Concrete Segmental Lining Research Articles

An analytical solution for shear bearing capacity of circumferential joints of precast concrete segmental tunnel linings considering dowel action

AbstractThe capacity of the circumferential joint of the precast concrete segmental tunnel lining (PCTL) structure in terms of shear performance principally includes the dowel action by connector/bolt as well as friction force, and it is a vital parameter to assess the mechanical response of the circumferential joint. Further, as there was no analytical model available for precise estimation of a circumferential joint in terms of the shear‐bearing capacity considering the dowel action of the bolt in the presence of axial/normal force, therefore in this investigation, an analytical model has been proposed to estimate the shear‐bearing capacity of the circumferential joint. Furthermore, the analytical model's precision and accuracy were validated via large‐scale experimental investigation on a circumferential joint of PCTL. Upon comparing analytical model outcomes with experimental results, absolute error varied between +5% and −9%, with an average value of the coefficient of friction at the yield point of the bolt. Moreover, the formation of hinges on the side of the bolt within the segment was considered a failure of the circumferential joint. Additionally, the parametric investigation revealed that with just a 1% change in axial force, the diameter, pre‐tightening, and yield strength of the bolt and concrete strength improved by 2.85%, 1%, 0.27%, 0.26% and 0.17% shear‐bearing capacity of the circumferential joint respectively. Axial force variation greatly influences the shear‐bearing capacity of circumferential joint followed by diameter, pre‐tightening, yield strength of the bolt, and concrete strength. Conclusively, with analysis of axial force distribution around the ring for the tunnel, the proposed model has been applied to a full ring to estimate shear bearing capacity.

Friction effects on connection systems in modern concrete segmental linings

AbstractOnce grouted into place and compressed by ground and groundwater, modern segmental linings do not need additional connections except when the lining is required to withstand unusual conditions. Connection systems are provided for two reasons: to assist in the segment‐building process and/or to prevent undesirable actions. A particular focus is on creating and maintaining sufficient gasket compression to seal the lining in the short and long term. The evaluation and design of these connection systems usually involve a simple 1D calculation, ignoring the actual geometry and the presence of other segments and connectors. This study looks at an actual design situation highlighting the role of friction between adjacent segments and the utility of guide rods in the radial joints.

Influence of structural form on hydration-heat-induced temperature rise of precast concrete lining segments for a metro transit station

This paper describes research on the effect of structural form on hydration-heat-related changes in large-scale precast concrete lining segments. These segments of mass concrete were used in the construction of an underground metro transit station in Changchun, China, a location where conventional cast-in-place construction is interrupted for months by cold winter weather. Heat of hydration analysis of closed-cavity and solid lining segments was conducted using the finite element method (FEM). Two ambient temperatures of 25°C and 10°C (77°F and 50°F) were selected to simulate segment manufacturing conditions in summer and winter, respectively. The FEM and selected parameters were found to be reasonable because the numerical predictions and experimental observations from the closed-cavity segment were consistent. At both of the ambient temperatures, the maximum internal concrete temperature of the closed-cavity segment was much lower than that in the solid one and the temperature distribution in the closed-cavity segment was more uniform. As a result, thermal stress was lower in the closed-cavity segment than in the solid segment, which reduced the risk for cracking. The closed-cavity form of the lining segments enhanced the structural integrity and durability, and the ability to prefabricate these segments in cold ambient temperatures effectively accelerated construction on this project.

Analytical and numerical study of one-dimensional and two-dimensional stress distribution within an elastic semi-infinite material under the action of an arbitrary rectangular uniform loading

