Detection Methods Of The Blasting Induced Damage Zone Based On Acoustic Parameters Of Rock Mass

Rock foundation excavation is one of the important links in the construction process of large hydropower projects, and drilling-and-blasting is a major means of rock excavation. While the excavated rock mass is crushed by the explosive energy, the remaining rock mass will inevitably be damaged under the action of the explosion load. It is the premise of the control of blasting induced damage to obtain the accurate blasting induced damage zone (BDZ) by field tests in engineering practices, and the BDZ detection based on acoustic test is the commonly used method recommended by the national standard.Combined with theoretical analysis, laboratory tests, and field tests in Baihetan hydropower station, the excavation blasting induced damage mechanism of rock slope is analyzed, the rock mass damage criterion based on P-wave rise time change ratio is built and the BDZ detection method based on P-wave rise time is proposed, and the improved acoustic transducer is invented, and the detection methods of the rock mass BDZ based on acoustic parameters are developed. The main research achievements are as follows.The partitioned extension mechanism of microcracks in rock of blasting induced damage is revealed. It is the necessary basis of understanding rock mass macroscopic damage to study the expansion mechanism of a microcrack. Under the action of explosive stress wave, the stress in the rock media will be in compression-shear or tensile-shear state with time order. In the field near blast hole, the microcrack extension is controlled by the compression-shear stress state; while the tensile-shear stress state plays a major role in microcrack extension when it is far from the blast hole; with the increasing of distance, microcrack extension is mainly controlled by tensile-shear stress state, which is primarily composed of a combination of radial tensile stress and tangential tensile stress.The rock mass BDZ detection method based on p-wave rise time is put forward. It is shown by the theoretical analysis and laboratory tests that influence of distance between the test hole wall and the acoustic transducer on the measured P-wave velocity is large enough to affect the accuracy of the test results; however, the measured P-wave rise time is nearly unaffected by the distance. Compared with the P-wave velocity, the P-wave rise time is more sensitive to the attenuation of the rock mass; and through analysis and comparison, it is considered more reasonable to take the change ratio as?10% to determine whether the rock mass is damaged based on P-wave rise time. Meanwhile, when the P-wave rise time change ratio is employed to detect the BDZ, the measured data are less variable and more stable than when the P-wave velocity is used.The improved acoustic transducer is invented. Because the traditional acoustic transducer cannot guarantee consistent coupling condition during field test, the measured P-wave amplitude cannot indicate the damage of rock mass well. Influence of coupling conditions on the P-wave amplitude is study by laboratory tests. The results show that, water coupling distance will affect the measured P-wave amplitude significantly; as a result, when the water coupling distance is relatively large, the measured P-wave amplitude cannot indicate the damage of rock mass attenuation completely. Meanwhile, coupling pressure also will affect the test result, thereby reducing the accuracy even correctness of the measured P-wave amplitude. The improved acoustic transducer can contact with the wall of the test hole in direct, and a device to control the coupling pressure is introduced in the acoustic transducer, thereby a consistent coupling condition can be strictly guaranteed during field test. As a result, it can be used to obtain the accurate P-wave amplitude.The testing techniques to improve the quality of acoustic waveform under broken rock mass conditions are put forward. When the rock mass is broken, water should be injected into the test hole to ensure the water coupling condition during field tests. It is shown by the laboratory tests that when the water flowing velocity changes in the test hole, the pulsating pressure will occur according to the momentum theorem. As a result, the acoustic signal received by the transducer will be affected by low-frequency interference. When a cushion made of low sonic impedance material is set above the acoustic transducer, the influence of low-frequency interference can be reduced. Moreover, when the acoustic waveform is influenced by the pulsating interference, the take-off point of the first waveform will be more obvious after filtering with an appropriate low-cut filtering parameter and the readability of the data will be enhanced as a result.