## 5 Discussions of Hydraulic Fracturing for Asphalt Cores and Earth Cores in Embankment Dams  
An asphalt core is a slender impervious wall with a width of 0.5-1.2 m in an embankment dam. It has to follow the deformations imposed by the embankment and foundation during construction, reservoir impounding, and (occasionally) earthquake shaking, without significantly losing watertightness. For impervious asphalt concrete without large strains, even when water pressure up to 7.5 MPa or 11.5 MPa was acting on the asphalt concrete, no hydraulic fracturing would take place [1; 2]. It was stated in ICMD Bulletin 84 [1], \"As, in comparison with earth cores, the main prerequisites are non-existent in the case of asphalt concrete, a similar effect to hydraulic fracture can be excluded in this instance. The result of these investigations and considerations also concurs with the behaviour of asphalt concrete facings where there has been no indication of a hydraulic fracture as yet, although there are considerably lower effective stresses in the asphaltic concrete layer of a few centimetres in comparison to the external water pressure\". However, when some cracks and/or large voids exist in the asphalt core caused by either severe earthquake shaking, large differential settlements during construction and operation, or poor construction quality, hydraulic fracturing might take place. If an asphalt core with deep cracks is significantly arched by stiff transition zones and rockfills in a narrow valley or a deep narrow trench, the displacements of the core are significantly restrained. The model test results under the vertical displacement restraint conditions suggested that low water pressure may trigger a fracturing through the deep cracks in the core. This may explain why no significant excess pore water pressure was needed to trigger fracturing in the earth core even though the tensile strains that cracked the compacted earth materials were 10 or 100 times less than those for asphalt concrete [25; 26; 27]. In such a case, the asphalt core should be designed to be more flexible to reduce tensile stresses in front of the cracks. This is also meaningful for earth cores in dams. The reservoir impounding level has to be slowly raised to obtain a balance of pore water pressure in the earth core. Otherwise, hydraulic fracturing or \"punch through\" will occur in the earth core. If large differential settlements between the upstream side and the downstream face of the asphalt core is predicted, the asphalt core has to be widened and made flexible to reduce the possibility of having cracks or dilations occur in the face layer of the core. The model test and numerical back analysis results for the asphalt specimens under the vertical displacement unrestraint conditions document that low water pressure through cracks can cause the asphalt concrete to fail. The water pressure caused large tensile strains in the crack front area and made it fail. The fracturing of the asphalt specimen was due to large tensile strains. This is in agreement with the findings in the literature [28]. The calculated extensional strains at failure in the centre part of the specimens were about 6% at 5 \({}^{\circ}\)C and about 10% at 20 \({}^{\circ}\)C, respectively. The direct tensile strains at failure were about 1.5-4.0% in the temperature range of 5-20 \({}^{\circ}\)C [12; 23; 24; 29]. The extensional strains at failure were about two to four times that of the direct tensile strains at failure for the asphalt concrete materials. Air porosities of 5% and 8% for the asphalt concrete significantly reduced the extensional strains at failure compared with that of the asphalt concrete with an air porosity of 1.0% (Figure 11). Pore water pressure in many of the voids distorted the asphalt concrete and caused cracks in the asphalt concrete, which contributed to the reduction in extensional strains at failure.