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Introduction
Heavy haul railways are the primary means of transporting bulk materials such as energy and minerals [1]. In China, coal resources have a reverse distribution between production and consumption, driving the rapid development of heavy haul railway technology. The axle load, train formation, and operating density of heavy haul railways have reached unprecedented levels, with the axle load of trains on the Shuozhou–Huanghua Railway reaching 30 t and the total length exceeding 4 km [2]. However, while the increase in axle load enhances carrying capacity, it also intensifies the interaction between the train and the infrastructure. During operation, the tunnel lining structure undergoes significant stress, increasing the probability of foundational structural diseases to 30%–70% [3], such as lining cracks, voids, basement subsidence, and mud pumping. Statistics show that the incidence of foundational structural diseases in heavy haul railways is 2.5 times that of ordinary railways [4].
The evolution of tunnel basement structural defects is a complex process involving the interaction of multiple factors [5]. Niu [3] pointed out that the early construction quality, design, and acceptance standards of tunnels were relatively low, resulting in initial defects in the tunnel lining structure. The evolution of initial basement defects is also related to service environments such as train axle load, water-rich conditions, and surrounding rock grade [6, 7–8]. Basement structural diseases typically evolve into cavities, voids, or empty spaces, with void heights reaching up to 15 cm, severely affecting the long-term operational safety of trains [9, 10–11]. Therefore, many scholars have conducted studies on tunnel basement void diseases. Liu et al. [10] demonstrated through numerical simulation that basement voids significantly increase the maximum tensile stress in the invert filling layer, leading to fatigue failure of the tunnel bottom structure under the long-term action of 30 t axle load trains. Meguid and Dang [12] analyzed the voids behind the tunnel lining using numerical calculations, revealing the influence patterns of different void sizes and locations on the tunnel structure’s stress.
Loose fill, as an intrinsic condition for the development of tunnel basement structural diseases, leads to initial defects such as pores and weak structural surfaces within the basement structure during the construction phase [13]. The underground water environment causes softening of the basement structure and surrounding rock, while...