| Written by Michal Wozniakowski-Zehenter
Tunneling: Planning and Feasibility Study
The construction process of a road tunnel starts with extensive planning and feasibility studies, the most critical stage, as it lays the groundwork for the entire project. Work begins with a general site investigation pertinent to understanding the geological conditions along the proposed tunnel route. This involves a detailed site survey with geological and engineering teams and embraces the drilling of boreholes, seismic surveys, and soil and rock samples. These studies determine the geotechnical properties of the terrain, in particular the parameters of rock environments and groundwater, which are crucial to the tunnel design.
Along with these geologic studies, an EIA - Environmental Impact Assessment is conducted to assess the potential effects of tunnel project activities on the environment, including local wildlife, water resources, and areas close to populated areas. Public consultations and stakeholder engagements are also conducted to get inputs and raise concerns, if any.
Once the site investigations and environmental assessments have been completed, engineering proceeds to the designing phase of the tunnel's alignment. The alignment is made based on several factors, including a route that is as short as possible, safety factors (read more about safety training equipment), and economic feasibility. Detailed engineering designs are prepared, which include the tunnel's dimensions, structural components, and construction methods.
Financing and Approvals
Securing funding and approvals are the most crucial stages in building a tunnel. Infrastructural projects large enough, like a road tunnel, present quite extensive costs. The funding may come from the government's budgets, private investments, or public-private partnerships. Financial planning is developed to include the project's financial estimate, the source of the finances, and the prospective revenue profitability.
At the same time, the project also needs to navigate numerous bureaucracies for approvals and permits, such as those for environmental, land acquisition, and construction. Bureaucratic procedures are cumbersome and time-consuming; typically, a delay in approval may affect project timelines. Close coordination with the regulatory bodies would be needed to meet the legal requirements and obtain the approvals smoothly.
Detailed Design and Engineering
Having secured financing and project approvals, the project then proceeds to the Detailed Design and Engineering stage. Here, designs are further developed, and detailed construction plans are prepared. Engineers do detailed analyses to determine the best methods, materials, and plants to be employed in the tunnel. This includes designing ventilation, drainage, and safety systems in the tunnel.
One of the critical considerations in detailed design is the choice of the appropriate tunnelling method. This would be dependent upon the geology, length, and diameter of the tunnel. Common methods of tunneling include tunnel boring machines (TBMs), the New Austrian Tunneling Method (NATM), and the cut-and-cover method. Each of these methodologies has some enlisting parameters of favour and limitation, and the choice has to be made based on a proper analysis of the requirements.
Procurement and Contractor Selection
Once the detailed design process is completed, the next step is procuring and selecting contractors. The process may involve an invitation to tender or negotiation of other forms of contracts with construction companies for bidding on the project. In selecting the contractors, the client is to use very rigorous selection criteria by considering the technical competencies of the bidding equipment, experience, and financial capability. Competitive bidding is administered to ensure that the contractors undertake the project at their disposal and avail the project within the stipulated budget and time.
The procurement stage also includes the acquisition of materials, equipment, and machinery needed for construction. For such big and complex tunnel projects, what's imperative is an assurance of the reliability of the supply chain and that delivery of materials will remain on schedule with no construction delay.
Tunneling: Excavation
Excavation is the initial actual construction phase of a tunnel. There are various ways to dig through earth and rock to create that all-important tunnelway. The selection of the method of excavation is based on the geology and the tunnel construction method that is selected. For soft ground, the cut-and-cover technique and/or TBM application are most in use. For hard rock conditions, drilling and blasting or NATM may be selected.
The TBM is assembled at the working place and set up to start boring. TBMs are very effective tools and can excavate large amounts of material while making tunnel linings at the same time. In this manner, tunneling over long distances might be very much preferred due to the reduction in surface impacts.
The cut-and-cover method is applied to construct shorter tunnels through the unfavourable ground. In this method, a trench is dug, and then the excavation structure within it is constructed and backfilled with earth. This method is easier but may cause considerable surface disruption.
The NATM is more flexible in its application to varying ground conditions. It involves the sequential excavation of small portions of the tunnel supported by temporary linings in series and then filled with permanent lining. This method is suitable for very complex conditions and allows real-time adjustments to be implemented based on ground behaviour.
Tunnel Support and Lining
Tunnel support and lining are installed to ensure stability and safety during the excavation process. Types of support or lining will obviously depend on the type of ground and method of construction undertaken. Pre-cast concrete segments used as tunnel linings are widely common when ground conditions are soft. They are placed right behind a TBM in its progress and give structural support to the tunnel's walls.
Rock bolts, shotcrete, and ribs provide tunnel support in hard rock conditions. In various other methods used through the NATM process, shotcrete and rock bolts are combined to temporarily line the excavated sections. Once the excavation is completed and the ground stiffens, the permanent lining is done.
At this stage, drainage is also essential. Groundwater inflow creates many problems since it poses a challenge in water build-up that exerts pressure on the tunnel structure. Establishing drains with pumps and pipes to allow drainage is vital for controlling groundwater that may result in flooding.
Installation of Systems and Services
Once the tunnel structure is in place, systems and services are installed. This consists of ventilation, lighting, communication, safety, or other installations a tunnel might need. Particularly in the case of making a long tunnel, ventilation is one of the major concerns because it ensures that the exhaust fumes are unblocked and that the airflow inside is fresh. A ventilation system comprises fans, ducts, and ventilation shafts located within the tunnel.
The installation of lighting systems provides illumination to ensure safe driving conditions. The lighting systems are designed to minimize glare and provide uniform lighting in the tunnel. Communication systems such as emergency phones and intercoms are installed to ensure smooth communication between users inside the tunnel and control rooms.
