## 4 Case Study
### Structural Diagnosis of the Arch Dam under Abnormal Uplift Pressure  
Generally, during the operation period, if the uplift pressure of this concrete arch dam exceeds the experience expectation, the seepage control effect of the dam may be poor. This reservoir is responsible for the water supply of nearby cities, and it can't stop operation wholly and immediately. Thus, it is necessary to clarify the safety state of this dam under abnormal uplift pressure as soon as possible, as well as provide advice for the operation. Applying the measured uplift, the stress and deformation of this arch dam are rechecked to clarify the influence of the abnormal measured uplift pressure. (1) Stress analysis of arch dam  
The stress analysis of the concrete arch dam is analyzed under the unfavorable condition of normal water level and temperature drop in this paper. The stress of dam and foundation in the riverbed, under design and measured uplift pressure, is shown in Table 3. The stress distribution is shown in Figure 12, in which the tensile stress is positive and the compressive stress is negative. It can be seen from Table 3 and Figure 12 that the stress changes are as follows: compared with the design uplift pressure, the maximum tensile stress of the dam body decreased from 8.44 MPa to 8.37 MPa under the measured uplift load, which appeared at the dam shoulder. The maximum compressive stress of the dam body decreased from 9.10 MPa to 9.05 MPa, which appeared at the foundation in the riverbed. Under the action of designed and measured uplift pressure, the stress distribution regularity of the dam body is very similar. The foundation in the riverbed is the most obviously affected structural part. Table 3 and Figure 13 present that the maximum tensile stress in the foundation of the riverbed increased from 2.73 MPa to 2.90 MPa, which appeared upstream of the dam foundation. The maximum compressive stress decreased from 6.06 MPa to 5.97 MPa, which appeared on the downstream face of the dam foundation. Under the designed and measured uplift pressure, the stress distribution regularity in the foundation of the riverbed is very similar, and the stress range changes slightly. With the increase of uplift pressure, the stress values in the foundation of the riverbed increase, which means the tensile stress increases, and the compressive stress decreases. The abovementioned major and minor principal stress are too large due to the local stress concentration phenomenon in FEM. The calculation results of FEM are usually converted into equivalent stress as a criterion in engineering design. A group of elements of the foundation surface of each dam section is selected to calculate equivalent stress in this paper. The equivalent stress of upstream and downstream faces of the dam foundation is shown in Figure 14. Under the design uplift pressure, the maximum equivalent tensile  
Figure 12: Contour map of dam stress under measured uplift pressure (Unit: MPa). Figure 13: Contour map of foundation stress in the riverbed under measured uplift pressure (Unit: MPa). stress of the dam foundation is 1.07 MPa, which occurs at the dam shoulder. The maximum equivalent compressive stress is 3.04 MPa, which occurs at the foundation of the riverbed. Under the measured uplift pressure, the maximum equivalent tensile stress of dam foundation is 1.17 MPa, and the maximum equivalent compressive stress is 3.90 MPa, in the same position. According to the Chinese design standard, with the corresponding load and concrete type, the equivalent tensile stress should not be greater than 1.5 MPa, and the equivalent compressive stress should not be greater than 5.71 MPa. Therefore, the equivalent tensile and compressive stress of the concrete arch dam, under the measured uplift pressure, still meets the standard requirements at present. With comparative design and measured uplift pressure, the equivalent stress at the foundation surface changes most obviously in the riverbed.