Monitoring Tunnel Constructions: Unlocking the Potential of Instrumentation
Instrumentation and monitoring (I&M) in tunnel construction have often been underappreciated within the broader construction processes. I&M refers to the systematic deployment of advanced sensors, measurement tools, and data management platforms to monitor deformation, stress, pressure, vibration, and other critical parameters during construction. With the advancement of measurement techniques and especially data communication and data management tools, monitoring has grown to provide a strong value—value that the tunnel industry is increasingly recognizing and implementing. Despite over 30 years of experience working on complex tunnel projects with major engineering and construction firms, we can report that there is still untapped potential that monitoring can offer. Building trust in monitoring data remains a challenge.
The benefits of I&M are far-reaching. I&M serves as an essential feedback mechanism, enabling stakeholders to validate design models, assess structural behavior, and make informed decisions to enhance safety and performance. I&M enhance risk management, and ensure the safety of construction personnel and surrounding communities. By detecting potential issues early, monitoring minimizes costly delays and helps maintain project timelines. Furthermore, these systems have been contributing to data-driven decision-making to improve stakeholder confidence, and offer long-term value by facilitating maintenance and operational planning for completed structures.
Tunnel Construction in North America: A Snapshot
Due to obvious and long waiting reasons like aging infrastructure and environmental concerns, the construction of tunnels for metro systems, combined sewage overflow (CSO) projects, railways, and water conveyance/flood protection have been booming across North America. Many of these projects rely on TBMs and involve large-diameter tunnels sometimes exceeding 30 ft and mining in populated areas. Ground disturbance due to construction activities constitutes large but manageable risks. With such massive undertakings, I&M plays a critical role in understanding the impact of support of excavation (SOE) and ground subsidence within the zone of influence. Many projects have successfully integrated monitoring systems to ensure safety and performance, demonstrating the industry’s ability to leverage these technologies effectively. Yet, despite its critical role, monitoring sometimes seems like an afterthought or, worse, an expense to be minimized.
The State of Monitoring: Tools and Techniques
Modern monitoring solutions include robotic total stations, conventional structural and geotechnical instruments for pressure, strain, and deformation measurements (piezometers, inclinometers, strain gauges, extensometers, etc.), terrestrial laser scanning, vibration sensors, wireless sensor networks, and even fiber optic sensing (a less common application) in tunnel monitoring. Our experience has showed that a well-defined monitoring scope typically account for around 1-2% of the overall project budget. As Red Book author John Dunnicliff aptly stated, “The lowest cost of an instrument should never dominate the selection.” The importance of qualified instrumentation providers cannot be overstated, as selecting the right partner ensures the monitoring program’s success. Using experienced monitoring provider is more critical than which instrument type or brand is selected.
The following are some challenges that a typical monitoring scope in a tunnel project faces along with some select solutions.
1. Monitoring is a team effort
Large, publicly funded projects place monitoring design and oversight squarely under the control of owners, regulators, and the owner’s engineers. While some contractors may see monitoring as a compliance requirement, many have embraced it as a key component of risk management. Defining the best monitoring application requires detailed planning and coordination among stakeholders. Instrumentation must be integrated in tandem with the construction sequence. This means instrumentation specialists being part of the project team who understand the construction stages and structural components.
Solution: Implementation of the I&M program is a critical path activity that must be thoughtfully considered and coordinated along with more traditional construction activities and milestones. It is a good practice to foster collaboration early in the project to align expectations between owners, engineers, contractors with the instrumentation specialist. Regular workshops and planning transparency can help bridge the trust gap and reinforce the value of monitoring. Assigning a monitoring coordinator role/function to act as a liaison between design and construction teams will ensure that instrumentation plans align with project timelines and goals.
2. Let’s be more specific
Instrumentation specifications prepared by consulting firms or designers are treated as the project “bible.” However, the specifications are often based on references from previous projects, some of which date back decades. This reliance on outdated specifications can hinder the successful implementation of monitoring systems by failing to account for project-specific needs or advancements in technology. The detail below is from a tunnel specification solicited in 2023, that requires a complicated set-up utilizing two antiquated uniaxial tiltmeters (prone to many issues), intended to take biaxial measurements, neglecting the fact that there are modern “all-in-one” biaxial tiltmeters available on the market.
