Another posting here about Farm house crawl space, etc. has discussion about instating drains to protect that area. I have posted several lengthy discussions there about the proper construction of drains for foundation protection and similar facilities. As you read the posts there pay attention to the misleading, yet well intentioned recommendations calling for washed stone, etc. backfill. Unfortunately that use of drain backfill with high void spaces has been around around a long time . Some of them work, but many fail in time.