Retaining Wall Drainage in Florida: Why Walls Fail

Retaining wall drainage decides whether a wall stands. A wall holds back soil, and water behind it adds pressure. Most retaining walls fail because of the build-up of pressure behind the wall. Redi-Rock, a block maker, says "it is best practice to include a drain within every wall." If you want a wall built, see our retaining wall and hardscape construction service.

Signs your retaining wall is failing

Look at the wall after a hard rain, not on a dry day. Western Interlock, a block manufacturer, says to look for "cracks in blocks, separation between courses, water staining on the wall face, or saturated soil behind the wall." It says to measure lean with a four-foot level held against the wall face, and that "Progressive lean indicates active failure requiring immediate attention."

The same Western Interlock article ties lean and outside loads to causes. The table below follows those lines.

Why drainage matters in a Florida downpour

Southern Loss, an association that trains property adjusters, traces wall failure to groundwater or soil-moisture accumulation and adds: "The hydrostatic pressure on the wall increases as the depth of the groundwater increases." Redi-Rock, a large block maker, says a drain gives water a direct outlet, because otherwise "trapped water can become impounded behind blocks, causing high pressure." It also says wall blocks, once placed, "are not naturally watertight."

Brevard County gets that water in bulk. The National Weather Service office in Melbourne found that May through October brings about 68 percent of the year's rain at Melbourne, with the wet season usually starting in the latter part of May. The ground does not help much. UF/IFAS Extension reports that Spodosols are the most extensively occurring soils in Florida, and that the majority of them are poorly or very poorly drained. That is a statewide soil survey, not a Brevard map.

Runoff from the roof matters too. Western Interlock says ensuring that downspouts and runoff divert away from the wall improves surface drainage, but that adding a proper drain pipe at the base of a finished wall "requires excavation that effectively means rebuilding the wall." Ask for the drain in the original build.

The retaining wall drainage detail: stone, pipe and outlet

The details below come from the Redi-Rock Installation Guide (V25), written for Redi-Rock's large blocks. Other block systems differ in the numbers, so the guide for your block governs.

• Drainage stone. The guide says to "Place drainage aggregate between and behind the blocks." For the leveling pad it prefers crushed stone "with no material passing the No. 200 (74 μm) sieve," which means no fines. Redi-Rock's engineering FAQ recommends "a clean crushed, free-draining rock similar to ASTM 57 or 67 crushed stone" to limit hydrostatic pressure.
• A drain pipe with outlets. "A drain is placed behind the Redi-Rock wall blocks at the lowest elevation where the pipe can safely outlet to daylight." A 4-inch perforated drain is commonly used, and the drain "needs to have proper outlets on the ends and at regularly spaced points along the wall." Solid pipe should be used for weep hole outlets through the face or under the wall.
• Fabric, where the plans call for it. The guide says to place non-woven geotextile fabric between the drainstone and the remaining backfill "if specified," and says the drain is often encapsulated in aggregate wrapped in the same fabric. It also calls for fabric in the vertical joints between blocks "if specified in the retaining wall plans." So fabric is a design decision, not an automatic part.

Redi-Rock also does not recommend waterproofing in a typical retaining wall. The goal is to let water out, not to seal it in.

The base and leveling pad under the wall

A wall can lean from the bottom even when its drainage is fine. Western Interlock says a lean at the bottom indicates base failure or settlement, and that a base that is too shallow, poorly compacted or built on unstable soil will settle and shift. It adds that this is particularly common when the first course is not buried below grade.

Redi-Rock's guide gives the numbers for its large blocks: the leveling pad thickness "shall be as designed by the wall design engineer," a minimum of 6 inches or 12 inches is common, and the stone goes in loose lifts a maximum of 6 inches thick, each one compacted. For a smaller wall, use that block maker's guide.

Geogrid, height, and asking your building department

Western Interlock describes geogrid as reinforcement "extending back into the soil mass," and says walls approaching or exceeding 36 inches need it, because without it "taller walls rely entirely on their own mass to resist soil pressure." The same article says walls exceeding 36 inches require engineering, and that walls near structures, utilities or property lines call for a professional. That is one manufacturer's rule of thumb, not a Brevard code. Redi-Rock's FAQ adds that expansive soils may need subgrade improvements and to "consult a local geotechnical engineer."

Ask your city or county building department whether your wall needs a permit or drawings, before anyone digs. Ask three things: the height at which a permit starts, whether anything sits above the wall, such as a driveway, a pool or a structure, and whether drawings are required.

What to ask each bid to list

Drain stone, pipe, outlets, a compacted base and any geogrid are buried in the finished wall, so a bid has to name them. Western Interlock's rebuild list is a good checklist: a 4-inch perforated drain pipe at the base, pipe sloped to outlet at daylight, filter fabric against the excavation wall, and at least 12 inches of three-quarter-inch-minus gravel backfill, "never native soil against the wall."

Ask each contractor three questions. Where does the drain pipe outlet? What stone goes behind the block? What is the base made of, and how deep is it? Compare bids line by line, and ask for the answers in writing.

Can a leaning retaining wall be repaired, or does it need rebuilding?

It depends on how far it has moved and why. Western Interlock gives this guide for a measured lean:

• Under 1/4 inch and not progressing: may be a cosmetic concern.
• 1/4 to 1/2 inch: "Drainage improvements may stabilize the situation if caught early and if drainage is the primary issue."
• More than 1/2 inch: "Reconstruction is almost always the only reliable solution."

Two warnings from the same article apply. "Temporary bracing doesn't address root causes." And attempting to force a leaning wall back "risks cracking blocks, breaking the base, and creating internal stress that accelerates failure." For a wall near a house, a pool or a property line, or one that is moving fast, have a Florida-licensed professional engineer assess it before you decide.

Where we fit

We build retaining walls, seat walls, planters and garden borders as part of our hardscape construction service in Brevard County. When a wall needs an engineer's design, that design comes from a Florida-licensed professional engineer, not from us. If your wall is leaning, bulging or staining, send us photos and the address. A site visit and a written estimate come before any work, and the estimate sets the scope, not this article.

See what our Patios, Retaining Walls & Hardscape service includes in Brevard County.