Many building associations have to deal with the creation of the shadow lines that form as we rise from a natural site, to a finished subdivision. The current greening of the design and construction world includes higher densification of homes and buildings to reduce the overall footprint of the sites. This density creates large urban corridors which result in very unfriendly maintenance environments. Consideration at the design stage, during construction, and after turnover for maintenance should be made by those parties involved.
Taking just the photograph above, lets analyze the situations that are common to the maintenance needs of such a dense configuration. Many readers will clearly understand how water management in a corridor like this requires considerable accommodation in the design phase. To begin with the site design, the development of the properties takes native land and creates hardscapes, buildings, and vegetated areas. This creation increases the amount of runoff from rain, snow, ice and frost, compared to the undeveloped site. The engineers must consider this increase and integrate Solutions Before construction, to provide a non-destructive drainage system. The most common Solution Before is often referred to as a point source discharge system. A roof collection system, with a gutter and downspout that collects the water from the roof and discharges it in a localized point at the downspout and splash block, is the most common point source discharge system. Water then is conveyed through a swale, ditch or pipe to a major storm sewer, detention or retention pond. The path that water takes, between the roof and the discharge area, creates challenges in maintaining the property.
The water in the areas between the buildings goes into the protective grading area, the backfill zone. This protective zone requires that the grade be between 5 and 10 percent in the first 5 to 10 feet, significantly reducing the amount of water that can impact the foundations or basements. The water then runs parallel to the building in the swale and must discharge to the pavement area. The grade of the soil must be higher than the asphalt or the water gets trapped. Many engineers refer to this as a bounded area. If the ground settles, common in backfill zones than the association must remove the landscaping and readdress the grade to achieve both the swale slope, typically at 2 to 3 percent and the backfill slope. This level of maintenance is reduced by good soil types and proper compaction. The water migration in the swale must be considered as water is the catalyst of soil movement and ponding water can exacerbate the way the soil behaves.
The water then flows from the swale onto the asphalt pavement. The construction of the pavement section, graded away from foundation structures, results in a construction joint in the center of the drive aisle (see the dark line, traveling and pooled water in the drive aisle). This construction joint must be prepared to reduce the potential for water leaking through the asphalt paving. Some builders and associations elect to seal coat approximately 4 feet of the center joint annually, to provide a minimum level of protection to the asphalt. The concentrated water will flow over it and the water will lift the binders out of the asphalt reducing its potential life. This is referred to as absorption. The loss of the asphaltic binders creates a non-durable asphalt product, that as it degrades creates a continuous and progressive damage to the street section. The damages associated with this can be aggregate exposure, alligator cracking, and raveling (loss of structural subgrade material). Increased protection to this concentrated flow area can be achieved with a higher frequency of full asphalt replacement, on perhaps a 5-year cycle. Another option would be to place a concrete pan in the center of the drive aisle, due to the much higher durability of the concrete. In new design, the use of center line valleys in drive aisles is not recommended. Best practices indicate that the crown of a street should be at the center, with curb and gutters used to collect, manage, and discharge the water. Due to the densification of sites, this configuration is not typical, which results in a more aggressive need for proper civil engineering design.
The other issues associated with the pictured design will appear in winter conditions. The creation of shadow zones, primarily in the east to west drive corridors, or shadowed streets from taller buildings, allows the formation of ice. The ice is typically not associated with larger snow events, but with the expected frost that occurs on the roofs due to solar radiation losses. Think of parking your car in the street and seeing frost form on your window. If you want to avoid this condition you park under a tree, and the canopy of the tree reduces the solar radiation (loss of earth’s heat to space). The warmer car window then does not develop frost as it is not cooled to dew point. The roofs, however, do not have that canopy above them and frost will form. The frost melts, runs to the gutter, down the downspouts, and to the drive aisle or walkways. Your association documents likely only address snow removal based on depth of the storm. These drive isles require a higher burden because the frost will melt, and reform in the shadow zones as ice. As the ice forms it acts as a dam and ice forms and grows behind it.
