The Role of Digital Monitoring Technologies in the Development
of Comprehensive Tunnel Maintenance Strategies
Anton W. Ackermann, Christopher Hunt
Amberg Engineering Ltd., Regensdorf-Watt, Switzerland
ABSTRACT
Owners and operators of infrastructure such as railways, roads, subway systems and water supply
systems are obliged to provide state-of-the-art, safe, dependable and economical services to their
customers in an ever increasingly complex environment. One key aspect is the maintenance of tunnel
infrastructure. The economic effectiveness of maintaining tunnel infrastructure can be increased
through implementation of comprehensive maintenance strategies which utilize modern digital
monitoring technologies. A wide range of digital monitoring technologies are currently available, two
of which include tunnel mapping and tunnel scanning. Both of these technologies can be
systematically applied within the framework of comprehensive long-term maintenance planning. This
paper introduces tunnel mapping and tunnel scanning technology and provides insight into how these
and other digital monitoring technologies may be successfully integrated into the development of
comprehensive tunnel maintenance strategies.
1. INTRODUCTION TO DIGITAL MONITORING TECHNOLOGIES FOR TUNNELS
The goal of managing tunnels in the most efficient and economic way possible has led to strategies
which apply digital technologies. Digital systems simplify information management by enabling
efficient collection and processing of raw data as well as many features which support data evaluation,
documentation and graphical output of results.
Ever more sophisticated digital monitoring technologies are being increasingly applied for a variety of
tasks related to the maintenance of tunnels. For example, phenomena such as tunnel damage can be
recorded directly into a digital system. Software programs enable the efficient evaluation of collected
data and the processing into a user-defined format that supports a given task.
In the following chapter, the digital technologies “tunnel mapping” and “tunnel scanning” are
introduced. These two technologies provide insight into the current developments to digitize tunnel
monitoring. It is important to note that there are many other digital monitoring technologies not
covered in this paper. The general statements regarding the role of digital technologies in the
development of comprehensive maintenance strategies will apply to many of them as well.
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2. TUNNEL MAPPING AND TUNNEL SCANNING
2.1 Tunnel Mapping
Tunnel mapping is a methodology used to chart and provide an overview of the condition of the tunnel
surface. Tunnel mapping is carried out from a special inspection wagon which typically has movable
platforms to enable inspection personnel to observe the entire tunnel surface up close. Inspection
personnel input observations of varying detail into tunnel maps. This information can include the type
and location of damage, construction materials, installations and other features of the structure.
Certain preparations are necessary prior to tunnel inspections using tunnel mapping. Tunnels are
divided into zones to enable orientation of inspection personnel in the tunnel and to ensure accurate
mapping. Zones are typically 10 m long, and are marked with spray paint or by hanging permanent
numbered markers before inspections.
Photos – Tunnel engineer inputting data into computer with help of portable equipment and software
Digital tunnel maps replace the recording of observed information by hand on to paper, thus
simplifying the mapping process in the field. Digital equipment and software has been developed to
rationalize data input and help manage information in the tunnel maps. The digital tunnel map is
prepared in the office during inspection preparations and copied into the field computer. The mapping
software has a number of preprogrammed tools which can be selected on the screen to simplify the
input of data. The tools enable fast and accurate input of information with the desired degree of detail.
Rates of performance for tunnel mapping vary widely, but the performance during past mapping
projects has ranged between 30 and 60 m’/hour. One modern method of tunnel mapping involves
equipping the inspection engineer with a sturdy and portable PC. The PC hard drive is stored in a
backpack, leaving the hands free to enter observations directly into the digital tunnel map using a
small touch screen.
The digitally recorded information can be exported directly into CAD or other programs for further
processing and editing. Color tunnel maps in CAD graphically indicate the location of the various
observed phenomena. For each inspected tunnel, the map is delivered with a detailed inspection report
written by the inspection engineer. The inspection report includes initial recommendations for repair,
further investigations and monitoring of deficiencies observed during the inspection. With the tunnel
map, the development of damage over a longer period of time can be periodically monitored.
