Shield Machine Classification

There are many types of tunnel boring machines (TBMs), and they can be classified in various ways. Based on the excavation method—whether manual or mechanical—they are divided into hand-excavated TBMs and mechanized TBMs. Additionally, depending on how the working face is shielded against soil pressure, TBMs can be categorized as open-type or closed-type. Finally, TBMs can also be classified into hand-excavated and mechanically operated types, based on their design and excavation approach.

I. Hand-Driven Shield Tunneling

Hand-dug shield machines are commonly used for excavating small tunnels in areas with favorable geological conditions. They feature a simple design and require minimal auxiliary equipment, as shown in the figure. Depending on the geological conditions of the working face, excavation can proceed either in an open-face manner or with front support—advancing as digging occurs. Its key features include:

1. The excavation face is open, allowing construction workers to observe changes in the geological layers at any time;

2. It's relatively easy to identify and address various underground obstacles;

3. Easy-to-achieve over-excavation requiring precise direction is beneficial for shield tunneling correction and also facilitates construction in curved tunnel sections.

4. The equipment is simple and low in cost. However, the drawbacks of hand-dug shield machines include high labor intensity for construction workers, slow construction speeds, and a tendency to cause quicksand and soil surges when used in unstable, water-bearing strata—resulting in poor construction safety.

II. Push-Type Shield Machines

Squeezing-type tunnel boring machines can be categorized into full-squeezing and semi-squeezing types. In the former, the working face is sealed off by a bulkhead, effectively blocking the surrounding soil and preventing water or soil from entering—thus eliminating the need for soil removal altogether. In contrast, the semi-squeezing type features selectively placed openings in the sealing bulkhead. As the shield advances, soil is squeezed through these openings into the machine itself, where it’s then loaded onto vehicles for external transport. A schematic diagram of the squeezing-type shield is shown here. This type of shield relies on powerful thrust to push the surrounding soil ahead of it, forcing it outward around the machine as it moves forward. It’s particularly well-suited for soft, plastic, cohesive soil layers. However, this method causes significant ground disturbance, so construction beneath existing surface structures should be carefully avoided whenever possible. The grid-type shield, meanwhile, represents a hybrid design that falls somewhere between the semi-squeezing and hand-excavation methods. Instead of a solid bulkhead at the front, it employs a steel mesh with strategically placed openings. As the shield advances, the soil is sliced into manageable chunks by the mesh before being collected inside the machine for removal. When the shield halts its movement, the mesh acts as a temporary barrier, helping to stabilize the excavation face and prevent potential collapse.

3. Mechanized Shield Tunneling

There are two types: semi-mechanized and fully mechanized. In a semi-mechanized shield, earth-moving machinery replaces manual excavation. Depending on the soil conditions, this machinery can include backhoe excavators, spiral cutters, or soft-rock tunneling machines. Since the cost of semi-mechanized shields is relatively lower than that of fully mechanized ones, and because they reduce labor intensity while maintaining high efficiency, they are widely used in underground construction projects. In contrast, a fully mechanized shield is equipped with a full-section rotating cutterhead—matching the diameter of the shield—at the cutting ring, eliminating the need for separate earth-transport equipment. This allows the entire process, from excavation to loading, to be fully automated. Fully mechanized shields are categorized into three main types: open-face shields, mechanically cut-type shields, and closed-face mechanized shields. Among these, closed-face mechanized shields have seen particularly rapid advancements in construction technology across countries worldwide. Currently, closed-face mechanized shields can be further divided into the following three subtypes:

⑴ Local Pressure Shield Tunneling Machine

Between the cutting ring and support ring of the open-face shield, partitions are installed to create a sealed chamber within the cutting-ring section. Compressed air is then introduced into the chamber to counterbalance the earth pressure at the excavation face, ensuring its stability. Compared to the conventional method of injecting compressed air throughout the entire shield tunnel during construction, the localized air-pressure shield approach helps eliminate the risks associated with workers operating under pressurized conditions. However, this method still faces several technical challenges as of today:

5. Due to the small volume of the sealed chamber, the compressed air capacity is limited. As a result, in formations with high permeability, it becomes impossible to maintain stable excavation-face air pressure.

6. The shield tail sealing system still cannot completely prevent the leakage of compressed air from inside the compartment.

7. There is an issue of compressed air leakage at the joints between tunnel segments, and occasionally, mud and water from outside are drawn into the tunnel along with it, further complicating construction. The image shows a schematic diagram of a partial-pressure shield machine; as a result, this method has yet to be implemented.

1. Atmospheric-pressure excavation and material transport system; 2. Belt conveyor; 3. Soil-discharging grab; 4. Excavation bucket; 5. Haulage vehicle; 6. Segment transport monorail; 7. Tunnel segments; 8. Lining segment assembler; 9. Expansion joint

(2) Slurry-Pressurized Shield Tunneling

In the shield's sealing chamber, slurry is injected to counteract the face soil pressure with its hydrostatic pressure. The process relies on full-face mechanized cutting combined with pipeline-based slurry removal, enabling seamless excavation and advancement of the tunneling shield. The image shows a schematic diagram of a slurry-pressure shield, which not only facilitates continuous slurry discharge through pipelines but also effectively prevents collapse at the excavation face, significantly reducing leakage around the shield tail. As a result, slurry-pressure shields have seen rapid development and are now widely used in tunnel projects across various applications.

