Concrete pavement over a vault 4

[roso]

There is a very important avenue to be re-built soon. The consultant wants it to be built using concrete pavement but, the problem is that there is a river channel running underneath. The river channel has a reinforced concrete vault placed at variable depths from 0.5m up to 2m. There will be an uneven support under the slab, because of the presence of the vault, and that may induce some longitudinal cracking somehow reflecting the location of the vault. Our concern is the elastic modulus of the subgrade (k). This value represents the stiffness of the underlying soil, and all the specifications recommend it to be reasonable uniform however, for this case the vault will increase the k value because it will behave as a bedrock but only for a portion of the slab, so this part of the concrete slab will have a very rigid support conditions, because of the presence of a very rigid vault underneath but, It will also be subjected to “normal” conditions (lower k value) in the portion of the slab that is not resting over the vault. This will create a different support conditions in the slab, so a different minor settlements may occur and the slab may crack.
We think we should use some type of reinforcement in the slab and we look for an advice to calculate it, or any other suggestions.

 

[JAE]

I'm not a pavement design guy - but I would think that if you have two different "K" values under a common slab of concrete, then you would end up with two different design results. And pavement design is mostly occupied with thickness of concrete and thickness/type of base.

Adding reinforcing will help keep cracks closed, but will not deal with differential movement or the initiation of cracking.

So part of the slab would have a thicker design than the other part, and you would transition one to the other on the vault side of the line to be conservative.

Another thought would be to bridge over the vault structurally but that might be cost prohibitive.

And speaking of that, can the vault take the added wheel loads (.5m isn't that much)?

 

[Ron]

JAE raises some good questions about the capacity of the vaults with little cover to dissipate the wheel loads.

Since you have at least 0.5m cover, you might consider placing a Controlled Low Strength Material (CLSM) over the entire vault area to provide a consistency in the k value. The CLSM would have a high enough modulus that it would provide good bridging. At the edges of the vault, you might have enough shear resistance difference to cause vertical displacement. In this condition you will need to provide reinforcement to carry the shear, much in the same manner that you would use dowels in a pavement joint. In this case don't worry about allowing one side to slip...it doesn't matter here.