On a La Mesa hillside I once took down a mortared block wall that had been standing for eleven years and failed in a single February. There was no weep, no drain, and eight feet of saturated fill behind it. The wall was well built. It was just asked to hold back something no wall of that type can hold.
The two families of wall
A dry-stack wall — segmental block, boulders, or stacked stone — holds soil by mass and friction and lets water pass through it. It flexes. Individual units can shift a little without the wall failing, and hydrostatic pressure never builds because the water simply drains out the face.
A mortared wall is a rigid structure. It has to be designed to resist the full lateral earth pressure plus any water pressure behind it, and it has to be founded below frost and on bearing soil. When it moves, it cracks, and the crack is permanent.
Neither is better. They fail differently and they suit different sites.
Choosing between them
- Height. Under 3 feet, either works and the choice is aesthetic. From 3 to 6 feet, dry-stack with geogrid is usually cheaper and more forgiving. Above 6 feet you are in engineered territory whichever you choose, and in most of San Diego County anything over 4 feet — or with a surcharge above it — needs a permit and a stamped design.
- Water. If the slope sheds significant runoff or has a spring line, dry-stack is far safer. Rigid walls in wet ground need a drainage system that is often more work than the wall.
- Movement. On expansive soils or a slope with any history of creep, a flexible wall will accommodate what a rigid one will not.
- Seismic. Rigid walls need to be designed for seismic earth pressure here. Flexible systems are more tolerant.
- Appearance and life. A well-built mortared stone wall reads as permanent architecture. Segmental block reads as engineering. Both last decades if drained.
The parts nobody sees, which decide everything
Whatever the face is made of, the wall is only as good as three hidden things.
Drainage. A perforated pipe at the base, wrapped in filter fabric, bedded in clean 3/4 inch aggregate that extends at least a foot behind the face, daylighted at both ends. Not a pipe buried in native soil, which silts up in three years.
Backfill. Free-draining gravel behind the wall, compacted in lifts, with fabric separating it from the native soil so fines do not migrate in and clog it.
Base. A compacted aggregate leveling pad, or a proper footing below frost depth for a mortared wall. On a slope this needs to be benched into the hill, not perched on the surface.
Costs in this area: segmental block with geogrid $35 to $70 per face foot, mortared stone $60 to $160 per face foot, engineering and permit $1,500 to $6,000 where required. Rebuilding a failed wall costs more than building it right did, because the slope has usually moved by then. When someone is comparing quotes I tell them to compare the drainage detail first — that is where San Diego hardscaping bids differ most, and it is the part that decides whether the wall sees eleven years or fifty.
Frequently asked questions
Do I need a permit for a retaining wall? In most jurisdictions here, yes above 3 or 4 feet measured from the bottom of the footing, and often at any height if there is a surcharge such as a driveway or structure above. Check with the local building department before design, because the answer changes the wall type.
What are weep holes actually for? They relieve water pressure behind a rigid wall. A mortared wall without weeps or a drain behind it can see hundreds of pounds per square foot of hydrostatic pressure in a storm, which is often more than the earth pressure it was designed for.
Can I build on top of an existing wall to gain height? Almost never safely. The original wall was sized for its original height and surcharge; adding courses increases the overturning moment on a footing that was not designed for it. Raising a wall usually means rebuilding it.