Can a geotextile filter be used in lieu of the 6-12 inches of granular filter that we commonly call for under the riprap?
The current specs require steps for geotextiles on steeper slopes. This is because of both the possibility that the riprap could slide on the fabric, and that the ground under the fabric might slough or fail. The type of geotextile fabric, characteristics of the soil and soil saturation are all important in determining if geotextile filter under riprap will be stable at a given slope. It should be stable at a 1:3 slope regardless of geotextile type and soil characteristics. At this time we could have to recommend the specs be followed. However, we're also interested in further investigation of this topic. Maybe we can determine which situations would be acceptable for a geotextile fabric to be placed on a 1:2 slope (without steps); let's keep each other informed if we find anything new.