Tunnels require maintenance and refurbishment to ensure their safe operation over long periods. Refurbishing a tunnel ventilation system presents various challenges, due to the demands of new standards and constraints by the existing structure.
INTRODUCTION
In road tunnels, the installations for operation and safety require regular maintenance and replacement. Replacement may require significant interruptions to normal tunnel operation. Although each sub-system may have a different design life, the replacement is coordinated to minimize tunnel closures. Some items may allow operation beyond their nominal design life; other items may require early upgrades.
In 2004, the 2km-long Tunnel Giswil was one of the first tunnels in Switzerland equipped with a local smoke extraction using remote controlled dampers1. After more than 20 years in operation, some parts of the tunnel ventilation system reached their nominal design life. In this article, we discuss design decisions regarding continued operation, early replacement and system upgrades for the tunnel ventilation system.
TUNNEL GISWIL: VENTILATION SYSTEM SINCE 2004
Most road tunnels in Switzerland are fully automated. There is no local control centre for tunnel operation. All systems are designed to operate autonomously. Following the disastrous fires in the Montblanc, Tauern and Gotthard Tunnels in 1999 and 20012, the Swiss Road Administration developed a new national design code for tunnel ventilation3.
Tunnel Giswil was designed based on an early draft of this design code. Experience gathered in the operation of the tunnel was then used to update the code.
Tunnel Giswil is operated with bi-directional traffic. It is one of the first tunnels in Switzerland equipped with a local smoke extraction using remote controlled dampers.
Smoke is extracted through an overhead smoke duct towards the fan building at the northern tunnel portal.
The smoke dampers are not evenly spaced along the tunnel. The damper actuators are controlled to allow open and closed positions, and an intermediate position.
The smoke dampers receive their control signals via optical fibre. The smoke extraction system is designed for a minimum flow rate of 144m³/s.
Two axial fans discharge the smoke through a vertical outlet. The axial fans have a 2m-diameter and each has nominal power of 260kW. Four jet fans are installed in a 400m-long tunnel section at the southern portal. This section is not equipped for smoke extraction. The jet fans are controlled to switch on and off based on the measured airflow in the tunnel. In normal operation, the extraction system is used as a longitudinal ventilation with midpoint extraction. The daily traffic volume is about 11,000 vehicles per day with 3.5% heavy goods vehicles. Traffic is increasing as the connection allows residents to commute from the rural area to the city of Lucerne in the north.
Tunnel Giswil has a parallel egress tunnel with seven cross-connections to the traffic tunnel. The egress tunnel is equipped with a pressurisation system. It is designed for moderate overpressure in normal operation and an airflow of 1m/s in three simultaneously open egress doors in emergency operation. The pressurisation system consists mainly of two axial fans and pressure relief dampers at the portals.
REFURBISHMENT PROJECT 2025
After 21 years of operation, some of the ventilation equipment reaches the end of its nominal design life:
- Control system – 15 years
- Jet fans – 20 years
- Smoke dampers – 25 years
- Axial fans – 30 years
The refurbishment project aims to renew and repair the tunnel ventilation system to such an extent that safe operation can be expected until 2040. The measures must be financially viable and meet the goals of sustainability.
In this project, this meant that changes to the civil construction shall be avoided or kept at a minimum.
Also, it must be evaluated if urgent measures must be fast-tracked as deviations from the safety targets may require immediate action. Some system components, such as the variable frequency drives for the exhaust axial fans, the airflow monitors as well as the visibility monitors in the tunnel have already been replaced as part of regular maintenance. It is expected that the tunnel sensors can be used until 2040. The exhaust fans are in good condition, but their capacity does not meet the requirement of the new design code; it requires an increase of 33% to a flow rate of 192m³/s. This equates to a flow velocity in the tunnel of 2m/s from both sides towards the extraction section. The total thrust of the tunnel jet fans does not comply with the current design code. For new tunnels, the code requires more conservative assumptions for barometric and wind pressure. Also, the design should consider increased traffic volume since 2004. New jet fans would have to have the thrust increased by 28%.
The ventilation control system has already exceeded the nominal design life. While the current operation is regularly tested, support by the original supplier is no longer available. The algorithm for airflow control in emergency ventilation is outdated. New systems are equipped with standard PI-controllers to allow faster and more reliable airflow control.
The design of the egress pressurisation system includes safety margins as it had to be based on assumptions regarding leakage. This causes higher power consumption. Since 2004, the requirements for egress pressurisation systems have been updated based on the experience of new egress tunnels in Switzerland.
DESIGN DECISIONS
One significant advantage of a refurbishment design for a tunnel ventilation system is that the design can be based on measurements in lieu of assumptions regarding friction losses and leakage. As part of a research project in 2011, measurements of pressure losses and flow rates have been conducted for Tunnel Giswil. These have now been used as a reference for the design. Three options have been investigated for the extraction capacity:
- 1:1 replacement of the fans to achieve the current exhaust flow rate 144m³/s;
- Design for an exhaust flow rate of 175m³/s, optimised according to the admissible pressure loads of the false ceiling and the smoke dampers; and,
- Design according to the current standard 192m³/s.
