Drains in Retaining Wall / Channel Linings
Drains in Retaining Wall / Channel Linings
(OP)
I have a question regarding drains in retaining walls that have water on one side.
Attached is a preliminary drawing for a channel lining my firm is designing.
My question is:
1) Should we put drains in this wall? I believe our worst case loading scenario is going to be a rapid draw down situation with the channel empty and fully saturated hydrostatic pressure on the dirt side of the wall. Since we are going to be designing for a fully saturated scenario, then why does it matter if it drains?
2) If we do put drains in the wall, then at what elevation? At the base? 6" inches up? 1ft up the wall? Higher?
Thank you.

Attached is a preliminary drawing for a channel lining my firm is designing.
- The sides will be designed as cantilevered retaining walls.
- A geotech engineer has performed a global stability analysis and said the proposed configuration will work.
- I'm hopeful that I can make the walls work with our large surcharge. I'm crunching numbers on this now.
My question is:
1) Should we put drains in this wall? I believe our worst case loading scenario is going to be a rapid draw down situation with the channel empty and fully saturated hydrostatic pressure on the dirt side of the wall. Since we are going to be designing for a fully saturated scenario, then why does it matter if it drains?
2) If we do put drains in the wall, then at what elevation? At the base? 6" inches up? 1ft up the wall? Higher?
Thank you.






RE: Drains in Retaining Wall / Channel Linings
RE: Drains in Retaining Wall / Channel Linings
If yoou use drains, then you need to get involved with suitable filters at the inside openings, meaning extra cost.
RE: Drains in Retaining Wall / Channel Linings
www.PeirceEngineering.com
RE: Drains in Retaining Wall / Channel Linings
https://res.cloudinary.com/engineering-com/image/upload/v1502156099/tips/screwy_dimensiooning_fupmpt.bmp
RE: Drains in Retaining Wall / Channel Linings
Oldestguy - The land department drew this up as a preliminary drawing so zero engineering has been done yet. Their CAD designers work in decimal feet for the most part. We will convert everything other than stationing to feet and inches when we do the structural design.
PEinc - I will check for buoyancy. We might have to thicken the walls or base, or add little heels to the wall to pick up some of the earth load in order to hold the channel down. Also, the surcharge load is so large that the walls might have to get thicker just to handle the moment.
The top of wall is at elevation 51.89 and the 100yr elevation is 51.82 which is essentially the same. I have to assume in a 100yr rain significant water could be behind the walls, thus I am assuming we will design for full hydrostatic and earth pressure behind the walls.
Do you think we should put drains in though? If I design for the full hydrostatic pressure then do I care if it drains? It's Texas so after the backfill we will have expansive clay behind the walls that is not that permeable.
The flap gates suggested by CVG seem worth investigating. But if they are below water with ~9 ft of head pressure above them, then how would slow seepage ever open them?
RE: Drains in Retaining Wall / Channel Linings
RE: Drains in Retaining Wall / Channel Linings
I would consider trying to make the channel watertight i.e. no weep holes. My last place never liked using weep holes as they are prone to blockages. We would run a perforated nova coil pipe wrapped in a filter sock parallel at the base of the wall. This would be back filled in 300mm width of filter material extending the full height of the wall. Discharge it to the downstream pond. I suppose CVG may argue the point of non return valve Weep holes.
Then you could design it with no hydrostatic pressures and be more confident that there would be none give a perforated pipe full length of wall is much less prone to blockages.
If you design the wall for empty channel and full hydrostatic that is the worst case scenario. Designing with full hydrostatic pressure may beef up your wall by what.... 30-40% (just a guess).
RE: Drains in Retaining Wall / Channel Linings
RE: Drains in Retaining Wall / Channel Linings
RE: Drains in Retaining Wall / Channel Linings
Also, suggest taking PEinc's comment seriously. My calcs indicate that neutral buoyancy for an empty channel occurs with water table at Elevation 48.7. For water table higher than 48.7 it definitely does float... might not pop up right away, but sooner or later it can happen.
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RE: Drains in Retaining Wall / Channel Linings
RE: Drains in Retaining Wall / Channel Linings
RE: Drains in Retaining Wall / Channel Linings
I think the nature of the foundation loads can be addressed using elastic theory.
I hate inches too!
f-d
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RE: Drains in Retaining Wall / Channel Linings
RE: Drains in Retaining Wall / Channel Linings
For State and/or municipal jobs, this bridge would be sitting on drilled shafts or piles, but this is a private job. Oftentimes, the private developers prefer spread footings for cost savings. They are not as concerned about long term scour or maintenance. Also, these are "constant level" man-made detention ponds that are being spanned. They increase in depth for a time period after a rain event and then drop back down.
I believe that when this job started, the size of the detention ponds was smaller. They have since been increased and the hydrologists needed more channel cross section area to move water across, which is how we got to the current channel configuration.
All I know is that I have the current cross section to work with. We had the geotechs modify their original report for the new proposed cross section and perform a global stability analysis. They have signed off and sealed their report addendum.
RE: Drains in Retaining Wall / Channel Linings
RE: Drains in Retaining Wall / Channel Linings
RE: Drains in Retaining Wall / Channel Linings