Evaluation of Light Falling Weight Deflectometer for In Situ Measurement of Secondary Deformation Modulus of Various Forest Road Pavements

Roads in forests are necessary for proper forest management and active protection of the natural environment. They facilitate tourism and recreation and have a very important function in firefighting. The cost of building roads in forest areas is considerable, even when relatively cheap materials such as aggregates of natural or anthropogenic origin are used. Therefore, any road investment must be well prepared and executed. Bearing capacity and compaction are among the most important and frequently used geotechnical parameters in road construction. The aim of this study was to determine the possibility of predicting the value of the secondary deformation modulus E2 (obtained from measurements with a static plate load test – PLT) based on measurements with a light falling weight deflectometer (LFWD) Zorn, type ZFG 3000 GPS with a drop weight of 10 kg. The regression analysis included 245 results of bearing capacity measurements carried out on 46 forest road sections with various road pavements. Different regression models were tested: linear, logarithmic, polynomial, exponential and power models, excluding polynomials of fourth and higher degree. Prediction of E2 (PLT) values from dynamic deformation modulus Evd (LFWD) values is possible. However, the reported unsatisfactory strength of the relationship between the two parameters is associated with a high risk of error (r=0.73, R2=0.54, Se=80.37 MN·m-2). Neither the use of more complex non-linear regression models nor the use of multiple regression by introducing an additional estimator in the form of the s/v ratio significantly improved the estimation results. The quality of the prediction of E2 values is not constant. It varies depending on the type of forest road pavements, the use of geosynthetic pavement reinforcements and the type of road subgrade. The study also found that the quality of E2 prediction can be improved by limiting the range of tested Evd values upwards. It is advisable to continue this type of research, as the results obtained could be the basis for developing national standards for the application of LFWD to control the bearing capacity and compaction of forest road surfaces in the future.