Tunneling Methods and Safety Hazards in Underground Construction

Underground construction represents one of the most challenging and hazardous operations in the construction industry. Tunneling work requires specialized techniques and rigorous safety measures to protect workers from multiple threats including cave-ins, flooding, toxic gases, and structural failures. Understanding the various tunneling methods and their associated safety hazards is essential for anyone working in or studying construction safety management.

Common tunneling methods used in underground construction 🔗

Modern underground construction employs several distinct tunneling methods, each suited to specific ground conditions and project requirements. The selection of an appropriate method depends on factors such as ground conditions, groundwater levels, tunnel depth, and the logistics of supporting the excavation.

Cut and cover method 🔗

The cut and cover method involves excavating a trench along the tunnel alignment, constructing the sidewalls and roof, and then backfilling the sides and top with soil. This technique is particularly suitable for shallow tunnels, especially in urban areas where underground stations need to be close to the surface. Prior to excavation, sheet piles, diaphragm walls, or pipe pile walls are installed to support the soil and neighboring structures. While this method causes significant surface disruption during construction, it remains cost-effective for shallow depth projects.

Tunnel boring machines 🔗

Tunnel boring machines are large, cylindrical equipment with rotating cutting heads that excavate horizontally through different types of soil and rock. These machines work like mobile factories, leaving a completely constructed tunnel behind as they progress. TBMs offer several advantages including continuous operation, reduced manual labor exposure to hazards, and minimal surface disturbance. However, they require substantial initial investment and skilled operators. Some TBMs have pressurized compartments at the front to work below the water table, balancing water pressure while operators work in normal air pressure behind.

Clay kicking method 🔗

Clay kicking is a specialized technique developed in the United Kingdom for digging tunnels in strong clay-based soil structures. In this method, the clay kicker lies on a plank at a 45-degree angle and uses a digging tool attached to their feet to excavate the clay. This method was notably used during World War I by Royal Engineer tunnelling companies because it was virtually silent and not susceptible to detection methods. While manual excavation is relatively slow compared to mechanical methods, it remains applicable for small works in hard clay strata.

Shaft method for deep tunnels 🔗

The shaft method involves constructing tunnels at greater depth from the ground surface, where a vertical shaft is built to the required depth. These shafts are permanent structures with concrete walls, circular in section, that facilitate the start and end of deep tunnels. Multiple intermediate shafts are provided along lengthy tunnels. After construction, these shafts serve dual purposes as ventilation systems and emergency exits. The shaft provides essential access for lowering tunnel boring machines and other equipment to the tunnel level.

Primary hazards in tunneling operations 🔗

Tunneling work presents numerous life-threatening hazards that require comprehensive safety management throughout all construction phases.

Side collapse and cave-ins 🔗

Cave-ins and groundfalls represent significant risks during tunneling operations, as unstable ground conditions or errors during construction can provoke catastrophic collapses. Cave-ins occurred frequently in historical tunnel construction due to inadequate reinforcement in tunnel linings. The threat becomes particularly severe in areas with varying geological conditions, where sudden changes in soil or rock composition can destabilize the excavation.

Falling materials and people 🔗

Materials and workers falling into excavations pose constant dangers during underground construction. Loose materials may fall from spoil heaps into excavations, while people working near edges risk falling into deep trenches. The risk intensifies when excavated soil and materials are stored too close to excavation edges, as their weight can contribute to collapse. Even falls from relatively shallow depths can result in fatal injuries.

Damage to adjacent structures 🔗

Tunneling operations can compromise nearby infrastructure and buildings. Many garden or boundary walls have very shallow foundations which are easily undermined by even small trenches, causing walls to collapse onto workers. Urban tunneling brings additional challenges including avoiding existing infrastructure and minimizing disruption to activities at ground level. Surveys of foundations and structural engineer assessments may be required before excavation begins.

Flooding from water ingress 🔗

Groundwater management is one of the major challenges in tunnels, as misalignment with underground water sources can cause potential flooding resulting in water ingress. Research indicates that water and mud inrush accidents account for up to 45 percent of tunnel construction incidents. Historical tunnel construction frequently experienced flooding due to lack of effective waterproofing techniques.

Preventing collapse and falls in excavations 🔗

Multiple protective measures must be implemented to prevent excavation collapse and protect workers from falls.

Battering and sloping techniques 🔗

Battering the excavation sides to a safe angle of repose can make the excavation safer, with the angle of slope in granular soils being less than the natural angle of repose. In wet ground, a considerably flatter slope becomes imperative to maintain stability. This proactive approach significantly enhances excavation safety by reducing pressure on the excavation walls.

Sheeting and support systems 🔗

Excavation sides can be protected by supporting them with shuttering and sheeting, the use of trench boxes, or hydraulic support systems. These protective measures include sloping, benching, shoring with planking or hydraulic jacks, and shielding using trench boxes. The necessary equipment including trench sheets, props, and baulks must be available on site before work begins.

Material storage and barriers 🔗

Excavated soil, materials, and effluent should be placed at a safe distance from excavations as the extra weight can contribute to collapse. Barriers must be installed if the depth of excavation exceeds two meters, though it is suggested for excavations of lesser depths as well. Edge protection should include guard rails and toe boards inserted into the ground immediately next to the supported excavation side, or trench box extensions.

Managing vehicle and utility hazards 🔗

Vehicle control near excavations 🔗

Stop-blocks should be placed approximately 1.5 meters from an excavation to prevent vehicles from falling or causing collapse due to weight pressure. Plant and vehicles should not be parked close to excavation sides, as extra loadings can make the sides more likely to collapse. Vehicle routes must be clearly defined with baulk timber and fencing. Where vehicles need to tip materials into excavations, barriers painted with fluorescent colors ensure visibility, especially during night shifts.

Protecting underground services 🔗

Many serious accidents have occurred when buried services have been damaged during excavation work, with contact with electricity cables potentially resulting in explosion and burns. Damaging underground utility lines can lead to gas leaks, electrical shocks, localized flooding, or water contamination. Cable, pipe, and service plans should be used to mark underground services before excavation begins. Mechanical equipment should be minimized near these zones, with manual tools like spades or shovels preferred over picks and forks that could pierce cables and pipes.

Ventilation and flooding precautions 🔗

Forced ventilation systems 🔗

Workers in tunnels need to be constantly supplied with 200 to 500 cubic feet of fresh air, as air after each explosion is full of fumes and dust unfit for breathing. Provisions must be put in place for proper ventilation to detect dangerous gases like methane and carbon monoxide, which can build up in tunnels. Auxiliary ventilation must be used to provide required airflow to all work areas of the tunnel, with fans arranged to prevent recirculation of contaminated air. The ventilation system must clear the tunnel of poisonous gas and dust within the 30-minute window between explosion and mucking process.

Efficient pumping systems 🔗

Proper pumping equipment must be ensured to prevent flooding by swiftly pumping seeping water out to a safe area. Water entering the excavation needs to be channeled to sumps from where it can be pumped out, though the effect of pumping on excavation stability must be considered. Effective water management strategies include dewatering systems and waterproof linings to maintain a dry and safe working environment. The supports on excavation sides should be designed to control groundwater entry, with additional water loading accounted for in the design.

What do you think? How can construction companies better balance the efficiency of modern tunneling methods with the critical need for comprehensive safety measures? What role should technology play in monitoring and preventing tunnel collapse incidents in real-time?

References

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