Retaining Walls Do One Job: Hold Back Earth Laterally
A retaining wall’s structural purpose is straightforward - resist the lateral pressure of soil on one side while maintaining a grade difference on the other. But the engineering behind it is less intuitive than vertical load-carrying columns and beams. Lateral earth pressure acts on the wall continuously, increases with depth, and depends on soil type and moisture content. For the ARE PPD, you need to understand the basic types of retaining walls, how each resists the earth pressure, and the critical role of drainage in wall longevity and stability.
Lateral Earth Pressure Basics
Soil exerts pressure on a retaining wall because soil particles have weight and friction between them. The two primary states are active pressure (soil pushing against the wall as the wall deflects slightly outward) and passive pressure (the soil in front of the wall being compressed, which resists the wall’s movement). Water pressure behind a wall is additive and significantly increases total lateral pressure - this is why drainage is not optional in retaining wall design.
Types of Retaining Walls
Gravity walls resist lateral earth pressure through sheer mass. They are made of concrete, stone, or masonry and rely on their weight to resist overturning and sliding. They are economical for low wall heights (under 3 to 4 feet) and require no reinforcement.
Cantilever walls are the most common type for walls in the 4-to-20-foot range. They consist of a vertical stem and a horizontal base slab (footing). The base slab extends behind the wall face (the heel) and in front of it (the toe). The weight of soil on the heel provides a resisting moment against overturning. Cantilever walls require reinforcement to resist bending in the stem.
Counterfort walls are used for taller walls where the cantilever stem would become excessively thick. Triangular cross-wall elements (counterforts) connect the stem to the heel at regular intervals, converting the stem into a horizontally spanning plate rather than a cantilever. This reduces stem thickness and is more economical at heights above 20 feet.
Sheet pile walls use interlocking vertical piles driven into the ground. They are used in temporary excavation support, waterfront applications, and tight-site situations where a gravity or cantilever wall is not feasible.
Drainage Requirements
Water buildup behind a retaining wall dramatically increases total lateral pressure. Hydrostatic pressure can exceed earth pressure and cause wall failure. Drainage is always required behind retaining walls and typically includes: a granular drainage layer (gravel or crushed stone) directly behind the wall face; a perforated pipe at the base of the drainage layer to collect and direct water away; and weep holes through the wall face at regular intervals to allow excess water to escape. Geotextile fabric often wraps the drainage layer to prevent migration of fine soil particles that would clog the drainage system over time.
Key ARE PPD Exam Points
- Active earth pressure increases with depth and is the primary lateral load on a retaining wall.
- Water pressure behind a wall is additive to earth pressure - drainage is a structural requirement, not optional.
- Gravity walls resist through mass; cantilever walls resist through the lever arm of the base slab and stem reinforcement.
- Counterforts reduce the required stem thickness for walls taller than 20 feet by adding horizontal spans.
- Drainage layer + perforated pipe + weep holes is the standard drainage assembly behind a concrete retaining wall.
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