A stacked-stone wall and a poured concrete wall can both hold back the same slope, but they do it in different ways. That difference matters when something starts to look wrong. A crack means one thing in a reinforced concrete stem and something else in a dry-laid rock wall. A bulge in a gabion basket has different suspects than a bulge in a row of timber lagging.
This post sorts retaining walls by how they’re built and walks through how each type tends to show distress. It’s written for homeowners in La Jolla citywide who want to know what kind of wall they have before they talk to anyone about it.
We have other posts on nearby questions. If you want the causes of a wall tipping forward, organized by mechanism, read why retaining walls lean. If you want a routine for watching a wall over time, use the retaining wall monitoring checklist. This one is about construction type.
Every wall has the same four jobs
Whatever it’s made of, a retaining wall has to resist the same forces. The San Diego Building Code, which adopts the California Building Code with local amendments, puts it in one sentence: “Retaining walls shall be designed to ensure stability against overturning, sliding, excessive foundation pressure and water uplift.”
In plain terms:
- Overturning is the wall rotating forward about its toe.
- Sliding is the whole wall moving out along its base.
- Excessive foundation pressure is the soil under the wall giving way because the wall presses down harder than the ground can take.
- Water uplift is water pressure working against the wall from below and behind.
The residential code uses nearly the same words, calling for walls “designed in accordance with accepted engineering practice to ensure stability against overturning, sliding, excessive foundation pressure and water uplift.”
What changes from one wall type to the next is how the wall resists those forces. That is also what decides where it shows trouble first.
Inspection manuals written for highway walls give a useful way to group them. The Wisconsin Department of Transportation’s structure inspection manual says “Gravity walls are considered externally stabilized as these walls use self-weight to resist lateral pressures.” It then notes that “Non-gravity wall are classified into cantilever and anchored wall categories”. Those manuals cover public road walls, not backyards, but the physics doesn’t change at a property line.
Gravity walls: weight does the work
A gravity wall stays put because it’s heavy. The Wisconsin manual describes it as “a large monolithic structure, which depends entirely on its self-weight and the weight of the soil that rests upon it for stability.” The New York State DOT’s retaining wall inspection program says the same and widens the material list: “Other materials utilized to construct a gravity wall include stone, stone and mortar, and gabions.”
On a residential lot, gravity walls include dry-stacked stone, mortared stone or rubble, rockeries, poured mass concrete and gabion baskets filled with rock. Concrete crib walls belong here too. NYSDOT describes crib walls as interlocking precast concrete members with soil-filled cells and says “Together, the backfill and the precast logs form a composite gravity wall.”
Where gravity walls give way. Because weight and width are the whole strategy, these walls are sensitive to anything that changes the balance between what’s pushing and what’s resisting. NYSDOT’s defect table for gravity walls lists misalignment that “May be caused by several factors including structural failure of the concrete mass; soil failure at toe or backfill slope; horizontal sliding, or seepage.” In practice, that can look like:
- The wall moving forward along its base, with a gap opening at the top behind it.
- Individual stones or blocks shifting out of line. The Wisconsin manual notes that in some gravity walls “The blocks of these gravity walls are freestanding with joints that may or may not be mortared,” so a dry-laid wall can move stone by stone.
- Soil loss at the toe. If the ground in front of the wall erodes or gets dug out, the wall loses some of the soil it was counting on.
Gabions have their own list. A gabion is a wire basket packed with stone. NYSDOT flags corrosion of the wire and notes “The rate of corrosion is dependent upon the oxygen concentration and moisture in contact with the wire.” It lists bulging that “May be caused by several factors including disruption to the internal drainage system and/or re-direction of surface drainage system; additional loads imposed on the wall; or soil failure at toe or backfill slope.” It also notes bulging can trace back to how the baskets were filled in the first place.
Cantilever walls: reinforcement and the heel
A cantilever wall is thinner and relies on steel and leverage instead of bulk. NYSDOT describes a cast-in-place cantilever wall as “a thin, reinforced stem cantilevered from a base slab.” The trick is the footing: “Stability of the cantilever wall is achieved by the weight of the wall system and the weight of the backfill above the heel projection of the base slab to resist lateral soil pressure.” The Wisconsin manual puts it another way: “The weight of the soil on the inner half of the footing slab (heel) provides additional vertical weight for stability.”
So the soil you’re holding back is also helping hold the wall down. The footing extends back under the fill (the heel), and that soil weight resists overturning.
