# Experimental Study on Hydraulic Fracturing of High Asphalt Concrete Core Rock-Fill Dam
## 3 Model Test with Press Sensor
### Test Parameters and Procedure  
The temperature during the test was 20 \({}^{\circ}\)C. The voids contents of the specimen were uniformly 3.0%, 3.4%, 3.5%, 4.0%, 5.2% etc. The applied water pressure was 0.13 MPa in the reserved hole of the specimen and stabilized during the test using a manual pump. The 0.13 MPa water pressure generated a 26 N upward force in the \(\Phi\) 16 mm hole of the specimen. The compressive forces on the upper surface of the specimen are provided in Figures 8-12. Figure 8: Test process for a specimen with a voids content of 3.4%. Figure 7: Cross-section of the model test device with pressure sensor. Figure 11: Test process for the specimen with a voids content of 4.0%. Figure 10: Fracture appearance of remains on the bottom steel plate. Figure 9: Test process for the specimen with a voids content of 3.5%. For the specimen with a voids content of 3.4%, the test process can be divided into two stages (Figure 8). The first is the steady process, where the force value gradually increased from 26 N to 131 N in 77.7 h, which afterward the specimen was still intact. In the second stage of the rapid fracturing, the force value increased rapidly to 1021 N within 23 h, wherein hydraulic fracturing damage occurred. If a point with maximum curvature was defined as the critical point of the test process curve, the critical time was 77.7 h and the critical force was 131 N. The inspection after the test showed that the specimen was horizontally broken; 2 to 3 mm thick remains stuck to the lower steel plate. In the first stage, for specimens with a voids content of 3.5%, the compressive force was increased from 49 N to 152 N in 0.56 h and the specimen was not damaged. In the second stage, the pressure value increased rapidly. After an up-and-down fluctuation, the compressive force increased sharply at 3.2 h from 327 N to 1014 N in less than 20 min and hydraulic fracturing occurred (as shown in Figure 9). The critical time was 0.56 h and the critical force was 152 N. Moreover, the inspection after the test showed that the specimen was horizontally broken. More importantly, there were 2 to 3 mm thick remains stuck to the lower steel plate. A more careful inspection to the fracture's appearance showed less marks of coarse aggregate on the surface remains of the 3.5% voids content than those of the voids content of 3.4% (Figure 10). This may explain the less critical time of this test, which compared the voids content of 3.4% to the test. Similarly to the test with a 3.4% voids content, the test with a 4.0% voids content had a compressive force increase to 1021 N at 56.9 h. The critical time was 43.3 h and the critical force was 93 N, as shown in Figure 11.  
In the test with a 5.2% voids content, the specimen was quickly broken in 1.8 h. There was no critical point in the whole process, as shown in Figure 12.  
The voids content of 3.0% was a critical value to the impervious asphalt concrete. For the test specimen with a voids content of 3.0%, the initial compressive force fluctuated greatly and gradually stabilized after 187 h. The stable value stayed around 30 N, as shown in Figure 13. During the first period of fluctuation, the maximum compressive force was 99 N and the minimum was 19 N. The test with a 3% voids content lasted for 477 h (nearly 20 days) and terminated with no sign of hydraulic fracturing. During the installation of the specimen before the test, the bolt nuts were intentionally screwed more tightly in order to give 50 N of force value. This ensured the full contact of the specimen to the pressure sensor, causing the initial fluctuation of force in the test for 187 h.  
Figure 12: Test process of the specimen with a voids content of 5.2%. The relationships between the critical time (t\({}_{\text{c}}\)) or force (F\({}_{\text{c}}\)) and the voids content are given in Figures 14 and 15, and Table 3, from which the larger of the voids content and the smaller of the critical time or force.