What is a retaining wall?

What this means for your home project

1. Ask the contractor to explain how the foundation will support the wall before you choose its visible facing. Source: USDA Forest Service, read 2026-10-29.
2. Ask how the soil behind the wall will be compacted; include that work in the written scope. Source: USDA Forest Service, read 2026-10-29.
3. Ask how water will drain behind the wall. Wisconsin DOT research identifies drainage as essential to preventing water pressure buildup and corrosion. Source: Wisconsin Department of Transportation, read 2026-10-29.

What is the purpose of a retaining wall?

Its purpose is to make usable flat space on a slope, to hold up a road or path cut into steep ground, and to stabilize a slope that would otherwise slump. USDA Forest Service guides give these uses, and say where a wall stops being optional: “It is commonly used to support a roadway or gain road width on steep terrain.” Source: USDA Forest Service, read 2026-10-29. “They can also be used for slope stabilization.” Source: USDA Forest Service, read 2026-10-29. “A crib wall is absolutely necessary where the terrain is steeper than the angle of repose for the fill material.” Source: USDA Forest Service, Missoula Technology and Development Center, read 2026-10-29.

How do EPA and the Forest Service define a retaining wall?

A retaining wall is a structure that holds a mass of soil at a face much steeper than the ground would keep on its own, often where a yard, drive or road changes level. The EPA and the USDA Forest Service define it this way: “A structure that supports earth masses having a vertical or near-vertical slope (such as 70 degrees).” Source: U.S. Environmental Protection Agency, read 2026-10-29. “A structure designed to resist the lateral displacement of soil, water, or any other type of material.” Source: USDA Forest Service, read 2026-10-29. “Retaining structures are designed to keep dirt and rock in place.” Source: USDA Forest Service, Missoula Technology and Development Center, read 2026-10-29.

What is a concrete block retaining wall?

A concrete block retaining wall is one facing among several kinds: federal guides list reinforced-soil walls faced with concrete blocks next to loose rock, reinforced concrete, timber cribs and gabions, which are wire baskets of 10 to 20 cm rock. The USDA Forest Service and the EPA name the kinds like this: “Retaining structures may be loose rock, gabions, reinforced concrete, piles, crib walls, soil nails, or mechanically stabilized soil walls with a variety of facings such as geotextile, welded wire, timber, concrete blocks, or tires (Photo 11.11).” Source: USDA Forest Service, read 2026-10-29. “The structure may consist of material such as concrete, gabions, steel sheet piles or timber (and may include a reinforcement element such as geosynthetic material) forming a gravity wall, cantilevered wall, anchored wall or mechanically stabilized wall.” Source: U.S. Environmental Protection Agency, read 2026-10-29. “Gabions - Baskets (usually made of wire) filled with rocks (or broken pieces of concrete) about 10-20 cm in size, used for building erosion control structures, weirs, bank protection, or retaining structures.” Source: USDA Forest Service, read 2026-10-29. “Gabion structures are very commonly used for walls up to 6 meters high, particularly because they use locally available rock and are labor intensive (Photo 11.9).” Source: USDA Forest Service, read 2026-10-29.

What is a crib retaining wall?

A crib retaining wall is built from stacked timber cribs that hold fill in place so it cannot follow gravity down a steep slope, and the USDA Forest Service trail notebook calls it the most common retaining structure. Its notes on crib walls say: “The most common retaining structure is the crib wall.” Source: USDA Forest Service, Missoula Technology and Development Center, read 2026-10-29. “Well-built crib is the most stable kind of uncemented retaining structure (except perhaps wire gabion).” Source: USDA Forest Service, Missoula Technology and Development Center, read 2026-10-29. “The crib wall keeps fill from following the call of gravity and taking the tread with it.” Source: USDA Forest Service, Missoula Technology and Development Center, read 2026-10-29. “They are often constructed of gabions, reinforced concrete, timber cribs, or mechanically stabilized earth.” Source: USDA Forest Service, read 2026-10-29.

What is an MSE retaining wall?

MSE stands for mechanically stabilized earth: layers of geotextile or welded wire are laid in the backfill 15 to 45 cm apart, so the reinforced soil itself becomes part of the wall behind its facing. A Wisconsin DOT report and the USDA Forest Service describe it: In Wisconsin, “MSE walls are composite earth retention systems that stabilize soil masses using tensile reinforcements embedded in engineered backfill.” Source: Wisconsin Department of Transportation, read 2026-10-29. “They are commonly constructed using layers of geotextile or welded wire placed in lifts 15 to 45 cm apart in the soil, thus adding tensile reinforcement to the soil (see Figure 6.3e).” Source: USDA Forest Service, read 2026-10-29. “Welded wire MSE walls are also commonly used.” Source: USDA Forest Service, read 2026-10-29.

Why does a retaining wall need drainage?

