Deep foundation pit engineering in urban rail transit control protection zones is one of the important risk sources for metro operation safety. During deep foundation pit excavation, changes in the stress state of the support structure, redistribution of surrounding soil stress, and changes in the groundwater seepage field may all adversely affect adjacent rail transit structures.
This scenario solution applies to monitoring projects with the following engineering conditions:
- Pit Depth: Excavation depth ≥ 5m, or < 5m but with complex geological conditions and sensitive surrounding environment
- Relationship with Rail Transit: Horizontal distance from pit edge to rail transit structure outer line ≤ 30m (Level I protection zone) or 30-50m (Level II protection zone)
- Geological Conditions: Soft soil layers, sand layers, confined aquifers, karst-developed strata, and other complex geological conditions
- Surrounding Environment: Adjacent existing buildings, underground pipelines, important municipal facilities, and other sensitive environments
| Challenge Category |
Description |
Response Strategy |
| Complex Spatial Coupling |
Variable spatial relationships between pit and rail transit structures |
Establish 3D geological-structure coupling model |
| Multi-source Data Fusion |
Multiple sources including manual monitoring, automated monitoring, and third-party monitoring |
Unified data platform for standardized access |
| High Real-time Requirements |
Real-time monitoring required during operation, minute-level warning response |
Deploy automated total stations, inclinometers, water level meters |
| Environmental Interference |
Continuous interference from construction machinery, vehicle traffic, dewatering |
Optimize monitoring point locations, use protective devices |
| Multi-stage Coordination |
Multiple stages including enclosure construction, excavation, support installation, base slab pouring |
Develop stage-specific monitoring plans |
The overall objective of deep foundation pit monitoring is to: through systematic monitoring work, real-time grasp of the deformation and stress state of the support structure and surrounding environment during deep foundation pit construction, timely detection of abnormal changes and early warning, provide data support for rail transit structure safety protection, and ensure the entire pit construction process is in a controllable state.
According to GB 50497-2019 and GB 50911-2013, deep foundation pit monitoring indicators are divided into four categories:
| Monitoring Item |
Monitoring Content |
Common Instruments |
Precision Requirement |
| Enclosure wall (pile) top horizontal displacement |
Horizontal displacement along pit direction |
Total station, GNSS |
±1.0mm |
| Enclosure wall (pile) top vertical displacement |
Settlement or heave of enclosure structure top |
Level, total station |
±0.5mm |
| Enclosure wall deep horizontal displacement |
Horizontal displacement distribution along depth |
Inclinometer |
±0.1mm/500mm |
| Monitoring Item |
Monitoring Content |
Common Instruments |
Precision Requirement |
| Surface settlement |
Vertical displacement of ground surface |
Level, total station |
±1.0mm |
| Deep horizontal displacement |
Horizontal displacement of soil along depth |
Inclinometer |
±0.1mm/500mm |
| Underground water level |
Water level change |
Water level meter |
±5mm |
| Building settlement |
Settlement of adjacent buildings |
Level, total station |
±0.5mm |
| Building inclination |
Inclination of adjacent buildings |
Total station |
±1/10000 |
| Pipeline settlement |
Settlement of underground pipelines |
Level, total station |
±1.0mm |
| Monitoring Item |
Monitoring Content |
Common Instruments |
Precision Requirement |
| Base slab heave |
Vertical displacement of pit bottom |
Level |
±1.0mm |
| Column settlement |
Settlement of support columns |
Level, total station |
±0.5mm |
| Support axial force |
Axial force of concrete/steel supports |
Strain gauge, load cell |
±1%FS |
| Anchor stress |
Stress of anchors |
Anchor stress meter |
±1%FS |
| Monitoring Item |
Layout Principle |
Spacing |
Quantity |
| Enclosure wall top displacement |
Along perimeter, at corners and midpoints |
15-20m |
~30 points |
| Deep horizontal displacement |
Along long sides, at corners |
20-30m |
6-8 holes |
| Support axial force |
Representative sections, each layer |
Each layer 3-5 sections |
24-40 points |
| Monitoring Object |
Layout Principle |
Spacing |
Quantity |
| Surface settlement |
Along perimeter, extending outward 2H |
10-20m |
30-50 points |
| Building settlement |
Corners, center, settlement joints |
6-8 points/building |
Per building |
| Pipeline monitoring |
Along pipeline, key nodes encrypted |
10-15m |
Per pipeline |
| Underground water level |
Pit corners and center |
20-30m |
4-8 points |
| Construction Stage |
Monitoring Frequency |
Basis |
| Enclosure construction |
1 time/day |
Normal period |
| Excavation (first support) |
1 time/day |
Normal period |
| Excavation (second support) |
1-2 times/day |
Key period |
| Excavation (third support) |
2 times/day |
Critical period |
| Base slab construction |
1 time/day |
Recovery period |
| Backfill |
1 time/2-3 days |
Stable period |
| Dewatering |
Real-time |
Continuous |
| Warning Level |
Trigger Condition |
Response Measures |
| Blue |
Displacement reaches 50% of control value |
Strengthen monitoring, notify construction party |
| Yellow |
Displacement reaches 70% of control value |
Encrypt monitoring to 2 times/day, hold coordination meeting |
| Orange |
Displacement reaches 85% of control value |
Encrypt monitoring to 3 times/day, start emergency plan |
| Red |
Displacement reaches 100% of control value |
Real-time monitoring, stop work, start emergency rescue |
The following values are typical values for reference. Actual control values should be determined based on design requirements and specific project conditions.
| Monitoring Item |
Control Value (mm) |
Warning Value (mm) |
| Enclosure wall top horizontal displacement |
30-50 |
20-35 |
| Enclosure wall deep horizontal displacement |
50-70 |
35-50 |
| Surface settlement |
30-50 |
20-35 |
| Building settlement |
15-25 |
10-18 |
| Building inclination |
1/500 |
1/700 |
| Pipeline settlement |
15-25 |
10-18 |
| Base slab heave |
20-30 |
15-20 |
| Support axial force |
80% design |
70% design |
| Report Type |
Frequency |
Content |
Format |
| Daily Report |
Daily |
Data summary, trend analysis |
PDF |
| Weekly Report |
Weekly |
Weekly summary, evaluation |
PDF |
| Monthly Report |
Monthly |
Monthly summary, stage evaluation |
PDF |
| Report Type |
Trigger |
Content |
| Warning Report |
Warning triggered |
Warning description, cause analysis, suggestions |
| Stage Report |
Stage completion |
Stage summary, evaluation, suggestions |
| Completion Report |
Project completion |
Complete summary, evaluation, experience |
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