Advanced Contouring University
Lesson: How Does Retaining Wall Drainage Work
Drainage is the system within or behind a retaining wall that helps manage water flow and prevent pressure buildup. It typically includes gravel backfill, perforated pipes, and weep holes. Drainage is vital because water is one of the most destructive forces acting against retaining walls.
Drainage works by allowing water to escape from behind the wall, relieving hydrostatic pressure. Without adequate drainage, water can saturate the soil, increasing its weight and pushing forcefully against the wall. Over time, this pressure can cause bulging, cracking, or even complete failure of the structure.
One common drainage method is to install a perforated pipe behind the wall at the base. This pipe collects and redirects water away from the structure. Builders also use coarse gravel to fill the area behind the wall to promote water movement and reduce saturation.
Drainage is important not only for wall integrity but also for preventing water-related damage to surrounding property. Poor drainage can lead to erosion, muddy landscapes, and flooding in nearby areas. Ensuring good drainage protects both the wall and the environment around it.
In summary, drainage is an essential part of retaining wall construction. It ensures that water doesn’t become trapped behind the wall, which could lead to structural and environmental problems. A properly designed drainage system is a safeguard against many of the common causes of retaining wall failure.
Key Terms
Hydrostatic Pressure
Hydrostatic pressure is the force exerted by a fluid at rest due to the weight of the fluid above it, increasing with depth. In the context of retaining walls, this pressure builds up behind the wall as groundwater accumulates, exerting a lateral force that pushes against the structure. If not properly managed through drainage or design, hydrostatic pressure can cause the wall to crack, bow, or even fail entirely. To prevent damage, retaining walls must incorporate effective drainage systems such as weep holes, gravel backfill, or perforated pipes to relieve the pressure and maintain structural stability.
Perforated Pipe
A perforated pipe is a type of drainage pipe that features small holes or slots along its length, designed to collect and redirect water from surrounding soil. Typically used in applications like foundation drainage, retaining walls, and French drains, the pipe allows groundwater to enter through the perforations and flow through the pipe to a designated discharge area. By channeling excess water away from structures, perforated pipes help reduce hydrostatic pressure, prevent soil erosion, and maintain dry, stable conditions around foundations and landscaping features.
Weep Hole
A weep hole is a small opening intentionally placed at the base of a retaining wall to allow water to escape from the soil behind the wall. As water accumulates due to rain or groundwater, it can build up hydrostatic pressure that pushes against the wall, potentially causing structural damage. Weep holes provide an outlet for this trapped water, relieving pressure and preventing water-related issues such as wall cracking, bulging, or failure. Typically spaced at regular intervals, weep holes are an essential component of an effective drainage system in retaining wall construction.
