What are Underdrains? And Why You Should Care

by Marlea Tichy

These unsung heroes save our homes and property from water damage by giving storm water somewhere to go other than your basement, under your foundation, or into your yard causing damage or even mudslides. Colorado is known for soil erosion, and it only gets worse when saturated, sometimes doing serious damage to building foundations. Underdrains often run below sewer lines and tie into storm drain systems, so imagine what happens when they quit doing what they were designed to do. While much has been required over recent years to deal with problems associated with underdrains, little has been written about the best ways to avoid the problems. Increasing failures and catastrophic events are forcing special district managers and their engineers to begin to proactively address the problem. Maybe this is a good time to start thinking about pre-funding future fixes before your constituents realize the damage.

Background

Perimeter drains have been employed for decades, but their use accelerated when Veteran Administration and Federal Housing Authority loan programs began requiring them in the late 1970s. As a result, developers began installing subdrains or underdrains on both private property and public rights of way to transport ground water from building perimeter drains to an outlet point. While special districts have had responsibility for drainage and for wastewater collection and treatment, ultimate responsibility for the maintenance and operation of underdrain systems in the public right-of-way still varies from district to district.

A number of efforts have been initiated over the past several decades to resolve the issue of underdrain installation and operation with limited success. Additionally, the exact extent of underdrain systems in public right of ways is unknown. While most installations have been included on wastewater plan submittals since the early-1980s, no mapping system or comprehensive records had been maintained prior.

Beneficiaries of the Use of Underdrains

Developers and special districts use underdrain systems that drain utility trenches to reduce the disturbance of the pipe bedding around wastewater collection lines. This reduces the potential inflow & infiltration of groundwater into the wastewater collection systems which requires treatment, thus also reducing the potential for illegal discharge connections from sump pumps to the wastewater system. With modern pipeline construction technology, there is diminished benefit to special districts from an inflow & infiltration perspective since new collection systems are sealed and air tested, resulting in a very tighter systems.

Developers benefit from dewatering around building foundations and subsurface infrastructure, allowing construction to proceed and owner financing to be obtained. An underdrain system may also be installed as insurance against future groundwater problems in a new subdivision if groundwater tables change due to drought or other developments.

Homeowners and businesses benefit from the reduction of potentially damaging water infiltration into their foundations and basements.

Despite almost forty years of discussion, these core issues of underdrains remain unresolved:

  • Who is responsible for maintaining and repairing underdrains in the public right of way?
  • How will maintenance and repairs be funded?
  • Who will maintain records on underdrains?

The remainder of this article will provide information on how some special districts have begun to address underdrains and are taking the lead in developing corrective actions in maintenance and repair programs.

Maintenance

Like all pipe infrastructure systems, underdrains need to be maintained in order to remain effective. Over time, underdrains become clogged or completely blocked by debris, root infiltration and in several areas in Colorado, severe calcium carbonate deposits. Additionally, , underdrains can be compromised by severe cracking or collapse.  This could lead to basements flooding with groundwater. There have been many lawsuits recorded to assign liability in these cases. Many between property owners and developers, and many others between owners and special districts.   Since most homeowner insurance policies exclude coverage for groundwater events, special districts eventually end up arguing in court over the extent of liability.

While relatively infrequent, these failures can be expensive to repair due to the depth of the installations and the location of other utilities like electricity, gas, water, wastewater, storm sewer, and telecom. A single spot repair excavation to the underdrain level often requires trench wall shoring and shoring or securing the other utilities encountered in the trench. If rodding or cleaning doesn’t resolve the problem, point repairs could easily exceed $10,000 and the replacement of sections of pipe could be much higher.

Success Story 1:

Problem: Certain Residents of Special District A experienced back up and flooding events on numerous occasions. The District eventually traced each occurrence to a failure in the underdrain system. From this investigation, the culprit was revealed to be blocked/collapsed lines and insufficient tie-ins to the underdrain mains. The District didn’t see the looming problems because clean-outs and access points were not required when the development was originally built. While current plumbing code requires clean-outs or access every 100linear feet for pipe 4” and smaller, and every 300LF for pipe greater than 4”, most older underdrain systems were installed without cleanouts. This lack of access led the District to inadvertently omit regular maintenance on the system.

Solution:

The District developed a corrective action program to address the blocked and collapsed lines. In many cases, this required line excavation and replacement, although in some instances, “pipebursting” was used to replace the original line without the need for a trench. Additional line was also installed to ensure that every line actually tied into the system. Finally, a prioritized program was developed to install clean-outs every 300 linear feet in the underdrain system over a period of years funded by an annual budget allowance. The District now maintains their underdrain system proactively. They’ve added cleaning (jetting) and inspection (video capture) to their annual Operations and Maintenance (O&M) budget, developed maps of their system, and no longer experience failures and property loss associated with any facet of their infrastructure.

Success Story 2:

Problem: Residents of Special District B also experienced back up and flooding events caused by calcium carbonate build up in the underdrain system. The District experimented with every conceivable method to clear the lines including dissolving chemical, mechanical rodding and high velocity chain spinning (chain knocker). Several techniques were eventually employed based on the severity of the build-up including pipebursting, auger and directional drilling.   All three technologies were trenchless, requiring only an access and exit pit on either end of each run. In some areas the calcium was drilled out and the original pipe was saved, while other areas required replacement. High Density Poly Ethylene (HDPE) pipe was used for replacement.

Funding Mechanisms:

Again, after almost 40 years of discussion the core issues of underdrains maintenance in the public right of way, funding and recordkeeping remain unresolved.   Some Municipalities and Special Districts, including Grant Ranch Water and Sanitation District and the City of Colorado Springs have established rate structures to fund an annual O&M budget specifically for underdrains or “groundwater systems” and have taken a proactive approach to solving the problems inherent in their use. What is certain, however, is that underdrains need to be maintained in order to remain effective.

Marlea Tichy, our guest author, is the CEO of Pipeline Industries, Inc. and has over 25 years of experience in the water, sewer and infrastructure industry. Pipeline Industries has helped many municipalities and special districts throughout Colorado solve difficult water, sewer, storm and gas line problems since 2000. The company specializes in emergency response and is available 24/7, 365 days per year.

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