Highway tunnels are occasionally built under difficult ground conditions and technical limitations, in a surrounding ground mass with weak mechanical characteristics, specifically the Cretaceous soil, which is a significantly weathered and deteriorated rock exhibiting a scaly composition. These tunnels are typically dug with wide cross sections and commonly in double-tube configurations, maintaining a distinct gap between them to promote traffic flow and safety services. Tunnels with high sections often cause considerable stress changes and distortions that can lead to ground collapse under misinterpreted conditions of the ground mass, especially in freeway twin-tunnel with defined spacing. This study examines a paired 3-lane tunnel, which is a component of a nationwide freeway initiative covering approximately 1200 km, characterized by a horseshoe configuration with a cross of 190 m2 each and a clear gap of 17 m. Considering the surrounding rock conditions, at a specific phase of the project, significant displacements peaked at 41 mm/day; additionally, concrete fractures were noted before a major failure extending up to 130 m toward the tunnel. This study suggests a back assessment for main disturbance factors before and after collapse, evaluates support force, and employs numerical modeling to reconstruct ground behavior. It has been observed that the decompression was quite significant, particularly above the tunnel's crown. The monitoring activities emphasized the effectiveness of both the original and enhanced support systems, showing a decrease in displacement from 47 to 64%. Given the surrounding rock's inadequate mechanical properties, the noticeable distance between the tunnels and the extensive excavation area raises concerns about support effectiveness. As excavation advances, the need for prompt responses is also highlighted to facilitate feedback contributions.

Research has been carried out to study the effects of new tunnelling on an existing adjacent tunnel to ensure the safety and serviceability of tunnels. Prior studies on twin-tunnel interaction have mostly centred on simplifying perpendicularly crossing tunnelling in a single-layered soil stratum. New tunnel excavation beneath an existing tunnel at different skew angles in two-layered strata can lead to different patterns of stress redistribution and adverse impacts on the existing tunnel. In this paper, results of three-dimensional centrifuge and numerical modelling carried out to study the twin-tunnel interaction with varying advancing orientations and layered soils will be reported. The influence of new tunnel excavation on an existing tunnel was simulated in-flight by controlling both the tunnel weight and volume losses. An advanced hypoplastic constitutive model that can capture stress-, path, and strain-dependency of soil behaviour is utilised for numerical back-analyses and parametric studies. Cases investigated include twin-tunnel interaction at three different skew angles (30 degrees, 60 degrees, 90 degrees) in a uniform sand layer and at skew angle of 90 degrees in two-layered sand with different relative densities and thicknesses. Distinct load redistribution patterns will be presented to explain deformation mechanisms of the existing tunnel at different tunnel advancing skew angles to highlight the effects of tunnelling orientation. The results of perpendicularly crossing tunnelling in twolayered sand will also be reported and compared to reveal the influence of layered soil. The findings and new insights can help engineers better estimate advancing tunnelling effects on existing tunnels and enhance the safety of tunnel construction.

For expedited transportation, vehicular tunnels are often designed as two adjacent tunnels, which frequently experience dynamic stress waves from various orientations during blasting excavation. To analyze the impact of dynamic loading orientation on the stability of the twin -tunnel, a split Hopkinson pressure bar (SHPB) apparatus was used to conduct a dynamic test on the twin -tunnel specimens. The two tunnels were rotated around the specimen's center to consider the effect of dynamic loading orientation. LS-DYNA software was used for numerical simulation to reveal the failure properties and stress wave propagation law of the twin -tunnel specimens. The findings indicate that, for a twin -tunnel exposed to a dynamic load from different orientations, the crack initiation position appears most often at the tunnel corner, tunnel spandrel, and tunnel floor. As the impact direction is created by a certain angle (30 degrees, 45 degrees, 60 degrees, 120 degrees, 135 degrees, and 150 degrees), the fractures are produced in the middle of the line between the left tunnel corner and the right tunnel spandrel. As the impact loading angle (a) is 90 degrees, the tunnel sustains minimal damage, and only tensile fractures form in the surrounding rocks. The orientation of the impact load could change the stress distribution in the twin -tunnel, and major fractures are more likely to form in areas where the tensile stress is concentrated. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY -NC -ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).