## 5 Overall Stability and Reinforcement Analysis of Xulong Arch Dam
### Discussion on Dam Stress Zones  
Based on past analytical experience and the analysis of the stress, displacement, and yielding region of the Xulong arch dam, five stress zones of the arch dam are proposed as follows. Figure 14 is a schematic diagram of the five stress zones. The five stress zones can better guide the crack prevention of the arch dam.  
Figure 12: Crown displacement under various analysis cases 1 to 6. (**a**) Crown displacement along river direction; (**b**) crown displacement cross river direction.  
Figure 13: The distribution characteristics of arch thrust of several high arch dams. (**a**) Right arch abutment thrust; (**b**) left arch abutment thrust.  
(1) Three-way compression zone of upstream surface  
This zone ranges from about 1/5 to 4/5 dam height, and the zone width is close to the height. The stress state in this zone indicates the structural state of the arch dam and it is important to control the compression stress in this zone. In general, the maximum compression stress is around the arch crown beam at 1/3 elevation of the arch dam. The compression stress results of the finite element analysis are around 6.2-8.0 MPa.  
(2) Tensile and compressive zone of upstream arch abutment  
The stress state may be tensile stress in the direction of both beam and arch or one of the directions is tensile stress. When upstream water pressure is considered, it is the state of double-tension single-compression or double-compression single-tension. More attention should be paid to control the tensile stress of this area to prevent cracking. The calculation results show that the tensile stress of the left arch abutment of the Xulong dam reaches 1.18 MPa of case 1. It is suggested to control the tensile stress of this area to less than 1.5 MPa when the FEM is adopted.  
(3) Tensile stress zone of upstream dam heel  
Based on the analysis of several super-high arch dams in China, it is suggested that the tensile stress should be strictly controlled within 1.4 MPa if the tensile stress of arch dams is based on FEM simulation. The dam heel tensile stress of the Xulong arch dam is 0.9 MPa. Although the upstream bottom joint can reduce the tensile stress of the dam heel, attention should be paid to the effect of hydraulic fracturing. The upstream bottom joint cannot affect the construction of the curtain grouting. The high tensile stress is related to the discontinuous geometric shape of the arch dam heel. The cracking of the dam heel should be paid more attention to.  
(4) Compression stress zone of downstream arch abutment  
This zone ranges from the bottom to the middle height of the dam. Normally, the largest compression stress is in this zone and it is important to control it. The compression stress of the left arch abutment of the Xulong dam reaches 8.88 MPa of case 10. It is suggested to control the compression stress of this zone to less than 14 MPa when the FEM is adopted.  
(5) Tensile stress zone of downstream surface  
The arch dam's downstream surface between the upper to middle elevation is a tensile zone, and the tensile stress is in the direction of the beam. The tensile stress may be large here due to the pier. When the upstream water level is low, this tensile stress zone will shift to the left and right arch abutments. The results of the geomechanical model test also show that the cracking of the downstream  
Figure 14: Five stress zones of high arch dam. (**a**) Upstream surface; (**b**) Downstream surface. Note: The number 1–5 represents compression zone of upstream surface, tensile and compressive zone of upstream arch abutment, tensile stress zone of upstream dam heel, compression stress zone of downstream arch abutment, and tensile stress zone of downstream surface, respectively.  
arch abutment basically extends to the center of the dam along the normal of the foundation surface, which is the failure of the tension and shear [2].