Shoring Concrete Slab on Grade

by Jim
(Canton, Michigan)

Question:

How should you shore a concrete slab, before cutting the slab, that is on grade beams when there is a load on the floor approximately 12" away such as a 8" block wall 10' high. In this location the grade beams are not directly under the trench area. The soils under the slab are poor and have settled by about 12" to 18". We need to repair the plumbing under the slab and then we will fill the void with grout

Answer:
Hi Jim,
This is a tough one because most slab cast on grade are unreinforced but the fact there is grade beam exist at this floor tells me that the slab might be heavily reinforced.

The 12" to 18" settlement is a huge problem to worry about too because once the soil settles, the plumbing beneath the slab will follow wherever the soil goes. The plumbing pipes need to be supported back to the slab so that when soil settles the plumbing pipes will remain at the same elevation that was intended.

This can be accomplished by using clamps that somehow tied back to the slab as you repour. Make sure the slabs that were cut need to be drilled and epoxied into the existing slab and need to be heavily reinforced i.e. #4 bars @ 12" on center each way.

As far as shoring the slab on grade, the slab should be ok without shoring if you're only excavating 12" into the slab especially if the slab is heavily reinforced.

You can shore the block wall to minimize the load going into the slab. If the block wall is bearing wall, chances are it is solid grout. You can drill holes thru the block wall every 4ft or so and use 5/8" diameter threaded rod with beams on both sides of the wall. You can rent beams for such application from concrete suppliers that supply concrete shoring that have either aluma or steel beam specifically for shoring.

If the block wall is not solid grouted, the drilling need to be done at the bond beam which usually at 4ft interval.

Hope this helps
Best of luck
Tonga