# Local Stiffness Assessment of Geogrid-Stabilized Unbound Aggregates in a Large-Scale Testbed
## 1 Introduction  
Geogrids have been widely used in flexible pavements and unpaved roads for an increased confinement by immobilizing unbound aggregates, increasing the bearing capacity of the layered system, and preventing excessive rutting. A geogrid, a form of geosynthetics, is typically a sheet featuring apertures of various shapes and is usually constructed from, or incorporates, polymeric materials [1]. When used in pavement unbound layers, geogrids could improve serviceability and extend service life. The geogrid functions performed on roads include reinforcement and stabilization. Geogrid reinforcement increases the strength and/or reduces the deformation of a material by carrying tensile forces that the material alone would not carry [2]. The effect of geogrid reinforcement on roads, embankments, or slopes has been extensively studied for the design of geogrid-reinforced earth structures [3; 4; 5]. The geogrid-stabilization function is mobilized by forming a geosynthetic/soil composite material, e.g., a geogrid-aggregate system that is less deformable than the soil [2]. By installing geogrids in unbound aggregate base/subbase layers, aggregate particles are laterally restrained under wheel loading. This is often referred to as a geogrid-aggregate \"interlock\" mechanism. Such mechanical stabilization creates a stiffened zone in the geogrid's vicinity. Previous research consistently exhibited reduced pavement surface deformations when geogrids were used to stabilize aggregate layers [6; 7; 8; 9; 10; 11]. Various research efforts have evaluated the stiffening effect brought by the geogrid-aggregate interlock. Dynamic cone penetrometer (DCP) tests have demonstrated the stabilization function of the geogrid as more blows are required to reach a certain depth when the granular material is stabilized with a geogrid [12]. Pullout tests have also been conducted to study the load-transfer mechanism and the pullout strength of the geogrid [13; 14; 15; 16; 17]. In addition, the stiffness of the soil-geosynthetic composite was evaluated through geosynthetic composite stiffness test using a large-scale pullout test device [18; 19]. Several research efforts have also utilized falling-weight deflectometer (FWD) and light-weight deflectometer (LWD) tests to examine the influence of geogrids, but the findings were generally inconsistent and dependable on the experimental conditions [8; 20; 21]. However, numerical studies using discrete and finite element methods (DEM and FEM, respectively) indicated that there is a stiffened zone in the vicinity of the geogrid [22; 23; 24; 25; 26]. Further, large-scale field and laboratory test section studies were commonly conducted to demonstrate the benefits of geogrids, which require significant research investments [8; 11; 21]. In addition to the evaluation techniques mentioned above, a direct quantification of the increase in modulus or stiffening effect, and the extent of the influence zone around a geogrid, are of immense interest to effectively design pavements with geogrids. A recent approach used at the University of Illinois has utilized shear wave transducers to quantify the stiffness enhancement brought by the geogrid and the influence zone. Shear wave transducers, in the form of bender elements (BE), have been widely used in laboratory settings to evaluate the stiffness characteristics of geomaterials, such as soils and aggregates, due to the good coupling between the sensor and the geomaterial [27]. Recent studies have proven that BE sensors can successfully quantify the local enhancement due to the geogrid-aggregate interlocking mechanism, both in laboratory and field settings [28; 29; 30; 31]. The objective of this paper is to quantify the stiffening effect and the extent of the stiffened zone due to mechanical stabilization by geogrids installed in unbound aggregate base courses.