Refraction seismics
Case study: Seismic investigation for the distinction of soil classes

During the construction of trenches along roads in unconsolidated and hard rock it is necessary to distinguish between soil class 6 (easy to fracture, use of excavators) and soil class 7 (hard to fracture, blasting is necessary) for the planning of an efficient use of equipment and the determination of the amount of hard rock to remove.

Seismic refraction surveys with small source and receiver spacings in combination with a tomographic travel time inversion of first breaks will result in the distinction between different soil classes. The result of this method is a depth section containing the wave velocities of the subsurface along the survey line. With the resulting velocity distribution it is possible to draw conclusions about the type of rock in the subsoil. The classification of the subsurface into soil classes is done by the velocity of the compression waves. The simultaneous determination of shear wave velocities and bulk densities will allow to calculate dynamic moduli used in geotechnics.

Figure 2 shows a part (150 m length, depth below ground level 10 m) of the velocity-depth section as the result of the tomographic inversion. Generally the velocities are increasing with depth which corresponds to the increasing consolidation of the soil with depth. The unconsolidated sediments with velocities lower than 600 m/s are clearly outlined. The abrupt rise of velocity characterise the transition from loose rock to hard rock. Within the hard rock the velocity is evenly increasing, representing the changes of the degree of alteration (deconsolidated to partially weathered). Due to the drilling results and in conjunction with the ZTVE-StB 94 velocities above 1700 m/s were assigned to soil class 7.

Fig. 1: View along the seismic refraction survey

Fig. 2: Velocity distribution in depth