Many buildings have enclosed below-grade spaces, which can either be exposed to permanent, temporary, or perched groundwater conditions. Typically, these groundwater elevations fluctuate several feet, if not more, due to precipitation or tidal influences. Groundwater infiltra - tion into these below-grade spaces, or worse, a buoyancy failure of the basement slab, can result in potentially costly problems to building owners and tenants, if not handled appropriately. Failure to adequately address the high groundwater conditions can lead to the disruption of building operations, damage to interior finishes, deterioration of structural and mechanical components, and potential interior air quality issues. Unfortunately, it is not always clear, which design discipline should lead the design efforts since the Plumbing Engineer of Record (EOR), Geotechnical EOR, Structural EOR, Civil EOR, or Architect, along with others, may be involved in the system design. Therefore, the Architect sometimes is left design- ing it themselves or coordinating the system design, which can be extremely challenging. Determining the appropriate overall approach for mitigating high groundwater requires consideration of the anticipated use of the space. The following three options, or a combination thereof, are typically considered for below-grade spaces: • Installing a watertight barrier and designing the foundation walls and slab to support the full design water pressure. • Installing a groundwater relief/dewatering system to drawdown the ground- water levels (e.g., dewatering wells either interior or exterior to the building). • Installing a sub-slab drainage system. Each of the options above have advantages and disadvantages, and this paper does not discuss the selection process. Instead, this paper focuses on the design elements of sub-slab drainage systems and the multidisciplinary design considerations that are critical to successfully executing its design and construction. What is a Sub-Slab Drainage System? A sub-slab drainage system is a series of below-grade water collector and conveyance elements that are installed below the interior space (Figure 1). It is preferable to install the drainage system during the original building construction; however, the system can be installed as a retrofit by performing the selective demolition of areas of the slab. Perforated or slotted piping is a typical method to provide both col - lection and conveyance means. Alternatively, designers use plastic drainage boards for collection along with solid piping for conveyance. Designers frequently specify clean, highly-permeable, crushed stone with minimal fines to increase the efficiency of the system either by in - stalling it in the trenches for the perforated piping and/or as a “blanket” under the slab. The designer determines if the piping is needed along the perimeter foundation wall, at regularly spaced intervals in the inte- SUB-SLAB DRAINAGE SYSTEMS Z.K. Boswell, P.E., B.P. Strohman, P.E., and A.R. Lewis, P.E.
rior, or both. These collector elements lower the groundwater levels in the proximity of the drain pipes by intercepting and directing the water either into a sump pit (Figure 2) or a gravity outlet, if the site grading allows. The latter is atypical because it is often difficult to daylight gravity drains due to the below-grade nature of the space. In between the collector pipes, the water mounds (i.e., elevation increases). As the spacing of the drains increases, the elevation of the mound between the pipes increases. The addition of a blanket of clean drainage stone with minimal fines is preferable for reducing mounding between the pipes.
Figure 1: Groundwater Drawdown Profile Below Slab
Figure 2: Common Sump Pit with Sump Pump
Elevated or Perched Groundwater Problems Elevated or perched groundwater conditions can create a myriad of problems for a building owner and their tenants. These problems can be as minor as incidental leakage that provides little disruption to build- ing operations to significant disruption and structural damage, such as slab heave due to the build-up of hydrostatic uplift pressures below the slab. Other examples include damaged or deteriorated equipment, contents, and finishes; injuries from walking on wet surfaces; and the formation of mold or mold-like substances in the below-grade space. Photos 1 and 2 show examples of groundwater infiltration. Figure 3 shows the typical groundwater infiltration pathways through the slab and foundation walls. Design Approach and Considerations The design of a sub-slab drainage system requires a thorough under- standing of the subsurface and groundwater conditions, surface and stormwater drainage features, and the as-built conditions below the
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