We all know what a failing retaining wall looks like. It seems that everyone and their neighbor has one falling over in the backyard. It's much harder to find a stable retaining wall that is truly well built. The difference between a successful wall and failing retaining wall has everything to do what isn't seen. The preparation and engineering determines the success of the wall, long before the final cap is laid. The keys to a stable wall are...
Use materials that are constructed and engineered specifically for retaining walls such as Versa-Lok, Uni-Lock, Belgard, and Anchor. If using natural stone, make sure it’s labeled as ‘Wall Stone’. Don’t use pavers, steppers, stone edging, or flagstone. These materials may be stackable, but they will not support heavy loads or have the same longevity.
The profile of the wall should step back ¾” with each course of block. When finished, the wall should look like it is leaning into the slope. This counteracts gravity and helps “push” back on the wall.
Mortar & Adhesive
Not every wall requires mortar or adhesive, but it can help bond the whole wall together to make it stronger.
Certain brands like Versa-Lok use pins to help align and lock the separate blocks together.
Depending on the size of the wall and the soils around it, geogrid may be necessary to install behind the wall. Geogrid is a synthetic material used to reinforce soils.
Gravel or coarse rock needs to be poured behind the wall for extra strength and fast drainage. Using plain soil as backfill
will let moisture to collect causing erosion problems, extra weight, and shifting during winter.
Install an erosion blanket between the rock backfill and soil. An erosion blanket is a heavy duty landscape felt that prevents soil from settling in with the backfill. This helps with drainage and stability of the soil behind the wall.
Drain tile redirects the water away from the wall through a perforated tube placed in the rock backfill. Water drains through it instead of settling behind the wall and eventually flows out through the front of the wall to keep moving downhill with minimal contact with the stone.
Install a compacted base material of coarse rock to support the wall and the heavy load pushing down on it. Sand and soil alone will not be sufficient. The amount of base depends on the size of the wall and its use. Don’t skimp on the depth as the entire load is dependent on the foundation.
A concrete footing may be necessary in the walls foundation. A footing is a concrete pad that the wall sits on to help lessen the effect of freeze-thaw expansion in the soil. The frost line in the Twin Cites is 42” deep.
All of these best practices should be common for a professionally built retaining wall. They add to the stability and longevity of any retaining wall; however, many times these items are skipped over in DIY projects and aren't factored into the final cost. If you're asking yourself "How much do retaining walls cost?", you aren't alone. Find out the typical costs of retaining walls in this Project Budgeting Guide.