Additional site investigations Study on long-term behavior of rock PhD project at Technische Universität München Chair of Engineering Geology Carola Wieser, Dr. Heiko Käsling hours / days / weeks

Background  Research project ABROCK  Improvement of penetration prediction model for tunnel boring machines (TBM) in hard rocks  Rock excavation largely dependent on rock strength

Background  Laboratory rock strength ≠ Rock strength at the tunnel face Rock excavation results in  relaxation of the rock mass  redistribution of stresses  deformation and cracking of the rock  time-dependent changes in rock strength → Degradation of rock quality before laboratory tests are performed

Site investigations Sampling: – m PH m SECTION, VIEW TO THE EAST

Site investigations  Sampling immediately after drilling  Sample preparation within 1 hour Sampling PH m

 P-wave velocity detection in 45° angles around the sample in 3 to 4 measurement planes and along the core axis Methodology Ultrasonic Testing

 AE = elastic waves caused by the release of stress energy (cracking)  Acoustic Emission Testing with 8 sensors attached to the sample surface Methodology Acoustic Emission Testing Data aquisition system Pre-amplifier Drill core + Sensors

Results: P-wave velocity Sample 1:-1242 m Sample 2: m  Rapid decrease in v P after drilling  Stable value after about 100 – 450 hours v P II core axis v P schistosity T

 v P is higher parallel than perpendicular to the schistosity  Strong variation in v P in the measuring planes (A / B)  Anisotropy within the measuring planes P-wave velocity v P core axis v P II schistosity B A A B T m

Acoustic Emission  Rapid increase of AE after drilling  Almost stable value after several days Event rate m m

Acoustic Emission Event rate mp-wave velocity

1 hour after drilling 147 hours after drilling Acoustic Emission Event localization

1 hour after drilling v P ≈ m/s 147 hours after drilling v P ≈ m/s Acoustic Emission Localization error

Ongoing work  received samples after core logging  ultrasonic measurements  uniaxial compression tests  including acoustic emission measurements during loading  try to gain the Kaiser effect for estimating primary stress level before core extraction  thinsection analysis

Conclusion  v P is reduced considerably within a few hours (up to > 1 km/s)  v P reaches a stable value after about 100 – 450 hours  Stable value is different for different samples  v P varies within the samples (up to > 2 km/s)  v P is highest perpendicular to the schistosity and lowest parallel P-wave velocity

Conclusion  v P is reduced considerably within a few hours (up to > 1 km/s)  v P reaches a stable value after about 100 – 450 hours  Stable value is different for different samples  v P varies within the samples (up to > 2 km/s)  v P is highest perpendicular to the schistosity and lowest parallel  Rocks are anisotropic even if schistosity is not clearly visible! P-wave velocity

Conclusion  Event rate is reduced considerably within a few hours  Event rate will probably reach a stable value after several days  3D event localization is not possible due to anisotropy of the rocks Acoustic Emission

Conclusion  Rock mass degradation takes hours to days  Ends up about 100 to 450 hours after core extraction from the drilling  Rock strength determined in the lab (days/weeks after drilling) may be lower than in situ  Reason might be degradation during extraction of the rock core due to high primary stress