________________ During mechanized tunnelling in hard rock, the construction ground always has surprises in store. Hardness and strength of the rock, the degree of fractures, fault zones and the karst may differ from the predicted values and information. Herrenknecht AG and the Helmholtz Centre Potsdam - German Research Centre for Geosciences (GFZ) have cooperated to develop a geophysical exploration system for early detection in the tunnel: ISP - Integrated Seismic Prediction.
Maintaining control of the construction ground with an eye on what lies ahead
A special feature of ISP is that the system can be installed on the TBM even after tunnelling has started. The high exploration reach fulfills the function of an early warning system for the TBM operations. Geological anomalies can be mastered quickly, investigated and verified at a very short distance using conventional methods. This eliminates some of the constantly required pre-drillings which lead to downtimes and thus high costs. ISP consists in principle of three modules:
Two pneumatically operated impact hammers are used as seismic sources to introduce energy into the rock. Accessibility to the rock is ensured by installing these on the grippers of an open hard rock machine or on an attachable console in the shield of a correspondingly prepared TBM. During rest phases in the tunnelling process when the cutting wheel is at standstill, during ring construction by a shield or after relocation of the grippers in an open TBM, the impact hammers are driven towards the tunnel wall, pressed against it and the impacts activated.
2. Receiver and data recording:
The seismic receivers consist of three-component geophones which are mounted at the tips of specially designed, reusable measuring anchors. Support pipes, which are stuck into the holes bored into the rock, provide for sufficient stability of the measuring anchors in the rock. Data recording itself is carried out using autonomous, battery-powered radio data loggers which are located directly on the measuring anchors. The data acquired are then sent to the computer unit for further processing.
3. Computer unit with steering, control and processing software:
A laptop with special system software is used as a computer unit which queries the navigation data and the machine status from the TBM's PLC. These data are necessary to reach a spatial definition of the overall measurement system with all components, for the impact release of the impact hammers and also for the correct subsequent processing and representation of the data.
The principle of wave conversion
By causing the tunnel wall to vibrate using an impact source, both space and surface waves (Rayleigh waves) are generated. The latter travel along a tunnel wall in the direction of the tunnel face where they are converted to an S wave (shear wave). If this wave comes up against an obstacle in the mountains, it is partially reflected. The relevant physical dimension here is the seismic impedance (acoustic impedance or wave impedance) which results from the product of the density of the sounded medium and the S wave velocity in each traversed zone.
A reflection of the S wave thus depends on a sufficient impedance contrast, which is to be expected in highly fractured and decomposed rock (faults) or in water-filled crevices or caves. The reflected S wave as such travels back to the tunnel face. There, it is again partially converted into a Rayleigh wave, which again travels along the tunnel surface where it can be registered by the geophones placed in the tunnel wall. This process is called RSSR wave motion.
The data loggers measure the respective travel times of the waves; this information is used to interpret the seismic signals using a special calculation procedure, which in turn makes it possible to determine where a wave originated.
The performance capacity of ISP
With an exploration range in the driving direction of 50 to 100 meters, and a maximum of 150 meters before the tunnel face, the resolution and/or position accuracy of the detected structures lies at 5 to 10 meters. Both water and air-filled cavities and geological faults or weakness zones in the rock can be detected by the system.
As with all geophysical exploration methods, the combination of different measurement techniques to verify suspected and/or detected facts is recommended. This means that, exploratory drills no longer need be carried out constantly, but only in a very targeted way.