Clog Away: Usage of Small-Slot Drainage Pipes

Looking at an industry-accepted practice, a Freese and Nichols study examined the use of small slotted drainage pipes in dams.

The Association of State Dam Safety Officials recently featured the findings of Marc Miller, PE, left, and Grady Hillhouse, PE, right, in the Journal of Dam Safety.

The Issue

Internal drains are commonly used to relieve pore pressure and control seepage in dams. These drain systems often include slotted or perforated pipes, and the pipe size openings depend on the type of filter medium used. When pipe openings become clogged, drain efficiency is reduced and can lead to pressure increases that introduce performance uncertainties and a reduction in dam safety.

Since smaller openings seem more prone to clogging, the dam industry has trended away from the use of small slotted pipes, particularly when sand is used as the filter medium. Although this concept is widely accepted, published case studies indicate that these findings are generally anecdotal in nature.

The Findings

The study suggested that small slotted pipes are prone to clogging from both adverse construction factors and seepage flows that carry soil particles. Given the improvement that two-stage filter systems are known to provide, the study suggested that their use is still preferred over single-stage sand filters with narrow slot pipes for primary or critical seepage collection systems.