It was decided to deviate from the design code and implement an optimised extraction capacity of 175m³/s.
With the reduced flow rate, exhaust fans can be selected with the same size as the current fans. Changes to the fan room layout can be avoided.
Also, the static pressure in the smoke duct does not exceed the design limits of the smoke dampers and of the false ceiling. The reduced flow rate achieves a tunnel airflow of 1.8m/s from both sides towards the extraction section, which is sufficient to limit smoke propagation. For the smoke dampers, three options have been investigated:
- 1:1 replacement;
- Replacement by dampers with increased cross-section – to match the increased extraction flow rate; and,
- Redistribution of the dampers in equal distances along the tunnel.
A redistribution of the dampers and an increase of the damper size would require a significant change to the civil construction and prolonged tunnel closures. While the current dampers may cause an increased pressure drop with the higher flow rate, the effect is acceptable. A close inspection of the smoke dampers and their maintenance records led to the expectation that the dampers can be operated safety until 2040 without major intervention beyond regular scheduled maintenance. The damper actuators must be replaced as spare parts are no longer available. Also for the jet fans, three options have been investigated:
- 1:1 replacement;
- Replacement of four jet fans with a total thrust to meet the new design code; and,
- Replacement of six jet fans with a total thrust to meet the new design code, including redundancy for one group of jet fans being unavailable.
Today’s four jet fans will be replaced by six jet fans to meet the requirements of the current design code regarding thrust and redundancy. This is possible due to the 400m-long section between the end of the smoke duct and the southern tunnel portal. While in a new tunnel only 300m would be acceptable, this section can now be used to accommodate three pairs of jet fans.
Two of the six fans will be controlled by variable speed drives to allow improved airflow control in emergency ventilation. This partly compensates the exhaust capacity not meeting the requirement of 192m³/s.
The axial fans of the egress pressurisation system will be replaced by fans better adapted to the measured operating conditions of the egress tunnel. The fans will be of the same size as the existing fans to minimise changes to ventilation ducts and installations. It is expected that the new fans will allow for a reduction of installed power and power consumption in normal operation by 50%.
The tunnel ventilation control system will be replaced completely. The new system will include an improved control algorithm for the jet fans in emergency ventilation.
It will be based on a PI-controller which – in combination with speed control on two of the six jet fans – will allow faster and more stable airflow control.
The ventilation in normal operation will also be changed from mid-point extraction to longitudinal ventilation with jet fans. Due to the reduction of vehicle emissions since 2004, the ventilation is rarely operated to maintain in-tunnel air quality. The change from midpoint extraction to jet fan operation makes the system even more sustainable.
The refurbishment project for the Giswil Tunnel is scheduled for completion in Autumn 2027.
IMMEDIATE ACTION
The annual tests of the tunnel equipment showed an increase in communication faults between the tunnel ventilation control system and the damper actuators. As the operation of the smoke dampers is a safety-critical component, it was decided to fast-track the resolution of these issues. The intervention had to be kept at a minimum not to interfere with the regular refurbishment of other system components. As time was critical, small contracts had to be awarded to specific suppliers complying to the regulations for contract tenders.
It was decided to replace the full signal chain from the communication gateway in the control system to the damper actuators. The actuators had to be replaced completely as there were no spare parts available for the communication modules. The damper supplier offered the installation of the replacement actuators complete with modifications to the dampers and new protective covers for the damper actuators. The intervention also included the replacement of the optical cable connecting the actuators to the control system. The replacement took place in November 2025 during night-time closures of the tunnel. During the closures, two actuators were exchanged including the modifications to the dampers and the installation of the new actuator covers. The dampers were connected to the new communication fibre and tested before the tunnel was re-opened to morning traffic. With all smoke dampers exchanged, the ventilation system was re-tested for all potential fire sections.
CONCLUSION
Refurbishing a tunnel ventilation system requires a multi-pronged approach:
- Analysis of the existing system;
- Deficiencies in reference to current standards;
- Remaining design life of system components (nominal and effective);
- Availability of spare parts;
- Availability of measurements of system performance;
- Coordination with other design disciplines;
- Constraints by the civil construction or other project specific constraints;
- Minimise interruption to normal tunnel operation; and,
- Immediate action for safety deficits.
In the refurbishment project for the ventilation system of the Tunnel Giswil, we had all the items above. But this is not an exception for a refurbishment project. It is what you expect. It is more complex than the design of a new system. Working in night-time shifts is also not to everyone’s liking. But a refurbishment project is also rewarding as you can learn from everything that worked smoothly the past 20 years.
