Heavy Section vs. Bracing 2

[Chewi00]

Is there an advantage to using a standard sheetpile (e.g. AZ-18) with a moderate section modulus and have to use interior bracing, rather than specifying a pile with a section modulus of 75-80 in^3 in order to achieve a cantilevered dig? I see a lot of designs that could be modified to get away with a very heavy cantilevered section but chose to use a lighter pile with tiebacks or cross-bracing.

Thanks in Advance
Adam

 

[PEinc]

It all comes down to performance and economics. Bigger, heavier, longer, cantilevered sheet piling may work but it may be expensive to buy or rent the sheets and the wall may deflect too much. Also, you may have trouble driving the heavy sheets deep enough.

On the other hand, smaller, shorter, lighter sheets with tiebacks or bracing are cheaper and easier to install but you have to add the price of the braces or tiebacks and wales. If the job is small, you may have trouble getting a tieback contractor inrterested enough to give you a price.

Remember, performance is more important than economics. However, you also need to decide what level of performance is needed.

 

[Chewi00]

great insight....thank you.

 

[GeoPaveTraffic]

I would add that most cantilevered walls will have significantly more deflection at the ground surface than barced walls. I have seen many cases where the calculated deflection was the controling factor.

 

[Chewi00]

Is deflection a problem in areas where there are no buildings or utilities and its a temporary structure for a hog and haul. I see deflections of 7-10 inches, but no yielding of the steel.

Thanks in Advance
Adam

 

[PEinc]

If you can allow 7" - 10" deflections with no expected problems, why can't you just open cut the excavation with sloped sides? A rough rule is that the vertical settlement behind a wall is about twice the horizontal deflection, therefore, 14" - 20" settlement. To me, that's a failure of the shoring system. Also, I would be surprised if workers would want to stand in front of a sheeting wall that has deflected 7" - 10". Large deflections can cause cracking at the ground surface behind the wall. Cracks can allow water to infiltrate behind the wall and cause a build-up of hydroststic pressure for which the wall probably isn't designed.

 

[GeoPaveTraffic]

If you truely believe that 7" of delfection will occur, then I would use a heavyer section or brace the wall. At that deflection you have to be close to yeild on the steel; and if the steel yields, then someone is likely to get hurt.

 

[Chewi00]

well there is a shallow water table and the dig is going to approximately 20'....so perimeter sheeting is being used for dewatering purposes and also the client is concerned to cut back slopes due to the potential for uncovering additional contaminants that they arent currently required to remove.

The current design shows a braced wall dig, but a peer review has recomended lowering the water table behind the perimeter sheeting to relieve the hydrostatic pressure in order to use a beefy canilevered section (AZ-36) instead of AZ-18 sheets with corner bracing and struts. The cantilevered section will work if the water table is lowered 10' with well points, but is showing deflections of 7" in the model at the surface. There is also the safety concern of lowering the water table to relieve pressure. (i.e. failure of the well point system and increase in hydrostatic pressure).

 

[PEinc]

"...the client is concerned to cut back slopes due to the potential for uncovering additional contaminants that they arent currently required to remove... There is also the safety concern of lowering the water table to relieve pressure. (i.e. failure of the well point system and increase in hydrostatic pressure)."

I agree that this may well happen. I usually design the cofferdam sheeting for water at subgrade in front of the sheeting. This is a worst case and gives the lowest passive resistance.

20' is a very high cantilever. 7" or more deflection is too much. How big is the excavation area? Is the excavation to be sheeted completely around the perimeter? Can you dewater behind the sheeting? Will extra dewatering require extra, expensive water treatment? Is there an impervious layer that would prevent water drawdown behind the sheeting? If so, you may need to design the sheting for water pressure also. Internal bracing or tieback anchors drilled from above the water table may be the better way to go.

There are many things to consider when designing sheeting. Your posts really do not give enough job information for me to give you more specific guidance or opinion.

 

[cap4000]

PEinc

I believe the typical settlement/deflection ratio is closer to 1:1 depending on the soils shear strength and relative density. This post sounds like an accident waiting to happen.

 

[PEinc]

I agree. Someone has to know when to say no or walk away.

 

[Chewi00]

I agree....all the client sees is the cost savings of using cantilevered system brought up by the peer reviewer, and cant understand the risks involved. I am likely to say either the peer reviewer can stamp a cantilevered design, or we can go with the current braced design. Thanks for all the insight.

 

[DRC1]

Although I don't follow it as a hard and fast rule, I try to keep deflections under an inch. Deflection calculations in sheeting are not terribly precise. Unlike calculations for bridges and buildings, the magnatude and distribution of loads on sheeting is not that well understood. When you get large deflections, like 7-10 inches, that is based on a model that may or may not be accurate. As you get large deflections, you start to get secondary bending effects that increases your stress and defletions.

The other problem with the cantilever system is that for a cantilever of that height, espically for a box dam, you will probably have to assemble a driving frame anyway to ensure proper alignment of the sheets.

If it were more econmical to use stiffer sheets, more contractors would. In general, however once you get over 12-14 feet, you are probably looking at a frame.

 

[PEinc]

Well said, DRC1.

 

[Chewi00]

Thank you all! these responses have helped validate our design teams thoughts and concerns and made me more confident that our initial braced design was indeed the best approach.