C+S September 2020 Vol. 6 Issue 9 (web)

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 groundwa- ter infiltration, the investigation often includes the following: • Exploratory openings through the building slab to document existing condi- tions (be careful performing openings if the slab is pressurized). • Supplemental borings and monitoring wells and/or test pits to collect sub- surface 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 dis- charge 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). There- fore, 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 judg- ment 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 responsi- bilities 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 ground- water fluctuations due to perched groundwater or surface flooding is es- sential. 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

Photo 1: Groundwater Infiltration at the Interface Between a Foundation Wall and Slab

slab, among other things. Below, we present a general design approach and the notable design considerations for sub-slab drainage systems. Photo 2: Ponding Water in a Below-Grade Space due to Groundwater Infiltration



september 2020

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