## 4 Measurement Results Analysis and Discussion
### Lateral Earth Pressure  
The attenuance of the lateral earth pressure (AOP) is introduced to characterize the attenuation characteristics of the soil pressure behind the structure. AOP is defined as the ratio of the attenuation of the earth pressure to the initially measured pressure.  
\[\text{AOP}= \frac{P_{att}}{P_{ini}} \tag{1}\]  
Here, \(P_{att}\) is the attenuation amount of the soil pressure and \(P_{ini}\) is the initial measured soil pressure.  
Figure 7 shows the variation in AOP with time. The greater the change in AOP (in units of time), the faster the earth pressure attenuates. During this period, the lateral earth pressure was measured 18 times. The variation of the lateral earth pressure with time was obtained.  
The earth pressure behind the prestressed supporting structure was greatest after the initial installation was complete. In Figure 7, the change in the earth pressure at each depth can be divided into four stages. The first stage was the rapid attenuation stage, which occurred within three days of the prestress application. The second stage was the fluctuation stage, which occurred between the third  
Figure 6: The schedule of the construction progress.  
day after the prestress application and the completion of shaft excavation. The third stage was the slow attenuation stage, which occurred within ten days of the completion of shaft excavation. During the last stage, the earth pressure hardly changed and was stable. Combined with the practical situation on site, it is concluded that the earth pressure increases rapidly by applying prestress (before it could dissipate) and then gradually dissipates as time progresses.  
In Figure 7, AOP is not consistent at the two monitoring points at the same depth. In addition, there are differences in AOP at different depths. The most stable AOPs were 0.32, 0.39 and 0.45 at depths of 1 m, 3 m and 5 m, respectively. By analyzing the geologic conditions, the plasticity index (\(l_{\rm p}\)  
Figure 7: Time series of the lateral earth pressure attenuance at two monitoring points: **(a)** Below the surface at 1 m; **(b)** below the surface at 3 m; and **(c)** below the surface at 5 m.  
at a depth of 5 m is larger than that at 1 m and 3 m. We conclude that AOP is related to soil conditions, and the more easily soil deforms, the more earth pressure is lost.  
Table 1 shows the ratio of the AOP after rapid attenuation to the AOP of the stabilization stage. When the rapid attenuation stage is completed, the attenuation of earth pressure has been completed by 50% or more. This finding indicates that a higher residual prestress can be obtained by improving the initial prestress to enhance the supporting force.  
The measured stable earth pressure and calculated Rankine earth pressure are presented in Figure 8, and the results show that the measured earth pressure values of both Ei1 and Ei3 increase with depth and are 108 kPa and 79 kPa at a depth of 5 m, respectively.  
In Figure 8, the measured pressure is located between the earth pressure at rest and the passive earth pressure. This result indicates that the prefabricated prestressed supporting structure applies prestress to the soil and has an active support effect. The earth pressure at different locations has a disparate value at the same depth. This phenomenon shows that the prestress applied by the prefabricated prestressed supporting structure was not uniform. Combined with the field tests, a conclusion is drawn that the single supporting structure layer asymmetrically expands while applying prestress, which leads to different pressures at the same depth.