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Q: I have a question about the geotextile placed under articulated systems in slope armoring. Based on my understanding, a drainage layer is recommended between the geotextile and the armoring structure, but, since the armoring structure has some spaces in between the armoring blocks, won’t the particles of the drainage layer be washed out through these spaces? How can we make sure that the drainage layer will not be eroded? Or is it assumed the water will flow in parallel to the drainage layer and will not leach out through the armoring blocks? I guess that a minimum thickness for the drainage layer should be considered in this case.
A: In extreme cases, one may need to build a graded filter (i.e., layered with multiple geotextiles and possible sand). Geotextiles have been used beneath erosion control structures since the late 1950s. Depending primarily on the care exerted by the contractor in placing the riprap, a sand cushion may be needed to protect the geotextile from impact damage during installation or abrasion damage during its lifetime (due to wave action agitating the rock riprap). If precast concrete blocks are being used, a sand layer is often used, not so much as a cushion (since these blocks are placed by hand or carefully lowered into position), but now as a pore water dissipator since a major part of the fabric can be directly covered by the blocks. This would be the geotextile filter in place with a riprap cover and also a failed system of an articulated precast block system.
The system you are describing may need a needlepunched nonwoven (NPNW) geotextile on the silt subgrade, uniform sand, a woven monofilament geotextile and then, finally, the slope-armoring system. Please make sure the armoring system has enough open area to dissipate pore water pressure from rapid drawdown. The U.S. Army Corps of Engineers has fantastic details on such a design.