The issue of large deformation mechanism in soft rock tunnels has puzzled tunnel scholars for decades. Previous studies have not evolved a clear and common understanding. Therefore, detailed on-site measurement, full investigation and statistical analysis have been conducted on the instability and failure of Muzhailing Tunnel since its construction, whose length is beyond 15 km. The study aims at systematically analyzing the failure mechanisms and modes of Muzhailing Tunnel in monoclinic and soft-hard interbedded rock strata. Study results show that the angle between strata strike and tunnel axis greatly determines the magnitude of deformation, the dip direction significantly controls the bias direction and maximum deformation direction, and the dip angle deeply affects the deformation form. The failure modes of surrounding rock mainly include four types: spalling and overturning failure, bending failure, shear slip failure and buckling failure. Large deformation characteristics are summarized from six aspects: failure form, groundwater, sensitivity to influencing factors, deformation degree, deformation speed and deformation duration. The instability modes of primary lining include in-plane (transverse) instability and out-plane (longitudinal) instability. Finally, the causes of large deformation are analyzed from geological, structural, engineering and human factors.

Reasonable evaluation and prediction of squeezing condition to avoid large deformation disaster in tunnels have been an important research issue. In order to accurately and rapidly predict the large deformation under complex geological conditions, this paper proposes a classification and prediction method for quick identification of large deformation in tunnel construction stage in a case study of Muzhailing Tunnel. Previous prediction methods of large deformation are usually based on the stages of geological survey and engineering design, and there are great difficulties in obtaining some prediction indexes in soft and fractured strata, such as uniaxial compressive strength. In this paper, the nonlinear deformation behaviors of surrounding rock were analyzed. The geo-stress condition, strata occurrence, rock strength, rock intactness and groundwater condition were chosen as evaluation indexes (including eight sub-indexes). The classification of large deformation was carried out based on the deformation statistics in Muzhailing Tunnel. And the fuzzy prediction was conducted and compared with other methods in engineering. The comparison results show that the new method has higher accuracy and applicability in predicting the large deformation of weak rock mass. This study provides a new approach for rapid identification and prediction of large deformation in tunnel construction stage.

Rock mass pressure has always been a research hotspot in the field of tunnel engineering, especially in the super large-span tunnel, which is characterized by flat section, large excavation span, and complex stress field. Based on the Letuan Tunnel (a bi-directional tunnel with eight traffic lanes) of Binlai expressway expansion project in Shandong Province, China, this paper focused on the calculation method of rock mass pressure and the evolution law of load release in the construction process of the super large-span tunnel excavated by upper-bench central diaphragm (CD) method. Based on field measured data of Letuan Tunnel, the deformation behavior of primary lining and the distribution state of rock mass pressure during the tunnel construction were analyzed. According to the bearing mode of supporting structure, the mechanical models of different construction stages were established. Then, the rock mass pressures in different construction stages were back-calculated using mechanical models and compared with the measured values, and the evolution law of load release during the tunnel construction was discussed. The study results show that the tunnel deformation and rock mass pressure were significantly affected by the construction process and support form, and the excavation span was the key factor affecting the stability of rock mass. For the shallow-buried super large-span tunnel constructed by upper-bench CD method, the primary support of upper bench was under eccentric pressure. The comparison between the back-calculated value and the field measured value indicated that they were similar, and the average relative error was 17.23%. According to the concept of load release coefficient proposed in this paper, the load release coefficient after the pilot tunnel ahead (Part I) excavation reached 63%, and the load release coefficient after the pilot tunnel behind excavation (Part II) was 37%, which means that the rock mass pressure of Part I is increased about 59% due to the excavation of Part II.

The central diaphragm is often used to reduce the span to maintain the stability and safety of structure in the construction of large‐span loess tunnel due to the structure complexities. In this paper, relying on the field monitoring and measurement for Wangcun tunnel in Huangling‐Yan’an expressway expansion project, the crown settlement and horizontal convergence of primary support steel rib and central diaphragm steel rib during the construction are analyzed by the upper bench CD method. According to the internal force transfer, deformation coordination, and arch foot displacement between the two structures, the support system is regarded as the arch‐beam fixed structure with three times of statically indeterminate and movable abutment under the loads, and the mechanical calculation model of sidewall steel rib and the central diaphragm structure bearing loads and deformation together is established. Finally, through the mechanical model mentioned above, the deformation characteristics of central diaphragm structure and the horizontal convergence in the upper bench of tunnel are calculated and analyzed. The research shows the following: (1) the accumulated settlement of sidewall steel rib in Part I is greater than that of the sidewall steel rib in Part II, and the accumulated settlement of each part at the support structure during the tunnel excavation is less than the reserved deformation of 150 mm specified in the tunnel excavation; (2) the settlement located at the waist and maximum excavation line position of central diaphragm is mainly affected by the excavation of Parts I and II in upper bench; (3) during the whole excavation process, the excavation of Part I and Part II has the greatest influence on the convergence at arch waist and the maximum excavation line position in Part I, and the convergence at the above two positions all experienced four stages of “convergence‐expansion‐convergence‐gradual stability”; and (4) the errors between the horizontal convergence and the deformation of central diaphragm obtained by the mechanical model and the field monitoring data are between 12.7% and 27.5%. The calculated results are in good agreement with the actual situation. The research can provide a theoretical basis for the study of deformation and mechanical properties for support structure in the construction of large‐span loess tunnel by the upper bench CD method.

In order to keep pace with fast urbanization and explosive growth of highway traffic volume in China, more and more super large‐span tunnels are constructed or will be constructed in the coming years. It is of great significance to employ a suitable construction method for ensuring construction safety when tunnelling in a complicated geological environment. Based on the reconstruction and expansion project of Binlai in China, this paper aims at studying deformation behaviors and mechanical properties of super large‐span and shallow tunnel constructed using the upper‐bench CD method. The study results showed that the crown settlement of the pilot tunnel was larger than that of the rear guide tunnel, and the deformation of rock mass tended to be stable after 60 days of excavation. The structural stress changed greatly in the construction stage, the stress distribution of steel rib was mainly under compression, and the maximum compressive stress of shotcrete appeared at the tunnel vault. The supporting structure was always kept in a safe state, and its safety factor was higher than 1.5. However, the upper‐bench CD method has poor performance in controlling surface subsidence and horizontal convergence. The study of this paper provides a new construction method and useful reference for similar super large‐span tunnels.

In the new construction or reconstruction of expressway projects, the number of highway twin tunnels with eight lanes is increasing. However, there are no corresponding design support parameters and measures in the current technical specifications for tunnel design and construction in China. In Laohushan superlarge‐span highway tunnel with single hole and four lanes, the deformation behavior and mechanical characteristics of support structures are measured and analyzed. The monitoring results indicated that the deformation of tunnel structure mainly experienced three stages: rapid deformation, slow deformation, and stable deformation, and finally reached a relatively stable state; the structure stress of primary support and secondary lining increases sharply at first and then tends to be stable gradually with the gradual construction of each excavation part in the tunnel; the stress of each measuring point at the steel rib is less than the yield limit of steel rib (235 MPa), and the support structure is safe and stable in the process of tunnel construction. Then, the structure safety of primary support under different support parameters is simulated and calculated by numerical simulation with Grade IV rock mass, and the reasonable support parameters for Laohushan highway tunnel are studied considering the structural safety and engineering economy. It is suggested to use the H175‐type steel sets with a distance of 80 cm and C25 shotcrete with a thickness of 26 cm. The results could provide reference for similar tunnel projects and provide a basis for the design specification and construction standards for superlarge‐span tunnels.