How is a segment ring structurally designed for a tunnel constructed using a TBM?

The a tunnel segment ring constructed using a TBM It is designed to ensure that the precast reinforced concrete lining can withstand the effects of the ground, water, thrust from the tunnel boring machine, grouting, handling, transportation, assembly, service, and potential accident scenarios. The structural design defines the ring’s geometry, thickness, number of segments, concrete grade, reinforcement, joints, bolts, connectors, tolerances, watertightness, durability, and assembly system.

In projects of segment tunnel, the segment is not designed solely for the final phase of service. It must also withstand extremely demanding temporary phases: demolding, stockpiling, transport, hoisting, jacking, back-up passage, backface grouting, and progressive loading from the soil.

What Is a Segmental Ring in a TBM Tunnel?

A segment ring is the structural lining assembled inside the tunnel boring machine’s shield using precast concrete segments, typically with longitudinal and circumferential joints. Each ring functions as a segmented, compression-bearing structure subjected to external loads, ground deformations, water pressure, and installation stresses.

Forces the ring must withstand

The structural design takes into account permanent, variable, temporary, and accidental loads.

Ring Geometry

The geometry is defined based on the effective inside diameter, the excavated outside diameter, the clearance, tolerances, structural thickness, the curvature of the alignment, and space for joints.

The following are determined:

• Inner diameter.
• Outer diameter.
• Segment thickness.
• Ring width.
• Number of segments per ring.
• Angle of each segment.
• Type of key segment.
• Ring taper, if the path has curves.
• Assembly tolerances.
• Space for a sealing gasket.
• Compatibility with segment erector.
• Clearance relative to the TBM shield.

The geometry must allow the ring to be securely mounted, maintain a tight seal, and conform to the contour without creating stress concentrations.

Structural Calculation Model

The ring can be analyzed in varying levels of detail depending on its diameter, depth, terrain, water pressure, and the project's criticality.

Standard models take the following into account:

• Equivalent continuous ring.
• Segmented ring with joints.
• Soil-structure interaction.
• Reduced stiffness due to joint articulation.
• Vertical and horizontal loads on the ground.
• Hydrostatic pressure.
• Soil reaction.
• Construction Phases.
• Three-dimensional effects in unique areas.
• Local loads on jacks and supports.
• Curvature conditions.

For complex tunnels, the model can incorporate nonlinear joint behavior, soil stiffness, excavation phases, face pressure, backfill grouting, and the actual construction sequence.

Structural Verification of the Segments

Each segment is checked against ultimate and serviceability limit states.

Flexural and Compressive Strength

The segment must withstand bending moments, axial forces, and combinations resulting from soil, water, installation, and service loads.

Cutting and local stresses

Areas near joints, supports, lifting points, jack thrust points, connector housings, and edges are inspected.

Cracking

The design must limit the formation of cracks to protect the reinforcement, ensure durability, and maintain performance in environments with moisture, chlorides, sulfates, or aggressive water.

Deformation and Ovalization

The ring must maintain deformations that are compatible with the clearance, watertightness, joints, interior equipment, and hydraulic or rail operations, depending on the tunnel's use.

Punching or local crushing

Concentrated loads from thrust jacks, mounting elements, or connectors can generate localized stresses that require specific assembly methods or load-distribution plates.

Design of Joints Between Segments

Seals are a critical part of the ring because they determine stiffness, leak tightness, force transmission, and assembly tolerances.

The following are studied:

• Contact Geometry.
• Compression capacity.
• Permissible opening.
• Permissible rotation.
• Shear.
• Force transmission between segments.
• Partial contact or eccentricity.
• Risk of chipping.
• Bolt or connector system.
• Elastomeric seal housing.
• Manufacturing and assembly tolerances.
• Water pressure compatibility.

The joint must balance three objectives: facilitate assembly, transmit forces, and maintain a tight seal throughout its service life.

Water Tightness and Water Pressure

In tunnels below the water table or subject to external pressure, watertightness is achieved through elastomeric seals and tolerance control.

The following is verified:

• Maximum water pressure.
• Minimum joint compression.
• Venue for the meeting.
• Support for rotations and translations.
• Position of longitudinal and circumferential joints.
• Risk of damage during assembly.
• Surface cleaning.
• Leak tests, if applicable.
• Durability of the elastomeric material.

