Types of Retaining Walls
Retaining walls are structures designed to hold back soil and resist lateral earth pressure. , and proper drainage systems for retaining walls should be verified during constructionGravity retaining walls rely on their own weight to resist overturning and sliding. These walls are typically constructed from mass concrete, stone, or concrete masonry units and are most economical for heights up to 10 feet. The wall cross section tapers from a wide base to a narrower top, with the base width typically 50 to 70 percent of the wall height. Gravity walls work well when adequate space is available for the wide base and when foundation soils have sufficient bearing capacity to support the heavy structure.
Cantilever retaining walls use a reinforced concrete stem and base slab to resist lateral forces. The base consists of a heel under the backfill and a toe extending forward from the stem. The weight of the soil on the heel adds to the wall’s stability. Cantilever walls are more economical than gravity walls for heights above 10 feet because they use less material. The reinforcement design must account for bending moments in the stem and base, with tension steel placed on the tension face of each component. ACI 318 provides detailed requirements for reinforcement detailing and development lengths in cantilever retaining wall design.
Counterfort retaining walls are used for heights exceeding 20 feet where cantilever walls become uneconomical. Counterforts are thin concrete walls spaced at intervals along the back of the wall, connecting the stem to the base slab. These counterforts act as vertical beams that reduce bending moments in the stem. The counterfort spacing typically ranges from 8 to 15 feet depending on the wall height and loading conditions. The structural analysis of counterfort walls is more complex than cantilever walls and requires consideration of two-way bending in the stem panels between counterforts.
Earth Pressure Theory
Understanding earth pressure is fundamental to retaining wall design. At-rest earth pressure exists when the wall does not move relative to the soil. Active earth pressure develops when the wall moves away from the soil, reducing lateral pressure. Understanding lateral earth pressure is essential for achieving quality results in this aspect of construction. Understanding geotextile drainage fabric is essential for achieving quality results in this aspect of construction.Understanding cantilever wall design is essential for achieving quality results in this aspect of construction.Passive earth pressure develops when the wall moves into the soil, increasing resistance. The Rankine and Coulomb theories provide methods for calculating these pressures based on soil properties and wall geometry. Most retaining walls are designed for active pressure on the back side and passive pressure on the front side below grade.
The magnitude of lateral earth pressure depends on soil unit weight, internal friction angle, and cohesion. Surcharge loads from adjacent structures, traffic, or stored materials increase lateral pressure on the wall. The equivalent fluid pressure method simplifies pressure calculation by treating the soil as a fluid with an equivalent density. Typical equivalent fluid densities range from 30 to 60 pounds per cubic foot depending on soil type and drainage conditions. Water pressure behind the wall can exceed earth pressure and must be relieved through proper drainage systems.
Drainage Systems
Proper drainage is critical for retaining wall performance. Hydrostatic pressure from water accumulation behind the wall can double the lateral force and lead to wall failure. Weep holes through the wall face at 4 to 6 foot spacing allow water to drain from the backfill. Perforated drain pipes at the wall base collect and convey water to suitable outlets. The drain pipe should be wrapped in geotextile fabric to prevent clogging by soil particles.
Granular backfill material with high permeability is placed directly behind the wall to promote drainage. The drainage zone typically extends 12 to 18 inches from the wall face and uses clean sand or gravel with less than 5 percent fines. Geotextile fabric separates the drainage zone from the general backfill to prevent migration of fine particles into the drainage material. The importance of drainage in retaining wall performance cannot be overstated, as inadequate drainage is the most common cause of retaining wall failures.
Construction Sequence
Retaining wall construction must follow a specific sequence to ensure stability at all stages. The foundation excavation must extend below the frost line to prevent frost heave damage. The base slab is placed on a prepared subgrade with proper compaction and reinforcement. For cast-in-place walls, the stem forms are erected and concrete is placed in lifts to control form pressure. Backfill is placed in layers and compacted to specified density, with drainage materials installed as backfill progresses. Construction joints between concrete placements must be detailed to prevent water leakage and provide shear transfer.
The backfill operation must proceed carefully to avoid damaging the wall before it reaches full strength. Heavy compaction equipment should not operate within 3 feet of the wall face to prevent excessive lateral pressure. Light hand-operated compactors are used in the zone adjacent to the wall. The geotechnical engineer should inspect the backfill operations to verify compliance with specifications regarding material quality, lift thickness, and compaction density.