Purpose. The study of stress distribution within an elastic semi-infinite material under the action of an external loading is of great importance in the theory of elasticity. In most cases, the lack of knowledge about the stress distribution within a material can result in incomplete and inappropriate engineering designs, leading to unsatisfactory consequences. The latter include cracks and fractures, created inside the concrete segmental lining in TBM tunneling, as well as indentations that occur in floors due to the lack of pillar design not only in underground mining, but even in civil projects. This study focuses on the one-dimensional and two-dimensional internal stress distribution induced by arbitrary rectangular–square loading, in other words, that applied to an elastic semi-infinite material. Methods. Firstly, this paper uses an analytical method and, subsequently, a numerical method. In the analytical study, the point load equations of Boussinesq and Westergaard are used. Extending these equations to the rectangular loading area, four new equations are introduced. Using the Abaqus finite element software, the numerical study is performed in 3D space. Findings. The results show that the answers from the introduced equations are in high consistency with numerical ones. However, the result of the extended Boussinesq point load equation is closer to the answer obtained by the numerical method. Originality. Four new equations, presented in this paper, describe one-dimensional and two-dimensional stress distribution. Practical implications. The presented equations can provide a simple and convenient way to solve rectangular load problems in many cases such as foundation, civil and mining projects. This study uses initial information on specific segments in the Tabriz Metro line-2 Project.

Thermal performance assessment of an energy lining for the Lyon-Turin base tunnel

The use of geothermal energy for heating and cooling purposes is an environmentally-friendly and cost-effective alternative with the potential to replace fossil fuels and help to mitigate global warming as well. The paper illustrates the geothermal potential evaluation of a portion of the Lyon-Turin base tunnel considering the thermal activation of the tunnel concrete segmental lining. The international infrastructure will connect Italy to France, crossing the Alps and meeting uncommon climate conditions, reaching a peak temperature of 47 °C, due to its significant rock overburden (up to 2500 m) under the Ambin massif. A thermo-hydraulic numerical model was used to simulate the heat exchange of the system and quantify the power achievable by thermally activating a 10 km-long section of the base tunnel. Sensitivity analyses were performed to investigate the influence of the heat carrier fluid and the air flow velocities as well as the inlet temperature on the heat exchange. Moreover, four different operational conditions were compared to allow for assessing the overall thermal performance of the energy lining in terms of heat exploited and of the capacity of cooling the tunnel.

Design guidelines for concrete segmental linings in transverse interaction with the TBM

Despite growing evidence proving that sealing pressures and transverse jack forces are transient load cases relevant for concrete segmental lining (CSL) design, these TBM transverse actions are consistently omitted by current design guidelines. In this paper, the TBM transverse actions are examined in detail and a theoretical framework is proposed to help tunnel designers incorporate them into the basis of design and the analyses required for structural verification. The design guidelines are formulated starting with a set of equations for the computation of sealing pressures and the geometrical quantities governing the jack forces. The proposed calculation methods are deployed to classify and describe global construction scenarios of CSLs in transverse interaction with the TBM, including expected behaviour and damage modes. Guidelines for the definition of relevant transient design situations and subsequent engineering models are finally provided.

Semi-analytical thermo-mechanical model for the shield tunnel segmental joint subjected to elevated temperatures

The segmental joint is the most critical component for determining the mechanical response of concrete segmental linings in a shield-tunnel structure. Existing segmental joint models have been developed mainly for the analysis of segmental lining design, but in the event of a fire, the conditions that a tunnel joint may experience could well exceed the normal design range. Consequently, the validity of the existing segmental joint models becomes questionable. In this paper, an improved semi-analytical thermo-mechanical model is derived based on the general plane joint model to consider the thermal effect induced by elevated temperatures. As a benchmark for the development of the model, a laboratory experiment was conducted on concrete lining segmental joints under elevated temperatures. The general heat transfer and special heat flow invading through the joint were deduced by theorical and numerical methods. A finite element (FE) model is developed and calibrated against experimental model tests, whilst the FE model is employed to generate the required parametric data, from which a fitting function is created. A temperature adjustment coefficient is introduced to improve the calculation of the joint section temperature field. The influence of elevated temperature and axial force on the bending moment of a segmental joint is also incorporated according to the stress state of the joint section. The validation of the proposed semi-analytical thermo-mechanical model is demonstrated through comparisons with the results of fire tests. Finally, the influences of the heating curves, bolt location, and axial load on the rotation stiffness of the joint are investigated using the proposed model.