Watertightness depends on the design as well as the manufacturing, storage, assembly, and inspection of each segment.

Assembly of Segments

The structure is designed to withstand global and local forces. It must take into account temporary phases as well as service life.

The following are considered:

• Main armor.
• Transverse bracing.
• Reinforced edges.
• Reinforcements at lifting points.
• Reinforcements in areas where jacks are used.
• Reinforcements around connectors.
• Crack Control.
• Nominal coating.
• Durability.
• Steel Congestion.
• Compatibility with molds.
• Manufacturing tolerances.
• Structural fibers may be used if the project allows it.

Some designs combine conventional reinforcement with fibers to improve crack control, residual strength, or performance under impact and handling.

Construction phases that influence the design

The ring must go through several stages before reaching its final state.

Manufacturing

The following are inspected: concrete, curing, formwork removal, tolerances, finishes, rebar placement, inserts, joints, and defect repair.

Transportation and Storage

The segments may crack or become damaged if the supports do not align with the intended points. The design must account for handling loads.

Assembly inside the TBM

During assembly, the segment may be subjected to localized contact, rotation, adjustments, and thrust from the erector.

Tunnel Boring Machine Advance

The TBM cats rest on the assembled ring. This phase generates concentrated loads that must be accounted for in the geometry and assembly.

Back-side injection

The injection pressure must fill the annular gap without excessively deforming the ring or causing joints to open.

Service

Once the ground has been stabilized, the ring withstands permanent loads, water pressure, environmental stress, and stresses inherent to tunnel use.

Durability of the segment ring

Durability depends on the environment, service life, coating, concrete quality, permeability, cracking, joints, and exposure to aggressive agents.

The following are reviewed:

• Lecture.
• Presence of groundwater.
• Chlorides.
• Sulfates.
• CO₂.
• Temperature.
• Chemical attack.
• Internal abrasion if there is flow.
• Reinforcement covering.
• Water-to-cement ratio.
• Strength and Permeability of Concrete.
• Crack Monitoring.
• Compatibility of gaskets and seals.

In water conduits, collectors, or tunnels in harsh environments, durability can be just as critical as mechanical strength.

Quality Control in Manufacturing and Assembly

The design must be accompanied by a control plan to ensure that the actual segments conform to the calculated geometry and performance specifications.

The following is monitored:

• Materials.
• Concrete Mix Design and Strength.
• Aged.
• Dimensions and Tolerances.
• Position of reinforcement and inserts.
• Condition of the seals.
• Edge finishing.
• Cracks and chips.
• Load tests, if applicable.
• Parts Identification.
• Assembly sequence.
• Tightening or locking torque for connectors.
• TBM thrust records.
• Injection pressure on the back side.
• Leak inspection.

Assembly monitoring allows for the detection of deviations, edge damage, improperly seated joints, or forces greater than expected.

Risks of an Inadequate Design

A poorly designed or poorly controlled ring of segments can cause:

• Excessive cracking.
• Chipped edges.
• Leaks.
• Deformation or ovalization.
• Opening of meetings.
• Damage caused by thrust bearings.
• Loss of tolerances.
• Assembly difficulties.
• Corrosion of reinforcing bars.
• Leaks.
• Repair cost overruns.
• Reduced service life.
• Risks During Tunnel Operation.

These risks typically arise from a combination of factors: geotechnical conditions, water pressure, joints, tolerances, TBM thrust, grouting, and manufacturing control.

Information Needed to Review the Project

To structurally design a segment ring for a TBM tunnel, the following information should be provided: inner diameter, outer diameter, tunnel length, intended use, plan and profile, radii of curvature, depth, overburden, geotechnical data, water pressure, water table, external loads, seismicity (if applicable), TBM type, maximum jack thrust, jack distribution, face pressure, backfill injection pressure, tolerances, number of segments per ring, ring width, service life, exposure class, aggressiveness of the water or soil, watertightness criteria, joint type, connectors, assembly procedure, segment logistics, manufacturing conditions, transportation, storage, cracking criteria, applicable regulations, and inspection requirements.

CTA: Request a Technical Review of the Structural Design of Segment Rings for TBM Tunnels Before finalizing the diameter, thickness, joint, assembly, leak-tightness, TBM, or RFQ documentation.