During tunnelling trials in the Lower Chalk at Chin-nor, Oxfordshire, two techniques for assessing rock quality based on measurement of P-wave velocity were evaluated. Ultrasonic pulse velocity measurements at 54 kHz were made over 0.2 m path lengths at the tunnel face and the conventional hammer seismograph method was used over 10 m path lengths along the tunnel side wall.
The ultrasonic measurements were obtained by placing transducers on the flat surfaces of circular grooves cut by the tunnel boring machine picks and provided an excellent method for in-situ evaluation of local variations in the mechanical characteristics of the chalk. In order to obtain the relation between acoustic velocity and geotechnical properties of the chalk in this region of complex fissuring, it was necessary to compare average values of velocity and chalk properties for successive readings over a distance of 1.6 m which is equivalent to ten times the mean distance between fissures. Linear relations were found for acoustic velocity versus both chalk strength and discontinuity spacing for the range of chalk variability that was encountered.
P-wave velocities from the hammer seismograph observations over 10 m path lengths showed considerably less scatter than the ultrasonic velocities because such a large number of discontinuities was included in each path length but a significant change in values from the range 1.08-1.27 km/s to the range 1.45-1.64 km/s occurred some 35-40 m from the tunnel portal. This increase is directly associated with the change in ground surface level and consequent increase in overburden stress. Moreover, the velocity ceased to increase significantly at an overburden stress of 0.4 MN/m2, a level that corresponds well with the stress at which a discontinuity became acoustically closed in laboratory tests. The measurements emphasize the fact that, when the P-wave velocity is used as an indicator of rock quality near the ground surface, it is important to take the effect of overburden stress into account.
- © The Geological Society, London 1980