# Experimental Study on Hydraulic Fracturing of High Asphalt Concrete Core Rock-Fill Dam
The asphalt concrete core wall has low voids content (less than 3%) and there is no interconnected pore inside. Therefore, the seepage cannot be formed under the external water pressure. However, the voids content can be changed by shear stress. The tri-axial test results have shown that asphalt concrete sample tends to dilatancy with decreasing bitumen content and decreasing lateral pressure. When the bitumen content is increased to 8%, dilatation does not occur, even when at a lower confining pressure [18]. Generally, the voids content of the core wall asphalt concrete is less than 3% and the construction test result is generally between 2% and 3%. However, if dilatation occurs, the voids content will increase and when the dilatation is large it may exceed 3%, which is a critical value of hydraulic asphalt concrete in view of its imperviousness. In practice, the low voids content of the core wall may also be increased during operation. Finstertal ACRD in Austria is 150 m high, the core wall is 96 m high, and the wall thickness is 50 cm at the top, 70 cm at the bottom. The aggregate of the core wall asphalt concrete has a maximum particle size of 16 mm and the content of the B65 bitumen is 6.3%. During construction, observation instruments were installed at three elevations of the main section of the core wall to measure the thickness change. After four years of operation, the three thicknesses were found to be increased by 4.8 cm, 2.4 cm, and 4.0 cm, respectively [19]. An indoor model test was conducted to study the effect of the increase on core wall thickness. The model was a cylinder with a diameter of 76 cm and a height of 96 cm. An asphalt concrete slab with a thickness of 15 cm was placed in the cylinder to simulate the core wall and the upstream and downstream transition zones materials were filled on both sides of the asphalt concrete slab to simulate the real situation of the upstream and downstream. Water pressure was applied in the upstream transition zone and 3.5 MPa compressive stress was applied on the top of the asphalt concrete slab. The horizontal displacement of the core wall was simulated by adjusting the support baffle at the downstream side. This test reproduced the thickness increasing phenomenon. After the experiment, the asphalt concrete slab was taken out for slicing and the voids content of the thickened core wall were tested. It was discovered that the voids content increased; the maximum value was 4%. The voids content of the center of the wall were still less than 3%. Therefore, the boundary area of the asphalt concrete core wall could be affected by the dilatation and the voids content could increase. However, whether a hydraulic fracturing will occur under such dilatation, no further research could be found in references. With the increasing voids content of asphalt concrete on the upstream surface, the impounding water could seep into the core wall and injure its resistance to hydraulic fracturing. The maximum height of Quxue ACRD in China is 164.2 m, which seats in a narrow valley with a steep left abutment of 1 V: 0.33 H. The maximum height of the core is 132 m. According to the numerical stress analysis of the Quxue dam, it has been found that the shear stress level could reach as high as 0.67 in the left upper zone of the core wall during the full impounding period, where the local water pressure is 0.13 MPa. Having taken the wetting deformation and the rheological deformation of the rockfill into account, the minor principal stress in this area could appear to be in tension of about 0.12 MPa. Furthermore, according to the tri-axial test of the asphalt concrete of the Quxue core wall, as the minor principal compressive stress is less than 0.1 MPa and the shear stress level is greater than 0.65, the shear dilatation to the volumetric strain will occu when the voids content is greater than 3%. In this paper, we use a model test to study if under this shear dilatation the hydraulic fracturing occurs in the core wall of the Quxue ACRD.