Aggressive Rehabilitation of Gravel Roads with and without Enzymatic Soil Stabilizers

Granular-surfaced roads built on weak subgrades or subjected to high traffic loads are susceptible to damage as the original roadway cross-section settles and spreads over time, which also reduces the effectiveness of the drainage ditches for removing water from the surface. Such roads may require significant maintenance yet still fail to reach their intended performance and lifespan. Their performance can be restored through aggressive rehabilitation, involving retrieving subgrade material from the ditches to rebuild the cross-sectional roadway and ditch profiles, while compacting and optionally stabilizing the materials to improve strength and stability. In the present study, four test sections were aggressively rehabilitated on a granular-surfaced road at a military base in Johnston, Iowa. The road experienced decades of high traffic from heavy vehicles, causing it to widen significantly beyond its original width of approximately 26 ft. This flattened the ditches and inverted the crown which retained water instead of draining efficiently. For the first two test sections, after aggressively rebuilding the profile, the surfacing aggregates and several inches of subgrade were treated with a liquid soil stabilizer derived from plant-based organic enzymes to improve strength and stiffness. Smooth-tired compaction and sheepsfoot compaction were used for the first and second sections, respectively. The third and fourth sections were also aggressively rebuilt and constructed with smooth tire compaction in one section and sheepsfoot compaction in the other, but without enzymatic stabilizer. A fifth section without any rehabilitation, stabilization, or maintenance was also monitored and tested as a control. Field Automated Dynamic Cone Penetrometer (ADCP) and Light Weight Deflectometer (LWD) tests were performed to measure the strength and stiffness of the road sections at five different construction stages over two winter-spring cycles: (1) before construction, (2) after construction in fall 2023, (3) after thawing in spring 2024, (4) in fall 2024, and (5) after thawing in spring 2025. The abrasion resistance of the roadway surfacing aggregates was also evaluated under simulated traffic loading using a laboratory gyratory compaction device. The gradation of the coarse fraction of surface material was analyzed before and after gyratory compaction tests using the Iowa DOT’s rapid Camsizer. The results of the tests showed a general improvement in the mechanical properties of the enzyme-stabilized sections compared to the untreated sections. The field California Bearing Ratio (CBR) values determined from correlations to ADCP tests and field LWD stiffness values both increased significantly over all time stages due to enzymatic stabilization, which can be attributed to increased cohesive bonding between the soil particles. These findings highlight the ability of aggressive rehabilitation to restore drainage ability, as well as the practical applicability and potential use of enzymatic chemical stabilizers to improve the performance of granular-surfaced roads.