Precast TBM (Tunnel Boring Machine) segments are prefabricated concrete or composite elements used in tunnel construction. They are specifically designed to fit together and form the lining of a tunnel excavated by a TBM.
Purpose: TBM segments serve as the primary support structure for the tunnel lining. They provide stability and structural integrity to the excavated tunnel.
Construction: Precast TBM segments are manufactured in a controlled environment, typically in a casting yard located near the tunnel construction site. They are produced using high-strength concrete or other materials such as fiber-reinforced polymers.
Shape and Size: The shape and size of precast TBM segments depend on the specific project requirements and the diameter of the tunnel being constructed. They are typically curved to match the shape of the tunnel, allowing them to be assembled into a circular lining.
Segment Connection: TBM segments are designed with specialized interlocking mechanisms to ensure a tight and secure fit during installation. These connections vary depending on the manufacturer but generally involve tongue-and-groove or bolted joint configurations.
Reinforcement and Waterproofing: Precast TBM segments may contain steel reinforcement bars to enhance their strength and durability. Additionally, waterproofing materials such as membranes or coatings are applied to the inner surface of the segments to prevent water ingress into the tunnel.
Installation: During TBM excavation, the segments are transported to the tunnel face, where they are assembled by a segment erector or robotic arms. The TBM pushes against the newly installed segments, creating a continuous tunnel lining as it progresses.
Advantages: Precast TBM segments offer several advantages in tunnel construction. They provide faster construction compared to traditional cast-in-place methods, improved quality control due to controlled manufacturing conditions, and reduced disturbance to the surrounding environment.
Inspection and Maintenance: Once the tunnel is complete, the TBM segments should be regularly inspected for any signs of damage or deterioration. Proper maintenance and repairs, if necessary, help ensure the long-term integrity of the tunnel lining.
It's worth noting that the specific details of precast TBM segments may vary depending on the project and the manufacturer. Different regions and countries may also have specific regulations and standards for their design and installation.
Shield tunnel apparently look a simple construction with solid cylindrical shape; it is in reality an articulated circular structure formed of segments arranged by consecutive rings in staggered layout. Segments are normally bolted together along longitudinal joints to form rings, which are then connected by dowels at circumferential joints. Joints in segment plays an important role in design and analysis. Several modelling techniques are available to simulate and account for the presence of joints, each with diverse degree of accuracy: given the repetitiveness of tunnel construction, built as a sequence of multiple rings, even little optimization in the analysis of the single component may lead to significant overall cost reduction. Mastering the available approaches thus allows tuning the analysis effort to the required design level, in order to satisfy Client’s requirements as appropriate.
Although precast tunnel lining is a modern construction technique but structural concept is much older. Concept of staggered layout to prevent longitudinal joints being in the same position, and this configuration allows mutual force transfer between rings, typical of 3D dimensional frame. These structures date back to Mesopotamian Era.
Different theories developed based on Strength of Materials and Theory of Elasticity. The unidimensional model concepts primarily based on either continuum rings or partially bedded based on Winkler Theory. Other hand, analytical solutions based on theory of elasticity based on assumption infinite plate with a hole. These theories are not really in practice now a days. For years, tunnel linings have been studied as solid rings with uniform flexural rigidity, thus ignoring the localized stiffness decrease at longitudinal joints. This simplification allowed the development of analytical solutions, under specific loading conditions, and is generally able to provide safe design for tunnel segments.
The “solid ring” approach with reduced stiffness is widely accepted by engineering community around the world adequately addressed in technical Standards and Guidelines, and refined by Muir Wood [1] via the introduction of a reduction coefficient for bending stiffness (an equivalent rigidity of the entire lining can reflect the reduction of rigidity at segment joints). This traditional approach is still valid for many applications, but does not allow a precise definition of section force distribution between segments and joints, which may be required for specific needs and, in general, for design optimization,
Evolution of different theories leads to an advanced approach by mitigating shortcomings by incorporating actual joints behaviour of segment joints. A further, possibly ultimate, evolution in tunnel design approaches may be the actual modelling of contact surface and the presence of mechanical fasteners, thus avoiding undesired analysis simplifications and definition of complicated adjustment techniques for spring stiffness.
