## 6 Test Results  
### Stresses of Soil Nails  
After excavation and installation of the soil nails and prestressed anchors, the data acquisition system was established. According to the stress-time curves, the stress on the soil nails increased slowly and tended to be stable after three months. The mechanical behavior of the soil nail-prestressed anchor retaining system was monitored for about three months. Figures 11-15 reveal the stress-date relationship obtained from the in situ tests during the period of three months, in which Tij presents the stress value of the j-th strain gauge calculated from the nail head in the i-th row of soil nails. Figure 11: Stress–date relationship of the first row of soil nails for testing sections. Figure 10: Location of instruments for testing set No.5. Figure 12: The influence of unloading on stress of soil nails for section No.1. Figure 13: Stresses of the upper two rows for sections No.4 and No.5. Figure 11 presents stresses of the first row of soil nails for sections No.1, No.4, and No.5 during the excavation process. It is evident that there is a non-linear increase in the nail force for section No.1 with each excavation step. The nail force tends towards a stable value when the excavation is completed. This means that there are effects of time and excavation on the internal force of the soil nail. This is consistent with the results reported in the literature [19]. Compared with section No.1, the effect of excavation is less significant for sections No.4 and No.5. The overall stress level monitored from the tests is relatively low. Corresponding to the excavation of the foundation pit, increments of earth pressure due to unloading are transferred to the soil nail through shear stress of soil-grout interface. For this testing project, the foundation pit is adjacent to Fengchan Road in the north, under which a gas pipeline is buried parallel to the foundation wall. The pipeline lies 1.0 m deep and about 1.5 m away from the side of the foundation pit. Due to this reason, the upper soil layers north of the foundation pit for sections No.4 and No.5 had been previously  
Figure 14: Stresses of the lower two rows for section No.5. Figure 15: The distribution of nail forces of the first row for sections No.4 and No.5. disturbed, which is considered looser compared to the undisturbed soil for section No.1. Therefore, the friction resistance of the soil/grout interface for sections No.4 and No.5 is lower, and the earth pressure increment transferred to soil nails is lower. According to the above analysis, a conclusion can be drawn that the stress distribution is affected by the density of the soil. The stress level and concentration is higher when the soil is dense; the stress level and distribution is uniform and lower when the soil is loose. The obtained results are consistent with observations reported in the literature [25]. As can be seen from Figure 12, the stress levels of soil nails for section No.1 decrease due to unloading, which only manifests that the first and second rows of soil nails close to the ground surface are predominantly affected; however, the third and fourth rows are less influenced. This is because the soil for section No.1 is dense, and the mechanical property is close to undisturbed soil. The unloading effect from the ground surface cannot reach the lower two rows. Figure 13 presents the stress history curves of sections No.4 and No.5. The stress distribution and levels are less affected by rainfall, which continuously lasted for 48 h. This is because the infiltration rate of the rainfall in the clay or silty clay is extremely low; for example, in clay soil, the initial infiltration rate is 2.21 mm/min, and the steady infiltration rate is 0.62 mm/min according to the test results [26]. Accordingly, the depth of infiltration is relatively low. In addition, the actual location of the first row of soil nails is moved down to keep away from the gas pipeline, and the exact locations for the two test profiles are \(-2.0\) m and \(-2.2\) m, respectively.