Developments in fibre reinforced concrete in tunnelling

AbstractThis paper will provide an overview of the state of the art of the use of fibre reinforced concrete in tunnelling. The major applications for fibres are for the reinforcement of sprayed concrete and pre‐cast concrete segmental linings, but fibre reinforcement is also increasingly used for other applications such as trackbed/trackslab and minor precast concrete elements such as cable ducts, drip shields, and drainage elements. As the technology has matured, a wide variety of fibre types, made from different materials, has emerged. Standards have been developed for the manufacture, testing and application of all these fibre types. Moreover, design guidelines and codes have been published and new design methods are still being developed. The International Tunnelling Association has published several guidance documents in this field. Last but not least, FRC has an important role to play in reducing the carbon footprint of tunnelling projects.

Structural response of concrete segmental linings in transverse interaction with the TBM. Part 1: Axisymmetric conditions

In both design and research, the model configuration of an isolated concrete segment working under the solely action of longitudinal jack forces is generally adopted as representative of the most onerous loading scenario borne by the segments of concrete segmental linings (CSLs) during tunnelling. The influence of TBM transverse actions, i.e. tail seal pressures and the transverse component of jack forces, on the ring deformations and structural damage typically recorded on site during the tail seal passage is thus systematically overlooked. In this paper, the segment interactions and stress concentrations experienced by the last erected ring, which is subjected simultaneously to the TBM ram loads and the advance of the sealing pressures, are analysed in detail via a suite of multiring finite element (FE) models. In this first part, the axisymmetric scenarios representative of concentric tail seal passage are examined. Non-axisymmetric scenarios are discussed in the second part. It was found that during the axisymmetric TBM advance, the segments rotate radially, i.e. about their tangential axis. Regardless of the cause of rotation, the maximum in-plane angularities, gasket gaps and gasket offsets in the longitudinal joints exhibit a strong linear correlation with the peak rotations and can become excessive under unfavourable loading or irreversible with joint plasticisation. The wider side of the keystone joints is the most critical area for groundwater leakage. The sealing pressures can increase spalling stresses at the packer extrados by 50% and hoop spalling stresses at the ram pad interspaces by threefold. The rear corners of the rotated segments are susceptible of radial bursting and shear failure in either convex-convex longitudinal joints or when differential radial rotations are very pronounced. The keystone joints can also undergo non-radial spalling at the rear of their bearing surfaces. Based on the numerical outputs, guidelines for tunnelling operations and joint design are provided.

Structural response of concrete segmental linings in transverse interaction with the TBM. Part 2: Non-axisymmetric conditions

In the second part of this paper, the behaviour of the last assembled ring when loaded simultaneously by the tail seal pressures and the jack forces is investigated in detail for eccentric scenarios of tail seal passage such as those encountered in the Crossrail’s Thames tunnel (CTT). In these non-axisymmetric scenarios, the sealing pressures grow linearly with depth and the ram pad restraints against the tunnel uplift are unevenly distributed over the ring circumference. The parametric study targeted the effect of uneven pad boundaries, tunnel unsupported length, Lu, and lining features on the numerical outputs of selected ten-ring finite element (FE) models. The numerical outputs of interest were ring deformations, internal forces, segment interactions and stress concentrations within the segment bodies. It was found that ring distortions can develop within Lu. The distortion mode is determined by the ram pad boundary. The ring deformations increase with the advance of the sealing pressures and decay with distance from the TBM pads. For a given pad boundary, Lu governs the magnitude of distortion, which, in the cases studied, raised up to 0.30% the tunnel diameter D by the end of the first advance. The tangential component of the jacks’ downward reaction can induce hoop tension at the ring front. The radial reaction forces result in significant hoop and torsional moments, differential radial rotations at transitional longitudinal joints and longitudinal moments in constrained segments. The total angularities of longitudinal joints can exceed recommended tolerances. The lower joints are at a higher risk of developing permanent angularities. Gasket offsets in joints undergoing differential radial rotations are sensitive to Lu. Temporary spear bolts can trim extreme offsets, e.g. 12 mm, down to moderate values. The transverse action of the pads increases the magnitude of peak tensile stresses in constrained segments: hoop spalling stresses at pad interspaces plus radial bursting stresses and ‘longitudinal’ spalling stresses near the rear corner of the longitudinal joints. Torsional shear stresses combined with hoop tension can cause excessive extrados surface tension near the front of constrained segments. Shear failure was identified in convex-convex transitional joints with considerable joint imperfections.

