Braced Excavation with Crossing Utility Pipe

[MrEngineerUS]

Hi all,

I designed a braced excavation (sheet piles with walers and interior struts) and I've just been notified that the utilities that weren't supposed to interfere are now an issue. Basically, they may need to connect the new 4" utility pipe and have it run through the braced excavation.

So, my question, does anyone have experience with staging such an operation and, more importantly, do they need any special details for cutting a 4-6" hole in a sheet where the new pipe will be run?

I figured they could drive the piles to a little above the pipe depth, locally. Excavate to the pipe depth, cut the existing line and mark the pipe elevation, drive the remaining length of the sheet pile, cut the hole in the pile and reconnect the pipe. Is it possible to cut a local hole like this or would any special support be required locally?

Thanks for any suggestions!

 

[KootK]

do they need any special details for cutting a 4-6" hole in a sheet where the new pipe will be run?

This probably isn't a big deal depending on the properties of your sheet.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[racookpe1978]

A simple hole through a single sheet of vertical steel plate is fairly easy = Burn it through with a oxy-acetylene torch. Brace the plate first nearby - perhaps on both sides - to the other side of the trench.

 

[RFreund]

I was wondering what the industry standard is here. I have seen before the hole burned through with no further "reinforcement" with holes up to 18" or so. I'm sure the load is distributed to adjacent sheets but I have not gone through the exercise of figuring out how to analyze that. If the holes will be substantial then H-Piles are of placed on each side of the opening. These can bear on the walers and lagging is placed between them (omitted at the openings). For such a small hole (6" or less) I wouldn't expect anything other than a torch cut hole.
Racookpe's suggestion is easy and effective but it most likely will be in the way of other forms and such so it most likely can't remain in place.

EIT

http://www.HowToEngineer.com

 

[jayrod12]

You also have to remember that the soil is going to bridge the hole as well. So if you are cutting a small hole it is unlikely you are going to get more than a handful of backfill coming through your hole, less when it is plugged by a utility pipe..

 

[PEinc]

Unless you are trying to retain ground water behind the SSP, when driving the SSP you should be able to leave out a SSP double where the existing pipe is. First, confirm pipe location with test hole. Above and below the pipe, you can install horizontal timber lagging as you excavate to subgrade.

http://www.PeirceEngineering.com

 

[RFreund]

PEinc - you are saying to put the lagging between adjacent SSP's?
Why not just torch a hole?

EIT

http://www.HowToEngineer.com

 

[MrEngineerUS]

Some more specifics: the soil being retained is sand and the water table is fairly high (although it should be lower, locally during construction due to pumping). So the system may need to retain water during construction and sheeting was the option desired by the client. Could this type of soil "bridge" an opening like that (even if it is small)? The sheeting is also a lightweight gauge which is why I was concerned about the load transferring adequately to the adjacent piles.

Maybe welding a plate or angle above and below the opening to distribute the load to the adjacent piles would work? Just not sure what is typically done in this situation. Another option: burn the hole and put in a small length of steel "casing" pipe through which to run the new connecting utility pipe?

I thought this would be a fairly common issue but it looks like H-piles with timber lagging is typically used if there are utilities. I haven't had much luck searching around online for a typical solution.

 

[LittleInch]

The issue as I see it is that once you've cut the pipe and then continued pushing the pile through you'll never get the sheet pile out again and in extracting the others could easily move the pile with a 4" pipe through it and break it. Pipes don't like having a potential hard point if any settlement occurs which sounds like what you're introducing.

Usually I've seen a gap in the structure and a wooden sheet substituted which can then be cut around the pipe and sealed with plastic.

Having a high water table and a pipe running thorough your piling is not good, but a 4" pipe shouldn't cause that much grief.



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MrEngineerUS]

Thanks for all the replies so far. Such a simple issue at first glance! The piles will remain in place and used as forms for concrete pit walls being installed in a basement. I expect that a casing pipe through the walls will be necessary.

 

[LittleInch]

Then a sleeve and something like link seal is what you need.

http://www.gptindustries.com/products/link-seal-modular-seals/sealing-plugs/link-seal-united-states

If you try any other simple seal it will fail after a short period of time.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[PEinc]

I thought that the pipe was already existing and would interfere with driving one or more sheet piles. Maybe I am wrong in my assumption. If the pipe is not already there, and if the sheets do not need to be extracted, cut an oversized hole.

http://www.PeirceEngineering.com

 

[MrEngineerUS]

PEinc, you were right. The pipe is there in-situ. Since the braced excavation requires piling the pipe will likely need to be cut then repaired. Presumably, through a hole in the piling which may necessitate a casing pipe to avoid a point load on the new pipe due to settlement. Or not depending on how large a hole I can get away with.

 

[dhengr]

MrEngineerUS:
Cut the existing pipe, back from the sheet pilling wall. You’ve already got a hole from which to work on the pipe and deal with the hole in the sheet pilling. Drive the full length pilling and cut it off 6" below the pipe. Pull the remaining pilling up 1' or 18", and you’ve got your opening for the pipe re-connection. Then maybe some reinforcing beams at the cut ends onto several sheet pillings on each side of the opening.

 

[SlideRuleEra]

MrEngineerUS - This is an interesting thread, and I have been watching it since you started it... but I will add my 2 cents. IMHO, PEinc's advice to leave out a pair of sheets is the best advice. Pile driving is not a precise operation, there is the potential that the installed sheets may not wind up exactly where intended. Say that plans are to cut the hole near the center of a sheet. What would you do if the installed sheets have to be cut at an interlock instead? Suppose the hole has to be enlarged for some currently unknown reason?

Unrelated to that, it is hard for many people, including engineers, to visualize the magnitude of forces on a sheet pile wall. Once it is in place and excavation is well underway, it is best to Leave It Alone. I doubt if any of us would start arbitrarily cutting into a beam, or other structural member, that was known to be loaded, but the magnitude of the load was not known. Why do it with sheet pile?

Consider that sheet pile failure can be sudden, without warning and catastrophic... and the most likely time of failure would be while there will be workers inside the excavation making the cuts.

If ground water flowing through the gap in piling is a problem, you could sink a well point outside the gap to help.

Doing all this is a lot of trouble and expense, that's why it is not a "textbook" answer.

Just something to think about.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[RFreund]

I'm curious - If you leave out the sheet at the pipe location and use wood lagging, is it possible to use/drive double sheets at the adjacent sheet locations? Is it reasonable to apply this 'extra' load to a few adjacent sheets? I suppose you could make your lagging longer to engage more sheets.

EIT

http://www.HowToEngineer.com

 

[SlideRuleEra]

RFreund - If the sheets are properly sized and braced, double sheet adjacent to the gap should not be needed. Again, IMHO, no sheet pile structure should be designed to "just barely" work. There are way to many unknowns.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[MrEngineerUS]

I've checked with the engineering department of a local steel pile manufacturer to see what they suggested or have seen in practice. Basically, they said to drive the sheet(s) above the pipe to pipe elevation and the rest to full depth; locally excavate and cut the pipe then drive the sheet the remaining depth and burn a hole to repair the pipe through the sheeting. I ran the idea of timber lagging past them and they were comfortable with that as well. They even said that driving the sheet pile to above the pipe and not the remaining depth would be fine for this excavation (pipe elevation, 4-5 ft and lowest dredge line at about 7.33'). I'll let you guys know what we end up doing.

Cheers!