Licensed and insured, building retaining walls across the Adelaide Hills since 2009
The number one reason a wall bulges or fails is water. Soil behind a wall holds water after rain, and that hydrostatic pressure pushes until the wall gives. How proper drainage, an ag-pipe, aggregate, weep holes and geofabric, stops it, and why a quote that leaves drainage off the page is the warning sign.
Most retaining walls that fail do not fail because the block was weak. They fail because of water. It is the most important thing behind any wall, and the easiest thing for a cheap quote to leave off. Here is what actually happens, and how proper drainage stops it.
Soil holds water. After heavy rain, the ground behind a retaining wall fills with water, and that water is heavy. It presses on the back of the wall. Engineers call this hydrostatic pressure. A dry wall only has to hold back soil. A wet, undrained wall has to hold back soil plus all that trapped water, and the load can more than double. Over a few wet winters, that is what makes a wall bulge, crack and lean.
Drainage is the cheapest insurance there is against the most expensive failure there is. The wall that is drained costs a little more than the wall that is not. The wall that is not is the one you rebuild.
Drainage is real work and real material, so it is the easiest line to drop to make a number look low. You cannot see it once the wall is finished and the ground is backfilled, so it is also the easiest corner to hide. That is exactly why a quote with no drainage line is a warning sign, not a bargain. We name the ag-pipe, the aggregate and the weep holes on the quote, as their own line, on every wall.
Ask this, exactly
“Is the drainage on the quote as its own line, the ag-pipe, the aggregate and the weep holes, and will it go in behind the wall as you build it?”
A wall that lasts is a wall that drains. A quote with no drainage line is the one that bulges in a few winters, because trapped water is the number one cause of failure.
The drainage goes in as the wall comes up, not bolted on at the end. The ag-pipe runs the full length of the base, the aggregate sits against the back of the wall, the weep holes go through the bottom course, and the geofabric keeps it all working for the long run. It is part of every wall we build, because it is the part that decides whether the wall is still standing in twenty years.
Tierline Retaining Walls