A material is generally considered free draining if its permeability is at least one order of magnitude greater than the material providing the water.

Therefore the gradation you list would be free draining if placed adjacent to a silt or clay, but not free draining if placed adjacent to a clean gravel. Additionally you have to consider filter effects. The drain material needs to be either sized as a filter for the adjacent material or a filter needs to be provided.

Mike Lambert

A good comment on filter requirements. Also a good rule of thumb that one order of magnitude greater permeablity.I guess it means 10 times being in a logrithmic sscale. However, what about if I put a clay of 10-6 near a clay 0f 10-7. Guess non would be free draining.

Please reread your posts for typos before hitting submit as they make your posts hard to read.

As for you comment about clay adjacent to clay, very theoretical since getting a clay permeability that close would be very tough. However, if you could then the 10-6 clay would be free draining (i.e. dry) if the only source of water it had was from the 10-7 clay.

Mike Lambert

I'd say forget the use of clay - river (concrete) sand would be a good source of MSE backfill - used it in India to good effect; used in dams as chimney drains. With respect to the above gradation in the OR, might want to consider if the material is self-filtering (see Terzaghi Peck and Mesri)

The clay was just an example for clarification on the rule of 10 time higher permeability. Certainly I would not be using clay for drainage.

The challenge is whether to use pure gravel (e.g. 100% retained on 4.75 mm) or sand and gravel with <5% fines (e.g. river alluvium). A colleague believes that if we have 47.5% sand and 47.5% gravel and 5% fines, the gravel acts as a guest in the matrix and in fact the sand would be the media through which water would seep. Hence the % of silt in the soil, which is actually subject to seepage will be 10% of the weight of sand.
This would then imply that we would have dirty sand, the permeability of sand with about 10% fines would thus diminish, compromising the free drainage criteria. This seems a theoretical paradox but I am looking for an answer. Nevertheless that is the reason that I am searching on this subject.

BigH is correct in stating that concrete sand is an excellent, often readily available, drainage material. Generally works for silts and clays.

As for you question, I would not allow any significant amount of minus 200 materail in my drainage layer.

Mike Lambert