The Aperture Stability Modulus (ASM) is also called In-plane torsional rigidity. It can be defined as a critical parameter that evaluates the effectiveness of geogrids. These are usually made of polymer materials such as polyester, polyethene, polyvinyl alcohol, or polypropylene. They have large openings or apertures that reinforce soil and aggregate layers, enhancing stability and performance.
Before the ASM was developed, geogrids were primarily assessed using tensile strength tests. Although tensile strength plays some role in geogrids, it doesn’t fully capture the ability to resist twisting or deformation under stress, which is vital for ground reinforcement.
This limitation came to light in the 1990s. Prompting Dr. Timothy Kinney’s research on more comprehensive tests. He realized that a test directly measuring a geogrid’s resistance to twisting could provide better accuracy in determining its suitability for specific reinforcement applications.
Geogrids depend on interlocking with aggregate particles to improve stability in foundations and unpaved roads. Unlike traditional and unreinforced layers, they are susceptible to sideways movement when positioned under heavy traffic loads.
This movement can result in the following:
The solution to these complications is geogrids, which provide a mechanism that interlocks with aggregate particles. This results in the following benefits:
The standard calculation method of Aperture Stability Modulus is still in the development phase of ASTM International (ASTM D7864 is frame method).
However, its basic principle includes measuring geogrid’s distorted behaviour in proportion to the junction of a biaxial geogrid.
Below is the breakdown of the testing process:
● Preparation of the sample: The first step is to test the Geogrid sample. A square section of the geogrid is taken as a test specimen to carry out this test.
● Fixing the secured perimeter: The next step is to fix a secured parameter to test the geogrid specimen. The specimen is then fixed along the edges to avoid movement during testing.
● Applying controlled moment: The next step is to apply a controlled twisting force or moment to the geogrid’s central junction.
● Measuring angular deflection: Once the moment is applied, the resulting angular deflection of the geogrid is measured.
Calculation of aperture stability modulus (ASM): The ASM is the ratio of change in a moment to angular rotation. The units are expressed as N-m/rad (Newton-meter per radians). The formula used for calculating ASM is
Aperture Stability Modulus (ASM) = M/θ
Where,
ASM = Aperture Stability Modulus (Units expressed in Newton-meter per radians)
M = Applied Moment (Units expressed in Newton-metre)
Θ = Angular deflection (Units expressed in Degrees)
Let’s assume a geogrid is positioned under an aggregate layer with a heavy traffic load. As traffic passes over the patch, the aggregate particles tend to shift sideways due to applied forces.
A geogrid is considered stiff when it has a high ASM value. It acts as a solid and interconnected net. The ribs and junctions of geogrid effectively lock the aggregate particles as they resist twisting and deformation. This mechanism avoids lateral movement, which leads to more stability and load bearing.
Understanding the application of ASM or Aperture Stability Modulus is integral to geogrid selection. It enables the engineers to make informed and proper decisions.
Here are some typical applications of ASM in geogrid selection:
● Unpaved/unsurfaced roads: Geogrids with high ASM are mostly preferred for unsurfaced roads with heavy traffic loads and weak subgrades. This is to avoid rutting and enhance bearing capacity.
● Enhanced soil reinforcement: Higher ASM geogrids are ideal for slopes as they enhance the soil reinforcement and prevent erosion.
● Retention of walls: Geogrids with proper ASM values can be used for retaining walls as they effectively distribute loads and improve the overall structural stability.
ASM plays a crucial role in selecting geogrids, and understanding ASM is essential for stable and prosperous structural projects.
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