Is Tunnel Safety Monitoring Only About Convergence?

Tunnel convergence is a common monitoring item, but it is not all of tunnel safety monitoring.

Depending on tunnel type, construction stage, surrounding rock conditions and operation status, monitoring objects may also include:

▪ Crown settlement;

▪ Peripheral convergence;

▪ Lining structure displacement and deformation;

▪ Displacement of portal cut-and-cover slopes;

▪ Ground surface settlement;

▪ Crack width and development trend;

▪ Mutual influence between existing and new tunnels;

▪ Pavement uplift, dislocation or settlement;

▪ Water seepage and groundwater changes;

▪ Displacement of hanging components and auxiliary structures;

▪ Changes in adjacent buildings and underground pipelines.

▪ Tunnel safety monitoring should be arranged around specific risks, rather than all projects uniformly deploying the same measuring points and instruments.

What Is the Difference Between Construction-Stage and Operation-Stage Tunnel Monitoring?

Construction-stage monitoring focuses on whether the construction process is safe

Monitoring during construction usually serves excavation, support and construction parameter adjustment, focusing on judging whether the surrounding rock and support structure are stabilizing.

The Ministry of Transport's interpretation of the Technical Specification for Construction of Highway Tunnels points out that the specification covers the whole construction process, including monitoring measurement and geological forecast, with requirements for measuring point layout, construction risk control and process management.

Common monitoring items during construction include crown settlement, peripheral convergence, surface settlement, support structure loading and portal slope displacement.

Operation-stage monitoring focuses on whether defects keep developing

After a tunnel enters operation, the focus gradually shifts to structural durability, defect development and traffic safety.

For example:

▪ Whether lining cracks keep propagating;

▪ Whether seepage causes structural or pavement defects;

▪ Whether the pavement has uplift, dislocation or fracture;

▪ Whether portal slopes are affected by rainstorms and geological conditions;

▪ Whether the lining shows permanent deformation;

▪ Whether hanging components have loosening, corrosion or fracture risks.

The current Technical Specification for Maintenance of Highway Tunnels requires periodic inspection of tunnels and maintenance plans based on inspection results, traffic operation status, structural technical condition, defect severity and surrounding rock geological conditions.

Note that the Ministry of Transport has issued the Technical Specification for Maintenance of Highway Tunnels JTG 5130-2026. It will take effect on September 1, 2026, when the original JTG H12-2015 will be repealed simultaneously. Relevant projects and publicity materials carried out after September 1, 2026 should follow the new official standard.

Which Tunnel Changes Deserve Key Attention?

1. Whether the Deformation Is Persistent

A single vehicle passage, construction vibration or environmental disturbance may cause short-term fluctuation. What truly deserves attention is whether the deformation keeps growing or fails to recover under similar conditions.

2. Whether the Change Speed Suddenly Accelerates

The cumulative displacement has not reached the control value, but the change rate has obviously increased over consecutive cycles, which may mean new changes in the surrounding rock, lining or external environment.

3. Whether Local Differential Deformation Appears

Tiny changes occurring synchronously across the whole tunnel and obvious displacement of a single cross-section or local component have different risk implications.

Local differential changes may correspond to local loading of the lining, construction disturbance, uneven strata or abnormalities at joints.

4. Whether It Relates to Rainfall, Groundwater and Construction Activities

Tunnel water seepage, portal slope changes and surface settlement may be related to continuous rainfall, groundwater level, drainage conditions or adjacent construction.

Therefore, monitoring data should be analyzed in combination with weather, construction conditions and on-site inspection records as much as possible.

Why Do Operating Tunnels Still Need Continuous Monitoring?

Periodic inspection can comprehensively grasp the structural condition, but changes may still occur between two inspections.

The key anomalies listed in the Technical Specification for Maintenance of Highway Tunnels include obvious permanent deformation of the lining with a trend endangering structure and traffic, large-scale groundwater inflow, severe pavement uplift or dislocation, and severe corrosion or fracture of hanging components.

These defects do not necessarily form suddenly. Some anomalies go through a process from tiny changes, continuous development to obvious defects.

Continuous monitoring of key cross-sections, existing defect locations or areas affected by external construction helps managers further judge:

▪ Whether the defect is in a stable state;

▪ Whether displacement or cracks are still developing;

▪ Whether the structure returns to stability after repair;

▪ Whether extreme weather or adjacent construction causes new changes;

▪ Whether special inspections need to be carried out in advance.

What Are the Field Difficulties of Automated Tunnel Monitoring?

▪ Dense measuring points

Tunnel structures are distributed linearly over long distances, with potentially many key cross-sections and defect locations. Equipment coverage and measuring point recognition capability directly affect system complexity.

▪ Limited reference conditions

Tunnel space is long and narrow, making an ideal stable reference hard to establish. Data misjudgment caused by the movement of the measuring equipment itself must also be prevented.

▪ Limited power and communication access

Long running tunnels, construction tunnels and remote highway tunnels may lack stable power or networks; power supply and data transmission schemes need to be designed according to site conditions.

▪ Lighting and environmental changes

Vehicle lights, construction lighting, dust, fog, water vapor and equipment vibration all affect long-term measurement stability.

▪ No interruption of operation

Installing and maintaining equipment in operating tunnels should minimize traffic impact, so the installation position, maintenance cycle and remote management capability of the system are very important.

How Is Vision-Based Displacement Monitoring Used for Tunnels?

vision-based displacement monitoringTargets can be placed on the lining, crown, portal or other key structures, with fixed equipment continuously recognizing target position changes.

It can be used for:

▪ Tunnel crown settlement monitoring;

▪ Convergence and deformation monitoring of key cross-sections;

▪ Local lining displacement monitoring;

▪ Relative displacement monitoring across both sides of cracks;

▪ Monitoring of portal slopes and support structures;

▪ Deformation monitoring of existing tunnels under adjacent construction;

▪ Long-term trend tracking of multiple defect points.

For long tunnels, multi-device zoning or flexible daisy-chaining can be adopted according to sight distance and occlusion conditions, so that data from different sections enter the same platform for management.

What Does the Haytham Tunnel Safety Monitoring Solution Focus On?

Haytham online visual trackeradopts non-contact vision measurement, continuously capturing the displacement of multiple targets without directly touching the measured structure.

According to tunnel site characteristics, the solution can be configured with:

▪ Multi-point identification by a single device;

▪ Multi-head synchronization and zoned monitoring;

▪ Long-distance flexible arrangement;

▪ Automatic numbering of targets;

▪ Reference movement identification and initial state recovery;

▪ Remote viewing of equipment operating status;

▪ Cumulative displacement and change rate analysis;

▪ Tiered alarms and automatic report generation;

▪ Data interfaces and third-party platform integration.

Before project implementation, the monitoring objects and risk sources should be clarified first. For example, adjacent construction impact monitoring and operational defect tracking have different requirements for sampling frequency, alarm thresholds and measuring point layout.

How Should a Tunnel Safety Monitoring System Be Designed?

The reasonable design order should be:

Clarify risk objects → determine monitoring indicators → select key cross-sections → determine measurement direction and range → set sampling frequency → establish alarm rules → clarify review and response processes.

Alarm values should not be copied directly from other projects either. They should usually be determined in combination with design documents, current standards, structural condition, historical data and expert opinions.

Tunnel safety monitoring ultimately needs to solve three problems:

▪ Whether the structure has changed;

▪ Whether this change is still developing;

▪ Whether the current change requires inspection, repair or safety control measures.

Only when continuous monitoring data forms a closed loop with on-site inspection, defect records and maintenance measures can the monitoring system truly serve the long-term safe operation of tunnels.