1. Drill bit; 2. Partition; 3. Pressure control valve; 4. Gang collection trough; 5. Inclined chute; 6. Agitator; 7. Shield tail seal; 8. Cement slurry; 9. Monu-type pump; 10. Gangue pump; 11. Telescopic pipe; 12. Emergency branch pipe

⑶ Earth-pressure balanced tunnel boring machine

Also known as the soil-cutter closed-type or slurry-pressure shield, this innovative type of shield has evolved from both partial-air-pressure and slurry-pressure shields, making it particularly well-suited for construction in water-saturated, weak ground conditions, as illustrated in the figure. The cutterhead of a soil-pressure balanced shield is equipped with a face-cutting disc, and a sealed partition is installed between the cutting ring and the support ring, creating a slurry-filled sealing chamber at the cutting face. A "slurry-forming material"—characterized by its fluidity and impermeability—is injected into the excavated soil, transforming it into a flowable yet watertight slurry that completely fills both the excavation-face sealing chamber and the connected long cylindrical screw conveyor. By precisely controlling the rotation speed of the screw conveyor based on the cutting rate, the shield ensures that the sealing chamber remains optimally filled with soil—neither too empty nor overly saturated. This soil-pressure balanced shield addresses the major drawbacks of conventional partial-air-pressure shields while eliminating the need for complex treatment equipment required in slurry-based systems. As a result, it stands out as one of the most promising and rapidly advancing pieces of underground construction machinery today.

1. Slurryed soil; 2. Pressure gauge for measuring slurryed soil; 3. Sealed chamber for slurryed soil; 4. Hydraulic motor to rotate the cutterhead; 5. Soil layer; 6. Segment; 7. Lining assembler; 8. Mixing blades; 9. Valve for grouting material injection holes; 10. Auger conveyor; 11. Cutterhead support equipped with cutting tools

(4) Micro Shield Tunneling Machine

Micro-tunnel boring machines (TBMs) refer to small-diameter TBMs with diameters below 2.0 meters. Since 1970, the growing demand for small-diameter tunnel projects—such as sewer and cable tunnels—has driven the development of micro-TBMs. In densely populated metropolitan areas, where underground utility networks are tightly packed and traditional trenching methods are impractical, micro-TBMs prove particularly advantageous when using an excavation-based approach. While the basic principles of micro-TBMs closely resemble those of conventional TBMs, they are not simply scaled-down versions of their larger counterparts. Instead, micro-TBMs possess several unique features tailored to their specific applications. For instance, they often operate under extremely shallow overburden—typically less than 2.0 meters—meaning the lining structure is subjected to concentrated loads. Additionally, since these operations frequently take place beneath busy urban roadways, micro-TBMs are designed to minimize site footprint by requiring fewer working shafts, while also incorporating noise-reduction measures to ensure minimal disruption to surrounding communities.

4. Shield tunneling for urban subways offers the following advantages:

1. With the exception of shaft construction, all construction activities are carried out underground, ensuring no disruption to ground-level traffic while also minimizing noise and vibration for nearby residents.

2. The main processes—such as shield propulsion, muck removal, and segment lining assembly—are carried out in a continuous cycle, making construction easy to manage while requiring fewer personnel.

3. The construction cost of the tunnel is unaffected by the amount of overburden, making it suitable for building tunnels with significant overburden depth.

4. Construction is unaffected by weather conditions (such as wind and rain, etc.);

5. When the tunnel passes beneath a riverbed or other structures, construction is not affected;

6. As long as the excavation face of the tunnel boring machine can be stabilized, the deeper the tunnel goes, the worse the ground conditions become—and the more underground utilities interfere with construction. In comparison to cut-and-cover methods, this approach offers significant advantages in terms of cost-effectiveness and construction schedule. However, there are also several challenges inherent in tunnel boring machine (TBM) construction.

7. When the tunnel curve radius is too small, construction becomes more challenging;

8. When constructing tunnels on land, if the overburden above the tunnel is too shallow, shield tunneling becomes significantly more challenging. Similarly, underwater construction poses safety risks with shield methods if the overburden is insufficiently deep.

9. When using the full-air pressure method during tunnel boring to dewater and stabilize the ground, labor protection requirements are high, and construction conditions are poor.

10. As the tunnel boring machine advances through the ground along the designed axis, surface settlement within the relatively wide area above the tunnel remains difficult to prevent entirely. In saturated, water-rich, and soft soil layers, stringent technical measures must be implemented to keep settlement within extremely tight limits.

11. In saturated aquifers, the assembled lining used in shield tunneling construction places high technical demands on achieving comprehensive structural waterproofing.