Surface Drainage Is More Than Just Stormwater Management

By Andrew J. Garner

With all of the regulations that govern stormwater management on construction sites, it’s easy to get caught up in maintaining silt fence, hay bales and other methods. Preventing soil from washing into streams and rivers is certainly an important issue and critical to the protection of the environment but too often the direct impacts of impounding this water are overlooked.

Everyone knows we need erosion control, but I’m not sure we’ve put enough thought into where we place it, or more precisely, where it will impound stormwater. From the geotechnical engineer’s perspective, silt fence has become a necessary evil. I have personally witnessed how silt fence often acts to create a pool of standing water directly behind curbs and over utility lines.

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As Denver began to dig out after the Christmas 2006 blizzard and the drifted snow started to melt, silt fences around the city were bulging. At one location, where pavements were badly cracked, we noticed water pouring out of a hole near the bottom of a silt fence. It was as if someone was holding a garden hose up to the hole in it. Instead of flowing over the curb and into the gutter, the silt fence had helped to form a snowdrift, and the melting snow turned slowly into a pool of water behind the curb. It’s not difficult to figure out that the subgrade soils below the pavement were saturated and could no longer support traffic.

I’m not sure anyone has fully thought about the potential impacts of this standing water, certainly not those in charge of the stormwater management regulations. In the example above, roadways around the site had begun to fail, utility trench backfill was being saturated and settling and where there were no utility trenches, curb and gutter, sidewalks and pavements were heaving due to the wetting of expansive clay subgrade. Repairing these features becomes a costly finger-pointing exercise.

What methods should we use to protect the environment and sustain our expensive infrastructure? Well-placed drainage swales, berms and water-quality/detention ponds could all be answers, or maybe it’s as simple as moving the silt fence away from these areas and features that could be affected and using other methods like seeding or blankets in the area between the silt fence and the curb.

The problems with surface drainage are not limited to erosion-control measures and pavement issues. Most geotechnical engineers recommend that positive surface drainage be maintained away from buildings throughout the construction process as well as after buildings are occupied. Water infiltration into the subgrade soils supporting buildings often causes unacceptable foundation and floor slab movement, especially in the more expansive clay soils found along the Front Range and other parts of our state.

The design and construction industry is only in control of this situation for a small portion of the useful life of a project, but the potential consequences of bad drainage during construction can take root before the end-users have even moved into the building.

Discussion of these issues in preconstruction meetings with design professionals and subcontractors might be a good place to start. Weekly inspections also help to better identify bad practices. Of course, the areas where poor drainage exists will become apparent after one good rainstorm. Foremen and superintendents should praise those who meet project budgets and deadlines while maintaining good drainage practices and demand better from those trades that let drainage slip to meet a demanding project schedule.

Civil and geotechnical engineers and landscape architects should be aware that these problems exist and work to educate the project team. We should lead the charge against allowing these problems to persist. Offer your plans to others on the team to review, plan ahead for possible drainage issues and be observant when visiting project sites. Speak up if you notice items that could affect the performance of the project and help to correct them.

Of course, it doesn’t end with design and construction, but if a project is completed properly, we have done what we can to reduce issues. Owners and end-users must also understand the risks involved with not maintaining drainage and allowing irrigation to go unchecked, and they should be on the lookout for potential problems. Helping to educate owners and users on the consequences of their actions, or inactions, as the case may be, is also one of the responsibilities of the project team.

In a sense, the success of the project and the long-term performance of buildings and property are dependent on maintaining good drainage and irrigation habits.

Andrew J. Garner is the geotechnical department manager and a senior associate in the Wheat Ridge office of Terracon Consultants Inc. He has more than 15 years of experience in geotechnical engineering and materials testing for land development, infrastructure and transportation projects. Garner is an active member of the Colorado Association of Geotechnical Engineers and the American Society of Civil Engineers. E-mail: ajgarner@terracon.com

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