## 3 Model Test Results  
### Model Test Results under Restraint Conditions  
When an asphalt core embankment dam is located in a narrow valley or a deep narrow trench is excavated along the central line of the asphalt core from the left bank to the right bank, the bottom of the core may be relatively restrained by the narrow valley and/or narrow trench owing to the arching effect of rockfill materials. The test was designed to investigate the possibility of hydraulic fracturing in the asphalt specimens with cracks or a large air porosity by increasing water pressure. The model setup is shown in Figure 1.  
Observations show that the temperatures in the asphalt cores inside embankment dams in the world are mostly in the range of 5-20 \({}^{\circ}\)C. Accordingly, the temperatures of 5 \({}^{\circ}\)C and 20 \({}^{\circ}\)C were selected for the tests. The bottom of the specimens was glued to the bottom of the steel mould (number 4 in Figure 1). The top 60 mm of the specimens was held by the steel sleeve, and the gap between the steel sleeve and the top of the steel mould was sealed with two rubble rings (number 6 in Figure 1). The side friction of the two rubble rings was calibrated, and the sliding friction force was to be about 80 N. Water pressure \(\sigma_{w}\) was increased step by step at 200 kPa per 0.5 h. The induced vertical force \(P\) at the specimen top was measured by the force sensor. The induced average vertical stress \(\sigma_{top}\) at the top of the specimen was the value of the measured vertical force \(P\) divided by the cross section area of the specimen with a diameter of 100 mm. Figure 2 shows the curves of applying water pressure \(\sigma_{w}\) and induced average vertical stress \(\sigma_{top}\) on the specimens with different ring crack depths versus time at 5 \({}^{\circ}\)C and 20 \({}^{\circ}\)C, respectively. Figure 1: Model tests for the asphalt specimen with (a) ring crack, (b) semicircle crack, and (c) air porosity of 8.0% under vertical displacement restraint conditions: 1� asphalt specimen; 2� ring crack; 3� pressure water; 4� glue binding; 5� steel sleeve; 6� rubber ring; 7� force sensor; 8� semicircle crack; 9� middle layer with large air porosity. Figure 2 shows that the relationship of the water pressure versus time was lineal with a regular \"saw thorn\" shape. The increase of 200 kPa of water pressure for each step loading was applied in a few seconds and then kept constant for half an hour. The specimen was subjected to 200 kPa of water pressure, which was quickly applied and was kept for half an hour in each step loading. Asphal concrete is a viscoelastic plastic material, and its behaviour is significantly time dependent and temperature dependent [20; 21; 22; 23]. The induced average vertical stress \(\sigma_{top}\) was smaller than the water pressure \(\sigma_{w}\). The stress \(\sigma_{top}\) was quickly increased and subsequently slowly increased with time thanks to the creep  
Figure 2: Curves of applying water pressure (200 kPa per 0.5 h) and induced average vertical stress at the top of the specimens with different ring crack depths vs. time at (a) 5 \({}^{\circ}\)C and (b) 20 \({}^{\circ}\)C, respectively. effect and stress redistributing in the asphalt concrete. That caused the \"saw thorn\" shape of the curve of the \(\sigma_{top}\), which was not as regular as that of the \(\sigma_{w}\).