Solution: Specifications must be carefully tailored to the specific needs of each project and updated to include the latest technological advancements. Engage experienced instrumentation specialists during the planning phase to review and refine specifications, ensuring they are relevant, precise, and aligned with current industry standards.
Once a project is awarded, the process to get newer technology approved as equal is long and uncertain. All too frequently, I&M providers must resort to deploying older, less reliable instrumentation on a project just to adhere to an outdated specification so that they can meet a project’s schedule. Some of the latest instrumentation on the market will typically meet or exceed the accuracy and precision requirements, while also offering competitive cost, increased reliability and decreased maintenance costs which is an overall benefit to the project.
3. Evaluating the data on what basis
Baseline monitoring is critical for understanding any structural behavior. How the structure behaves under various seasonal conditions shows significant variation and lack of, or short baseline periods can lead to incomplete data, making it difficult to establish reliable trends. How feasible is it to understand structural behavior with just one week of baseline monitoring data for a project with five years duration?
Solution: In recent years, some owners have instinctively called for baseline monitoring to take place well in advance via separate contract prior to tunneling contract and collect adequate data which can be handed over. It is advisable to extend baseline monitoring periods whenever possible and design them to capture meaningful trends. Early investment in quality baseline data pays dividends later.
4. Manual vs. Automated Readings
The natural tendency of project staff favors manual readings because personnel are already onsite. However, automated measurements provide more data, greater accuracy, and reduced human error. Eliminating human related variation in a deformation monitoring data (survey prism) that is targeting 2mm vertical accuracy is a better approach. Manual readings also are not near-real time, the process of collecting the data, processing, checking, uploading to a web-based monitoring platform typically can take from a few hours up to a day. An automated monitoring system is customizable and can reduce this time to a few minutes.
Solution: It is always good to establish automated measurements as the standard, supported by manual readings only for redundancy and critical checks. Currently available data acquisition and transmission systems have considerable battery life (wireless nodes with 5 years), which eliminate frequent routine maintenance visits.
5. Redundancy is the key
Data redundancy in instrumentation monitoring is crucial for ensuring reliability, accuracy, and confidence in the collected data. Redundancy is collecting more than one data point to monitor specific location for targeted impact, and allows for cross-verification of measurements from different instruments monitoring the same parameter. If one instrument reports an anomaly or error, data from another instrument can confirm or refute the result. When an instrument shows a concerning trend or exceedance, redundancy is crucial to confirm or refute the observation. For example, if deformation is measured by an optical prism installed at the top of Support of Excavation (SOE) system, an in-place inclinometer can corroborate the movement, and provide additional insight by mapping the deformation profile of the SOE. Accurate, corroborated data helps stakeholders make informed decisions quickly, reducing delays and preventing unnecessary interventions.
Solution: Implement complementary monitoring methods as a standard practice to ensure reliable data interpretation. Data redundancy is not just a safeguard but a proactive strategy that strengthens the reliability of instrumentation monitoring. It builds resilience into the system, ensuring that decisions are based on verified, high-quality data, which ultimately enhances safety, efficiency, and project success. Given the high risk associated with tunnel projects, redundancy should be key I&M feature to ensure the integrity of the monitoring system.
6. Data Management
In current practice, data management platforms have reached an advanced stage, offering tools to display, share, and analyze information in real time. Common data management platforms are proven to be useful to not only simplify data visualization but are also evolving to integrate advanced engineering analysis and design features, enabling stakeholders to make informed decisions quickly. Despite these advancements, these tools are often underutilized or confined to a small subset of the project team.
Solution: Make data management platforms a core component of the monitoring strategy. Train all relevant stakeholders, including contractors, engineers, and owners, to use these platforms effectively. By leveraging these tools to their full potential, teams can ensure real-time collaboration, enhance project efficiency, and improve risk mitigation. Regularly update and customize dashboards to reflect the specific needs of each project.
A Call to Action
Based on our long experience we recommend that I&M should be recognized as a critical component of risk management and project execution. By embracing advanced monitoring technologies, fostering collaboration, and prioritizing data quality, the tunnel industry can continue to enhance its reputation for safety and innovation.
Many projects have already set an excellent example by successfully implementing robust monitoring systems. By building on these successes and further integrating best practices, the industry can unlock the full potential of instrumentation and monitoring to safeguard our projects, investments, and communities.
Authors: M. Melih Demirkan, Peter Laskaris / Wang Technology
