# Local Stiffness Assessment of Geogrid-Stabilized Unbound Aggregates in a Large-Scale Testbed
## 6 Conclusions  
The quantification of the local stiffness and the geogrid-stiffened zone in the vicinity of the geogrid in geogrid-stabilized aggregate systems can provide important inputs for the mechanistic-empirical designs of flexible pavements. In this study, gender element (BE) field sensors were utilized to measure the local stiffness properties of unbound aggregates near an installed geogrid through shear wave velocity measurements. The study used a large-scale laboratory testbed with dense-graded crushed aggregates placed over a geofoam layer. Five static surcharge loads were applied to provide different confinement levels. Two punched and drawn geogrids with different triangular aperture sizes were installed, one at a time, near the bottom of the aggregate layer. On the other hand, the control sections were compacted without a geogrid. At the five surcharge load levels applied on the surface, shear wave signals were collected using a BE field sensor installed at three different heights above the geogrid. In addition, lightweight deflectometer (LWD) and dynamic cone penetrometer (DCP) tests were performed on the constructed aggregate layers. Further, a repeated load triaxial test instrumented with shear and compressional wave transducers was conducted to determine the Poisson's ratio of the aggregates for calculating the small-strain elastic modulus. Using the BE field sensor in the large-scale testbed, researchers successfully determined the small-strain modulus and the extent of the stiffened zone in the vicinity of both geogrids. Based on the small-strain modulus profile throughout the thickness of the test section, the study found that the geogrid-stiffened zone with up to 16.2% stiffness improvement in dry dense-graded aggregates extended between 15.2 cm (6 in.) and 25.4 cm (10 in.) above the geogrid. The extent of the geogrid-stiffened zone was from 8 to 13.3 times the geogrid aperture size. The results showed that for this type of dense-graded aggregate material, the geogrid with a smaller aperture size was more effective for geogrid stabilization. This is consistent with previous research, which used triaxial testing of BE-instrumented aggregate specimens. Further, DCP test results revealed that the geogrid-stabilized sections needed up to 26% more blow counts to penetrate to the geogrid location, and the section with the smaller aperture geogrid was stiffer than the one with the larger aperture. However, the moduli obtained from LWD tests conducted on the surface were inconclusive for quantifying the effect of geogrid stabilization, showing comparable average LWD moduli for geogrid-stabilized sections and the control sections, likely due to the insufficient influence range for detecting local stiffening effect near geogrid. In summary, this research study has shown that the BE field sensor and DCP tests are effective tools for determining the extent and magnitude of stiffening above the geogrid location, which is crucial in determining the mechanically stabilized layer profiles for mechanistic pavement analysis. The laboratory approach to quantify the geogrid stabilization effectiveness could eventually be adopted for in situ stiffness evaluation and the modulus input required for pavement design. Further investigation is recommended to evaluate the effectiveness of geogrid stabilization on different aggregate and geogrid types. Conceptualization, E.T., I.L.A.Q., J.S.T. and M.K. ; methodology, E.T., I.L.A.Q., J.S.T. and M.K. ; experimental investigation, M.K. and H.W. ; data analysis, M.K., E.T., and I.I.A.Q. ; writing--original draft preparation, M.K. and H.W. ; writing--review and editing, E.T. and I.I.A.Q. ; project administration, E.T. ; funding acquisition, E.T. All authors have read and agreed to the published version of the manuscript. This research was funded by the US Army Corps of Engineers under Contract No. W912 HZ19P0047, entitled, \"Development and Use of Bender Element and SmartRock Advanced Sensor Technologies for Field Instrumentation--ERDC SMART Runway Initiative (BAA 18-0104)\".