Calculating loading 2

[roadwhore75]

Now, this is out of my area of expertise so bear with me. I want to install a 22" concrete pad over buried instrument/electrical conduit. The top of the conduit is about 48" below ground. This leaves a fill space of about 26" between the bottom of the pad and the top of the conduits. I am not sure what the diameters are of the conduit in the duct bank. The soil is clay or rock. If you go down 5' you're supposed to run into sandstone. This is Lemont, Illinois. It may be be that the duct either runs through rock or right above it.

The pad will contain a 11,000 gallon liquid nitrogen tank and vaporizer: 175,000 lbs. for the tank, equipment, and pad. Add about 57,000 lbs. for liquid when filled.

How can I work this out. My first thought was to construct a sleeve around the duct bank made of steel with the foot carved into the rock below. Any ideas? Am I worried for no reason? Will the soil disperse the weight. The pad is about 30'X18'. Another idea was to cast the concrete around the conduit. It just means more concrete and that's cheaper than steel.

 

[msquared48]

I would sleeve it and either take all the foundation to the sandstone, or float the foundation over the soil mass. I would not locally etend the concrete to the sandstone in the vicinity of the conduit as that would create a hard point in the foundation, making it react differently, creating a support for the equipment slab over the conduit that you do not want. And, yes, the soil will disperse the load

Mike McCann
MMC Engineering

 

[roadwhore75]

Thank Mike,

So, what would you use as a sleeve. I should add that I think there is a cross over duct underneath so the sleeve might be a regular shape.

You said that the soil will disperse the load: 232,000 lbs.? Is that possible if the soil is only 26" deep above the duct bank?

Here's another idea I had: support the 22" pad on 4-6 steel columns, pylons, sunk into the rock below 5'. That would support the weight directly on the rock and relieve the pressure on the soil. What do you think?

Dirk

 

[msquared48]

Currently bearing is around 3000 psf off the cuff from your pad size and loads (my calculator is downstairs and I am laid up for a few days following ankle surgery). If you draw 45 deg433 lines from the lower edges of the pad, you can extend the effective pad size for bearing to about 21.6' X 33.5'.

Rather than the steel columns, I would consider concrete piers to the sandstone and designing the slab as structural.

At a questileonable four feet to the sandstone, overexcavation may be too much. Then again, if you used CDF for backfill with the regular concrete slab over it, it could be feasible. Just verify the allowable bearing pressures to the CDF and sandstone layers.

Mike McCann
MMC Engineering

 

[civilperson]

Less than 500 psf is a nominal load and would not cause me any heart burn regarding the buried conduits. Putting the conduits in a sleeve is a good idea for access after the pad is built.

 

[MiketheEngineer]

Move the pad??

 

[cvg]

most duct banks are backfilled with flowable fill which also provides additional protection to the conduit. If it is encased, it will be difficult to put this in a steel casing. If you are going to dig it up and install a sleeve, why not just relocate the conduit so it is not under your slab? Alternatively, a concrete blanket which is basically a protective layer over the conduit is a typical way of protecting existing utilities from increased loads.

 

[bimr]

Why don't you move the conduit? The concrete pad should not be built over the conduit as the conduit was most likely not backfilled with structural materials. Every strutural engineer will tell you to build on undisturbed soil.

 

[roadwhore75]

Relocating the pad
Unfortunately, relocating the pad won't work. I've presented 6 alternative locations in a crowded refinery. Besides, now that they've fallen in love with this location they want me to make it work for them. It's one of the dangers of the business we're in. You guys should know from working with architechs.

Moving the duct bank
Moving the conduit means shutting down the plant to pull the wires. This is definitely not going to happen.

What do you guys think of Mike McCann's solution? I was thinking about concrete pylons or piers (symantics to me) before he responded. Obviously, I don't know the load that will be bearing on the rock below. How much can it bear? What kind of footing do I need to each column? Should I design for 4 or 6, or even 8 columns? How do I connect the column to the concrete beam? Do I modify the concrete beam for support off from the column?

