Weep Pipes in Reinforced Segmental Retaining Wall Post Construction

[JErvin87]

Hey all,

My firm recently designed a segmental retaining wall using #57 stone for the full length of the geogrid reinforcement. The builder did not have a third party testing firm inspect the wall as it was being built, and as a result, the continuous drain and all outlets through the wall face were omitted during construction. Now the city inspector has put a stop work order on the project and has requested input from my firm on how to proceed. It is my understanding the wall is nearly complete and the contractor does not want to completely dismantle the wall to install the drainage elements. My question is, have any of you designed or worked on a project where weep pipes were installed by coring the facing units after wall construction was completed? If so, I would like to know how you accomplished this without losing the open-graded stone through the hole before the pipe was installed. Right now, we are just looking for some ideas to assist our client in getting back to work and getting a CO for the house.

 

[oldestguy]

First off why the open graded stone? That is not a filter of fine soil. Under significant water flow there can be erosion of the finer materials through the openings of that (non) filter. I'd drill the holes in the wall, fill some of that hole and beyond with ASTM C-33 fine concrete aggregate (concrete sand). Then backfill the hole with some of that stone, of suitable gradation that the sand is held back. Then wedge in a galvanized screen, say 1/2" openings. Why a pipe? One of these weep holes about every 4 feet ought to be sufficient to keep any water back there from causing any significant pressure on the wall. This is about what many times is built without a continuous drain behind the wall. If you want your job to plug some day, use open graded stone that is not a filter.

 

[JErvin87]

oldestguy,

In our area we use open-graded stone because A. its relatively cheap and B. the fine-grained soils don't meet the requirements of NCMA for use a reinforced backfill. However, to address your concern about the fine-grained material clogging the open-graded we reduce this risk by wrapping the open-graded stone with a non-woven geotextile fabric to allow water to pass through but not the fines. I know it doesn't stop all the fines, but it drastically reduces it. We have started to use dense-graded stone with just a face drain of open-graded in some areas as we have some contractors who save a buck or two doing it that way. I really appreciate the input on how you would do that as we haven't run across this before and I was hoping someone had and could provide some help. Thanks again and have a great day.

 

[oldestguy]

In case you wonder My Master's degree specifically studied under-drains in highways and backfill to walls. At that time 1950's no filter fabric was available. Since then, I've seen many a job with filter fabric and stone. If done right, it works. Too many times it is done wrong. You can't go wrong with the info I gave and I can't believe you can't get concrete sand or correct stones to do the job. Good luck.

 

[cvg]

havent seen any details, but unless you have a filter layer preventing migration of fines, than you have problems as OG has said. However, drilling the holes and trying to place sand in there will not work because unless the gravel lifts were fully wrapped in geotextile filter fabric, than the sand will just migrate into the voids also. at this point, any sand you put in the hole needs to be wrapped.

"MSE wall failures vis-à-vis the lack of filtration design"

https://www.geosynthetica.com/mse-wall-failures-geotechnical-filters/

 

[oldestguy]

OG once more. Regardless of which way it is done, using a fabric and then some more open graded stone or holes in a pipe, etc., the experience of how well this job is run tell me use of fabric covering some drain material requires FULL TIME EXPERIENCED INSPECTION. Do it right this time.

 

[bridgebuster]

I'm on a design-build highway project with 1400 feet of precast wall. The backfill is #57 stone. The supplier said this is common; also it had to be designed for a rapid drawdown condition; the 57 stone is a plus.

Anyway, yes, I've drilled weep holes into a bunch of existing walls. A four inch PVC pipe wrapped in filter fabric. I'll post a spec tomorrow.

 

[bridgebuster]

here's a spec

 

[bridgebuster]

here's a detail