###### Abstract  
Hydraulic asphalt concrete is virtually impervious. Hydraulic fracturing due to the generation of pore water pressure can be generally excluded for asphalt facings and asphalt cores in embankment dams. However, when some cracks and/or large voids exist in the asphalt core in dams, hydraulic fracturing could take place during reservoir impounding. Cracks and/or large voids may be caused by either earthquake shaking, large differential settlements during construction and operation, or poor construction quality. Therefore, asphalt specimens with either cracks of different depths or large air porosity were prepared, and a model test apparatus was developed to investigate the possibility of hydraulic fracturing. Model tests were conducted on cylindrical asphalt concrete specimens of 100 mm in diameter and 180 mm in height. The top boundary of the specimens was either prevented (restrained) from moving in the vertical direction or free to move (unrestrained). The model test results for asphalt concrete with cracks under unrestraint conditions were numerically back-analysed. The model test results indicated that the cracks in the asphalt concrete under the restraint conditions could experience an \"open-close\" progress with increasing water pressure. That suggested that if the asphalt concrete were long, low water pressure would trigger a fracturing through the cracks. Under the unrestraint conditions, low water pressure could cause the asphalt specimens with either cracks or large air porosity to fail. The extensional strains at failure were reduced, and the reductions depended on the magnitude of the crack depth or air porosity. Suggestions are given for designing the asphalt core dams to reduce the possibility of the occurrence of cracks and/or dilations in the core, especially for dams in unfavourable geological and topographical conditions. A specialty contractor is preferable to do the asphalt core job to rule out the possibility of a weak bond between the layers and an air porosity larger than 3.0% in the core. embankment dam; asphalt core; hydraulic fracturing; crack; large air porosity; reservoir impounding; model test; suggestion 202331352333/2023/20  
is virtually impervious because the aggregates are \"supersaturated\" with bitumen and because the air porosity is less than 3.0%. The bitumen viscidity is very high in the asphalt concrete under normal conditions. The bitumen reaction inside the asphalt concrete to water pressure could be determined only at a temperature of 40 \({}^{\circ}\)C and under a pressure level of 7.5 MPa (750 m water head) [1]. It can be inferred that there would be no reaction of the bitumen to water pressure in the realistic temperature range of 5-25 \({}^{\circ}\)C and a reservoir water pressure of up to 3.0 MPa for the asphalt core in very high embankment dams. Therefore, the only preconditions for hydraulic fracturing to take place for an asphalt core in dams are that some cracks exist and/or a large air porosity exists in the core. The cracks and/or large air porosity may be caused by either earthquake shaking, large differential settlements during construction and operation, or poor construction quality. None of the asphalt core embankment dams built so far have been subjected to severe earthquake shaking that can cause cracks in the asphalt core. However, shaking table tests, dynamic centrifuge model tests, and seismic analyses on asphalt core embankment dams did show that the permanent differential deformations between the core and the adjacent transition zones were large during severe earthquake shaking [6; 7; 8; 9]. Cracks may occur in the asphalt core because of the reciprocation bending motions of the top asphalt core wall during severe earthquake shaking. [PERSON] et al. [10] conducted a risk analysis on asphalt core embankment dams when the core was sheared off and concluded that the asphalt core behaviour was likely to prevent a catastrophic failure even in the event of an extreme earthquake.