Full scale tests and a progressive failure model to simulate full mechanical behavior of concrete tunnel segmental lining joints

In the context of segmental concrete linings for tunneling, segmental joints are the most vulnerable parts of the lining. Most existing segmental joint models cannot evaluate the full mechanical behavior of segmental joints from the elastic deformation to the inelastic damage and failure. However, in some actual field cases, segmental joint damage and failure can occur. In this study, full-scale tests on the segmental joint adopted by a gas transmission tunnel are presented. The segmental joint is continuously loaded until it reaches its ultimate capacity. The internal force, deflection and failure characteristics of the segmental joint during the loading process are reported and compared with those of previous studies tested with flat segments. The relationships between the internal force, deflection and damage to the segmental joint are systematically analyzed. The tests reveal that the full mechanical behavior of the segmental joints can be divided into three progressive phases with four critical points. Based on the test observations, a progressive failure model that consists of a multiple stress states is proposed to simulate the full mechanical behavior of the segmental lining joints. The proposed model is ultimately compared with the full-scale test results.

The role of temporary spear bolts in gasketed longitudinal joints of concrete segmental linings

Temporary spear bolts in modern concrete segmental linings (CSLs) are inherited features whose current role is unclear to tunnel practitioners. In the absence of specific design guidance and detailed quantitative studies, tunnel designers opt for their deployment on the basis of historic precedence. Understanding the functions delivered by the spear bolts during tunnel construction can help designers in their decision making and lead to project cost savings and a more efficient use of structural materials. In this paper, the functions of temporary spear bolts in gasketed longitudinal joints working under serviceable conditions are thoroughly investigated with the aid of analytical rotational models of isolated longitudinal joints with flat and curved geometries, a simplified calculation method based on two-segment models and 3D numerical models of a ring subjected to sequential loading with radial pressures. It is found that the spear bolts are not effective in holding the joints closed under gasket loads and become redundant features for this particular function in CSLs equipped with dowelled connectors in circumferential joints. The spear bolts can, however, delimit initial longitudinal joint misalignments during ring assembly and mitigate the severity of permanent in-plane angularities and joint misalignments arising near the tail skin due to differential radial rotations between adjacent segments, which in turn can increase the risk of concrete damage around longitudinal joints and gasket underperformance. Temporary spear bolts may be needed at the ring front in tunnel sections where the sealing and grouting pressures are expected to be high or unevenly distributed, e.g. deep tunnels or tunnel sections with curved alignment. In tunnel sections with moderate depth and straight alignment, the use of spear bolts at the front of keystone joints only may be sufficient.

Safety format for the flexural design of tunnel fibre reinforced concrete precast segmental linings

Fibre reinforced concrete (FRC) is increasingly used in tunnel segmental linings constructed with tunnel boring machines. Several codes include the FRC as a structural material and specific guidelines cover the design of FRC precast segmental linings. All of these consider the traditional limit state format, applying partial safety factors to the loads (γL) and material strengths (γM). Often, the γM applied to the residual tensile strength of FRC is assumed the same as for concrete in compression. However, the variability in the determination of the residual tensile strength may be several times bigger than that of the compressive strength. In this context, the application of the same γM should be revised as it could lead to reliability indexes lower than the established for traditional reinforced concrete structures. The objective of this paper is to estimate safety factors for the residual flexural response of FRC used in tunnels segments, considering the variability of the material in both the material characterisation test and in the real-scale structure. After a study about the influence of the element size on the variability, an evaluation of the structural reliability of the segmental linings made with FRC is performed using the FORM method and a database of real-scale test results compiled from existing literature. Based on that, values of γM are proposed according to different failure consequences levels and relative economic costs required to increase the structural safety.

