An Accuracy Problem of the Fwd Time History with Regard to the Application of the Sasw Method

A Falling Weight Deflectometer (FWD) represents one group of the devices used for diagnostics of pavement bearing capacity. Usually, the FWD dynamic load is substituted by a static load in the evaluation process to determine the equivalent modulus of a pavement structure or modulus of pavement layers. However, the data recorded during a bearing capacity test by FWD can be used to reveal the propagation of a dynamic impulse generated by FWD. It gives a possibility to use them in an evaluation method based on the propagation of waves generated by dynamic impulse. Therefore, the FWD time history data was assessed with regard to possible using in the method of the Spectral Analysis of Surface Waves. Basically, the possibility to determine the velocity of a generated surface wave was evaluated. It was found out the same deflection values exist at consecutive time intervals in the relevant part of the time history data (the arrival of the front of the wave or the area of maximum deflection value). Two methods were used to determine the exact time of the wave occurrence at receivers. It was concluded the differences between the used methods exist. It means the calculated velocities of a wave and shear modulus are also different. Importance of the shear modulus differences were estimated using the Slovak bearing capacity classification based on elastic modulus values. Taking into account the range of modulus in one classification class it can be stated the differences in the shear modulus determined according to used two methods could be very significant if the values calculated for short distance of the receiver are used. In the case of longer distance of the receivers the differences are not so high and significant.