Drainage behind retaining walls: why walls fail in San Diego

The most common reason retaining walls fail

Ask any experienced retaining wall contractor in San Diego what causes most wall failures and the answer is almost always the same: drainage. Not bad block. Not poor installation technique. Not even earthquakes. Drainage.

A retaining wall does not just hold soil. It holds soil that is sometimes saturated with water after a wet season. During San Diego’s periodic atmospheric river events, slopes and hillsides can absorb several inches of rain in 24-48 hours. All of that water has to go somewhere. If there is no path for it to drain through and away from the wall, it builds up as pressure against the back of the wall. That pressure is called hydrostatic pressure, and it can be surprisingly large.

A wall holding saturated soil is holding something much heavier and much more dynamic than dry soil. Walls that would hold dry hillsides for 30 years can push out, crack, or topple within a few seasons if they are built without drainage.

How San Diego soils make drainage more urgent

The county’s soils are not uniform, but they share some characteristics that make drainage management particularly important for retaining walls.

Much of San Diego’s coastal and inland residential land sits on decomposed granite (DG) or sandy loam that drains reasonably well when dry but can become unstable and heavy when fully saturated. In the eastern valleys from El Cajon through Santee and into the back country, clay soils are more common. Clay expands when wet, which creates pressure on walls not just from water weight but from the soil itself swelling against the back of the wall.

Hillside lots in neighborhoods like Skyline, Encanto, and the canyon-adjacent communities in Mission Valley were often graded and developed rapidly during San Diego’s postwar growth. Some of those cuts were stabilized with walls that were built without drainage systems that meet current standards. When those walls start to lean or crack, poor drainage is often a contributing factor.

What proper drainage behind a retaining wall looks like

A properly drained retaining wall has three components working together.

Drain rock. The area directly behind the wall, called the drainage zone, should be filled with clean crushed rock (typically 3/4-inch angular gravel) rather than the native soil that was excavated. Drain rock creates a permeable zone that water can move through freely instead of building up pressure against the wall face.

Perforated pipe. At the base of the drainage zone, a perforated drain pipe (typically 4-inch schedule 40 or SDR 35) collects water that moves down through the drain rock and carries it to a point where it can exit. That exit is usually a weep hole through the wall face at grade level, a solid pipe extending out past the wall face, or a daylight drain that exits at a low point below the wall.

Weep holes or through-wall drains. For shorter walls without a full drainage pipe system, weep holes at intervals in the wall face let water bleed through before pressure builds. They are not a substitute for drain rock and perforated pipe on taller walls, but they serve as a pressure release on smaller projects.

Signs that an existing wall lacks drainage

If you are buying a property in San Diego with an existing retaining wall, or if you have a wall that has been in place for 10 or more years, look for these signals.

White staining on the wall face. Efflorescence, the white mineral deposits you see on concrete block and poured concrete walls, forms when water moves through the wall material and deposits minerals as it evaporates. It is not dangerous by itself, but it is evidence that water is getting behind the wall and moving through it because there is nowhere else to go.

Leaning or bowing. A wall that has developed a visible lean outward from the slope is under sustained pressure. In most cases that pressure is water-related. A small lean can become a structural failure given enough time and enough wet seasons.

Cracking near the base. Cracks that appear at the bottom section of a concrete or block wall are often caused by soil pressure concentrated at the toe of the wall, which is where hydrostatic pressure is highest.

Mounded soil at the wall base. If soil has been accumulating against the face of a wall over time, drainage exits may be blocked. Water that cannot exit will find another way, usually by forcing the wall outward.

What to ask before a new wall is built

Before any retaining wall project starts, get clear answers to these questions.

What is the plan for drainage behind the wall? The answer should describe drain rock, perforated pipe, and where the water exits.

What is the exit point for the drain? The pipe needs somewhere to go. If there is no natural low point, the contractor needs a plan for daylight drainage, a dry well, or a connection to an existing drainage system.

Is the drainage included in the quote? On some bids, drainage is listed as an add-on. Make sure you know whether it is in scope before you compare prices.

If a contractor does not have a clear answer to these questions, that is useful information before the wall is built rather than after.

Wall Pro SD connects San Diego homeowners with insured local retaining wall crews who build drainage systems correctly the first time. Call (858) 925-5546 to get connected with someone who can assess your site.

For existing drainage problems, see our drainage solutions for retaining walls and emergency retaining wall repair service pages.