Project Background
The Luzhou–Gulin–Jinsha Expressway (Gulin to the Sichuan–Guizhou border section) is a major interprovincial corridor connecting Sichuan and Guizhou. The route starts at the Gulin East interchange of the Xuyong–Gulin Expressway and ends at the Sichuan–Guizhou provincial boundary, with a total length of 38.559 km. The alignment traverses steep mountainous terrain with a high bridge-and-tunnel ratio, making construction conditions extremely complex.
For this project, structural health monitoring systems were built simultaneously for three super-large bridges — the Chishui River Bridge, the Shuiluo River Bridge and the Gulin River Bridge — with data integration into the Sichuan Provincial monitoring platform and the Ministry of Transport's Structural Health Monitoring Data Platform for Long-Span Bridges. The implementation contractor was Sichuan Jintong Engineering Testing Co., Ltd., and the report was compiled in April 2024.
Overview of the Three Bridges
Gulin River Bridge
Located in Yongle Town, Gulin County, Luzhou City, the bridge crosses the Gulin River. It is an 86+160+86 m prestressed concrete continuous rigid-frame bridge, built as separated left and right carriageways, with a total length of 332 m. The main girder uses a separated single-cell, twin-box cross-section with a side-to-main span ratio of 0.538. Design speed is 100 km/h, the live-load class is Highway Class I, deck width is 48.5 m, and the seismic peak ground acceleration is 0.05g.
Shuiluo River Bridge
The bridge crosses the Shuiluo River, a tributary of the Gulin River, at the boundary between Yongle Town and Longshan Town in Gulin County, about 10.7 km from the project start point. The site features steep mountains and a remote location. Its start-side approach connects to the Longshan Service Area, while its end-side approach connects to an embankment section. The bridge adopts an arch structure, with transition piers 9# and 10# and tall columns 1# and 10#.
Chishui River Bridge
A large cable-stayed bridge, this is the largest monitoring contract section of the project. Monitoring points are distributed across the mid-span deck and at the tops of both towers on either side, making it a typical representative of mountain expressways crossing deep valleys.
Monitoring Scope: Full Coverage of 17 Metrics
Based on the structural characteristics of the three bridges, the system deploys a complete health monitoring framework covering the following 17 metrics:
| Category | Monitoring Content | Typical Number of Measurement Points |
|---|---|---|
| Environmental | Ambient temperature and humidity | Distributed uniformly along the bridge length |
| Load | Vehicle loads | Dynamic weigh-in-motion (WIM) |
| Structural response | Main girder deflection | Gulin River 12 / Shuiluo River 14 / Chishui River 16 |
| Structural response | Tower deviation and lateral girder displacement | Chishui River 3 points + 1 reference point (GNSS) |
| Structural response | Column deviation | Shuiluo River 8 points (inclinometers) |
| Structural response | Girder-end rotation angles | Gulin River 4 / Chishui River 4 (inclinometers) |
| Structural response | Longitudinal girder-end displacement | 4 points per bridge (magnetostrictive displacement transducers) |
| Structural response | Structural strain | Gulin River 48 / Shuiluo River 40 / Chishui River 60 (FBG strain gauges) |
| Structural response | Vibration acceleration of main girder and towers | Gulin River 20 / Chishui River 25 (accelerometers) |
| Structural response | Arch rib vibration acceleration | Chishui River 12 points at quarter points of the arch rib |
| Structural response | Stay-cable force and vibration | Key cables of the Chishui River stay cables |
| Structural response | Arch crown deviation and arch springing displacement | Shuiluo River arch bridge |
| Structural response | Structural cracks at key sections | Key sections of each bridge |
| Environmental | Structural temperature | Sharing locations with strain measurement points |
| Environmental | Wind load | Chishui River: mid-span + two tower tops, 3 points total (ultrasonic anemometers) |
| Environmental | Ground motion | Seismometers in the bridge site area |
| Video | Video surveillance of key areas | Covering the deck and tower areas |
Key Sensor Selection
FBG strain gauges — the absolute mainstay solution for strain measurement points, offering high accuracy, strong interference immunity and long-distance signal transmission, making them highly suitable for the harsh electromagnetic environment at mountain bridge sites. Compared with conventional vibrating-wire sensors, FBG gauges exhibit less signal attenuation when multiple measurement points are connected in series, making them the first choice for structural strain monitoring on this project.
