pit is recommended to provide redundancy in the event that the primary pump fails, the inflow rate exceeds its capacity, or is taken offline for main- tenance. Both the primary and secondary pumps should have emergency backup power in the event of a power outage. In critical situations, a sump pit controller capable of alternating the pumps will help to identify an issue immediately by regularly exercising both pumps. The number of pump starts per hour is critical to the system design. Most pumps require about ten starts per hour or a cycle time of 6 min. to prevent burnout. This requires the designer to size the pumps and sump pits appropriately. If the pit is too small, it fills too fast between the off and on elevations in the pit, instigating short cycle times. Periodic testing of the pumps is also critical. The designer may also consider using high-water sensors in the pit or basement to provide early notification to the owner in the event of unexpected conditions. • Discharge Options: Prior to finalizing a sub-slab drainage system design, the designer must evaluate where the water will be discharged after the water is collected, and in many cases, pumped by the sub-slab drainage system. Options for discharging the water include tying into the existing site stormwater system, discharging the water at the ground surface, or discharging the water into a sub-surface infiltration system. An analysis of jurisdictional requirements or restrictions is required. For all options, a backflow preventer should be included in the interior plumbing design to minimize the risk of water flowing back into the building and the increased cycling of the pumps. • Demolition and Temporary Shoring: When installing a sub-slab drainage system in an existing building, it may require temporary shoring of the exist- ing foundation walls and footings. • Radon Systems: An existing radon system introduces additional require- ments for the sub-slab drainage system, since the piping below the slabs can become pathways for the radon. Sealing and venting of sump pits are typical in these situations. Closing Remarks Groundwater infiltration into below-grade spaces is a problem that many designers, contractors, and building owners, and their tenants regularly battle. A thorough understanding of the subsurface and groundwater conditions, surface and stormwater drainage features, and the as-built conditions of the building and the below-grade space is required to achieve a successful design. This paper presents a general approach for the design of sub-slab drainage systems and describes notable design considerations. The authors have successfully applied this approach at several building sites, where the owner was experienc - ing systemic groundwater infiltration issues.
Figure 3: Typical Groundwater Infiltration Locations
rate, spacing of pipes, and the size and number of pumps and sump pits for the sub-slab drainage system. The pipe layout and the number of pumps and sump pits will vary significantly based on the subsurface conditions at the site. In addition, the geotechnical engineer determines if the natural soils may migrate (known as soil piping) into the drainage stone used for trenches and drainage blankets due to the water flow of water. The geotechnical EOR may implement design enhancements, including increasing the factor of safety on flow capacity, among other things, to address iron ochre or sedi- mentation buildup in the pipe. • Civil: The civil engineer analyzes the existing site topography and storm- water runoff at the site. This includes minimizing surface ponding adjacent to the buildings and evaluating where and how to legally discharge and treat (if applicable) the water from the below-grade space. • Environmental: The environmental engineer will provide recommenda- tions for mitigating contamination movement or treating groundwater, if any limitations exist. • Mechanical/Electrical/Plumbing (M/E/P): Although it varies, an M/E/P engineer or licensed plumber, typically determines the size and layout of the interior piping and cleanouts following the relevant building codes using the flow rates determined by the geotechnical engineer. The M/E/P engineer selects the sump pits, primary and secondary pump sizes, on/off sequenc- ing and elevations of primary and secondary pumps, the type of water level sensors, and the control panel. The M/E/P engineer or licensed electrician assesses the electrical demands for the sump pumps following the relevant building codes. • Structural: The structural engineer designs the building slab, footings, foundation walls, and superstructure based on the design water pressures provided by the geotechnical engineer, including evaluating the impact of hydrostatic uplift on the structure. Failure of the sub-slab drainage system and the resulting water pressures on the structure should be considered, particularly, if there is no cutoff wall below the structure. The design team must account for a number of additional consider- ations, including the following: • System Maintenance: Replacement or repair of the components is inher- ently challenging since the system is below-grade; therefore, maintenance considerations are essential. The sump pits and pumps need to be in loca- tions that are easily accessible for maintenance, but not in areas, where they impact building or tenant operations. The design should include cleanouts throughout the subsurface piping system, in particular, at bends and along long pipe sections to allow for adequate access to maintain the system. • Mechanical Considerations: The use of a secondary pump at each sump
Z.K. BOSWELL, P.E. is a consulting engineer at Simpson Gumpertz & Heger Inc. They can be reached at zkboswell@sgh.com. B.P. STROHMAN, P.E. is Senior Project Manager at Simpson Gumpertz & Heger Inc. They can be reached at bpstrohman@sgh.com. A.R. LEWIS, P.E. is Senior Principal at Simpson Gumpertz & Heger Inc. They can be reached at alewis@sgh.com.
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