Conduits Under Footing

[EngStuff]

All,

We are in a bit of a situation where we have an existing building with conduits below where we have to design a footing with a decent size loading(see pic below). Even if we try moving the column around, it won’t make a difference since there are conduits all over the ground. Moving the conduits was discussed in depth and that option was determined to be out.

In the sketch below we have a 6” slab above soil that has a k=100 pci, and top of conduits are located about 8" to 12" below the slab (slab slopes). Conduits are extremely close to each other so I cannot penetrate between them.

Only idea I have would be to design a steel beam that is buried and spans about 15’-0”(unconfirmed, have to cut more slab). The ends of the beam would be supported by footings.

Any Ideas or advice?

Note: I did look for similar threads on here, but most were about conduits in the slab. There was one thread almost similar to my situation, but said to move the conduits, but we aren't able to do that on this job.

 

[BAretired]

Why on earth would anyone place conduits in that fashion? Can you excavate below the conduits, locate a footing below them and separate the conduits sufficiently to get a pedestal between them?

BA

 

[EngStuff]

BAretired,

Apparently They should have been outside, but it was a last-minute change that had happened. We did try to see if separating them would work, but unfortunately, the conduits are too tight together that the contractor wasn't able to separate them.

 

[Agent666]

Would excavating out all the soil above and below and clumping some conduits together enough to use a flowable fill (low strength flowable concrete basically) to bridge through the conduits to bear direct on virgin soil below be an option?

What is in the conduits, any chance of removing whatever is in a few and feeding down another existing one to allow cutting out of a section of conduits for the new footing pedestal?

https://engineervsheep.com

 

[EngStuff]

Agent666

The issue comes back to them being tight together. There is no space between the conduits and orientated in a row that is about 13 to 14 feet wide. see sketch below. majority of the conduits are electrical spanning over 400 feet, we had a discussion of moving or rewiring a few. Code doesn't allow them to splice, they would have to rewire them over the length, the cost was out of the question for the owner. Digging below the conduits might be difficult due to the way they are laid out, see the sketch below. we would only be able to dig below from the sides of the conduits. If we cut about 20 feet of slab to get them to start digging around the ends and inward towards the center, the other issue is how long(Length of the cut LxW) would we have to cut the slab and dig to loosen up the conduits.

EDIT: added "(Length of the cut LxW)"

 

[BAretired]

Only idea I have would be to design a steel beam that is buried and spans about 15’-0”(unconfirmed, have to cut more slab). The ends of the beam would be supported by footings.

That won't work because the conduits are too close to the surface. A better solution would be to use a column each side of the conduits with a beam spanning between them at a higher level where a deeper beam can be used. That assumes it is architecturally acceptable.


BA

 

[dhengr]

EngStuff:
Depending upon the soil conditions, you might consider injection grouting under and around the conduits and in the area around the new footing. Some soils do not take well to injection grouting, while others are well suited for this soil strengthening method.

 

[mmarlow]

Could you use a concrete beam to span over the conduits with a void form between the beam and conduits? The beam could span between 2 footings and could be at or below the slab elevation?

-MMARLOW EIT

 

[EngStuff]

That won't work because the conduits are too close to the surface. A better solution would be to use a column each side of the conduits with a beam spanning between them at a higher level where a deeper beam can be used. That assumes it is architecturally acceptable

It was an option that was turned down due to the Architectual limitations and steel already in the process of being fabricated. We are stuck with something to do with the ground. it would require a whole redesign of the area and load path as well.

Depending upon the soil conditions, you might consider injection grouting under and around the conduits and in the area around the new footing. Some soils do not take well to injection grouting, while others are well suited for this soil strengthening method.

Can you explain this a little more, I am not following. I feel like we would still be stuck with the same situation.

Could you use a concrete beam to span over the conduits with a void form between the beam and conduits? The beam could span between 2 footings and could be at or below the slab elevation?

This was our original option, concrete beam over the full length and create a footing on both ends. Dowel reinforcement into the center of the existing slab thickness and the new concrete beam every so often. so, if beam deflects, the slab will deflect with it and also somewhat help restrain it a bit. Width of the beam can be pretty much anything we want; however, we are limited to depth of probably 12" if not less on one end. Spanning a concrete beam about 15'-0" we were having a decent deflection, and we still have not including creep in the deflection.

BAretired, mmarlow,

Thats why we were thinking of doing a steel beam(s) even if it's 2 to 3 steel beams side by side that are welded together. Encase the beams with concrete, i don't think we would need to analyze a composite beam section. We just make sure the steel size works for strength/deflection and encase it with concrete to match the floor slab.

If we had to design for worst case 10" steel beam(s), no foam underneath. We encase it with concrete and have a min of 4 inches of slab above one side of the beam(s) and We would restrict the deflection to about 1/8". This would eliminate changing the structure above, digging under the conduits, rewiring/moving conduits.

 

[dik]

If you are changing the loading regime, I would be reluctant to encapsulate the conduits in any sort of 'local' rigid material.

How many conduits, and how closely spaced, and what it the conduit material? The conduits are located about 16" below the top of the slab?

A 6" slab on 100pci material may be a little thin for the prescribed loads.

What is the soil type? Can you use friction piles?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[TLHS]

Can you not put in some sort of vault or something where you could house electrical splices? One vault on either side of the area where you need to get access for your footing?

 

[HTURKAK]

I did not look to the previous posts in detail..

If i were , i would break the SOG corresponding area (say 15 ft X 5 ft ) hand excavate and make bare the conduits and provide concrete protection ( tanking ) for the conduits ..

 

[bridgebuster]

EngStuff - Maybe this helps; it's similar to your situation. I'm part of a team on a design-build proposal for a highway-bridge project. There are several utilities - 10" dia oil-o-static lines & 5kV lines - & an interceptor sewer that can't be relocated. Below is an illustration from the RFP for pile supported slabs to span the utilities where bridge substructures and retaining walls will be constructed.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1639924122/tips/Protective_Covers_over_Utilities_and_Drainage_for_Optional_Work_k5axzo.pdf[/url]

 

[hokie66]

This is a classic case of someone else's problem being foisted onto the structural engineer. Even just with a slab on grade over the conduits, the services installation should never have been done in that manner.

 

[bridgebuster]

$#!+ happens; gives us billable hours though.

 

[dik]

To me, this is what engineering is about.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik