The Significance of Asperity Height and Asperity Density in Real-World Applications

Regardless of your line of work, you’ll often find yourself asking, “just how important is this feature?” Sometimes, through regulations, the decision has been made for you. Not always. We’ve gathered results from the literature to provide you with an overview of asperity height and asperity density (concentration) as well as its significance in various real-world applications.

An overview of asperity height and asperity density on shear strength

In the context of textured geomembrane liners, asperity height is a known factor in improving shear strength in relation to the interfacing material. In the past, asperity height was assumed to have a linear relationship with shear strength: Increase asperity height to improve shear strength. During the last decade, however, a greater effort has been made to quantify the impact of asperity height on various interfaces including soil, sand, textiles, and clay. Thus far, the data suggests that for each material there is a point at which an increase in asperity height no longer yields a linear benefit.

In one study, over 100 shear strength measurements were conducted in a shear box apparatus per ASTM D5321, involving different geotextiles, clay soil, and sandy soil. It is shown that for the given test conditions, the key factors influencing the shear strength properties of these interfaces are the nature of the materials involved first, and then the asperity height (1). An asperity height of approximately 20 mils was also observed to be a threshold value above which an increase in asperity height would begin to lose its linear relationship with shear strength.

The conversation to nonlinear relationship, however, should not be dismissed. At an asperity height of 50 mil under normal loads, shear strength increases by up to 20 kPa—a significant improvement, especially for high-demand projects. The interfacing material is the most significant variable when determining the impact of asperity height. Whether the liner is in contact with granular or cohesive soils is an essential factor in determining expectations and explaining results (2, 3).

The importance of the interfacing material properties cannot be overstated. In fact, it has been found that the optimal asperity height can be determined for each kind of material. For geotextiles and geogrids, for instance, there is an ideal asperity ratio which is defined by the asperity spacing of the reinforcing material and the mean particle size of the interfacing material. The highest interfacial strength in these instances have been measured when the asperity ratio is equal to one (4). This ratio suggests that for some interfacing materials, an asperity height greater than 50 mil will continue to demonstrate a liner increase in shear strength.

Asperity density (also referred to as asperity concentration), on the other hand, is a measurement of the number of asperities per unit area. It can also be calculated as the measurement of the distance between asperities. In general, increasing asperity density will increase shear strength performance. However, for textured geomembranes interfaced onto a textile, it is possible to increase the density of asperities to a point where the high concentration actually disrupts the interlocking mechanism between the liner and interfacing material (5).

Balancing strengths and weaknesses in real-world applications

From these findings, it is clear that pushing any one product quality to the extreme in a blind effort to increase performance is a mistake. Increasing asperity density, for instance, will affect other properties such as weight, strength, installation complexity, cost of shipment, cost of manufacturing, and more (6). For this reason, it is important to take each product as a whole within the context of a specific application.

It is also important to regard historical or test databases from manufacturers as basic barometers to guide the overall design. As mentioned earlier, even the microscopic characteristics of the interfacing material can go on to affect the performance of asperity height and shear strength (4). Therefore, regardless of what you find in the literature, it is considered best practice to conduct rigorous site-specific testing to verify whether the results are in line with expectations (2, 6).