## 3 On-Site Monitoring  
Real-time monitoring of the foundation pit and its surrounding environment is carried out to ensure the safety of foundation pit construction, which consists of conventional foundation pit monitoring and tunnel deformation monitoring. The foundation pit monitoring mainly includes deep soil displacement, pavement settlement around the foundation pit, horizontal displacement of the ring beam, and the axial force of the inner support. The displacement of the deep soil mass is measured by the inclinometer pipe embedded in the supporting pile in advance, and the buried depth of the inclinometer pipe is the same as that of the supporting pile. The settlement of the pavement around the foundation pit and the horizontal displacement of the ring beam are measured through pre-arranged observation points. Considering the existence of tunnels under the western pavement of the foundation pit, the monitoring range of the western pavement of the foundation pit is enlarged. The axial force of the inner support is measured with a rebar gauge. The layout of monitoring points for foundation pit monitoring is shown in Figure 5. The tunnel deformation monitoring includes horizontal deformation monitoring and vertical deformation monitoring. The specific values will be compared with the finite element simulation results in Section 4.  
Figure 4: Support layout in section B of the foundation pit.  
Figure 5: Monitoring point layout.  
There are 44 road settlement monitoring points (R1-R44) laid out on the pavements around the foundation pit. The allowable value of settlement of the surrounding pavements is 20 mm. When the change speed of settlement exceeds 2.5 mm/d, the monitoring system will automatically issue an alarm. The point R34 (east side of the foundation pit) has the largest cumulative settlement, and its value is 14.9 mm. The smallest cumulative settlement occurs in the point R15 (the west side of the foundation pit), and its value is 6.7 mm. From the monitoring results, the foundation pit construction has a slight impact on the surrounding pavements, but the impact is within an acceptable range, and no cracking is observed on the surrounding pavements.  
A total of 34 horizontal displacement monitoring points (D1-D34) are arranged on the ring beam of the foundation pit, of which the point D32 has the largest displacement with a value of 9.5 mm, and the point D4 has the smallest displacement with a value of 2.5 mm. The limit allowable value of the horizontal displacement of the ring beam is 20 mm, which is about twice the measured maximum displacement. From the monitoring results, during the removal process of the support, there are two sudden changes in the displacement, which is a normal situation, and all the observed values are within a reasonable range. There are 26 support axial force monitoring points on each layer of the support system. The monitoring results show that most of the support axial force does not exceed 80% of the design value, and only a few parts reach about 90% of the design value, which indicates that there is still a certain safety reserve in the support structure system.  
There are 15 monitoring points (CX1-CX15) in the deep soil displacement of the foundation pit, among which the maximum cumulative displacement of each inclination hole is the point CX10, whose value is 15.5 mm, and the minimum cumulative displacement is the point CX3, whose value is 11.5 mm. The limit allowable value of deep soil displacement is 30 mm, and the maximum deformation on site is only close to half of the limit value. The cumulative displacement of monitoring points CX3 and CX10 is shown in Figure 6. From the monitoring results, the maximum deformation of the inclinometer hole is generally at a depth of 7-10 m, and the observed value is within a reasonable range.  
Figure 6: Deep soil displacement curve in CX3 and CX10.