Meta title:Revolutionizing Tunnel Construction: Advanced Full-Section TBM Excavation Techniques
Meta Description: Explore advanced full-section TBM excavation techniques and key terminology for efficient, cost-effective tunnel construction in challenging geological conditions.
Key words: TBM project, tunnel projects, steel structure
Revolutionizing Tunnel Construction: Advanced Full-Section TBM Excavation Techniques and Key Terminology
1. Scope of Application
The full-section tunnel excavation method employs a Tunnel Boring Machine (TBM) to develop tunnels through continuous excavation. In this process, the TBM utilizes a rotating cutter head to continuously shear the face of the rock mass, with an integrated muck removal system that immediately conveys excavated material out of the tunnel. Behind the machine, equipment such as segment erectors and rock bolt installation systems are employed to immediately install pre-cast lining segments and steel sets, thereby enabling a continuous cycle of excavation and support. This method is particularly effective for long tunnel projects. Successful implementation of TBM-driven excavation depends not only on favorable geological conditions and robust machine design but also on the coordinated efforts of skilled personnel, comprehensive planning, and efficient project management to achieve both rapid and cost-effective outcomes.
When designing a TBM, factors such as the hardness and abrasiveness of the rock must be considered to select appropriate cutting tools (cutter disks), cutting motors, bearings, and other rotary cutting system components. The machine’s performance is inherently influenced by the properties of the rock being excavated, necessitating designs tailored to specific geological conditions or even localized modifications. Ideally, the rock mass should exhibit moderate strength, high homogeneity, low hardness (to minimize wear), minimal jointing, and absence of faults or significant groundwater inflow.（Welcome to learn more at Glory Rail!)
TBM schematic drawing
TBMs can generally be classified into two categories based on the design of the cutter head and shield: open type and closed type.
Open-type TBMs are best suited for tunnels with favorable geological conditions where expansive or squeezing ground and high water inflow are not expected. Closed-type TBMs, on the other hand, are employed in environments characterized by variable geology, expansive or squeezing conditions, or significant water inflow.
Closed-type TBMs feature a steel shield that encloses the machine’s perimeter to protect against loose rock and are typically used in conjunction with concrete segmental linings. The shield configuration may be either single-shield (integrated with the main machine) or double-shield (or telescopic shield). In a single-shield TBM, the shield and support installation equipment move as a single unit with the machine, necessitating a temporary halt for segment erection. In contrast, a double-shield TBM comprises two distinct shields: the front shield advances with the cutter head, while the rear shield provides reaction force by bracing against the tunnel walls until a full cycle of excavation and segment erection is completed.
When the TBM shield encounters significant shear zones or fault materials, the surrounding rock may become unstable and collapse, potentially increasing the cutter head’s torsional load beyond safe limits and hindering machine progress. Under such conditions, localized ground improvement measures are typically required. However, the challenges associated with ground improvement—including technical limitations, effectiveness, high project costs, and potential downtime of the TBM—are among the most critical issues in TBM operations.
For instance, in the Pinglin Tunnel, an 11.74-meter diameter full-section double-shield hard rock TBM was utilized. Including its support systems, the machine has a total length of approximately 240 meters, weighs 2,500 tons, and is equipped with 83 cutter disk mounting positions and two independent propulsion systems. Under favorable geological conditions, the gripper system can engage the tunnel walls to provide the necessary reaction force, enabling simultaneous excavation and segment erection. In poorer ground conditions, a single-shield operation is adopted, wherein the tail shield assists with jacking to push against the segments, after which the segments are erected.
TBM Bottom View
2. Key Terminology and Components
Cutter Head:
A rotatable, integral steel disc positioned at the very front of the TBM, outfitted with dozens of cutter disks to press against and shear the rock face.
