Trench in Existing Slab on Grade

[bookowski]

I'm sure this is a typical detail but I haven't dealt with it and could use a point in the right direction.

I have an industrial building with an existing 6in. slab on grade. They want to put in a trench for electric, about 75 linear feet. The trench is supposed to be 6" deep x 6" wide, and have a flush removable cover. I took a look online and saw some precast systems but they seemed like overkill.

I've never done this but I was thinking of the attached sketch. Basically overcut the existing slab and provide a concrete beam, with a formed trench in the top and expansion/movement joint at each side. This will have forklifts traveling over it so it will see some decent point loads.

Is this overkill? Is there a typical detail for this that makes more sense? Do the joints at the side make sense or would it be better to provide only a normal control joint in the top 1/4 of the joint. Thanks.

 

[hokie66]

I would want to cast the new trench with about a 2" ledge for the slab to bear on. Just requires a bit of detailed hand excavation.

 

[bookowski]

I like that idea. So hand excavate out a bit and cast the trench below the ex. slab on each side. What about joints between the new/old?

 

[bookowski]

Like this?

 

[hokie66]

Yes, that's what I would do.

 

[bookowski]

What about the joint between new and old? Pour new against existing and then provide a normal control joint where the two meet?

 

[dik]

Yup... but, I would slope the excavation a bit and make the trench a bit wider at the top to accommodate the sloped sides... less of a hassle of granular base sloughing in.

Dik

 

[bookowski]

Dik - You are talking about something like 'Option 2' in the attached? Better than Option 1?

 

[hokie66]

Hang on, Option 2 lost the seat for the slab. You can slope the sides a bit if necessary, but keep the seat.

I would keep the joint as you showed it before. The compressible material full depth is not really necessary, but a bond breaker is. Maybe just a sheet of plastic. Then form the recess at the top for a backer rod and sealant. High modulus polyurethane sealant to manufacturer's specification as to joint geometry and backer rod.

 

[bookowski]

Seat is back, with sloped sides - how about this?

 

[bookowski]

Ooops, here it is...

 

[hokie66]

I like it.

 

[dik]

Pretty much the detail I would use, except the soil slope may be a little more towards 60 degrees for drawing just to accommodate whatever sloughing occurs.

Dik