Q: I just watched your recorded video of mechanically stabilized earth (MSE) wall inspection. Thank you for the presentation; it was very informative. I do have a follow-up question for you. I was recently the site engineer for a precast panel MSE wall and now have the opportunity to inspect construction of the wall that has just begun. The question I have for you is on industry standard MSE wall drainage systems.
The wall vendor shop drawings propose an underdrain pipe that’s installed at the back of the wall straps. This type of wall drainage is typical to what I’ve seen in Connecticut and California, and is what I detailed in the wall plans. During the design, the geotech has included another wall drainage detail that proposed an 8-inch (20-cm) thick sand blanket underneath the wall along with a slightly different underdrain. I attached both details for reference. In your experience, is there an industry standard or do the wall drainage systems vary based on the geotech/region?
A: The short answer is yes, we see slightly different MSE underdrain systems (base, back outlets) depending on availability, geotechnical condition and region of the United States. I have attached a PDF and PowerPoint presentation that I think are expansions of what you saw earlier. The three National Concrete Masonry Association (NCMA) scenarios are informative and probably realistic. Unfortunately, one only really knows once the cut (excavation) is open. It is often a tough call for a young engineer or technician.
In the northeast, we see base drains of geotextile-wrapped gravel with embedded perforated polyvinyl chloride (PVC) pipe. Back drains are almost exclusively drainage geocomposites