Of course, I will have to get the vendor of the tank involved. They will want their structural engineer involved.

I want to thank all of you for your help so far. If you ever need any environmental or chemical engineering help, look me up.

 

[cvg]

as stated by civilperson, your loading is less than 500 psf which is fairly light. Even poor soils should be able to support this light loading. Putting in piles to support this seems to be overkill to me. However, I am not a geotechnical engineer, and there is not enough information to make an informed decision. You have not provided data on the soils other than a reference to "clay or rock". You also have not investigated whether the ductbank is above the rock, on the rock or in the rock. These are important details, necessary to make a decision. Seems like you need to do a little more homework before deciding on a method. You really might want to post this on the geotechnical engineering or foundation engineering forum to get better input.

 

[bimr]

The first thing that you need to do is a soils investigation. Contact SEECO over in Tinley Park. They will do a good job for you in just a few days. For only $6K to $10K. They will be able to make a recommendation as to the best foundation for the soil type. You may also have previous soil investigations in your files.

I doubt that piers will work. You probably have bedrock just several feet below the surface. The duct bank is probably laying right on top of the bedrock. Over in Lockport, the bedrock is 3-4 feet down. The bedrock is typically capable of 10,000 psf.

You might put the tank on the pad portion where it will bear on the bedrock, while the evaporator may be placed over the duct bank.

Investigate reducing the size of the unit if that will help.

 

[roadwhore75]

Thanks for the response. Here's some information on ground conditions:

Pad above ground: 3"; Pad thickness: 22"; Fill depth: ~ 2'8" below pad.

Ground level: 591'; Top of conduit duct bank: 587';
Bottom of conduit duct bank: 583'; Bedrock: 585'.

Obviously, the problem is not the loading. At only 432 pounds per square foot (psf), this is less than the 500 psf mentioned earlier. The problem is that the load is over a bank of old electrical conduit. The electrical engineer is not sure what the loading capacity is. I, for one, would not want to match old rolled steel, even in a bunch, against concrete and a filled 10,000 gallon tank pressing down against bedrock.

Another issue came up. I looked at one so-called exploratory excavation report. I guess that's what they do around here rather than using a ditch witch. The report says there is a sidewalk ductbank less than a foot down. This makes me nervous. Still, I can't believe there would be a duct less than a foot below the surface. I am going to recommend another dig where we want to put the pad.

Right now, I am still leaning towards piers. I planned 6 and estimated spirally reinforced piers at 6" diameter; I drew 12" on my CAD drawing. This is now something I will leave to the civil engineers. I just wanted to make sure I left at least 2 ft clearance for the footings. I want to go back to my other assignments.

10,000 psf for bedrock? That's pretty high. I'll have to remember that one.

Thanks guys

 

[bimr]

In a typical electrical conduit installation, concrete will be placed over the ductbank as a safety precaution. Sometimes this concrete is formed, other times it is just poured on top. The purpose of the concrete is to prevent future excavation from hitting the electrical wiring by accident.

It is wise to do a geotechnical investigation for any tank, especially in an area with disturbed soils. The cost is minimal and it will proabably save money in the design of the foundation. Do yourself a favor and talk to SEECO.

 

[chicopee]

My suggestion is to expose the conduits, then protect them with corrugated piping such as used for culverts. Cut the corrugated piping in half and rest it on a grooved or keyed 12" wide poured concrete footing and backfill inside to support the conduits and outside. Outside backfill s/b compacted with graded material before resting the concrete pad.
Hopefully the concrete pad is properly reinforced.

 

[bimr]

It is also appropriate to have a competent electrical engineer review this project for you. The solution is going to depend somewhat on what type of electrical ductbank is buried as well as how old it is. For example, does this ductbank contains high voltage transmission cables? Is the site in a hazardous area?