To the editor:
Thank you for the article “Junction-strength requirements for roadway design, construction” (Geosynthetics, February/March, 2007), defining and categorizing the use of junction strength as a specifying property for geogrid reinforcements. The excellent summary of performance requirements for geogrids that junction strength might assess, along with industry’s and research attempts to quantify those requirements, directly addresses an issue that has perplexed the geosynthetics industry for more than 20 years.
Simply stated, engineers and specifiers seem to be intuitively more comfortable with stiff, rigid geogrids exhibiting high junction strength, even though research and empirical performance data fails to conclusively confirm that correlation (Berg, Christopher, and Perkins, 2000). Consequently, based on that intuition, geogrid specifications are then mistakenly written with product specific properties, substituted for the minimum properties (necessary) to ensure performance.
Outlined below are two simple reasons why the intuition that requires high junction strength is faulty:
1. The specifier’s exaggerated importance of a single junction to the overall performance of the geogrid.
As a tensile reinforcement the geogrid must be able to engage the soil, by transferring the generated tensile loads into soil shear stress and/or passive resistance. It does this over a pullout length that is significantly in excess of a single junction, and controlled more directly by the available soil shear strength. The direct measurement of this load-transfer efficiency is the pullout coefficient of interaction.
Generally, even with this direct measure, an additional length (safety factor usually 1.5) is used to determine the actual pullout [development] length installed. Therefore, even when minimum pullout or overlap lengths of 300mm (12 in.) are specified, a large number of junctions are engaged in transferring the tensile load to soil. Even for large aperture geogrids 37mm x 37mm (1.5 in. x 1.5 in.) there are about 64 junctions per square foot, meaning each junction carries about 1.5% of the total load.
2. The effects of soil confinement are being ignored.
This article elaborated on confinement effects as they related directly to the junction strength test and measurement of junction properties. However, the effects of confinement are equally crucial to modulus of the geogrid [increasing], and more importantly the composite reinforced soil mass modulus.
It is this composite modulus of the reinforced soil mass that controls behavior, which as of yet has been difficult to predict, particularly when using geogrid properties from index tests performed in isolation (air), and hence lack of correlation to performance. Soil confinement is why all geogrids work well.
I concur that geogrids should continue to be used on roadway projects, and offer the following alternate junction-strength specification to be utilized, “as a conservative value that allows product usage without concern” (see Geosynthetics, Vol. 25, No. 1, p. 41). Until such time as the research and testing recommended [in this article] catches up to the marketplace with quantifiable and conclusive performance properties for specification use.
The “sum of the junction strength” criteria was presented (Elias and Christopher, 1997) to ensure; “For geogrids, … junction strength … adequate to prevent … failure of the grid joint throughout the design life of the structure.” This criteria which has been used in DOT specifications; requires significantly more junction strength than the minimum survivability value recommended by GMA (35N, 8 lbs.), is self-adjusting to the geogrid strength requirements identified by design calculations, and appropriate for all geogrid applications.
The “sum of the junction strength” criteria is: The minimum junction strength shall be greater than the ultimate unit strength (kN/m or lbs/ft) of a product, divided by the number of junctions present in a unit area (sq. m. or sq. ft.) of the same product. The ultimate tensile strength shall be defined by ASTM D-6637 and the junction strength by GRI-GG2.
This is conservative specification criteria because the vast majority of geogrid designs have working loads less than 50% of ultimate, and most are less than 33%, providing a safety factor of 2 to 3 on junction strength.
While, as a designer, I too would prefer site-specific testing for pullout and survivability properties, it may be appropriate, cost effective, and conservative on some projects to substitute the “sum of the junction strength” criteria in specifications when either the pullout resistance, survivability, stiffness, and/or flexural rigidity is considered important to the specifier. The “sum of the junction strength” criterion is a fair and unbiased manner to specify a minimum junction strength requirement for any project.
Letter submitted by: Michael Adams
Research Geotechnical Engineer
Federal Highway Administration
Berg, R.R., Christopher, B.R. and Perkins, S. (2000) “Geosynthetic Reinforcement of the Aggregate Base / Sub-base Courses of Pavement Structures,” prepared for AASHTO Committee 4E, prepared by GMA, 176 p. (spec. pg. 63-64).
Elias, V.E., Christopher, B.R. and Perkins, S. (1997) “Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Design and Construction Guidelines,” prepared for Federal Highway Administration, Demo 82, Contract No.: DTFH61-93-C-000145, 371 p. (spec. pg. 325).