# Experimental Study on Hydraulic Fracturing of High Asphalt Concrete Core Rock-Fill Dam
However, [PERSON] [12] used uniform samples to simulate hydraulic fracturing under the above conditions, but hydraulic fracturing did not occur, which indicates that these conditions were not necessarily sufficient. Furthermore, during the study of the failure of Wister dam, [PERSON] [13] noticed that concentrated leaks could enter cracks in the impervious core, as the reservoir level is rising only a meter or two above the elevation of the entrance to the crack. This shows that no significant excess pressure is needed to initiate the fracturing action. It is considered that all impervious sections of embankment dams contain a multitude of cracks (fissures), which are formed through either drying during construction, deformation under wheels of compact equipment, or the incomplete bonding of layers. These cracks are squeezed shut by the weight of the overlying embankment but they remain nevertheless as closed cracks in the fabric of the compacted soil that the reservoir water can enter when the pressure is greater than the earth pressure, starting the hydraulic fracturing process. After the initial penetration of the water into these centimeter-sized, closed cracks or fissures at the upstream face of the impervious section, the water pressure starts to act on the surface of least resistance (which has total stress acting on it less than the water pressure) and the advancing water jacks open a crack (fissure) that had not existed before the reservoir was raised. Therefore, it has been speculated that internal defects of the impervious soil (such as closed cracks) may also be a precondition for the occurrence of hydraulic fracturing. Based on this conjecture, [PERSON] [14] has developed a soil sample containing an infiltrated weak surface in a hydraulic fracturing test. The test showed that if there is an initial infiltrated weak surface in the soil sample, the applied water pressure will induce hydraulic fracturing. Moreover, [PERSON] believed that in addition to the arching effect of rock-fill to the core wall, the defect factors can lead to hydraulic fracturing of the core wall, such as the infiltrated weak surface that may exist in the core wall, and the wedge effect of water pressure that is due to the rapid impounding of the reservoir. The properties of asphalt concrete core walls are quite different from those of clay core walls. In comparison to an earth core with a water content of 10% and a voids content between 10 and 20% vol, there is no system of pores (less than 3%) inside the asphalt concrete core wall that would allow the entrance of water and thus the generation of a pore pressure. In the earth's core, the fluid medium of water is present from the beginning and has a very low viscosity. However, the \"fluid medium\" of bitumen in the asphalt concrete core at normal temperatures has a viscosity so high that it does not behave like a fluid. Tests performed by Haas have shown that the bitumen, even at a temperature of 200 \({}^{\circ}\)C (which is well above temperatures possible in a core wall, around 10 \({}^{\circ}\)C) and under a pressure of 115 bar (equivalent to a head of 1150 m), does not move inside a hydraulic asphalt concrete specimen. At 10 \({}^{\circ}\)C bitumen, viscosity is hardly measurable. Since the main preconditions of pore water pressure and pressure transmission do not exist, the hydraulic fracturing of asphalt concrete can generally be ignored [15]. Despite the above fact, hydraulic fracturing tests have been carried out in the constructions of some high ACRDs in China due to cautious considerations, such as the Maopingxi dam and Quxue dam. In these tests, cylindrical models with diameters of 100-150 mm and heights of 200-250 mm were adopted and water pressures were applied to the inner cavity of the specimen to verify whether it will be cracked under project loading [16; 17] However, without considering the seepage characteristics in the process of hydraulic fracturing, these test results had little difference from those of failure tests by mechanical loading.