Geogrid Aperture Stability Explained

What is it?

Geogrid Aperture Stability is also known as the ‘Geogrid In-Plane Torsional Rigidity’, it relates to the stiffness of the geogrid and its ability to interlock with the aggregate particles of the gravel material placed on the geogrid. Therefore, Aperture Stability (Torsional Rigidity) can be defined as the resistance of a geogrid against torsion where the grade of geogrid stiffness is reported in units of N-m/degree being tested in accordance with ASTM D7864.

How does it work?

The trafficking of gravel material develops shear stress in the aggregate on the plane of the geogrid, which differs with regard to the intensity and direction as the wheel passes. This in turn causes a twisting motion in the geogrid. The magnitude is the highest in the outward direction and the stress may not be high enough to cause slippage in all directions, but the stress and the potential for movement change their direction, which results in a twisting stress in the geogrid. Geogrids with high torsional stiffness provides a greater resistance against the twisting stresses which allows a more effective interlock and lateral restraint of the gravel material than reinforcing elements with lower torsional stiffness.

Why is it important?

Due to the result of high shear stresses between the gravel material and geogrid reinforcement, the use of geogrids with a high stiffness improves the bearing capacity of the gravel material, thus minimising pavement deformations. The effect of reducing the aggregate particle movements, absorbing the radial shear stresses and the positive influence on the degree of load distribution all contribute to the development of an optimal rigid reinforced geogrid structure.

Several tests by Kinney & Xiaolin (1995) showed a particularly good correlation between Aperture Stability and the US Army Corps of Engineers series of large-scale tests results for base reinforcement under paved light aircraft runways at the Waterways Experiment Station (WES). Kinney & Xiaolin interpreted from their gained test results, that geogrids with high Aperture Stability (Torsional Rigidity) behaved superior to geogrids with less torsional stability due to more effective interlock and lateral restraint of the granular base course material.

Aperture stability is an important geogrid parameters being used directly in some pavement design methods such as Giroud and Han (2004), where higher aperture stability ends up with a less pavement thickness:

How does Secugrid® and Combigrid® measure up?

Both Secugrid® and Combigrid® have been independently tested to have the highest Aperture Stability/Torsional Rigidity. Based on the Kinney test method, Secugrid® has very high test values.

According to test trials carried out by WES and Kinney, a geogrid with a high aperture stability (also referred to as torsional stability) will allow more traffic passes than products with lower aperture stability. Therefore, they have a higher Traffic Benefit Ratio (Relationship between number of truck passes over an area with geogrid compared to an area without geogrid).

In summary, Secugrid®/Combigrid® geogrid structures have proven to be an effective total system with high aperture stability, providing a high level of safety for stability against stresses caused by dynamic traffic loads.

To design your Secugrid®/Combigrid® reinforced pavement using Giroud and Han (2004) calibrated software, please click here (SecuCal® website). Our Technical Team is on hand to guide you through the software if needed.