Metrological Assessment of Three German-type Light Falling Weight Deflectometers for Evaluating the Load-bearing Capacity of a Forest Road with a Crushed-stone Surface

Rational forest management requires an efficient road infrastructure that provides safe access to forest stands and supports non-productive functions. The technical condition and bearing capacity of forest roads are crucial for timber harvesting logistics and emergency response, as confirmed by numerous studies. With increasing demand for fast and reliable quality assessment methods, lightweight falling weight deflectometers (LFWDs) are an important complement to static plate load tests. However, structural differences between devices highlight the need for objective metrological evaluation. The aim of this study was to analyse the consistency and usefulness of three German LFWDs in assessing the bearing capacity of crushed-stone forest roads. Two diagnostic parameters were evaluated: the dynamic deformation modulus (Evd) and the deformation rate index (s/v). The results revealed statistically significant differences between the devices, affecting both mean values and measurement variability. The ZFG 3000 GPS (10 and 15 kg) exhibited the highest measurement consistency, whereas the HMP LFG 4 and especially the Terratest 4000 Stream produced more variable Evd and s/v values. Parametric and non-parametric analyses confirmed the lack of metrological equivalence between the devices (p < 0.001). The findings indicate that interpreting LFWD data requires the use of homogeneous device types and further standardisation of measurement procedures, while combined analysis of Evd and s/v can support a comprehensive assessment of forest road bearing capacity.