SLAB ON GRADE SUPPORT

[Erasa]

Hi Engineers,
I am looking for the best way to support an existing slab (300mm thick) on grade before excavation under the slab to pass my services. See attached; assuming the slab is weak and not reinforced. Also assume the soil to be weak.

 

[dik]

What are you putting in the trench and are you backfilling it?

Dik

 

[hokie66]

A slab 5m x 3m, just take it out, build a services pit, and put the top slab back.

 

[dik]

With the services pit that large, why not use it to support a structured slab?

Dik

 

[Erasa]

Dik, i am putting ducts in the trench and planning to backfill this with concrete to support the existing slab.
And with the large pit to support the slab; I want to support the slab first before excavating to the required depth.

Hokie66, how i wish i can do as you proposed but i am afraid it is an existing slab that contain other service which will complicate issues when i do not want to touch.

 

[dik]

A 12" slab is capable of supporting very large point loads, but is predicated on a good granular base. Why a CLSM fill? If backfill, then, possible to use good compacted granular base.

Dik

 

[oldestguy]

Concrete slabs get almost no added support benefit from base course under them. This is well documented in studies done for the USA Interstate highways at Ottawa, Il. Chances are your cost and time required for the job will be much less if you remove the slab first, either by actually moving it or demolishing it. The quality of the job will likely be much better also.

 

[dik]

OG is correct for granular base... didn't know the advantage of CLSM...

Dik

 

[oldestguy]

Is this job suitable for pipe jacking? Could the ducts be pre-cast and then jacked under the slab?

 

[hokie66]

Yes, oldestguy, if he can't just remove the slab and replace, I agree that jacking a pipe through would be my first option, especially if the soil is granular. Tunneling under the slab looks to me like it would be unsafe unless you really know what you are doing.

 

[Erasa]

Thank you all for your input.