Provision for safety systems such as fire and suppression, emergency exits, and signage is also incorporated. These systems ensure that all tunnel users are safe should an emergency occur. The systems must be installed to ensure total operability before the tunnel is actually put into use.
Tunneling: Testing and Commissioning
Before admitting traffic into the tunnel, very comprehensive testing and commissioning must be done. This involves thorough and elaborate testing of all systems and components to achieve the set safety and performance criteria. Partial-depth track slabs are subjected to a structural integrity test to ensure the stability and strength of the tunnel lining and support systems.
Check the ventilation and lighting systems to meet the required standards of air quality and illumination measurement, respectively. Test the fire detection and suppression systems for conformance to ensure that they work just fine in an emergency. Test the communication and control systems for proper functioning.
This is followed by a final inspection undertaken by the regulatory agencies to ensure that the tunnel complies with all safety and regulatory requirements. The deficiencies noted during the testing and inspection stages are acknowledged and corrected before the tunnel is declared fit for operation.
Challenges and Hazards in Tunneling
Constructing road tunnels brings with it several challenges and hazards whose management, therefore, needs to be keenly followed to protect the workers and ensure the project's success. One of the biggest challenges in tunneling is dealing with continually varying geological conditions. Unexpected geological conditions, such as weak rock, high groundwater pressure, or fault zones, can give rise to significant delays and high construction costs. Detailed site investigations and monitoring during and after excavation are the key to their effective management.
One of the major tunneling problems is groundwater inflow. Excess groundwater would cause flooding, put pressure on tunnel linings, and further complicate excavation. Therefore, adequate, well-planned, and well-operated groundwater management systems, including dewatering pumps and drainage systems, should be implemented to mitigate water problems.
Ventilation is a key aspect that applies particularly to long tunnels. The need for proper ventilation in long tunnels is high in order to reduce the accumulation of exhaust gases and to ensure that there is a supply of fresh air in the tunnels to workers and tunnel users in general. Inadequate ventilation results in an improper atmosphere, causing health hazards for workers and drivers. Design and efficient ventilation systems need maintenance in various ways to guarantee air quality and safety.
Fire risk remains an ever-present danger during tunnel construction. Equipment malfunction, electrical faults, and accidents all work together to generate a fire. Fire detection and suppression systems must be effective and reliable in quickly detecting an outbreak and extinguishing it. Emergency response plans and required drills help workers prepare in case of outbreaks.
Another key issue to establish in a tunnel is its structural stability. The design made for the tunnel lining and support systems must be able to resist the different developing forces and stresses from the ground around it. An inadequately low level of support is responsible for tunnel collapses, which are associated with grave risks to workers and users. In the long run, proper monitoring and routine maintenance are key to attaining long-term stability in the tunnel.
Limited access and egress, poor ventilation, and potential entrapment add another dimension to hazards in confined spaces. With this knowledge, proper training and safety measures are to be deployed to protect personnel working in confined spaces. Safe working in confined spaces can be achieved by good ventilation, monitoring potentially hazardous air quality conditions, and providing carefully located emergency escape routes.
In the interview for the California Department of Water Resources, Dr Greg Korbin, a veteran of numerous big tunnel projects, including the Bay Area's BART extension to San Jose, the Port of Miami tunnel under Biscayne Bay, and the State Route 99 tunnel in Seattle said: "The gas wells are a hazard, and they're recognized as such. There are extensive maps of the gas wells. We'll try to avoid as many as possible. Some have been abandoned. If, perchance, and there's always a chance, something isn't found or hasn't been correctly abandoned, and the machine was to hit one of these wells, there are safety devices on the machine that will immediately shut it down and lock everything off to avoid allowing the gas into the tunnel. Of course, this would lead to a significant delay because they'd have to now abandon the well correctly before advancing the machine. But, again, worker safety would be handled."
Another big problem is handling and transporting materials and equipment in tunnel construction. Delivery of materials and transportation of equipment and machinery is doubtful because the tunnel is a narrow space with few entry and exit points. Careful planning and coordination have to be made to ensure the timely and safe delivery of materials at the construction site.
Delve deeper into safety in tunneling and how to approach it.
TAKEAWAY
Road tunnel construction is a complex and demanding process that involves meticulous planning, advanced engineering, and skilled labour. From the initial planning and feasibility studies to the final testing and commissioning, each phase of tunnel construction presents unique challenges and hazards. Effective management of these challenges is essential to ensure workers' safety and the project's successful completion (continue reading: Tunnel construction safety).
Despite the inherent difficulties, road tunnels play a crucial role in modern infrastructure, providing essential connectivity and contributing to economic growth. Advances in tunneling technology and construction methods continue to improve the efficiency and safety of tunnel construction. By addressing the challenges and hazards with careful planning and innovative solutions, engineers and construction professionals can successfully build road tunnels that stand the test of time.
Sources:
(1) https://water.ca.gov/News/Blog/2021/October/QA-Building-the-Tunnel
(2) https://tunnelsmanual.piarc.org/en/strategy-and-general-design-strategic-issues/steps-tunnel-life
Author
Michal Wozniakowski-Zehenter, Marketing Manager
Michal Wozniakowski-Zehenter is an experienced marketing and project management professional. He spent most of his career on projects with a strong focus on digital marketing and event management. He is a very active voice representing offshore and mining industries through social media channels. Michal writes mainly about offshore oil and gas, renewable energy, mining and tunnelling. Compiling and sharing the knowledge within industries is one of his goals.
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