“MSE wall back drainage design” is the topic for the October webinar presented by the Geosynthetic Institute—Wednesday, Oct. 12, 11:30 a.m.–1 p.m. (ET).
Within GSI’s database 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.
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
(i) Appreciate that the large number of MSE wall failures are hydraulically driven
(ii) Understand that back drainage must accompany the use of fine grained backfill soils
(iii) Learn “flow through porous media” concepts in the form of finite difference
equations to result in a required flow rate
(iv) Learn about the necessary transmissivity procedure to obtain an allowable flow rate
(v) Examine how the flow rate factor-of-safety varies with the different types of
geosynthetic drainage materials
(vi) Learn about various outlet options for water collected in the wall’s back drain
Owners of MSE walls, berms and slopes in both the public and private sectors; federal, state and
regional geotechnical, transportation, and environmental engineers; engineers from municipal
districts and townships; private and municipal land developers, architectural and landscape
designers; general civil consulting engineers; testing laboratories servicing these organizations;
manufacturers and representatives of geosynthetic materials; contractors and installers of MSE
walls, berms and steep soil slopes; academic and research groups; and others desiring technically
related information on this important aspect of our constructed infrastructure.
Specific Topics Covered
1. Introduction and Background
2. Seepage Forces
3. Avoiding Seepage Pressure
4. Flow Rate Factor-of-Safety Calculations
5. Back Drainage Comments
The instructor is Dr. Robert M. Koerner, professor emeritus of civil engineering at Drexel University and director emeritus of the Geosynthetic Institute.
11:30 a.m.–1 p.m. (ET), Wednesday, Oct. 12, 2016
GSI members: $200 USD
Non-GSI members: $250 USD