MSEW drainage?

Editor’s Note: From the Geosynthetics article: “Pipeless drainage system” (April/May 2008)

GMA Techline question

From: Carlos
RE: Drainage

What is the most used design method for drainage systems in MSEW with geosynthetics? Or what can I do for designing a system for drainage in the MSEW?

Response

From: October/November 2010 Geosynthetics
RE: GMA Techline answer

Editor’s note: A question-and-answer from the GMA Techline column that ran in the October/November 2010 issue has been updated. Bob Koerner’s revised answer regarding MSEW drainage follows:

Dear Carlos,

Sorry to have answered your GMA Techline drainage question from the perspective of landfill liners instead of the requested mechanically stabilized earth walls (MSEW), as you requested.

Regarding MSEW drainage of the reinforced soil zone, water potentially comes from:

1. beneath, as in a rising water table,
2. behind, as in drainage from the retained earth zone,
3. above, as from surface water drainage, and
4. within, as in drainage from the backfill soil itself.

Regarding the answer to each of these, the following applies:

1. A water table within the reinforced soil zone must be accommodated by a gravel backfill to the maximum anticipated elevation. The gravel size must accommodate the rapidity and quantity of the rise. Both of these issues are site-specific considerations.
2. Flow from the retained soil zone can be estimated using Laplace’s analysis and a flow net construction as a worst-case scenario (see Koerner, R. M. and Soong, T.-Y., “Drainage System Design Behind Segmental Walls,” 18th PennDOT/ASCE Conference on Geotechnical Engineering, Hershey, Pa., 2000, 28 pgs.; also Proceedings GRI-14 Conference, December 2000, pp. 323-351).
   This process will invariably lead to design of a back (or chimney) drain behind the reinforced soil zone so as to intercept the flow from the retained soil zone. Its bottom must, of course, be connected to a base drain or pipe outlet system beneath the reinforced soil zone and from there be daylighted from the front of the wall.
3. Flow from above the reinforced soil zone must be prevented by using a geomembrane or geosynthetic clay liner above this zone and even extending over a portion of the retained soil zone.
4. Dissipation of water from within the reinforced soil zone will usually be a natural process but if it is of concern strips of nonwoven geotextile can be added or (better) a geogrid/geotextile composite used for combined reinforcement and drainage. There are several products available in this regard.

Robert M. Koerner, Ph.D., P.E.
Geosynthetic Institute and & GMA Techline

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