As the need for effective stormwater treatment and volume control grows, so does the number of available stormwater control measures (SCMs) such as filtration systems, hydrodynamic separators, bio-retention systems, green roofs and pervious pavement. As part of their due diligence, engineers go through an evaluation process to determine if the proposed system will meet some basic criteria, one of which is structural considerations. Below are some of the structural considerations. The engineer should not assume that because a SCM is approved and/or detailed in a manual that all of the structural issues have been addressed.
Structural integrity is critical
Many underground systems are designed to handle traffic loads; therefore, it is important to evaluate the structure for integrity and design life. Make sure the structures are reviewed by structural engineers to ensure expected traffic loads can be handled. Placement of larger covers and manhole lids in the travel way are subject to dynamic loads. Sub base preparation is critical for both structural solutions and for supporting expansive solutions such as pervious pavements. Underground storage solutions are now available in many types of materials and configurations. Evaluate the background, and ensure structural integrity is demonstrated; especially in a world where installation, backfilling and loading are not always in line with the theory.
Water tightness is required by many agencies
Evaluate vertical and horizontal joints for design integrity. Vertical joints are more difficult to control because of differential settlement. Some agencies require a water tightness test prior to acceptance. All joints below the permanent pool elevation need to be watertight. This is especially critical for harvesting projects where the water will be subject to reuse and will be stored for long terms.
Buoyancy measures need to be considered
In areas of high groundwater, take measures to prevent system floatation. Ballast wings or straps are frequently employed to maintain structures. However, the increased use of impervious liners and surface infiltration technology warrant higher scrutiny than in the past.
Constructibility considerations are important
What appears simple on the plans can be difficult during construction. Consider construction loads, back filling, etc. Does the contractor have a track record of constructing and installing similar facilities?
Consider materials of construction
Stormwater runoff can be highly corrosive, and stored water can become anaerobic. The presence of numerous fasteners, differential metals contact, pivot points and hinges, galvanized parts, and adhesives are all potential candidates for corrosion and ultimate failure.
Consider long term issues
Long term all SCMs will be subjected to maintenance. Structural components such as plastic standpipes, impervious liners and sub drains can all be damaged during construction or as part of a maintenance cycle.
James H. Lenhart, PE, D.WRE
Jim Lenhart is the Chief Technology Officer for Contech Engineered Solutions. He was founder of Stormwater Management Inc, and was also owner of Stormwater Northwest where he consulted with companies that provide products and services in the stormwater related markets. Jim has authored over 45 papers on the subject of water quality and stormwater treatment. Jim is a professional Agricultural and Environmental Engineer with over 25 years of experience in consulting engineering and research and development. He served as an adjunct instructor of Civil Engineering at Portland State University where he taught hydraulics and water resources engineering. Jim currently is an active member of the Water Environment Federation, the Water Environment Research Foundation, ASCE EWRI, and serves as Vice Chair of the Urban Water Resources Research Council. Jim holds a BS in Plant Sciences, a BS in Agricultural Engineering and MS in Water Resources Engineering.
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