## 3 Analysis of the Onsite Monitoring Data and Emergency Measures
### Influence of the Construction Quality of the Construction Method Pile  
The particle gradation curves of the silt, medium sand and coarse sand layers through the construction method pile are shown in Figure 5. Compared with medium sand and coarse sand, the silt layer contains more fine particles (particle sizes less than 0.075 mm). Previous research has shown that three-axis mixing piles easily produce construction quality defects under the influence of fine soil particles [16, 36], resulting in a large permeability coefficient of the pile section of the construction method, and the water-retaining effect of the water-resisting curtain is poor. Figure 4: Section diagram of the foundation pit supporting structure (unit: mm): (**a**) main supporting structure of the foundation pit; (**b**) elevator shaft support structure. The change in the deep horizontal displacement monitoring of the foundation pit is shown in Figure 6. Prior to the inrush accident, the horizontal displacement values of the CX1 and CX2 (CX represents the monitoring point of inclination measurement) monitoring points were significantly greater than those of CX3 and CX4, indicating that the stability of the supporting structure on the sides of CX1 and CX2 was lower than that on the sides of CX3 and CX4. After the accident, the horizontal displacement values of CX3 and CX4 are still small, whereas the horizontal displacement values of CX1 and CX2 increase strongly in the depth range of 9-12 m (the shadow part in the figure). The horizontal displacements at the 9 m and 12 m monitoring positions increased by approximately 34% and 63%, respectively. This depth is the depth of the above silt layer, which also shows that the construction quality defects of the construction method piles monitored by CX1 and CX2 are generated near depths of 9-12 m. When an accident occurs, groundwater and sand particles flow into the foundation pit through the defects of the supporting structure in this depth range and strongly affect the deformation of the supporting structure. After the inrush accident, the water levels of CYS1 and CYS2 (CYS represents the monitoring point of the confined water level) holes on the northeast side of the foundation pit water-resisting curtain decreased significantly. The geological survey reveals that the thickness of the silty sand layer in each borehole around the foundation pit is approximately 2.4-3 m, whereas the CYS1 and CYS2 holes are located at the thickest position of the silty sand layer (consistent with the above CX1 defect position). Figure 7 shows that the two holes  
Figure 5: Gradation curve of each sand layer in the foundation pit. Figure 6: Changes in the deep horizontal displacement monitoring of the foundation pit: (**a**) preaccident monitoring value (15 May 2023); (**b**) postaccident monitoring value (11 June 2023). still show relatively large water level changes after the accident treatment was essentially completed, again confirming that the groundwater at the defect location continues to leak and that the treatment cannot effectively stop the incoming water. The displacement changes at each monitoring point on the top of the retaining wall of the foundation pit prior to the accident are shown in Figure 8. The cumulative displacement amplitude \(\Delta S\) (\(\Delta S^{2}=\Delta X^{2}+\Delta Y^{2}\)) at points JC2 and JC4 (JC represents the horizontal displacement monitoring point at the top of the retaining wall) is close to the warning value, and the maximum value is 29.68 mm. The warning value of 30 mm can be easily exceeded owing to the effects of construction disturbances. The two points are located near the CX1 monitoring point in Figure 6, and the displacement increases abnormally, which is also caused by the construction quality defects of the construction method pile.