Mechanical behaviour of semi-rigid connections coupling two concrete segmental linings

This paper describes the use of segment joints in shield-driven tunnel linings. The opening of segment joints due to the application of a bending moment under different levels of axial stress is studied. The main focus is the mechanical behaviour of the joints, which are considered to be semi-rigid, and the moment-rotation behaviour of the joint. A physically based mathematical model is proposed based on the joint dimensions and material properties. Considering joints connected by two long curved bolts as an example, the model parameters are evaluated, and the moment-rotation curves for the joint are presented. The model distinguishes joint behaviour before and after opening, and agrees well with the real mechanical behaviour of the joints. A comparison with previous experiments on segment joints and with Janssen's and Gladwell's models is provided, and the proposed model is found to be accurate and universal.

Hybrid reinforcement design of longitudinal joints for segmental concrete linings

AbstractIn mechanized tunneling, the performance of segmental tunnel lining segments is closely bound to the longitudinal joint design. The guiding local effect is partial area strip loading with predominantly plane load distribution. Current normative standards only inadequately cover this case, which leads to an underestimation of the actual bearing capacities and thus offers considerable optimization potential. A hybrid material concept is proposed to improve the joints' performance, which is based on the results of robust combined trusscontinuum topology optimization. It combines high‐performance steel fiber reinforced concrete in the area of load application with conventional materials used for the remaining regions. A relocation of the gasket to the segments' center proves to provide better protection, and a double hinge neck leads to a more efficient load transfer. The effects of these measures on the structural performance are analyzed experimentally and compared against conventional designs. The results show pronounced increases in bearing capacities by using hybrid reinforcements and provide a quantification of optimization based design modifications.

Thermally-induced longitudinal decompression of simultaneously backfill grouted tunnels

The longitudinal compression of concrete segmental linings (CSLs) can enhance the durability of the tunnel structure. The compressive force reduces the risk of water ingress through ring joints and the differential displacements between adjacent rings. The hydraulic jacks of tunnel boring machines (TBMs) compress longitudinally the tunnel tube during construction, but subsequent longitudinal movements in the lining can cause the migration of the prestressing forces to the ground. One significant, yet disregarded, contributor to the longitudinal decompression of CSLs is the time-varying lining temperatures induced by tunnelling, particularly in CSLs that are simultaneously backfilled with cementitious grouts. This paper proposes analytical solutions for the prediction of longitudinal mechanical strains thermally-induced in CSLs during and after tunnelling. The solutions are compared with field data collected from the Crossrail’s Thames tunnel. It is proved that time-varying lining temperatures in simultaneously backfill grouted CSLs are responsible for the substantial loss of longitudinal compression exhibited in the months following construction. Tunnels subjected to moderate overburden pressures may experience a total loss of compressive forces at thermal equilibrium. The magnitude of the thermal decompression depends mainly on the temperature profile developed behind the TBM, which is in turn governed by the grout exothermic properties and ambient temperatures. A method for the assessment of longitudinal forces over the CSL design life is proposed for asset owners of future tunnels.

Longitudinal beam response of concrete segmental linings simultaneously backfilled with bicomponent grouts

The longitudinal beam response of concrete segmental linings (CSLs) to modern shield tunnelling with tunnel boring machines (TBMs) can impair ring joint performance. The opening and shearing of ring joints may lead to groundwater ingress. The differential displacements between rings, owing to either longitudinal shear forces or the ovalisation of rings subjected to unevenly distributed longitudinal forces, can increase the risk of structural damage around ring joint connections. Sequential beam-spring models have been deployed in the past for the prediction of the longitudinal response of CSLs during construction. However, the calibration of the models was limited to CSLs simultaneously backfilled with mortar grouts and usually required the amendment of the lining bending stiffness to obtain good approximations to the field data. In this paper, a sequential beam-spring model based on Bogaards’ and Bakker’s (1999) approach is proposed for the construction loads and boundary conditions representative of CSLs simultaneously backfilled with bicomponent grouts (BGs). The model is applied to the Crossrail’s Thames tunnel (CTT) case study to provide the complete history of the CTT longitudinal behaviour near the instrumented rings during tunnelling and the residual response as a result of tunnelling. The calibration is conducted satisfactorily without the need for tweaking the lining bending stiffness. It is shown that the lining responds to the TBM steering around curves mainly through longitudinal bending and to cumulative uplift forces through shear. The long term longitudinal internal forces are highly dependent on the sequence of TBM moments and TBM transverse actions near the tail skin and may lead to deferred structural damage with the progressive loss of longitudinal tunnel prestressing. An assessment method for the early detection of tunnel sections susceptible to ring joint damage is proposed for asset owners of future tunnels.

