Retaining Wall Drainage 1

[Redacted]

Hi there,

Background :
I was recently asked by my boss to design a new retaining wall to replace an existing stone retaining wall that failed due to a tree pushing out stones in a concentrated area. The tree root propagation created various cavities under the road surface that need to be filled.
I designed the new wall to act as a prop whilst the cavities are filled (designed to take the wet weight of concrete for the entire height from the base) and to also resist the horizontal force as surveys have shown that the existing wall is deflecting or sliding in this area as well.

Now for the drainage :
The existing wall stones are dislodged, what looks to be 100 mm or so in areas. To make it easier for construction I specced 200mm of fill for that uncertain area.
I originally specced for 200mm of fill (free-draining granular) behind the new wall pour to facilitate drainage, however, my boss commented that the wall backfill should just be a monolithic pour with the new wall stem. This is because it would be easier as you won’t need a back face of formwork because it can be poured right against the existing stone wall. This is fine, however, I am trying to determine how this will affect the drainage? Can I still have weep holes even if they end up going straight up against the existing wall? Or what is the standard approach for drainage in this situation, where there is no backfill?

 

[r13]

Agree with your boss on this to save formworks. I suggest to core through the existing wall and insert weep hole pipes. But I would still make sure the retaining wall can resist the expected hydrostatic pressure behind the wall.

 

[r13]

Another way to do it, is to install drainage fibric on the existing wall, assume the existing wall is loose enough that allows free flow. Then you only need short weep holes on the new wall.

 

[Redacted]

@retired13, thanks that's helpful.

 

[STrctPono]

I concur with retired13's 2nd piece of advice. If the existing wall rocks are ungrouted, install geocomposite drainboards on the front face of the existing wall.... Not sure how much rain you get but I would go approx. every 10'-0" o.c. Hydraulically connect the bottom of the drainboards to a PVC pipe and extend it through the fill all the way to the front face of the new wall.