Seal (mud) Slab or CSS Pad?

[JoelTXCive]

I have a large cast-in-place multibox culvert that will be constructed with varying fill on top of it. At the shallowest point, it will have ~5ft of fill on top, and at the deepest it will have 15ft of fill on top.

We have called out BOTH a seal slab and a Cement Stabilized Sand (CSS) pad underneath. (There is also a CSS blanket on the sides and top, but the bottom is the area of interest.)

The contractor has asked if we can delete one of the subgrade items. I'm inclined to agree with the contractor, but cannot decide which one we should keep. We are starting our excavation right next to a large drainage ditch, so it's safe to assume the contractor will hit some water during excavation.

Our nearest borings report fat clay and stiff clay alluvium soils.

Our options are:

Option 1: 4" Seal (mud) Slab of 1500psi concrete? Or......
Option 2: 12" thick CSS pad (~200psi)

What do you think? Keep them both? Or Option 1 only? Option 2 only?


Thank you in advance.

 

[HTURKAK]

Our nearest borings report fat clay and stiff clay alluvium soils.

So you don't have borehole specific for the culvert location. My vote would be 300 mm compacted crushed stone ( ballast ) bedding + 75 mm blinding layer.

I will suggest you at least ; see the excavation and decide as per site conditions. If the soil stiff clay , alluvium soil , extend excavation 100 mm below then compact the surface before 100 mm blinding.