One residential version in San Diego is reinforced concrete block (CMU) built as a cantilever. The City’s Information Bulletin 221 offers “prescriptive reinforced masonry cantilevered retaining wall designs” for limited conditions. The bulletin’s exclusions show what makes cantilever walls work harder. Its standard designs may not be used to retain “expansive soils, surcharge loads” or on sites with listed geologic hazards. It adds that surcharge “imposed by building foundations, driveways, parking areas, fences, etc., shall not be allowed on the wall’s upper level” for those designs, and that “Retaining walls not conforming to the design criteria in this bulletin shall be designed by a civil engineer, structural engineer, or architect licensed in the State of California.” It also says “A geotechnical investigation may be required depending on the project location.”
What to watch on a cantilever wall. The stem is the part you see, and it’s the part that bends. One detail from NYSDOT’s manual is worth knowing: flexural cracks “would occur on the back side of the wall and would not be visible to an Inspector.” The face you can see may look calmer than the side against the soil. The same manual tells inspectors to “consider width, spacing, location, orientation of the cracking and determine whether the cracking is structural or nonstructural in nature,” because shrinkage and temperature also crack concrete.
Things that change a cantilever wall’s math include added load on top (a new patio, parked cars, a structure near the edge), saturated backfill, and anything that disturbs the heel or the soil in front of the toe. Our retaining wall vs. foundation interaction post covers the case where the house itself sits close above the wall.
Segmental block walls: two very different walls that look alike
Segmental retaining walls are the dry-stacked concrete block walls you see in landscaping. They look similar from the front, but there are two kinds. The National Concrete Masonry Association’s inspection guide says “Segmental retaining walls (SRWs) are gravity retaining walls which can be divided into two groups.”
- Conventional SRWs resist the soil “solely through the self-weight and batter of the SRW units”. That’s a gravity wall made of small pieces, leaning slightly back into the slope.
- Geosynthetic reinforced SRWs are “composite systems consisting of SRW units in combination with a mass of reinforced soil stabilized by horizontal layers of geosynthetic reinforcement materials.” Here the blocks are a face. Layers of grid buried in the backfill turn a block of soil into the real wall.
The building code also has a line for the units themselves: “Dry-cast concrete units used in the construction of segmental retaining walls shall comply with ASTM C1372.”
Failure modes for block walls. NCMA’s design manual lays out failure modes that map onto the two kinds.
For the whole wall, external stability: “To be stable, the gravity wall structure must form a coherent mass (weight) that has sufficient width to prevent both sliding at the base and overturning of the mass about the toe”. That covers sliding and tipping, the same as any gravity wall.
Between the blocks, internal stability: “Conventional SRWs must have enough internal shear capacity between SRW units to withstand lateral pressures without shearing failures occurring between units”. Visually, that can show up as one course sliding out past the course below it.
For reinforced walls, the manual adds checks for the grid pulling out of the soil or breaking, and for the connection between the grid and the blocks. It notes the top layer of grid has to sit close enough to the top “to preclude local overturning failure of the upper, unreinforced section of wall units (crest toppling).” When the top few courses rotate on their own, that’s the pattern it describes.
NCMA also reminds owners that the wall isn’t the only question. The site’s civil and geotechnical engineer should evaluate “surface drainage, groundwater, excessive foundation settlement, and potential global/overall slope instability.” Its inspection guide is blunt about installation: to work as intended, “SRWs must be properly designed and installed.” From the face alone, you can’t tell whether grid was installed, or how long it is. Drawings or the original installer can.
Timber walls: the material sets the clock
Timber walls show up as stacked landscape ties, timber cribs, and post-and-lagging walls where horizontal boards span between posts set in the ground. NYSDOT’s wall-type list includes timber sheeting and soldier-pile-and-lagging walls, which are cantilever types; a timber crib uses the same cell-and-fill idea as the concrete crib walls above.
What sets timber apart is that the material itself breaks down. The USDA Forest Products Laboratory’s Wood Handbook explains why soil contact matters: “Only when previously dried wood is contacted by water in the form of rain or condensation or is in contact with wet ground will the fiber saturation point be reached.” That’s the condition decay fungi need. The handbook calls for “using pressure-treated wood for the high hazard situation associated with ground contact,” and notes “Decay resistance may be less for members placed in contact with the ground and/or used in warm, humid climates.”
