Information for lesson "MSE Wall; Back Drainage Design - Dr. Robert Koerner - 1.5 PDH"
Description: Within GSI’s data base of 171 failed mechanically stabilized earth (MSE) walls reinforced with geogrids or geotextiles, 60% have failed due to water. Furthermore, 61% have been constructed with fine grained silt and/or clay soils. Taken together, these low permeability backfill soils do not drain easily and instead build up hydrostatic pressure leading to the large number of failures.
In order to avoid such pressures, back drainage is necessary between the reinforced and retained soil zones. This webinar is focused entirely on the proper design of this back drainage zone and its outlet system beneath the reinforced soil zone. The design is applicable to sand as the back drainage material, however, its near vertical orientation lends toward geocomposite drainage systems. Ten different geocomposites will be tested for their transmissivity and illustrated in the four numeric examples that are presented. In all cases a flow rate factor-of-safety is developed and analyzed accordingly.
Objectives: Participants will understand how internal and external water enters and acts upon the reinforced soil zone of MSE walls when constructed with low hydraulic conductivity silt and clay soils. The avoidance of hydrostatic pressure using geocomposite drains is the key in this regard. Technically, proper flow rate factor-of-safety calculations will be formulated by comparing allowable flow rates (using a transmissivity test) with required flow rates (using a finite difference approach). Numeric examples are presented to specifically illustrate the design process.
• Appreciate that the large number of MSE wall failures are hydraulically driven • Understand that back drainage must accompany the use of fine grained backfill soils • Learn “flow through porous media” concepts in the form of finite difference equations to result in a required flow rate • Learn about the necessary transmissivity procedure to obtain an allowable flow rate • Examine how the flow rate factor-of-safety varies with the different types of geosynthetic drainage • material • Learn about various outlet options for water collected in the wall’s back drain