Geosynthetics help improve soft soils and stabilize aggregates in pavement foundation layers. But how can engineers evaluate how much improvement geosynthetic material may provide?

Seeking the answers are Illinois Center for Transportation and Illinois Department of Transportation in a joint project, “R27-234: Effectiveness of Geosynthetics in Soil/Aggregate Stabilization — Evaluation Using Bender Element Sensor Technology.”

Photo Credit: Han Wang

Erol Tutumluer, Abel Bliss Professor in Engineering, led the project with Heather Shoup and Andrew Stolba, IDOT’s central office geotechnical engineer and chief geologist, respectively.

Engineers use geosynthetics in roadways, railways and airfields that have weak soil foundations, such as those that are too soft, are highly compressible or have high water content.

Geosynthetics increase the load-bearing capacity and extend pavement life by restraining base and subbase deformation under traffic loading. Unbound aggregate particles are immobilized by geosynthetic material, allowing them to interlock and not spread out laterally.

The ICT-IDOT project aimed to quantify how much improvement may be provided by a geosynthetic material when this unbound aggregate stabilization occurs.

“The interaction between the soil aggregate / different types of geosynthetics was something we had to capture, because the aggregate movement retrained by the geosynthetic increases load-bearing capacity and the life of the pavement,” Tutumluer said. “But how can we measure this interaction and associated stiffness increase that creates stabilization?”

Photo Credit: Han Wang

 For Tutumluer, the answer lies with bender-element sensors, a seismic testing technology that he and his students have pioneered the use of in pavement applications. The sensors send a shear wave through an aggregate medium as a test specimen and measure its speed, allowing researchers to evaluate a pavement layer’s stiffness and strength.

Tutumluer’s team evaluated 12 types of geosynthetics, including geogrids and geotextiles, to quantify their performance with six aggregate materials common to Illinois. They used laboratory triaxial testing, which predicts a soil’s stability by testing its shear strength, equipped with bender-element sensors to evaluate their interactions.

To compare how the geosynthetics would perform while in use, they built an unbound aggregate layer in a large-scale test bed to evaluate the four best-performing geosynthetics. The researchers applied loads to the test bed and used sensors to measure changes in shear wave speed to quantify the geosynthetic stiffening of stabilized aggregates.

Photo Credit: Han Wang

Their findings allowed them to categorize the tested geosynthetics into three tiers based on their ability to enhance the stiffness, or modulus, of typical pavement structures.

“From all the geosynthetics tested, we found three performance categories: one that was the best overall, one that was average, and then one that was not as good as the others,” Shoup said. “We (IDOT) can use that knowledge to help us with our specifications and what materials to use for stabilizing aggregates with geosynthetics.”

Tutumluer’s team created proposed guidelines for IDOT to design and construct underlying pavement layers like subgrades and subbases using geosynthetics. Their recommendations include when to select various geosynthetic materials as well as their benefits and guidance on when to reduce the thickness of aggregate layers.

Allowing engineers to construct thinner aggregate layers when using certain geosynthetics will help lower construction costs while optimizing performance.