The asphalt and concrete drive aprons share other issues. They, like the swale, are subject to the supporting soils behavior. Likely the drive aisle was installed first, as part of the horizontal construction. Subsequently, the buildings and garages are excavated, and foundations installed. This requires a tight area of backfill to be placed between the asphalt and the foundation. This soil is subject to movement and impacted by the introduction of water. That impact can be a settlement, heave or frost heave. In the winter at that heave a lip can occur between the garage slab and the apron. As water melts off the cars it moves to the door, and freezes against the lips. This, in some cases, will actually freeze the door to the concrete slab. If the garage has no door to the interior, a dangerous slip and fall condition is created in that walk area. The water entering the soil can be reduced by sealing the joints between the slab and the apron, and that between the apron and the drive isle. If your drive aisle is concrete instead of asphalt, then the systems can be dowelled structurally. NOTE: Never connect the drive isle or apron directly to the foundation, as the inherent movement will damage the foundation system. Building cladding is another area where it pays to be cautious of the impacts of upward movement of the drive isle or aprons. The cladding should be located at least 2 inches above these surfaces, properly flashed, trim primed and painted. The garage door trim should be cut at 45 degrees, primed and painted to allow for movement of the slabs without structurally compromising the systems.
The downspout that drops onto the system as discussed results in the formation of ice in the winter and algae blooms in the summer. The consideration of a proper system to reduce the potential issues of the urban canyon would include storm drainage placed below the frost line of the soils in the drive aisle. The downspouts can be connected directly to the system with a freeze inspection port located on the bottom of the downspout for maintenance. This type of system allows that water to migrate to the ponds or public storm sewer with little impact on the drive aisle, thus reducing maintenance burdens, providing an environment in the development with less potential for slip and fall claims, better soil behavior and thus a longer life, more durable system.
Whether searching for Solutions Before construction, to integrate into the design of your next high-density project, or Solutions After, to remediate drainage issues with an existing urban canyon, your long term success, in terms of client satisfaction with the physical environment, and with the long-term maintenance and upkeep costs, will come from properly assessing the soil quality and conditions, and developing an integrated solution which will control drainage water, and convey it to a proper drain or storage site. This, coupled with periodic site inspections and evaluations, from qualified civil engineering experts, to identify new or developing issues, should serve to keep drainage issues off your radar, for years to come.
Edward L. Fronapfel, MSCE, PE, CBIE, CFCC, EDI, CBCP, Senior Member of NAFE, owner of SBSA: Solutions Before Solutions After.
Edward holds a Bachelor of Science in Civil Engineering, and a Master of Science in Civil Engineering with an emphasis in forensic and structural engineering. He is a Certified Level 2 Infrared Thermographer, a Certified 3rd Party EIFS Inspector, a Certified Forensic Claims Consultant (CFCC), and a Board Certified Building Inspection Engineer (CBIE) a Post Tension Level II Inspector, a NACHI Home Inspector. Ed’s experience includes geo-hydrology, hydrology, hydraulics, civil engineering, structural engineering, thermography and extensive work in construction forensics for building sciences that utilize his knowledge across the entire construction process. Clients include construction companies, management companies, insurance companies, attorneys, homeowners, and homeowner associations.
Ed’s work has included deposition testimony, expert witness testimony, mediations, and arbitrations. He is a registered engineer in Arizona, Colorado, Florida, Georgia, Kansas, Louisiana, Michigan, Minnesota, Mississippi, Montana, Missouri, Nebraska, North Carolina, North Dakota, New Mexico, Nevada, Oklahoma, Oregon, South Carolina, Texas, Utah, Virginia, Washington, and Wyoming. Ed’s dedication to the community is demonstrated by his active membership in many professional organizations including RCI, AIA, NSPE, Lorman, CABO, RESNET and others. He is a published writer in “The Journal of RCI,” “Building Integration Solutions,” “The Forensic Engineering Task Force,” “The Investigative Engineer,” “The I-ENG-A Report, “Inframation, Proceedings Volumes 4, 5, 6, 7, 8,” “Colorado Claims Guide,” and “Building Integration Solutions.” Adjunct Professor Fronapfel teaches at the University of Denver in the Burn’s School of Real Estate and Construction Management, teaching courses on Estimating, Project Management and Control, Sustainability in Construction and Residential Construction Systems.
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