

http://www.schnabel.com).

http://www.schnabel.com/schnabel/nada.html

Also, these tiedback walls are usually designed using active earth pressures, not at-rest pressures. Monitoring of tiedback walls by companies like Schnabel shows that rarely does the actual tieback load significantly increase more than its lock-off load which is usually 70% to 100% of the design load based on active pressures. I know that there are soft, creep susceptible soils and landslide applications where the wall loads will be higher and that there are walls which need to be designed for higher pressures in order to limit movements when supporting supporting adjacent structures. But this is not usual.

It looks like we agree on the turnkey or performance specification approach to these walls.

 

[Focht3]

Hmmm,

I think the big difference between my approach and [blue]PEInc[/blue]'s comes down to a couple of key areas:[ol][li]Is load, or deflection, the more important criteria for evaluating wall performance?[li]How should wall performance be measured?[li]How much reliance should be placed on specialty contractor data and claimed experience?[li]Who really bears the risk should a wall fail to perform as expected. and what role should the assessment of risk play in the structure's design and construction?[/ol]In my view, the key measurement of wall performance is deflection. After all, it's the wall deflection that will allow loss of ground and damage to adjacent structures to occur. The magnitudes of the loads on the wall don't matter if the wall experiences excessive movement. We have to focus on the effect that really matters -

Wall performance should be measured using a combination of devices, including inclinometers, weldable strain gages, load cells on anchor rods, and Mustran cells. Precision surveying is also key, and it must be referenced to a stable benchmark that is unaffected by the construction work. Calibrated jacks don't provide very good data, and anchor loads alone are not a sufficient basis for evaluating wall performance.

I am very careful about which specialty contractors I trust, and how much I rely on their "experience". I think a healthy dose of skepticism is needed in evaluating the contractor's opinions and data. Hell, I do the same thing with data 'promoted' by my peers, so I'm not picking on the "poor contractor."


Who bears the risk of failure? Generally, the owner is exposed to the greatest financial risk. The GC and specialty contractor also bear some risk, although they frequently avoid much of their responsibility for a variety of reasons. The design team may also suffer even when the specialty contractor has provided their own sealed plans.

The key idea is to avoid problems - and the courthouse, if possible. How does one do that? Carefully choose your clients, do first-rate work, and - of course - be lucky!



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[PEinc]

Another important thing to remember is that soldier beam and lagging walls, even with tieback anchors, are not the preferred method of choice for supporting adjacent structures. The are used only when other, better methods have been eliminated for various reasons. And, as Focht3 said, the resulting increased risk must be evaluated - jointly by the owner, GC, and wall contractor.

Soldier beam and lagging walls are not usually equal substitutes for underpinning, tangent caisson walls, slurry walls, and other stiff walls. Neither is steel sheet piling. Soldier beams and lagging and steel sheet pile walls are flexible walls. I agree with Focht3 that deflections can be a problem if a flexible wall is supporting a nearby structure. Also, a stiffer wall with sloppy construction methods can also cause problems.

 

[Focht3]

AGREED.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[DRC1]

A forty foot wall is a pretty tall wall. If you have some doubts, I would go with steel sheetpiling. Even if you are above a true water table, the clay will be very wet and weep into the excavation. The lagging would need to be properly packed with straw and monitered to be sure that fines were not migrating into the excavation. The floor will also be a mess and hard to work on. If the clay is soft you will be fighting heave as you get deeper and should be carefully checked. The steel sheetpiling will be a more forgiving system to install. Also the sheet pile will allow you to have some penetration to reduce the possibility of heave. At forty feet you will have multiple rows of tiebacks (probably 3) and the soil pressure will be based on emperical pressure envelopes. I also agree with PEINC and Focht3 that sheeting should not be the first choice for underpinning.
Let us know how you make out.