Signs of trouble in timber. The Wisconsin manual describes decay signs: “Signs include discolored wood with a soft rotted texture. Look also for fruiting bodies and depressed areas of the wood surface.” It adds that insects “can be detrimental to the integrity of a timber wall and can cause significant internal damage.”
The weak spot is hidden. NYSDOT’s manual says some wall types, “timber pile and lagging, for example,” may need an inspector “to randomly select a few posts where an excavation below groundline can be progressed to determine if piling is deteriorated at the soil level.” A post can look fine above grade and be soft just below it. For the boards, the same manual says “Lagging or cribbing should be checked for excessive deflections. Excessive deflections may allow the soil behind to spill or wash out, causing settlement in the retained material above.” A sagging board, or soil leaking through gaps, is worth a closer look.
Movement that shows up in every type
Some movement patterns cross every category. The Wisconsin manual says differential settlement may cause the “opening of joints or cause wall cracking or transverse tipping.” Lateral movement “may occur when the lateral soil pressures exceed the resisting soil frictional and shearing forces.” And “Rotational movement, or overturning, is generally the result of asymmetrical settlements or lateral movements; however, it may result from increased soil pressure behind the wall.”
Water is the shared suspect. Saturated backfill raises the pressure on any wall type, which is why drainage matters regardless of material. Our drainage behind retaining walls post covers weep holes and backdrains, and our foundation drainage page covers the house side.
How to tell what you have
Before calling anyone, it helps to describe the wall accurately. A few observations help:
- What is the face made of? Loose stone, mortared stone, poured concrete, concrete block, wire baskets, or wood.
- Does it lean back into the slope? A deliberate backward lean (batter) points toward a gravity or block wall.
- Are the blocks small and dry-stacked with a lip? That’s a segmental wall. Whether it’s reinforced isn’t visible from the front.
- Is it thin for its height? A thin concrete or block wall points toward a cantilever design with a footing you can’t see.
- What’s above it? A driveway, patio, fence, pool or part of the house near the top edge adds load.
- Is there paperwork? Permit records, plans or a soils report tell you more than the face can.
Photos of the wall face, both ends, the top edge and the ground at the toe are useful to whoever looks at it next. Our when walls need structural evaluation post covers when it’s time to bring in a professional.
Questions worth asking
- What type of wall is this, and how was it designed to resist the soil?
- Is the movement I’m seeing in the wall itself, in the soil under it, or in the slope as a whole?
- If it’s a block wall, is it reinforced, and is there any record of the grid?
- If it’s timber, has anyone checked the posts below grade?
- Has anything been added above the wall since it was built?
- Does an engineer need to look at this, and does the site need geotechnical input?
Cost factors, not prices
We don’t publish prices. What shapes the cost of evaluating a retaining wall includes the wall type and length, access to both faces and the top, whether excavation is needed to see a footing or posts below grade, whether plans or permit records exist, and whether the slope itself needs engineering or geotechnical review.
Where we fit
We’re a foundation and structural repair company serving La Jolla citywide, and retaining walls come up in hillside work. We can look at a wall, document what it’s doing, and help you understand what kind of wall it is before decisions get made. We aren’t engineers. We work with verified contractors and coordinate with licensed engineers as needed. This post is general education, not an assessment of any property. See our retaining wall repair, hillside foundation repair and structural engineer pages for how that coordination works.
Next step
If a wall on your lot is cracking, bulging or leaning, request a Free Quote for an on-site visit anywhere in La Jolla, or call (650) 739-6537.
Sources (accessed October 30, 2026)
- San Diego Building Code (2025 California Building Code as amended), Chapter 18, §1807.2, via UpCodes
- San Diego Residential Code (2025 California Residential Code as amended), Chapter 4, §R404.4, via UpCodes
- City of San Diego Development Services, Information Bulletin 221, Standard Cantilevered Retaining Walls
- Wisconsin Department of Transportation, Structure Inspection Manual, Part 4, Chapter 4: Retaining Walls
- New York State Department of Transportation, Retaining Wall Inventory and Inspection Program (December 2025)
- National Concrete Masonry Association, TEK 18-11, Inspection Guide for Segmental Retaining Walls
- National Concrete Masonry Association, Design Manual for Segmental Retaining Walls, 3rd edition
- USDA Forest Products Laboratory, Wood Handbook (2021), Chapter 14: Biodeterioration of Wood