## 3 Test Results  
### Mechanical Characteristics and Acoustic Response  
Figure 4 shows the time-stress-AE curves of two kinds of samples, and the AE data include AE count, signal amplitude, and energy. The main results are as follows:  
(1) Compared to the cavern-free sample (peak strength 19.38 MPa), the cavern-existing sample shows a smaller peak strength (14.38 MPa, 25.82% reduction). The invisible cavern illustrates a strong negative impact on the strength of the tunnel-surrounding rock.  
(2) The AE events densely occur at 275-315 s in the cavern-free sample, the maximum AE count during this period approaches 25,000, and the corresponding peak signal energy is over 10\({}^{3}\) J; the above evidence proves that the typical local failures appear in the rock. AE events maintain a calm period in the following 80 s. The high-energy-intensive AE events occur again from 400 s and continue until the test end; the highest AE signal amplitude (almost 99 mV) is obtained in this period, and the average amplitude is over 50 mV. Therefore, the initiation of local failure is calibrated at about 275 s (z-axis loading is about 8 MPa), and the dominant secondary failure starts at about 400 s (z-axis loading is about 15.5 MPa).  
(3) The relatively high-energy AE events (peak energy exceeding 20\(\times\) 10\({}^{3}\) J) start at 267 s in the cavern-existing sample; the obvious stress fluctuation appears at 280-310 s, the signal amplitudes are relatively high, while the AE counts and signal energy remain low. The above evidence indicates that the typical secondary failure appears in the rock, which can be recognized as a local failure because the loading curve keeps a stable increase in the following period. The maximum AE count occurs at 386 s (31,372 events), and  
Figure 3: True three-axis loading test machine and supporting monitoring system layout diagram: (a) the loading system and AE monitoring system in this test; (b) AE sensors are distributed along x-axis; (c) monitoring position of the internal camera.  
the second peak count occurs at 401.6 s (28,974 events), accompanied by the peak z-axis loading; the dominant secondary failure thus starts at 386 s and continues until the test end. In addition, the AE signal amplitudes show gradual decrease during the second round of preloading (125\(\sim\)150 s); the relatively low AE amplitude remain stable until the stress fluctuation appears; the above phenomena may cause by the asymmetric distribution of prefabricated cavern, which lead to the different levels of compactions on both sides of tunnel surrounding rock; therefore, the stable low-frequency signals did not appear in this sample.  
(4) Comparing the mechanical characteristics of two kinds of samples, the start time of local failure is almost the same and the initiation time of secondary failure is mainly controlled by the triaxial loading status and the material properties. The major difference between the two kinds of samples is the development speed of the dominant failure. The cavern-free sample takes about 75 s from the first stress fluctuation to the peak stress status, and several rounds of AE bursts (relatively high AE counts, energy, and amplitude) appear in this period. On the other hand, the dominant secondary failure lasts about 20 s in the cavern-existing sample, and the corresponding failure intensity is larger than that of the cavern-free sample. Subsequently, the fast-developing secondary failures induced by the adjacent cavern definitely present a non-negligible negative influence on the tunnel construction.