Z.K. Boswell, P.E., B.P. Strohman, P.E., and A.R. Lewis, P.E.
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 infiltration 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 designing 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 groundwater 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 collection 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 installing 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 interior, 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.
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 building 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 understanding of the subsurface and groundwater conditions, surface and stormwater drainage features, and the as-built conditions below the slab, among other things. Below, we present a general design approach and the notable design considerations for sub-slab drainage systems.
The first step in designing a sub-slab drainage system is to perform a thorough review of the subsurface and groundwater conditions, and the interior and exterior as-built conditions of the building and site. For a new building, this information is typically required and readily available as part of a new building design. For an existing building, where a sub-slab drainage system is being added to address groundwater infiltration, the investigation often includes the following:
- Exploratory openings through the building slab to document existing conditions (be careful performing openings if the slab is pressurized).
- Supplemental borings and monitoring wells and/or test pits to collect subsurface and groundwater information.
- Laboratory testing or field testing to determine the permeability of the existing soils.
- An as-built survey to understand the surface drainage features and discharge locations around the building exterior.
A fundamental understanding of seepage flow through soils is critical for the design of sub-slab drainage systems. Seepage flow rates (e.g., gpm per foot of pipe) are dependent on the soil material property called permeability (reported in units of length per time). The higher the soil permeability, the larger the radius of influence for the dewatering system (i.e., the extent of drawdown and required flow rate). Therefore, for sites with higher permeability soils, the pipes can be spaced further apart, but the pipes will carry higher flow rates. Unfortunately, soil permeability values vary by up to 12 orders-of-magnitude and can vary by several orders-of-magnitude in the same soil stratum. This requires the EOR to perform careful calculations and exercise judgment in designing such systems to be appropriately designed but not too conservative. In addition, sites frequently have varied soil layers, which further complicates the design.
The designer can develop a seepage analysis for the proposed sub-slab drainage system using the subsurface conditions, groundwater data, and the as-built information collected during the investigation. The seepage analysis consists of either hand calculations or commercially available computer software. Using the analysis results, the designer optimizes the location and spacing for the subsurface piping system to reduce the groundwater levels sufficiently below the bottom of the slab. The designer uses the calculated flow rate for the system design.
Design Roles and Responsibilities
The design of a sub-slab drainage system can be complex and requires a variety of multidisciplinary considerations to achieve a successful design. Below, we describe the following general roles and responsibilities by discipline:
- Geotechnical: A geotechnical engineer evaluates the existing subsurface and groundwater conditions at the site. The selection of the design high groundwater conditions is critical and may require deep hole test pits, groundwater observation wells, or other methods. Assessment of groundwater fluctuations due to perched groundwater or surface flooding is essential. Soil testing may include grain size testing or performing laboratory permeability or field pumping tests for use in estimating permeability of the soils and tests to identify the risk of iron ochreii clogging. From this data, the geotechnical EOR may perform analytical analyses to determine the flow 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 sedimentation buildup in the pipe.
- Civil: The civil engineer analyzes the existing site topography and stormwater 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 recommendations 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 sequencing 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 considerations, including the following:
- System Maintenance: Replacement or repair of the components is inherently challenging since the system is below-grade; therefore, maintenance considerations are essential. The sump pits and pumps need to be in locations 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 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 maintenance. 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 existing foundation walls and footings.
- Radon Systems: An existing radon system introduces additional requirements 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.
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 experiencing systemic groundwater infiltration issues.
Z.K. Boswell, P.E. is a consulting engineer at Simpson Gumpertz & Heger Inc. They can be reached at firstname.lastname@example.org.
B.P. Strohman, P.E. is Senior Project Manager at Simpson Gumpertz & Heger Inc. They can be reached at email@example.com.
A.R. Lewis, P.E. is Senior Principal at Simpson Gumpertz & Heger Inc. They can be reached at firstname.lastname@example.org.