How do you account for water pressure and drainage in retaining wall design and construction?

1 Water pressure and retaining walls

Water pressure is the force exerted by water on a surface. In retaining wall design, water pressure is usually measured as lateral earth pressure, which is the pressure applied by the soil and water behind the wall. Lateral earth pressure can be calculated using different theories, such as Rankine, Coulomb, or Terzaghi. The main factors that influence lateral earth pressure are the soil properties, the wall inclination, the backfill slope, and the water table. Water pressure can cause the wall to deform, crack, or fail if it is not properly resisted by the wall strength and stability.

2 Drainage and retaining walls

Drainage is essential for reducing the water pressure and preventing the buildup of hydrostatic pressure, which is the pressure caused by the weight of water. Drainage can also improve the soil strength and prevent erosion, seepage, and frost heave. It can be achieved by using different methods, such as perforated pipes, gravel layers, geotextiles, or weep holes. The choice of drainage method depends on the site conditions, the wall type, and the design objectives.

3 Retaining wall types and water pressure and drainage considerations

There are many types of retaining walls, such as gravity walls, cantilever walls, sheet pile walls, anchored walls, and reinforced soil walls. Each type of wall has its own advantages and disadvantages, and requires different design and construction techniques to account for water pressure and drainage. For example, gravity walls rely on their own weight to resist lateral earth pressure, and need adequate drainage to prevent overturning or sliding. Cantilever walls have a stem and a base that act as a lever to balance the lateral earth pressure, and need reinforcement and drainage to prevent bending or cracking. Sheet pile walls are thin walls driven into the ground to retain soft soils or water, and need anchors or bracing to resist lateral earth pressure and drainage to prevent corrosion or buckling.

Anchored walls are walls that are supported by cables or rods attached to anchors in the soil or rock behind the wall, and need proper design and installation of the anchors and drainage to prevent failure or loss of tension. Reinforced soil walls are built with layers of soil and reinforcement materials, such as geogrids or metal strips, and need adequate compaction and drainage to prevent settlement or bulging.

4 Retaining wall design and construction steps

To account for water pressure and drainage in retaining wall design and construction, you need to follow some basic steps. First, you need to conduct a site investigation and a soil analysis to determine the soil properties, the water table, and the groundwater flow. Then select the type of wall that suits your project requirements, such as the height, the load, the aesthetics, and the budget.

You'll need to calculate the lateral earth pressure using the appropriate theory and the design parameters, such as the soil friction angle, the cohesion, and the unit weight. Then design the wall dimensions, the reinforcement, the drainage system, and the safety factors, according to the relevant codes and standards. Finally, construct the wall following the design specifications, the quality control measures, and the environmental protection practices.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?