Abstract：Ultrasonic testing is one of the most frequently used measurements in geotechnical rock mass evaluation. The square of the ratio of longitudinal wave (P-wave) velocity of rock mass to that of rock block is defined as intactness index of rock mass. As we know，the P-wave velocity of in-situ rock mass is believed to be lower than that of relevant rock block. The more broken the rock mass is，the lower its P-wave velocity is than that of rock block. However，according to the result of ultrasonic testing carried out at Runyang Highway Bridge，it is surprised to find that most P-wave velocities of rock mass are higher than that of rock block. Many different measurements are applied in the comparative research，which is based on hundreds of rock cores collected from over 40 boreholes in the bed rock of the Bridge site，and the reliability and causes of the results are analyzed. The concepts of rock core unloading disturbance and unloading-sensitive rock mass are then put forward，with relevant engineering countermeasures.
