It is well known that limiting rutting in pavements should consider the actual load-associated plastic (permanent) strains. Therefore, the Swedish transport administration have paid a particular attention to find a simple in-situ time saving technique that can be used to predict the permanent strains for unbound road materials to provide a powerful material assessment and pavement design tool. Such a design tool can be utilized to establish the risk level of permanent strains in the unbound road layer during the design and construction stages something which may result in an increase in pavement service life. To fulfill the objective of the study, cyclic (repeated) light weight deflectometer tests have been conducted at a test pit located at the backyard of VTI in Linköping. A silty sand subgrade soil has been chosen for testing. Initially, the selected subgrade soil was placed in the test pit and thoroughly compacted using a small vibrator, then, series of in-situ cyclic LWD were conducted at different ranges of water contents (namely 8%, 10% and 15%) and various applied stress levels (namely, 50 kPa, 100 kPa and 200 kPa).

Prediction models for accumulated permanent strains based on the cyclic LWD measurements are suggested in this study. These models correlate the measured accumulated permanent strains using the new developed LWD to the number of loading cycles, stress levels and water contents. It is expected that the suggested models will provide supporting data for an increased understanding of future pavements ruts. For the tested subgrade soil, the suggested models have showed good matching to the accumulated permanent strain (εp) data measured by the cyclic LWD tests except for the cases of p=100 kPa and 200 kPa at 15% water content due to the excessive accumulated permanent deformations reported for these cases.