Every retaining wall needs drainage. The slightly more complicated answer though…is what kind?
Most wooden retaining walls are not built with drainage, but they should be. For a wooden wall you want to limit the amount of water behind the wall for 2 reasons.
The first reason would be to slow down the rate at which the wood will rot. Constant moisture is not good for wood long-term. The 2nd reason is water is heavy, and reducing the weight of the soil pushing on the wall will help to make your wall last longer, and stay straight while doing so. The worst place for wet heavy soil is at the top of the wall is this is where it has the most leverage to overturn the wall.
For a concrete retaining wall, specifically Allan block water effects it differently. Unlike wood concrete is almost impervious to moisture. For all intents purposes water will not affect concrete… So why is water so bad for it?
Well for the same reason as the wood, water is heavy and will put a huge unnecessary surcharge on the wall. But unlike wood this concrete retaining wall will have to resist this pressure for 50-100 years. Maybe even longer. The added lifespan of the concrete means you also have to take into account the wear and tear of mother nature year after year.
Water if not drained away, will also create problems during freezing temperatures as the water freezes it expands and will cause the wall to move or shift. This may not structurally affect the wall, but it won’t look straight anymore. This reduces the aesthetic look of your focal point.
Water present around a concrete retaining wall will also encourage root growth from nearby trees or shrubs, all of which can upheave or cause a wall to bow outwards.
These types of aesthetic blemishes may not be a big problem for a wooden retaining wall if it’s being replaced frequently, but for a Allan Block retaining wall that will last a lifetime an early blemish should be avoided!
Fortunately, the Allan Block contractor training program teaches us contractors how to avoid these problems, and how to build strong structurally sound retaining walls.