Tunnel Classifications and Construction Techniques

Tunnel Typology

General Tunnel Classifications

• True tunnels: Typically constructed by boring or mining.
• Cut-and-cover tunnels: Involve slope excavation and the construction of retaining walls, with the tunnel then covered.
• Submerged tunnels: Constructed underwater, often by sinking pre-fabricated sections.

Classification by Ground Type

• Soft ground tunnels: Require specialized soft excavation techniques and robust support systems.
• Rock tunnels: Involve hard rock excavation methods and typically require moderate support.

Classification by Purpose

• Without inner pressure: Examples include hydraulic, railway, road, pedestrian, and hybrid tunnels.
• With inner pressure: Such as pressure galleries where the ground's stiffness significantly influences design and stability.

Classification by Construction Method

• Without Tunnel Boring Machine (TBM) in soil or rock: Examples include full-face excavation, heading and bench methods, and multiple-phase construction.
• With TBM in soil or rock: Utilizing rock TBMs, mechanized shields, or double shields for excavation.

Classification by Transverse Cross-Section

• Horseshoe: This is a conventional construction shape. It typically features a round arch vault, straight walls, and a partial circular or flat floor (invert arch).
• Circular: Commonly constructed using a Tunnel Boring Machine (TBM).
• Rectangular: Often associated with cut-and-cover tunnels, sometimes referred to as "false tunnels."
• Other shapes: Various other cross-sectional shapes may be used depending on specific requirements.

Classification by External Conditions

• Interurban tunnels: Located outside of city areas.
• Urban tunnels: Construction in urban environments can potentially cause effects on nearby structures and infrastructure.

Classification by Relative Position

• Single tunnel: Consists of only one tunnel bore.
• Twin tunnels: Two parallel tunnels situated either in the same horizontal plane or in the same vertical alignment (piggyback configuration).
• Multiple tunnels: More than two tunnels in close proximity.

Tunnel Construction Methods

Methods Without Tunnel Boring Machines (TBM)

These methods generally involve three main stages: excavation, installation of primary and temporary support systems, and construction of the permanent lining (often cast-in-situ concrete).

Excavation Techniques (Non-TBM)

Common non-TBM excavation techniques include:

• Drill and blast
• Roadheader
• Impact hammer
• Mechanical excavator

Drill and Blast Method

The drill and blast method can be used in various ground types. However, for soft ground conditions, adequate ventilation is essential due to the potential release of toxic gases from explosives. The explosives generate a compressive pressure wave that causes the rock to fracture. Key irregularities encountered with this method include transverse irregularity due to rock bedding and longitudinal conicity resulting from the obliquity of contour drill holes.

Process:

• Perforation (drilling of blast holes) and execution of the blast.
• Scaling (saneation, i.e., removal of loose rock) and mucking (removal of debris).
• Installation of rock bolts and other support elements to sustain the excavation.
• Application of final projected concrete (shotcrete) or other lining components.

Roadheader Excavation

With roadheader excavation, the harder the rock, the greater the force the machine will need to exert. The selection of the appropriate roadheader is a critical part of the process, and the rock's uniaxial compressive strength and abrasion index are key factors in this choice. Roadheaders are typically used for excavating medium-hard rocks, such as limestones. Rock formations harder than this usually require excavation with a TBM or the drill and blast method. This technique consists of the mechanical cutting or wearing of the tunnel face by the roadheader's rotating cutter head and, simultaneously, the collection and removal of the excavated material.