Cutting Hole in Slab for Manhole or Similar

[pmsometimes]

We have a precast slab which sits on a cast-in-place vault. A manhole (with appropriate reinforcement) is cast in the slab. I think it is precast just so it could be set in place so formwork didn't have to be built and removed out of a manhole. It is not prestressed. The slab with the following dimensions:
12'x12'
13" thick
12" bearing all sides
#4 @ 12" OC EW top
#6 @ 12" OC EW bot
Designs for HS20 loading (which I have little experience with). We may be able to put up bollards to reduce the load requirements.
It was cast with a manhole in it, but the manhole is misplaced for the activity required. It will require the owner increased time/cost due to the location. Workers will have to get inside this vault more frequently since it is offset from where it needs to be.

I'd like to cut a hole that will fit within the 12" spaced reinforcement, but this seems unlikely to meet the owner's needs. Alternatively, I can just "extend" the manhole if we can find a rectangular cover that will work. My thought is to support w-shapes off the ring beam of the vault, which shouldn't be an issue. This does lower the head height if someone has to get in the vault - the head height is currently only about 4'.

Any other ideas? If I would cut a bigger hole, how would you treat the exposed ends of the reinforcement since cover will now be zero? Any other ideas?

 

[amlinerrichard]

Is it possible to pick up the slab and rotate it?

 

[pmsometimes]

Is it possible to pick up the slab and rotate it?

We considered that! The manhole is at a quarter point and the access inside is at the center, so no good there. Thanks for the idea though!

 

[pmsometimes]

Also, I need to figure out of we can even get a 10' long beam in place through the manhole.

 

[nackra]

can you get a new slab precast with required dimensions and just swap slabs

 

[pmsometimes]

can you get a new slab precast with required dimensions and just swap slabs

This may be the only viable solution, but we were hoping for something at lower cost/effort.

 

[TehMightyEngineer]

How deep is it below grade? Can you post a sketch of the opening configuration?

You should be able to make this work if you place bollards so that the H20 load requirement isn't needed and it can just be considered for light vehicles and pedestrian loads (300 PSF).

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[pmsometimes]

How deep is it below grade? Can you post a sketch of the opening configuration?

The vault extends 15' below grade with precast grease trap sitting inside. The slab in question sits on top of the vault and is even with the pavement. I can work on getting a sketch uploaded.

 

[canwesteng]

I've had contractors paint exposed bar with interzinc 52. I can't say this is ideal but it's better than nothing. Hopefully the bar inside the concrete remains well passivated and the portion with no cover is galvanically protected by the paint.

 

[KootK]

Also, I need to figure out of we can even get a 10' long beam in place through the manhole.

You could surely field splice two shorter pieces if you needed to.

Once you relocate the manhole, is there anything preventing you from casting a slab to cover the hole where the manhole used to be? This wont't create flexural continuity (at least no without great effort) but may solve your durability issue.

 

[pmsometimes]

is there anything preventing you from casting a slab to cover the hole where the manhole used to be?

That shouldn't be an issue, but the small piece of slab between the 2 manholes probably won't work.

 

[KootK]

but the small piece of slab between the 2 manholes probably won't work.

I didn't suggest that it would and, absent a sketch, really have no way to gauge that. I thought you might use the infill strategy to address your corrosion concern and perhaps couple that with steel beams etc.