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2.2 Tunnel Scanner Systems
The development of laser scanning systems enables continuous, rapid and objective digital data
collection in tunnels. These systems scan the entire tunnel surface to provide full-coverage digitized
visual images and profile measurements.
The visual scan measures the surface reflectivity of an object as well as the coordinates of each point
of measurement. The result is a scaled high resolution photo-quality black and white visual image.
Using viewer software, the visual images can be viewed as needed back in the office.
Photos - scanner visual image results as viewed with special viewer software
Profile scanning measures the coordinates of each measurement point and creates a file consisting of
continuous cross sections of the object. The distance between adjacent profiles varies according to the
speed at which the scanning equipment passes through the tunnel as well as the rotational speed of the
laser, but typically one or more profiles per cm along the tunnel are measured. This is a major
advantage of the scanning technique, as profiles are available at any location in the tunnel. With the
collected data, clearance conditions between the existing tunnel profile and various clearance profiles
can be analyzed. Profile scans can be made in combination with visual image scans in a single run.
Scanning results are typically delivered with special software that allows viewing of visual images and
profile data.
Photos - scanner profile results
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3. TUNNEL MAINTENANCE STRATEGIES
3.1 Development of Comprehensive Tunnel Maintenance Strategies
The development of comprehensive maintenance strategies to manage tunnel assets involves the
systematic planning of tunnel maintenance tasks over a defined period of time. The main goal of such
strategies is to maintain tunnels through preventative maintenance. This results in many benefits,
including the elimination of unplanned interruptions in operations, increased safety, the development
of acceptable solutions to environmental considerations and last but not least, the minimization of total
costs and efficient use of maintenance personnel.
In order to realize such benefits, it is important to carry out the right maintenance tasks and not just the
most urgent tasks. Technical know-how and experience with tunnel structures are therefore required.
Priorities must be set based on long-term planning so that maintenance tasks can be carried out at the
most opportune time. A tunnel management system consists of several important interacting elements,
which are shown in Figure 1 and summarized below.
3.1.1 Policies, Goals, Requirements, Constraints
This element involves the determination and communication of global company goals and policies,
goals, requirements, technical standards, and system constraints with regards to structural safety,
financial resources, traffic or operational conditions, environment, safety, and execution of
maintenance works.
3.1.2 Global Maintenance Strategy
The Global Maintenance Strategy involves the evaluation of maintenance requirements for the
determination of monitoring procedures, creation of maintenance handbooks, scheduling and financial
planning for routine and major maintenance tasks, and prioritization of maintenance tasks including
optimization by evaluating various maintenance alternatives and their effects upon system constraints.
It may, for example, involve the development of a long term maintenance plan (for example 20-30
years) and detailed 5 year program of maintenance tasks as well as the development of detailed 1-year
work programs including scheduling, manpower and resources needed for each task.
The global maintenance strategy requires periodic control to update changes to the established system
based on changing needs and experience gained during tunnel operation.
3.1.3 Monitoring and State Assessment
Monitoring and state assessment involves the tunnel inspection, detail investigations, control
measurements as well as the evaluation and judgement of structural safety and serviceability
requirements.
3.1.4 Normal Maintenance Tasks
Normal maintenance tasks involve daily or planned maintenance tasks and are typically carried out by
permanent maintenance personnel.
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3.1.5 Detail Planning for Major Tasks
Detail planning includes specialist engineering tasks such as studies of alternatives, detail design,
permitting, tendering, scheduling, and cost estimations for complex tasks which are not normally
carried out by permanent maintenance personnel.
3.1.6 Major Maintenance Tasks
Major maintenance tasks include refurbishment, modification, renewal, major repair, or
demolition/replacement works or even emergency measures.
Policies, Goals,
Requirements and
Constraints
Information Management
Global Planning of
Maintenance
Strategies
Detail Planning Monitoring and
State Assessment
Major Normal
Maintenance Maintenance
Tasks Tasks
Figure 1 – Flow diagram of tunnel maintenance system elements
3.2 The Role of Digital Monitoring Technologies to Maintenance Planning
Digital technologies can be effectively applied to carry out tasks associated with each of the elements
of a given tunnel maintenance system, such as introduced in Chapter 3.1.
