The damping ratio in the condition "0" for the slabs P1, P2, P3 and P5 was approximately constant for the amplitude of vibrations between 0.001 mm and 0.01 mm and varied between 0.8% and 2.5%, see Fig 8a. For the slabs characterised by the smaller stiffness of the contact layer, i.e. the slabs P4 and P6, the factor ζ, for the amplitudes of between 0.001 mm to 0.01 mm, was also assuming approximately a constant value between 0.8% and 2.2%. Then, the value ζ for the amplitudes of between 0.01 mm and 0.15 mm was growing to over 5% for the slabs P4 and P6, and suddenly started to fall after reaching a maximum value, see Fig. 8a