GNSS displacement monitoring — used for tower deviation and lateral girder displacement monitoring on the Chishui River Bridge. In mountainous canyon environments, GNSS delivers centimeter-level real-time kinematic positioning without requiring line-of-sight, resolving the pain point that conventional total stations struggle to maintain intervisibility across canyons.
Ultrasonic anemometers — one measurement point each at the mid-span deck and the tops of both towers of the Chishui River Bridge, recording wind speed and direction time histories to evaluate the wind field characteristics at the bridge site and the effect of fluctuating wind on the structure.
Inclinometers — used for column deviation and girder-end rotation monitoring. They are simple to install and highly reliable, well suited to tall arch-bridge columns and girder-end deformation monitoring in mountainous terrain.
Magnetostrictive/tensile displacement transducers — used for longitudinal girder-end displacement monitoring. Their measuring range accommodates the deformation amplitude of expansion joints, directly measuring the change in distance between fixed points at the main girder end and the adjacent approach spans.
Technical Highlights
Multi-section deployment of FBG strain gauges
On the Gulin River Bridge, 12 sections with 48 strain measurement points were arranged at the mid-span of the side span of the main arch, the quarter points of the main span and the pier tops; on the Chishui River Bridge, 9 sections with 60 strain measurement points were arranged at critical girder sections including the mid-span of the side span and the quarter points of the main span. Series-connected optical fiber wiring reduced the number of signal cables, lowering the wiring difficulty of construction in mountainous areas.
GNSS dynamic monitoring in mountain canyons
The Chishui River Bridge site features steep mountains and deeply cut canyons, where conventional surveying methods struggle with intervisibility. Using GNSS receivers for real-time kinematic positioning, with measurement points at the mid-span deck and the tops of both towers, continuous all-weather monitoring of tower deviation and lateral girder displacement was achieved.
Multi-source data fusion analysis
The system fuses and analyzes multi-source monitoring data — strain, deflection, vibration, temperature, wind load and vehicle loads — and combines the results with finite element model calculations for structural condition assessment, upgrading from "single-point alarming" to "comprehensive assessment."
Data Analysis and O&M Services
Analysis and reporting mechanism
- Within one month after the completion of each quarter's monitoring, a quarterly analysis report is compiled, comparing finite element model calculations with measured data
- Within the first month after the completion of each year's monitoring, an annual structural safety assessment is performed, an assessment report is compiled, and maintenance recommendations are provided
- Before and after special events (red-alert high winds, earthquakes, vessel collisions, major traffic accidents, etc.), dedicated structural condition assessments are carried out
Four-level structural health rating
The system adopts a four-level health rating system — Level I: essentially intact / Level II: minor anomalies / Level III: moderate anomalies / Level IV: severe anomalies — combining multiple parameters such as strain at key sections, vertical girder displacement, bearing displacement and girder vibration frequency for overall structural evaluation. When the health rating reaches Level III or above, an expert assessment mechanism is activated.
7×24 O&M response
Around-the-clock telephone support (7×24 hours) is provided throughout the defect liability period, with on-site response within 24 hours for general faults and arrival on site within 2 hours for emergency faults. During system outages, alert messages continue to be pushed to safeguard bridge operational safety.
Implementation Summary
No safety or quality incident occurred during the construction period of this project. The quantity, specifications and technical parameters of the sensors on the three super-large bridges all met the design requirements; installation positions were correct and secure, and signal cable connections were in good condition. Data acquisition equipment was in normal working order, with tidy wiring, clear labeling and reliable grounding inside the cabinets. Monitoring data were received, processed, stored and forwarded normally, and all software modules met their design requirements. The system met the acceptance criteria and formally entered its O&M service period.
The main challenges during implementation included limited power and network transmission conditions at the bridge sites, difficult construction in canyon terrain, and inconvenient equipment transportation. Through thorough preliminary surveys, scientific management and reversed-schedule planning, project quality and progress were both ensured.