Assessing the Hydrogeological Conditions Leading to the Corrosion and Deterioration of Pre-cast Segmental Concrete Linings (Case of Zagros Tunnel)

Underground excavations are naturally under deteriorating environments due to corrosive ground water, type of minerals, and rock mass structures. The abundant of water (e.g., humidity and the inflow of groundwater), and the presence of chemical materials are among the main reasons creating such environments. Rock formations also contain minerals, which may be dissolved in the stream of groundwater and react with support measures (e.g., rock bolts, shotcrete and concrete linings). Deterioration is the main factor leading to the decay of the support systems and consequently affecting their functioning. The deterioration of support measures may result in significant consequences such as the failure of supports and the collapse of underground excavations, in long-term. It is, therefore, vitally required to investigate the long-term deterioration and behaviour of supports, and to find suitable approaches for mitigating the undesirable consequences of this natural phenomenon. Hence, the main scope of this study is to investigate the hydrogeological factors resulting in the deterioration of the segmental concrete linings at Zagros water transfer tunnel, located in the West of Iran. The key factors governing the deterioration mechanisms are identified and discussed based on a combination of field observation, laboratory testing and the literature studying of similar cases. The outcomes of these studies indicate that sulfate attack has been the crucial mechanism leading to the deterioration of the concrete linings in the Zagros tunnel. The hydrogeological condition of the tunnel well satisfies the four risk factors previously found by the UK Thaumasite Expert Group. The tunnel segmental concrete linings are, therefore, highly prone to sulfate attack. This study also shows that tunnelling through H2S and water bearing grounds has been the main reason providing such a deteriorating environment. The existence of organic materials (e.g., oil and coal) in the surrounding sedimentary rocks is expected to be the main source of H2S gases. The study is eventually complemented by proposing a few practical techniques to prevent further damage of the segmental linings, due to chemo-physical deteriorations.

A comparative study of two meta-heuristic algorithms in optimizing cost of reinforced concrete segmental lining

In this work, we tried to automatically optimize the cost of the concrete segmental lining used as a support system in the case study of Mashhad Urban Railway Line 2 located in NE Iran. Two meta-heuristic optimization methods including particle swarm optimization (PSO) and imperialist competitive algorithm (ICA) were presented. The penalty function was used for unfeasible solutions, and the segmental lining structure was defined by nine design variables: the geometrical parameters of the lining cross-section, the reinforced feature parameters, and the dowel feature parameters used among the joints to connect the segment pieces. Furthermore, the design constrains were implemented in accordance with the American Concrete Institute code (ACI318M-08) and guidelines of lining design proposed by the International Tunnel Association (ITA). The objective function consisted of the total cost of structure preparation and implementation. Consequently, the optimum design of the system was analyzed using the PSO and ICA algorithms. The results obtained showed that the objective function of the support system by the PSO and ICA algorithms reduced 12.6% and 14% per meter, respectively.

Research on Hybrid Fiber Reinforced Concrete Lining Segments

Numerous cases on tunnel fires emphasized the importance of fire protection of tunnel lining and indicated that fire may cause severe damages to tunnel lining in term of concrete spalling, reduction of physical-mechanical characteristics and load bearing capacity. Hybrid Fiber Reinforced Concrete (HFRC) as a new type of composite building materials has excellent physical and mechanical properties. Compared with ordinary concrete, HFRC’s tensile strength, flexural strength, abrasion resistance, impact resistance, corrosion resistance, fatigue resistance, toughness and cracking resistance, antiknock properties can be increased. Due to the problems existed in the traditional shield tunnel concrete lining segment, the research of new kind of lining segment becomes an important field of the shield tunneling research. The research of hybrid fiber reinforced concrete segment is lacking in application example in China, still in starting stage. The probability for the application of the fiber reinforced concrete segment and some other related questions are discussed in the paper, and it will promote the research and the application of new lining segment in our country.