Quantifying Improvements in Geogrid Stabilized Pavement Unbound Aggregate Layers
Advisor: Professor Erol Tutumluer
Abstract
Understanding the stiffness behavior of unbound aggregate layers in pavement foundations is fundamental
to mechanistic-empirical (ME) pavement design. This dissertation presents a comprehensive investigation
into the stress-dependent modulus characteristics of geogrid-stabilized and non-stabilized unbound
aggregate layers subjected to static and dynamic loading environments. Through a combination of fieldscale
experiments with embedded instrumentation, laboratory testing innovations, and numerical modeling,
this work addresses critical challenges in quantifying geogrid-induced mechanical improvements in
pavement systems.
The field component of this study was conducted along a 750-ft long road segment paved during the US-
20 reconstruction project in Elkhart County, Indiana. The road segment was divided into three 250-ft long
instrumented research test sections: a control section with no geogrid, Geogrid Section 1 (GG1) with the
geogrid installed at the base-subgrade interface, and Geogrid Section 2 (GG2) with the geogrid embedded
at mid-depth within the aggregate base. The geogrid used was biaxial punched and drawn type
polypropylene having a square aperture of 1.30 in. (33 mm). Each test section was instrumented with
Bender Element (BE) field sensors and earth pressure cells to measure shear wave velocity and vertical
subgrade stress, respectively. Additionally, repeated-load Automated Plate Load Testing (APLT) was
performed to capture resilient modulus and permanent deformation characteristics of the instrumented
pavement foundation layers. Field test results demonstrated that geogrid-stabilized sections exhibited a
measurable increase in resilient modulus, with modulus enhancement ratios ranging from 1.07 to 1.35
relative to the control. BE field sensor data indicated higher shear wave velocities measured, indicating
considerable local stiffness enhancement near geogrid locations, thus validating the formation of a
mechanically stabilized layer (MSL). The reduction in stress transmission to the subgrade in GG2 further
confirmed the role of geogrid placement in improving structural response. Seasonal monitoring over a 16-
month period showed that geogrid-stabilized sections maintained their stiffness profiles more effectively
than the control, particularly through freeze-thaw and spring-thaw transitions. These observations confirm
the role of geogrids in providing long-term structural benefits under the influence of traffic and
environmental loading.
To enable BE field sensing under dynamic field conditions, a laboratory-scale testing program was
developed using a redesigned BE sensor frame and a synchronized data acquisition protocol. Tests were
conducted on control and geogrid-stabilized specimens using standard aggregate base layer material and
included both static monotonic and repeatedly applied deviator stress applications. Dynamic testing
revealed that while static loading captured stress-hardening effects, the dynamic BE measurements can
detect real-time stiffness fluctuations associated with traffic-simulated loading. In geogrid-stabilized
specimens, the difference in stiffness profiles observed between loading and rest phases was minimal,
highlighting the stabilizing influence of the geogrid-aggregate interlock.
The final phase of the research developed a base sublayering approach for modulus backcalculation using
the recently developed U.S. Army Corps’ flexible pavement finite element analysis program C-FLEX.
APLT field-measured data were used to calibrate the ME Pavement Design Guide (MEPDG) resilient
modulus model parameters (k₁, k₂, k₃), which were then applied in a piecewise linear distribution across the
depth of base layer with sublayer-specific adjustment factors (α, β, γ). This was done to account for the
localized influence of the geogrid. The developed sublayered modeling approach demonstrated that
stiffness gains near the geogrid could be attributed not only to an increased constant stiffness model
parameter (k₁) but also to reduced stress sensitive model parameters (k₂, k₃), an important distinction that
modulus-only adjustments factors alone fail to capture. Model predictions aligned well with APLT
measured deflection basins, validating the proposed framework. The model was then implemented to
analyze a range of pavement configurations for varying traffic loading considerations. The results indicate
that the relative benefits of geogrid stabilization are more pronounced in thinner pavements, such as lowvolume
roads, where geogrid mechanical stabilization directly influences stress and strain distributions
within the aggregate base.
The field instrumentation data collected and analyzed in this research study clearly demonstrates the
feasibility of quantifying stiffness changes in geogrid-stabilized aggregate materials and provide a pathway
for integrating sensor-based measurements into ME pavement design and analysis. The findings support
the development of geogrid-specific modulus adjustment protocols and enable data informed design
practices that incorporate local stiffness improvements into pavement foundation design. This work
represents a significant step forward in bridging field measurement technologies, laboratory investigations,
and computational modeling to enhance the design and performance prediction of stabilized pavement
foundations.