## 3 Results
### The FEM Model Results
#### 3.2.3 Comparison between Monitoring Results and Calculation Results  
As mentioned previously, the concrete surface strain was compression strain from the calculation results (see Figure 10). In the engineering, the strain sensors could only measure the strain development after the installation time. As a result, the strain of time 40 h (see Figure 10) was set as a baseline. Then the calculation strain subtracted the baseline value during the ILRT. Obviously, the strain was positive after subtraction. In the FEM model, elements 10,801, 10,426, 9426 and 14,401 were close to measuring points 1 to 4. The results of these elements are plotted in Figure 12. In each direction, the strain discrepancy was small. The maximum discrepancy value is 12 \(\upmu\epsilon\) in the circumferential direction at 0.483 MPa (Figure 12b). In order to testify the calculation results, the comparison between monitoring results and calculation results was made (Figure 13). Because there was little difference between the calculation results of four measuring points, the calculation results of MP 4 were chosen as a representation. The monitoring results and calculation results were plotted in Figure 13.  
It turned out that the calculation results were consistent with monitoring results of four measuring points during the first 110 h. However, the discrepancy became larger and larger after 110 h. During the time from 200 h to 230 h, the decrease rate of monitoring results became faster, mainly due to the abrupt temperature drop (see Figure 8b). The huge discrepancy was caused by temperature change. In the future, temperature change simulation should be done for more precise simulation. Compared to vertical direction strain, circumferential direction strain was not sensitive to abrupt temperature drop during 200 h to 230 h (see Figure 13b). The calculation results showed good consistence with monitoring results. Figure 14 shows the concrete strain-inner pressure relationship. Comparing the calculation results with monitoring results, it was clear that both monitoring results and calculation results showed a similar trend. During the pressurization process, the monitoring results kept almost linear growth until 0.4 MPa. When it came to 0.45 MPa and 0.483 MPa, the monitoring results even decreased while the calculation results were still linear. The calculation results maintained almost the average value of the monitoring results of four measuring points during pressurization. During the depressurization process, the monitoring results were almost below the calculation results. This phenomenon might be caused by glue relaxing of FBGs. Figure 12: The calculation results in two directions. (**a**) Vertical direction strain; (**b**) Circumferential direction strain. Figure 14: Concrete strain-inner pressure relationship. (**a**) Vertical direction during pressurization; (**b**) Circumferential direction during pressurization; (**c**) Vertical direction during depressurization; (**d**) Circumferential direction during depressurization. Figure 13: The comparison between monitoring results and calculation results. (**a**) Vertical direction; (**b**) Circumferential direction. Then, on comparing the vertical direction monitoring results with circumferential direction, it seemed that there were more fluctuations in vertical direction monitoring results. In order to verify this conclusion, linear regression was used in data fitting of each measuring point because the trend of monitoring results was almost linear. At the same time, the ratio of residual sum of squares (RSS) to total sum of squares (SYY) [20] could be calculated as shown in Table 3. The smaller the ratio is, the more accurate the linear regression will be and fewer fluctuations will exist. From Table 3, it can be concluded that the circumferential monitoring results had fewer fluctuations both in the pressurization process and depressurization process. The reason of this phenomenon should be discussed.