Effects of Separation Geotextiles in Unpaved Forest Roads on Control Measurements Using the Light Weight Deflectometer

Geosynthetics are increasingly used in forest road construction for their potential to improve structural performance and reduce material consumption. However, little is known about their influence on dynamic modulus measurements derived from Light Weight Deflectometer (LWD) testing. This study investigates how different geotextiles affect stiffness measurements immediately after construction and two years later. Five reconstructed forest roads in the Czech Republic were divided into control and geotextile-reinforced subsections (PP150 with geogrid, PP200, and PP800). Modulus differences between the surface and subgrade (S–SG) and differences after two years (S2–SG) were analysed using permutation ANOVA, Cohen’s d, and linear mixed-effects models. The results showed significant short-term reductions in measured modulus for PP150 and PP800, which diminished over time. Only PP800 maintained a strong effect at the two-year mark. Interaction effects with base material types revealed potentially adverse synergies, particularly between PP800 and vibrated gravel. These findings suggest that as LWD is commonly used during road construction for quality control, these early misleading readings may lead to unnecessary over-compaction or increased layer thicknesses, resulting in elevated construction costs and a higher carbon footprint, which counteract the sustainability goals often associated with geosynthetic use. The study highlights the need for long-term monitoring and method refinement in evaluating geosynthetic-reinforced unpaved roads.