3.2.1 The Evaluation of Digital Monitoring Technologies Within the Scope of Global Planning
The decision to apply specialized digital technologies should be determined within the scope of global
planning of maintenance strategies. It is important to determine where benefits of the digital
monitoring technology are to be realized. The advantages/disadvantages should be considered with
regard to the data collection, data accuracy, the ease of managing the collected data, the speed of data
processing and the application of the data to subsequent tasks.
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The application of tunnel mapping or tunnel scanning, for example, should be based on an initial
clarification of several factors, e.g.
• compatibility with system constraints and maintenance goals
• which assets are to be monitored (i.e. structural elements and electromechanical equipment)
• technical details of digital technologies including costs, capabilities, accuracy, availability etc. of
systems/services
• which tasks the gained information is to support
• the frequency in which the system under consideration is to be applied
• compatibility of suitable technologies with existing systems and strategies of client
• clarification of all software capabilities, requirements, and costs (inclusive demonstration)
• clarification of data management requirements and current resources of client
• determination of how data will be accessed or provided to involved parties (internal / external)
After these factors have been analyzed, the correct choice of technologies can be made and integrated
into the maintenance strategy over a defined period, or applied to provide information for a specific
predefined task.
3.2.2 The Application of Digital Technologies to Tunnel Monitoring and State Assessment
Tunnel mapping and tunnel scanning technologies are examples of digital monitoring systems that
have been widely and effectively applied during the execution of tunnel monitoring and state
assessment tasks. These include:
• localization of structural elements
• recording of location and quantity of damage and installations
• locating boundaries and quantities of construction materials of linings
• recording of profile conditions
• comparing changes in structural state over time
• periodic tunnel inspections
• evaluation and prioritization of damage
3.2.3 The Application of Digital Monitoring Technologies to Other Maintenance Tasks
The data collected by digital monitoring systems provide a basis of information for the other elements
of tunnel management systems as well, such as the detail planning of major maintenance tasks and to
support various tasks during execution of normal and major maintenance tasks. Infrastructure
managers, engineers, contractors, and maintenance personnel, and can apply the collected data in
carrying out numerous engineering and infrastructure maintenance tasks such as:
• acceptance procedures for newly constructed or refurbished tunnels
• preliminary and detailed project design tasks (tunnel rehabilitation, renewal, modifications)
• tendering
• determination of material quantities
• clearance analysis in rail systems, profile analysis for new wagons, new catenary equipment,
verification of clearance for oversize loads et. al.
• support of further long term, mid-term, and short-term maintenance planning
• documentation of particular structural features
• support documentation for maintenance handbooks and normal maintenance tasks
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3.2.3 The Role of the Tunnel Specialist
It is important to note that digital technologies currently provide improved methods that significantly
simplify certain tasks. They do not, however, eliminate the need for project involvement by qualified
specialists. The unique conditions of the tunnel environment with regards to limited time, light, space,
and access, health and safety, special equipment, uncertainty associated with geology and hydrology,
variable loading of structure, variability in structural design, effects of adjacent structures and many
other factors make the integration of a specialists essential.
Each digital technology has its own need for a specialists of some sort, whether in the study of
available technologies, preparation and/or execution of data collection, or in the interpretation,
analysis or application of collected data.
Tunnel inspections involving laser scanning, for example, although providing very high resolution
images of the tunnel at a particular point in time, should be supplemented by an on-site inspection of
the facility by a qualified expert. The role of specialists is in this case is to:
• interpret and evaluate the digitally collected data based on valuable on-site observations
• evaluate risks based on site observations, study of documents, and experience
• analyze problems and technical solutions
• make recommendations of for further testing, monitoring and construction measures
4. CONCLUSION
The significance of digital monitoring technologies such as tunnel mapping and tunnel scanning in
carrying out diverse tunnel maintenance tasks will continue to increase. The results obtained from a
specific technology may provide a basis of information which can be applied for a large number of
subsequent and interdependent tasks. Efficient data management and access for the involved parties
represents an important criteria for successful application. The choice of digital monitoring technology
for a given problem should be carefully considered to ensure the technology achieves the desired
performance, provides the expected results and is compatible with existing maintenance systems and
strategies of the client.
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