Cutter Disk:
A tungsten carbide steel annular disc, typically several tens of centimeters in diameter, mounted on the cutter head. Its number and configuration are designed based on the rock characteristics. When the cutter head rotates, adjacent cutting marks are typically spaced 8 to 10 centimeters apart. Cutter disks on the outer ring often extend beyond the cutter head edge to produce an excavation profile slightly larger than the cutter head, reducing friction between the shield and the rock face. As a consumable, cutter disks require frequent replacement; however, they can be reconditioned after re-steellining, with carbon content adjusted according to rock hardness.
Conveyor System:
A set of components including power cars, muck cars, tracks, and auxiliary parts that transport the excavated material (muck) from the TBM out of the tunnel.
Drive System:
The primary power system that rotates the cutter head, typically powered by high-voltage electrical systems, although hydraulic drive options also exist.
Gripper:
Devices used when the rock is sufficiently competent to support TBM advancement. Grippers, aided by hydraulic jacks, press against the tunnel walls to provide the necessary reaction force for TBM progress. The size, number, and placement of the pressure plates depend on the strength of the rock; in softer ground, alternative jacking systems may be required to supplement the reaction force.
Advancing Jacks:
Multiple jacks distributed around the rear of the cutter head that apply pressure against the excavation face, propelling the TBM forward. Once a stroke (typically 1.5 to 2 meters) is completed, the jacks retract to pull the rear shield forward.
Conveyor Belt:
A belt system that collects the rock fragments dislodged by the cutter disks, which then fall through gaps between the cutter head and the rock face, are directed by scrapers into a hopper, and subsequently transported out of the tunnel.
Invert Segments:
Pre-cast lining segments installed at the tunnel invert. In open-type TBM operations, the rear part of the TBM is equipped with invert segments and rails. To accommodate the weight and stability requirements of muck cars, larger invert segments are used, with PE foam strips affixed at the front edge to serve as water stops.
Rotary Muck Inversion Device:
Located on the external work platform, this device rotates the muck car 180° to facilitate the efficient unloading of excavated material without the need for detachment from the truck.
Penetration Rate (meters per hour):
The ratio of the TBM’s excavation advance (in meters) to the cutting time (in hours).
Utilization Rate (%):
The ratio of the TBM’s actual cutting time (in hours) to the total working time (in hours), expressed as a percentage.
Excavation Rate (meters per hour):
The overall advance of the TBM (in meters) per total working hour, calculated as the product of the penetration rate and the utilization rate.
Full-Ring Steel Support:
In adverse geological conditions, when the rock behind an open-type TBM’s cutter head is exposed, full-ring steel supports along with steel mesh are immediately installed to secure the tunnel and prevent further rock degradation.
U-Shaped Channel Steel:
For moderate geological conditions where the rock is exposed behind an open-type TBM’s cutter head, U-shaped channel steel and steel mesh are anchored to the rock using rock bolts to lock in small rock fragments and prevent the collapse of larger rock wedges.
Rock Bolt:
Installed in pre-drilled holes at the TBM’s rear, rock bolts enhance the support of the surrounding rock mass. Typically mechanical rock bolts are used for ease of installation and immediate support, serving both to secure U-shaped channel steel and to provide arch support.
Shotcrete:
Applied in a dry mix process, shotcrete generates dust and rebound material; therefore, the spray application must be positioned at a safe distance from the TBM main body. Additionally, a 1-meter wide, 5-centimeter thick layer of shotcrete is applied along both sides of the invert segments to prevent seepage from the tunnel walls or drainage channels, which could lead to rock softening or even derailment of muck cars.
3. Conclusion
The full-section tunnel excavation method leverages the continuous operation of TBMs to combine excavation and support installation seamlessly, making it particularly well-suited for long tunnel projects. The detailed understanding of TBM components and key terminology presented here underscores the significant advantages of this method in addressing complex geological challenges, thereby offering a rapid, economical, and reliable solution for modern tunnel construction.
Glory Rail not only provides the relevant steel structures and rails for TBMs prior to construction, but also offers customized services. We bring the quality products to your project, adds fuel to your project completion.