Shotcrete for seepage control 2

[BaronENG]

I am designing a shotcrete application for an exposed rock face that is mostly vertical. The rock is currently covered in shotcrete from many years ago. It is currently showing stress cracks pressumably from expanding shale, water pressure, or freeze/thaw cycles.

I am aware of using a geo material as a drainage system, but am concerned over allowing the shale to get wet/dry cyclically through over-draining. Also, allowing the water to flow to easily seems like it would weaken the shale. Allowing the water pressure to build behind the surface (~20'x80') seems out of the question as well.

How do I control the amount of drainage and is this the best course of action. This is my first experience with an open rock facing seepage control.

Also, what are your expericences with wet vs dry mixes?

Thanks

 

[Prepakt1]

Suggest you permeate grout the shale to block flows. If you still think that you need to allow drainage. Drill through the grouted area wherever you want the drainage points. If necessary you can then pipe the water to any location

Intrusion Prepakt /marineconcrete.com

 

[InDepth]

I agree with Prepakt1. Is the shale expansive when exposed to air? May have to seal off air intrusion using asphaltic bases?

 

[PEinc]

Consider drilling and installing some horizontal or slightly upward-sloping slotted drain pipes to collect water before it gets to the face?

 

[BaronENG]

Thank you guys very much for the feedback. It turns out that the currently stressed shotcrete is very old and has succumbed to freeze/thaw cracks. It doesn't seem like this is very expansive shale. It is currently in a 1:1 slope.

Since water is going to be behind the shotcrete we are going to be using a limited amount of surface area of drain sheet in vertical strips underneath the shotcrete. This should allow it to retain a "natural" amount of moisture content and hopefully avoid degradation.

The senior guys aren't too worried about this stuff. I'm simply trying to be extra thorough as the new guy here.

Good idea PEinc, but this is an entire formation of shale. It's not just the face that's shale.

Also, we don't know the reactivness of the shale as it has been covered for over 20 years. All we know is that it is cracking slightly and some light vegetation has taken root and is growing through.

I'll keep you guys updated.

 

[PEinc]

BaronENG,

The horizontal, drilled-in drains can be installed in all shale. You want the drain to intercept any water flowing through the rock joints in order to keep it away from the shotcrete. These drilled-in drains could also be installed behind the vertical chimney drains that you place behind the shotcrete.