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SOE with existing Utility Pipe

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DirtSmuggler

DirtSmuggler

Geotechnical
Sep 29, 2021
29
Client wants us to design the SOE but also guide in the installation of the SOE. We've calculated that the sheets have to go down to 72 feet so that we can excavate down to 42 feet for the mat foundation. However, within the proposed area, there's an existing 60" RCP that runs through the proposed area at about 38 feet deep. The portions of the existing pipe that's within the proposed area will be demo and replaced with a pump chamber, but the portion of the existing pipe outside the proposed area has to stay intact as the pump chamber will connect to it. The area is within a coastal plain deposit with mostly all SP. We also need to install piles for the mat foundation. So a portion of the area would be difficult to install piles with an existing 60" RCP going through it. Area is roughly 30'x30' and borders on neighboring properties.

How exactly would we be able to get the sheeting down to 72 feet if there's a pipe at 38 feet? I don't imagine the sheeting being able to do a clean slice of the RCP like a knife as it goes down. We have an exact similar situation at another location for this project but the excavation only needs to go 12 feet deep. So we may or may not need to employ the same strategy for that one. Any tips or idea for the strategy on how to do this?

We asked if there were any existing sheeting in place from the previous construction, they said they have no idea.
 
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Why would SSP need to be 72 feet deep for a 38 feet deep excavation? Sounds excessive. Please explain. Is dewatering required? You will need to jump the SSP across the 60 inch pipe twice and install lagging above and possibly below the 60 inch pipe. Lagging cannot retain ground water or prevent seepage or a quick condition below subgrade. If you need to dewater,you are looking at a couple deep wells or possibly a multi-stage wellpoint system capable of dewatering inside the 30x30 pit and around its outside perimeter. The sheeting walls will also need internal walers and bracing. The bracing locations, installation, and removal will need to be coordinated with the construction of the new chamber and its construction joints. The sheeted pit will need to be large enough to allow chamber wall construction without interference with the walers and braces. Check if you will be allowed to have any braces penetrate the new concrete walls.

 
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DirtSmuggler - A hammer that can drive sheeting to 72' depth should be quite capable of crushing RCP as long as the sheet pile impacted on top of the pipe, not a glancing blow. The question would be will the sheet pile survive and how much damage to the pipe. Even if it was "clean slice", this would not do you any good. The portion of the pipe needed for the tie-in would be inaccessible, underground outside the sheeting.

If high ground water is an issue, excavation may need to be substantially deeper that 42' to construct a seal.
If a seal is required the permanent piles would have to driven with a spud thru water before before seal placement.

Have you looked into ground freezing instead of a cofferdam?



 
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I do not have any idea for the local availability of WPS in your region.
I would prefer to install well point system so you will not need SOE ( sheep piling etc ) anymore . Then detour the 60 inch pipe and drive the install piles .



Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM


 
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Groundwater is at 8 feet bgs. Dewatering is planned.

I've never heard of ground freezing before. Did a quick search, looks very interesting. Will research more into it. Thanks for the info.

Even with wells, SOE will still be needed due to boundary constrains. Can not do a sloped evacuation.
 
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