When one retaining wall is constructed above another retaining wall, the two walls together are called a Tiered Wall System.  Tiered walls look like this:

Distance Between Tiers:
Extra loads on a retaining wall are called surcharges.  Retaining wall contractors understand that as the Upper Wall gets closer to the Lower Wall there is more load (more surcharge) put on the Lower Wall.  If the Upper Wall is far away, then there is less load (less surcharge) put on the Lower Wall.

Question: How far away must the Upper Wall be so that it will not surcharge the Lower Wall?  Answer: Take the height of the Lower Wall and double that number.  That is how far away the Upper Wall must be to apply little to no surcharge on the Lower Wall.  For example, if the Lower Wall is five (5) feet high, then the Upper Wall must be at least 10 feet back from the Lower Wall so it won’t overload the Lower Wall.  (This assumes that the Lower Wall is taller than the Upper Wall.)  See this drawing:

Question: Can the Lower Wall be specially designed for the surcharge of the Upper Wall?  Answer: Yes, definitely!  A retaining wall engineer can design the Lower Wall for the surcharge load applied by the Upper Wall.  In the NCMA* Design Manual (2010, Third Ed.) use Figure 5-6, “Surcharge Approximation for Tiered Walls”.Question: Can I have a single wall that “splits” apart, with the bottom half curving away from the top half?  Yes!  Retaining wall contractors and engineers have become very creative, with splitting (or merging) walls.  The most important thing to remember is that you must provide support below the upper blocks when the upper part of the wall curves away from the lower part of the wall!  See this sketch for a concept drawing of burying blocks to support that upper wall:

Remember, tiered walls can be a beautiful way to improve a retaining wall design and layout.  But the tiers have to be designed properly to be structurally sound.

Joseph Kowalski, P.E.
www.RetainingWallNetwork.com
Current State PE Registrations

*National Concrete Masonry Association, Design Manual for Segmental Retaining Walls, Third Edition, Fifth Printing, 2010, Copyright NCMA, Herndon, VA.