Preliminary thickness for slabs

[dbuzz]

We are doing preliminary design (for cost estimate purposes) of slabs for a proposed underground workshop, about 1000m below the surface.

Some areas will be subject to pedestrian and light-vehicle traffic. Others will be subject to heavy-vehicle traffic.

The slabs will be cast directly against the rock floor.

Does 100mm slab thickness for light-vehicle areas and 150mm for heavy-vehicle areas sound about right?

 

[boo1]

That looks a little thin. Consider 6" (.15 M)for light traffic and 8" for heavy loading.

 

[dik]

Just finished up a car dealership with lightly loaded slab in the display area using 3-1/2" slab. OK as long as the sub-base is good... in the service bay areas 6" was used. For heavier loadings, often use 8" as noted...

I use an earlier PCA program 'Airport' that I used for aircraft pavements to determine stress under wheels and racks... not what the program was intended for... but, I use it with caution...

 

[dbuzz]

The site is an underground mining operation, 1km below the surface. The slabs are going to be cast directly against hard rock (E~80000MPa, q~9000kPa) not against granular fill (E~40MPa, q~200kPa) or clayey material (E~20MPa, q~100kPa)

On the surface I would be using 150mm light-vehicle traffic and and 200-300mm for heavy-vehicle traffic (depending on vehicles and sub-base material).

Given the stiffness of the hard rock as the sub-base material, I would have thought that some reduction in thickness would be acceptable.

 

[RiBeneke]

I think that some reduction in thickness would be acceptable.
However there are other considerations :

There will be some variability in the thickness of the floor depending on the accuracy of blasting or cutting etc. You must design for a minimum thicknes on the high spots of the rock, or use a weaker concrete levelling course.

Even hard rock can have weak patches faults etc. You must allow for these.

If the bond between the concrete and the rock is good, then thickness reduction is in order. If the bond is poor then the concrete will function independently and may tend to curl or creep.

The point loads from jacks stools etc in a workshop are quite high. Concrete strength design and curing should be as high as you would require on surface.

 

[daqingwa]

I imagine that overbreak due to blasthole misalignment and blasting are going to make a mockery of any estimate based on thickness.

Your best bet is estimate based on 200mm -300mm thick, and then place concrete as best you can, giving just 25-50mm cover over the high spots.

Make sure it all gets blown clean before placing concrete. A layer of mesh could be handy at the heavy tracked areas. If youre using CAT tracked equipment wear could need to be considered.

 

[dbuzz]

daqingwa,

Thanks for the input. We used an "over-fill" allowance of 30%-by-volume in the estimate to account for the inevitable uneven casting surface. The slabs will have mesh reinforcement, if the project even gets to the detailed design and construction phases.

Thanks to all participants.

Cheers,

dbuzz