Water that builds up in the soil behind a wall raises the pressure on it and corrodes metal reinforcement, so drainage holes, weep holes and the drainage system behind them are working parts of the wall. A January 2025 Wisconsin DOT research report on highway walls puts it plainly: In Wisconsin, “Proper drainage is essential to prevent water pressure buildup and corrosion.” Source: Wisconsin Department of Transportation, read 2026-10-29. In Wisconsin, “For example, clogged drainage holes may increase hydrostatic pressure, but if the root cause is an inadequate drainage system, clearing the weep holes only provides temporary relief.” Source: Wisconsin Department of Transportation, read 2026-10-29.

What goes behind a retaining wall?

Behind the wall sits backfill: soil or granular fill placed in thin layers and compacted, and the type of soil matters because clay pushes harder than sand and gravel. USDA Forest Service road guidance and Wisconsin DOT practice give the numbers: “Compaction is important behind any retaining structure or fill.” Source: USDA Forest Service, read 2026-10-29. “Wall backfill is typically compacted to 95% of the AASHTO T-99 maximum density.” Source: USDA Forest Service, read 2026-10-29. “For finer or clay rich soils, earth pressure on the wall will increase and the wall base width (B) will have to increase for each height.” Source: USDA Forest Service, read 2026-10-29. In Wisconsin, “Lighter equipment and thinner lifts are required near the wall face.” Source: Wisconsin Department of Transportation, read 2026-10-29.

Why do retaining walls fail?

Most retaining walls that fail do so at the bottom, because the ground under them gives way, and Wisconsin DOT also links failures of highway walls to drainage problems, corroded reinforcement and poor building or upkeep. The USDA Forest Service and a Wisconsin DOT study report: “Most wall failures occur due to foundation failure.” Source: USDA Forest Service, read 2026-10-29. “All retaining structures, either mechanically stabilized earth (MSE) walls or gravity walls, require a good foundation.” Source: USDA Forest Service, read 2026-10-29. In Wisconsin, “These failures are often linked to drainage deficiencies, reinforcement corrosion, construction quality issues, and inadequate inspection or maintenance.” Source: Wisconsin Department of Transportation, read 2026-10-29.

How long does a retaining wall last?

Engineered highway walls are designed for 75 to 100 years, but drainage problems and corroding reinforcement have ended some in as little as 30 years; these figures are for state highway walls, not a promise for a yard wall. Wisconsin DOT research on reinforced-soil walls reports: In Wisconsin, “MSE walls are typically designed for a service life of 75 or 100 years.” Source: Wisconsin Department of Transportation, read 2026-10-29. In Wisconsin, “Despite a 75 or 100-year design service life, premature deterioration has been observed nationally and within Wisconsin, with some failures, defined as a total loss of service, occurring in as little as 30 years.” Source: Wisconsin Department of Transportation, read 2026-10-29. In Wisconsin, “This design life is primarily governed by the durability of the reinforcement elements, especially metallic components subject to corrosion.” Source: Wisconsin Department of Transportation, read 2026-10-29.

What materials are used for a wood retaining wall?

Wood in a retaining wall sits in ground contact, and a federal test shows why the treatment matters: untreated posts failed in under 3 years while treated wood lasted decades; old creosote railroad ties are a common reused material. A USDA Forest Products Laboratory test in southern Mississippi and the EPA report: “In contrast, the untreated posts failed in less than 3 years.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “As a whole, the post and lumber specimens indicate an expected durability of over 50 years for creosote-treated wood and over 60 years for wood treated with pentachlorophenol, copper naphthenate, ACZA, or CCA.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “In relating these findings to treated commodities, it should be noted that these test specimens have not been subjected to the same mechanical loads or wear and tear associated with in-service structures.” Source: USDA Forest Service, Forest Products Laboratory, read 2026-10-29. “For example, creosote-treated railroad ties are sometimes recycled as landscaping timbers.” Source: U.S. Environmental Protection Agency, read 2026-10-29. “Reuse of creosote-treated wood is not subject to regulation by EPA under pesticide laws.” Source: U.S. Environmental Protection Agency, read 2026-10-29.

Sources

• Remedy Resilience: Resilience Measures (engineered structures table) — U.S. Environmental Protection Agency; read 2026-10-29.
• Low-Volume Roads Engineering Best Management Practices Field Guide (copy hosted by University of California ANR) — USDA Forest Service; read 2026-10-29.
• Trail Construction and Maintenance Notebook: Crib Walls and Other Retaining Structures — USDA Forest Service, Missoula Technology and Development Center; read 2026-10-29.
• Long-Term Durability of Pressure-Treated Wood in a Severe Test Site — USDA Forest Service, Forest Products Laboratory; published 2013; read 2026-10-29.
• Creosote — U.S. Environmental Protection Agency; read 2026-10-29.
• State of Practice for Specifying and Repairing Mechanically Stabilized Earth Walls (WHRP 0092-24-06) — Wisconsin Department of Transportation; published 2025-01; read 2026-10-29.
• Low-Volume Roads Engineering Best Management Practices Field Guide: glossary and Chapter 11 (second capture; copy hosted by University of California ANR) — USDA Forest Service; read 2026-10-29.