Retaining Wall Drainage: Why It Matters in Iowa

Water is the single biggest threat to any retaining wall. It does not matter how nice the stone looks or how carefully the blocks are stacked. If the wall does not have proper drainage, it will fail. Water builds up in the soil behind the wall, creating hydrostatic pressure that pushes against the structure. In Iowa, where we get heavy spring rains and our clay soil holds water like a sponge, that pressure is enormous. A wall without drainage is a wall with an expiration date. We have seen it happen too many times in Cedar Falls. A beautiful retaining wall that starts leaning, bulging, or cracking within a few years, all because the water had nowhere to go.

The drainage system behind a retaining wall has three main components, and each one plays a specific role. The first is gravel backfill. Instead of putting the native clay soil directly against the back of the wall, we backfill with clean, washed gravel. The gravel creates a void space that water can move through freely instead of building up in the clay. The second component is a perforated drain pipe at the base of the wall. That pipe sits in the gravel and collects water that drains down through the gravel. The pipe is sloped to carry that water to an outlet at the downhill end of the wall. The third component is an outlet, either a weep hole through the wall face or a pipe that carries the water to a safe discharge point away from the wall.

Iowa clay soils make drainage even more critical than it would be in sandy or loamy ground. Clay is dense and has tiny pore spaces that water moves through very slowly. When it rains, the clay behind the wall absorbs water and swells. That swelling creates lateral pressure against the wall that adds to the hydrostatic pressure from the water itself. In winter, the wet clay freezes and expands even more, pushing against the wall with tremendous force. A wall built without gravel backfill and drain pipe in Iowa clay will almost certainly fail within a few years. The first signs are usually a slight tilt at the top or a bulge in the middle. By the time you notice the movement, the wall needs to be rebuilt.

The gravel backfill needs to be the right kind of gravel. We use clean, washed crushed stone, typically 3/4 inch or 1 inch diameter. The key word is washed. Gravel that contains dust, sand, or fine particles will clog over time and stop draining. The fines fill the voids between the larger stones and trap water instead of letting it flow through. The gravel should extend at least 12 to 18 inches behind the wall and go from the base to within 6 inches of the top of the wall. That column of gravel acts like a vertical French drain, collecting water from the full height of the wall and directing it down to the perforated pipe at the bottom.

The outlet is the part of the system that people forget about most often. You can have perfect gravel and a well-placed drain pipe, but if the water has nowhere to go once it leaves the pipe, it backs up and the system fails. The outlet needs to discharge at a point that is lower than the drain pipe and far enough from the wall that the water does not soak back into the ground behind it. That often means running the outlet pipe to a drainage swale, a ditch, or a downhill area away from the wall. In some cases, a pop-up emitter that releases water at the surface works. For walls built on a slope, the outlet may daylight naturally at the downhill end of the wall. We always plan the outlet location before we start digging.

The cost of adding proper drainage to a retaining wall is a small fraction of the cost of rebuilding a failed wall. The gravel, pipe, and fabric add maybe 10 to 15 percent to the total project cost. That is cheap insurance. A retaining wall that is built with proper drainage will last 50 years or more. A wall that was built without it will start showing problems in 3 to 5 years, and it will need to be torn down and rebuilt by year 10. We build every retaining wall in Cedar Falls and the Cedar Valley with full drainage. It is not an upgrade you have to ask for. It is part of how we build walls that last. If you are thinking about a retaining wall, make sure drainage is part of the plan from day one. Give us a call and we will walk through what your project needs.