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Is it magic?

August 1st, 2010 / By: / From Our Readers

Editor’s Note: Prof. Dov Leshchinsky, a regular contributor to Geosynthetics magazine, authored an intriguing article for the June/July 2010 issue.

In “Geosynthetic reinforced walls and steep slopes: Is it magic?” Leshchinsky urges a critical review of the apparent magic before it becomes a “fact” adopted in design.

Several readers responded:

Response

Dov,
I just finished reading your latest article in the June/July Geosynthetics issue and wanted to let you know the article really hits home. It has been my experience through review of MSE wall designs and wall failures that some engineers rely on the “magic” of apparent cohesion in fill soils by using a liberal design approach and minimizing the reinforcement layout with respect to spacing, strength, and length.

There are likely many MSE walls and slopes designed without any attention paid to internal and compound internal stability. While many of these walls are still standing, indicating their factor of safety (Fs) is greater than 1.0, their actual Fs is likely well below engineering standards. Failure may only be a rain event away … just add water and the magical “apparent cohesion” is gone.

Sincerely,
Blaise J. Fitzpatrick, P.E.
Fitzpatrick Engineering Associates P.C.,
Lawrenceville, Ga.

Response

Hi Dov,
Just wanted to say I enjoyed your article in Geosynthetics (June/July issue).

I always make a special point concerning the issue of “apparent cohesion” to my students. One of the problems that we deal with in our more glaciated soils up north is the problem of “real but light cohesion” mostly from reprecipitation of carbonates from the grinding action of the glacial garbage can of materials.

It is not uncommon for a homeowner to make a vertical cut in these soils, which does stand vertically for a while, and then build a house very near it. Generally, over time, what happens is that the slope will slough and a natural angle of repose will result. This is OK as long as they own the property behind their house to allow the slope to form by simply removing the sloughing soils allowing the slope to cut backwards. But I suspect that some day someone’s house will get taken out!

Stan Vitton
Department of Civil & Environmental
Engineering, Michigan Technological
University, Houghton, Mich.

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