Without proper drainage, water saturates the soil behind a retaining wall and turns it into dead weight the wall was never built to hold, so the wall starts to bulge, lean, or crack. In a freeze-thaw climate that trapped water also freezes and expands, prying the wall apart from the inside, and a poorly drained wall commonly shows real damage within a few seasons instead of lasting for decades.
A wall's design assumes the soil behind it drains normally. When it can't, saturated soil can weigh substantially more than the same soil dry, and that extra weight adds lateral load the wall's footing and mass were never sized for. The pressure doesn't ease off between storms either, it stays elevated until the water finally finds somewhere to go, which without drainage might not happen until it pushes through the wall itself.
In the Blue Ridge foothills, that saturated soil goes through repeated freeze-thaw cycles all winter. Trapped water freezes and expands, adding a second source of pressure on top of the hydrostatic load, then thaws and refreezes again the next cold snap. A wall on flat, well-drained ground in a milder climate might tolerate poor drainage for a while. A wall here usually can't.
When we're called out to look at a wall that's showing damage, drainage is one of the first things we check, because patching the visible crack without fixing the water problem behind it just buys a little time before the same failure comes back. Depending on how far the damage has gone, that can mean adding proper drainage to a section as part of a repair, or rebuilding that section correctly from the footing up.
More on this from Chelo's Brick & Stone.
Chelo's Brick & Stone serves Hillsville, Galax, Wytheville, Fancy Gap, and Independence, VA. Call for a straight answer and a free on-site estimate.
