{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T01:35:21Z","timestamp":1760060121787,"version":"build-2065373602"},"reference-count":45,"publisher":"MDPI AG","issue":"8","license":[{"start":{"date-parts":[[2025,8,4]],"date-time":"2025-08-04T00:00:00Z","timestamp":1754265600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["52404094","52404085","202203021222088"],"award-info":[{"award-number":["52404094","52404085","202203021222088"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]},{"name":"Fundamental Research Program of Shanxi Province","award":["52404094","52404085","202203021222088"],"award-info":[{"award-number":["52404094","52404085","202203021222088"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Symmetry"],"abstract":"<jats:p>Crack initiation, propagation, and slippage serve as the key mesoscopic mechanisms contributing to the deterioration of deep tunnel surrounding rocks. In this study, a secondary anisotropy of deep tunnels surrounding rocks was proposed: The axial-displacement constraint of deep tunnels forces cracks in the surrounding rock to initiate, propagate, and slip in planes parallel to the tunnel axial direction. These cracks have no significant effect on the axial strength of the surrounding rock but significantly reduce the tangential strength, resulting in the secondary anisotropy. First, the secondary anisotropy was verified by a hybrid stress\u2013strain controlled true triaxial test of sandstone specimens, a CT 3D (computed tomography three-dimensional) reconstruction of a fractured sandstone specimen, a numerical simulation of heterogeneous rock specimens, and field borehole TV (television) images. Subsequently, a novel SSA (strain-softening and secondary anisotropy) constitutive model was developed to characterise the secondary anisotropy of the surrounding rock and developed using C++ into a numerical form that can be called by FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions). Finally, effects of secondary anisotropy on a deep tunnel surrounding rock were analysed by comparing the results calculated by the SSA model and a uniform strain-softening model. The results show that considering the secondary anisotropy, the extent of strain-softening of the surrounding rock was mitigated, particularly the axial strain-softening. Moreover, it reduced the surface displacement, plastic zone, and dissipated plastic strain energy of the surrounding rock. The proposed SSA model can precisely characterise the objectively existent secondary anisotropy, enhancing the accuracy of numerical simulations for tunnels, particularly for deep tunnels.<\/jats:p>","DOI":"10.3390\/sym17081234","type":"journal-article","created":{"date-parts":[[2025,8,4]],"date-time":"2025-08-04T15:30:06Z","timestamp":1754321406000},"page":"1234","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Study on Mechanism and Constitutive Modelling of Secondary Anisotropy of Surrounding Rock of Deep Tunnels"],"prefix":"10.3390","volume":"17","author":[{"given":"Kang","family":"Yi","sequence":"first","affiliation":[{"name":"College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, China"}]},{"given":"Peilin","family":"Gong","sequence":"additional","affiliation":[{"name":"College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, China"}]},{"given":"Zhiguo","family":"Lu","sequence":"additional","affiliation":[{"name":"CCTEG Coal Mining Research Institute, Beijing 100013, China"},{"name":"State Key Laboratory of Intelligent Coal Mining and Strata Control, Beijing 100013, China"}]},{"given":"Chao","family":"Su","sequence":"additional","affiliation":[{"name":"College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, China"}]},{"given":"Kaijie","family":"Duan","sequence":"additional","affiliation":[{"name":"College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, China"}]}],"member":"1968","published-online":{"date-parts":[[2025,8,4]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"37","DOI":"10.1016\/j.jrmge.2022.11.004","article-title":"Mechanical behaviors of coal measures and ground control technologies for China\u2019s deep coal mines\u2014A review","volume":"15","author":"Kang","year":"2023","journal-title":"J. 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