Backfill and filter for clay subsurface drain?

[fsolomon]

I am building a subsurface drain (french drain)in a high clay soil. I am planning on backfilling the trench with sand to filter the clay and using a drain sleeve filter on the perforated drainage pipe. What sand and AOS filter should I use?

I have seen C33 sand is recommended. If I use a ASTM D6707 filter with an AOS of 0.6mm, then almost half of the sand will be smaller than the filter. Will the filter clog?

 

[blueoak]

The NRCS has some of the best filter design criteria. A filter for a pipe is similar to under riprap where you may need 2 or more layers of filter material between the soil and the riprap. It may be listed under their dam design info or waterway. I am not a huge fan of the sleeve filters. You may want to consider grading up to a gravel to surround the pipe. If you are worried of piping material from a structure you need a better filter rather than a sock or if not then it is best to let the clay go rather than plug the sock.

 

[redneckengr]

The NRCS reference Blueoak is talking about about is the National Engineering Handbook part 633 chapter 26. You will need some soil gradations and then you can follow the 12 step process outlined in Chapter 26. There are some very good examples in there and also some basic ASTM gradations in the back. I suggest you also research your state DOT aggregate gradations as those are often the most readily available at local quarries.

 

[cvg]

if you are draining subsurface water through a clay layer, you may not have very high flow rates. Are you sure you need a pipe?