The structural behaviour of the tunnel lining subjected to bending is mainly determined by the supporting effect of the surrounding soil. Only in special cases with unusual loads or extremely low bearing capacity soils must the tunnel lining made up of individual segments bear load as a rigid ring. In these cases the coupling of the rings to each other in the ring joint is of crucial importance for the load transfer. The calculation for the tunnel lining can be done using both the truss model and the continuum model. In the calculation with a 2D continuum model the tunnel lining is also shown as a truss – in a 3D continuum model correspondingly as a shell. The behaviour of the subsoil can be determined more accurately using a continuum model and corresponding constitutive laws. For this purpose it is necessary to realistically mirror all structural elements (tunnel shell, interaction of the segments, annular gap grouting, soil) in order to realistically register the effects resulting from them and their interaction with each other.
To take into account the torsional rigidity in the longitudinal joint and to detect the resulting moment transfer, non-linear torsion springs are arranged in the computational model at these locations The associated non-linear spring characteristics reproduce the moment in the longitudinal joint as a function of the existing torsion and the effective ring normal force. The determination of these spring characteristics is taken after the calculation model of Leonhardt/ Reimann.
In a continuum calculation the interaction of the tunnel shell with the soil is not detected through the bedding and the specification of slack loads, as in the truss models, but through the discretization of the soil itself. This also includes the fundamental possibility of better mirroring of the nonlinear stress-strain behaviour of the soil. In the mirroring of the tunnel lining as continuous members, a bowl or as a continuum, it is necessary to also reproduce the load bearing mechanisms of the interaction of the individual segments.
explained in detail under truss modeling. If a coupled ring system is to be mirrored, a spatial continuum model with shell elements to simulate the segmental lining is recommended. For control purposes the continuum calculations are always be checked for plausibility by simple analytical calculations. They are only of limited use for the segment design. To determine the actions from rock pressure and to calculate ring deformations under special actions, a continuum calculation can provide relatively realistic results.
The requirement, nature and extent of structural fire protection for a tunnel lining is always to be considered in association with the operational protection measures (e. g. traffic management and control, possibilities to drive into safety/evacuation zones, escape routes design, fire detection, smoke removal, ventilation, cooperation with emergency services) in accordance with the relevant regulations and determined accordingly. In the relevant regulations, ensuring adequate stability and if necessary, also serviceability (water tightness, limitation of permanent deformations) during and after a fire are named as the key protection goals of structural fire protection in tunnels.
The fire design based on the method according to EN 1992-1-2:2010, chapter 4.2 and Annex B.2 “Zone Method”. By using the “Zone Method”, the crosssection damaged by fire is represented by a reduced cross-section ignoring a damaged zone of thickness az at the side exposed to the fire. The load case “fire” leads in statically indeterminate beam models, such as bedded tunnel linings, to internal forces due to constraint forces caused by for example temperature (linear and constant temperature difference), imperfections and support displacements, are sufficiently taken into account within the design.
The durability of the tunnel structure assumes its planned load bearing capacity and serviceability – with reasonable repair expenses – over its entire useful life. Service lives of 100 years or more are common Inspection and repair options are usually available only to a small extent. This is especially true for the ground contact surfaces, seals and side surfaces no longer accessible after installation. The following requirements must always be met.
Reinforced concrete segments are exposed to ongoing aging from processes attacking concrete and steel. The individual mechanisms occur mostly in combination and may reinforce each other. They include Carbonation, Sulphate attack due to sulphate -containing water or subsoil with dissolving or driving effect, Chloride attack. Other reasons may be;
We evaluate ground environment and design appropriate concrete to ensure durability by controlling water ingress and density. Our detailing ensure that no part of reinforcement is exposed to harsh environment. Use latest tools like Life-365 to predict concrete life for our proposed design.