The use of steel fiber reinforced concrete (SFRC) in tunnel lining construction through tunnel boring machine has gained popularity across the world for the past few decades due to its high toughness and improved durability performance compared to conventional reinforced cement concrete (RCC). In addition to the soil surcharge during the serviceable life, the precast tunnel segments experience bending stresses during the transient stage involving demoulding, stacking, and handling of the segments which could potentially lead to unforeseen critical cracks in the segment. The large-scale testing of precast segments is not always viable and can be very expensive. In this study, the SFRC tunnel segment is tested numerically under three-point bending using DiANA software. The uniaxial stress-crack opening law in tension is obtained through inverse analysis which is performed by comparing experimental and numerical results of the notched beam modelled in DiANA using a discrete crack approach. To simulate the fracture behaviour of SFRC tunnel segment under three-point bending, non-linear finite element analysis is performed using the smeared crack approach and the SFRC constitutive law obtained from the inverse analysis. Finally, the model is validated by comparing the results of numerical analysis with full-scale experiments performed on the tunnel segments.

In recent years, prefabricated open caissons constructed by VSM method have been widely utilized around the world, for the advantages of high construction precision and speed, low disturbance to the surrounding environment, and adaptability to various stratum. Compared with open caisson foundations, the stiffness of prefabricated open caisson is small; Compared with shield tunnels, the direction of prefabricated open caisson is vertical, and the external load for each ring is mainly uniform pressure. Therefore, the seismic response of prefabricated open caisson constructed by the VSM method requires further study. To address the above problems, this paper developed refined 3D finite element models to investigate the seismic response of precast open caissons. Continuum shells with rebar layers are utilized to simulate the shaft segment, equivalent nonlinear springs are used to simulate the joint bolts, and viscous-spring artificial boundaries are utilized to apply the earthquake waves. Seismic responses of three structural forms, including integral models without joints, models that only consider ring joints and models that consider ring and longitudinal joints, are analyzed and compared under different ground motion directions and stratum. Conclusions are obtained as follows: 1) Structural seismic response is mainly controlled by the horizontal ground motion, while the impact of vertical ground motion on the structure is low; 2) The structural lateral deformation type is related to the connection stiffness. When the connection stiffness rises, the structural lateral deformation will close to bending deformation. On the contrary, it is closer to the shear deformation; 3) Internal force and damage to the structure are mainly concentrated in joints. In addition, the internal force and deformation of ring bolts are significantly higher than that of longitudinal bolts. In other words, the stiffness of ring bolts takes control of the seismic response of prefabricated open caisson constructed by the VSM method.

A combined experimental-computational approach is used for predicting stresses in a segmented tunnel lining [1]. Vibrating wire sensors equipped with thermistors were used to measure strains and temperature inside (i)~plain concrete specimens undergoing uniaxial creep tests over one year, exposed to the environmental conditions of the Koralm tunnel, and (ii)~precast reinforced concrete tubbings consituting the lining of the Koralm tunnel. A Boltzmann-type thermo-viscoelastic model is used to translate measured strain histories into corresponding stress histories. The used creep function consist of a power law describing short-term creep and a logarithmic law describing long-term creep of concrete. The stress histories are inserted into a Drucker-Prager failure function in order to quantify degrees of utilization. The latter stabilizes some four months after installation of the analyzed segmental tunnel ring. Seasonal temperature fluctuations are found to have a rather insignificant effect on the stresses and the degrees of utilization. Extrapolating the measured strain histories up to 150 years allows for long-term predictions of stresses and degrees of utilization. It is found that stress levels are expected to remain at some 40% of the concrete’s strength, throughout the service life of the tunnel.

[1] A. Razgordanisharahi, M. Sorgner, T. Pilgerstorfer, B. Moritz, C. Hellmich, B. Pichler: ``Realistic long-term stress levels in a deep segmented tunnel lining, from hereditary mechanics-informed evaluation of strain measurements.'' Tunnelling and Underground Space Technology, 145, 105602 (2024), https://doi.org/mcxv.

As effective solutions for subsurface projects in high-density urban underground spaces, non-circular shield tunnels have been successfully employed in numerous projects. However, given the various types of cross-sections available, engineers are always confronted with pivotal decisions regarding the selection of different types of cross-sections. Furthermore, there exists a notable research gap concerning differences of the structural behavior between tunnel linings with different cross-sections. With continuous cross-section geometry transformation, the evolution of structural behavior, ultimate bearing capacity, and distribution of weak parts of non-circular tunnels have not yet been thoroughly investigated, as well.

In this study, a parameterization method for cross-section geometry of different non-circular tunnels is proposed. This method establishes geometric connections between different non-circular tunnels, such as typical quasi-rectangular segment tunnels (QRST tunnels) and double-o-tube tunnels (DOT tunnels). A test-validated macro-level nonlinear structural model is then proposed to reveal the elastoplastic failure processes of different tunnel structures. Additionally, for the quantitative evaluation purpose, an energy-based robustness evaluation method considering the entire structural bearing process is proposed. Finally, the evolution process of structural mechanical behavior during the continuous transformation of geometric parameters of non-circular tunnel structures are revealed and analyzed. The results show that the influence of changes in cross-section geometric parameters on the mechanical behavior and failure mechanism of non-circular shield tunnel structures are effectively captured, thereby facilitating the selection of an appropriate non-circular tunnel regarding the specific engineering conditions.