# Improvement Methods for Reduction of the High Stress of Ultra-High Asphal Concrete Core Dams
## 2 Investigation on the Shear Stress Feasibility of UACC  
### Shear Failure Control Standard of ACC  
Although the literature [7] indicates that the strength of asphalt concrete materials has a nonlinearity behavior, asphalt concrete materials are dispersive materials with obvious particle-like characteristics comparing with other homogeneous or hydrogel knot materials. The strength of asphalt concrete materials is mainly associated with the cohesion of asphalt, the internal friction resistance of aggregate, and the adhesion between aggregate and asphalt binder. Therefore, Mohr-Coulomb strength theory can be applied to asphalt concrete materials successfully [8; 25; 26], and it is considered that the shear strength of asphalt concrete materials obeys the following relationship:  
\[\tau=C+\sigma\cdot\tan\varphi \tag{1}\]  
where, \(C\) is the cohesion and \(\varphi\) is the internal friction angle.  
The values of \(C\) and \(\varphi\) can be determined by Mohr-Coulomb strength theory and the conventional triaxial test of cylindrical specimens. The confining pressure is applied while the axial pressure is gradually increased until the specimen is failed. The intercept between strength envelope and longitudinal axis is the cohesion \(C\), and the dip angle is the internal friction angle \(\varphi\), as shown in Figure 1. The Mohr-Coulomb strength theory takes \(C\) and \(\varphi\) values as material strength indices, and the expression is given by:  
\[\sigma_{1}-\sigma_{3}=(\sigma_{1}+\sigma_{3})\sin\varphi+2C\cos\varphi \tag{2}\]  
where, \(\sigma_{1}\) is the major principal stress and \(\sigma_{3}\) is the minor principal stress.  
Figure 1: Strength envelope of asphalt concrete triaxial test.  
The ACC is the anti-seepage barrier of ACCRDs, which can determine the safety of ACCRDs directly. The shear safety of ACC can be determined by the stress level \(S\) (the ratio of current shear stress to failure shear stress); the expression of stress level \(S\) is as follows:  
\[S=\frac{\sigma_{1}-\sigma_{3}}{(\sigma_{1}-\sigma_{3})_{\mathrm{f}}} \tag{3}\]  
where \((\sigma_{1}-\sigma_{3})_{\mathrm{f}}\) is the failure shear stress.  
The \"Design specification for rolled earth-rock fill dams\" (DL/T 5395-2007) stipulates that the minimum anti-sliding stability safety factor of dam slope for 1 type of dams should not be less than 1.30. The ratio of ultimate stress 1.0 to stress level is defined as the shear safety factor of ACC in this paper. It is assumed that the shear safety factor of ACC is consistent with the anti-sliding stability safety factors of dam slope, i.e., 1.30 is the yield shear safety factor of ACC (the corresponding yield stress level 0.77 is the yield shear failure control standard (YSFCS) of ACC) and 1.0 is ultimate shear safety factor (the corresponding ultimate stress level 1.0 is the ultimate shear failure control standard (USFCS) of ACC). If the stress level of ACC is less than or equal to 0.77 (the shear safety factor is more than or equal to 1.30), the ACC is considered to be safe and will not experience shear failure. If the stress level of ACC is more than 0.77 and less than or equal to 1.0 (the shear safety factor is less than 1.30 and more than or equal to 1.0), it is considered that the ACC has the probability that experiences shear failure. If the stress level of ACC is more than 1.0 (the shear safety factor is less than 1.0), it is considered that the ACC experiences shear failure.