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Is it magic? author offers more insight

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

Editor’s Note: The issue of dormant reinforcement due to apparent cohesion was described in “Geosynthetic reinforced walls and slopes: Is it magic?” by Prof. Dov Leshchinsky, Geosynthetics, June/July 2010.

Among the many responses to this article was a relevant question about the new “K-stiffness method.”

Inquiry

RE: Requested information for implementation of the K-stiffness method in program MSEW (3.0)
August 23, 2010

Response from Prof. Leshchinsky

Although such implementation in the framework of MSEW is simple, we do not plan to perform the task since the current method appears to be invalid, very possibly rendering unsafe results.

The reasoning behind this conclusion is the formulation of the K-stiffness method, which is a statistical analysis based on a compilation of field data of retaining walls from various, unrelated researchers. The formulation is not based in physics or statics, but in statistics, ignoring long-term design stability essential to the safe function of a structure.

This approach empirically links stiffness, spacing of the reinforcement layers, facing properties, batter, and shear strength of the soil using little more than statistical correlations. However, the method ignores the vital inclusion of statics in design for the sake of rendering less conservative reinforcement tensile forces that are scientifically unsubstantiated over the long-term. Seasoned engineers would, and should, be skeptical about the feasibility of such statistical shortcuts.

The field data that serves as a foundation for the K-stiffness formula, deemed as a comprehensive basis for solutions, was very likely for situations where soil matrix suction rendered apparent cohesion. Cohesion reduces, or even eliminates, the reinforcement load. However, apparent cohesion is dependent on moisture content (or degree of saturation of soil) and should not be counted on in design as its value may periodically diminish over the life of the structure.

In fact, the K-stiffness method ignores the apparent cohesion in its statistics (the magnitude of this cohesion is impossible to determine in-situ) and, essentially, attributes the low “measured” force in the reinforcement to what amounts to “magic.” That is, the method bypasses basic statics, ignoring simple global equilibrium in favor of uncritical acceptance of field data combined with statistics, and gives no scientific justification for the lower tensile forces.

Indeed, apparent cohesion may render reinforcements dormant, but when the cohesion vanishes, the reinforcement will be activated and failure may occur. The issue of dormant reinforcement due to apparent cohesion is explicitly described in: Leshchinsky, D., “Geosynthetic reinforced walls and slopes: Is it magic?Geosynthetics magazine 28(3), 2010.

The indications that suggest current design is conservative do not transitively imply that the remedy offered by the K-stiffness method is correct. In fact, without a mechanistic benchmark, its use may lead to overly reduced conservatism, an unsafe conclusion that could result in failure.

Additional material relevant to this issue is published in refereed engineering journals, including:

Leshchinsky, D., “On Global Equilibrium in Design of Geosynthetic Reinforced Walls,” ASCE, Journal of Geotechnical and Geoenvironmental Engineering, 2009, 135(3), pp. 309-315.

Leshchinsky, D., Zhu F., and Meehan, C.L., “Required unfactored strength of geosynthetic in reinforced earth structures,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 2010, 136(2), pp. 281-289.

Leshchinsky, D., Imamoglu, B. and Meehan, C.L., “Exhumed geogrid-reinforced retaining wall,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 2010, 136(10).

Dov Leshchinsky
ADAMA Engineering
Newark, Del.

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