Shock Absorption Layer Materials for Tunnel Engineering: Classification, Performance, and Future Directions

Abstract

Damage to tunnel structures under seismic action severely affects engineering safety and post-earthquake rescue, making it crucial to enhance the seismic capacity of tunnels. Current seismic approaches for tunnel engineering mainly include seismic isolation (shock absorption layer technology), damping, and anti-seismic, among which shock absorption layer technology has attracted considerable attention due to its economic efficiency and effectiveness. However, existing research has primarily focused on single shock absorption layer materials, lacking systematic classification frameworks and multi-dimensional comparative analyses, making it difficult to provide comprehensive guidance for material selection and engineering applications. This paper systematically reviews the research status of tunnel shock absorption layers. First, it elucidates three core mechanisms through which shock absorption layers function: wave-impedance mismatch and energy reflection, material damping and energy dissipation, and system stiffness reduction with natural period elongation. This study proposes categorizing the existing materials for tunnel shock absorption layers into five main types: foam concrete, other types of concrete, polymer materials, asphalt materials, and porous metallic materials. A detailed introduction is provided for each material category, covering their physical properties, shock absorption performance, advantages and disadvantages, as well as relevant optimization studies conducted to address material limitations. By comprehensively comparing the mechanical properties, shock absorption performance, durability, constructability, recyclability, and economy of these five types of materials, revealing their unique advantages and applicable limitations in tunnel shock absorption. Finally, the limitations of existing research are summarized, development directions for tunnel shock absorption layer materials are proposed, and the future research trend of tunnel damping layer technology is envisioned. This paper provides a reference for the research, selection, and standard formulation of tunnel shock absorption layer materials.

1. Introduction

Increasing seismic damage cases have underscored the critical importance of seismic performance for underground structures [1,2]. Seismic damage to tunnels poses severe threats to operational safety, incurs substantial economic losses, and may interrupt critical post-earthquake lifeline corridors, potentially triggering secondary disasters [3,4]. Consequently, intensive research on tunnel seismic protection and enhancement of tunnel seismic resistance capabilities is of paramount importance. As illustrated in Figure 1, engineering seismic protection generally encompasses three approaches: (a) seismic isolation, (b) damping, and (c) anti-seismic. From the perspective of the dynamic process of seismic energy transmission and control, these three approaches follow a logical sequence: first, the shock absorption layer functions to absorb, block, and alter the input characteristics of seismic energy; second, structural dampers activate to further dissipate the transmitted energy; and finally, the structural seismic resistance capacity withstands the remaining seismic effects. These three components act synergistically to form an integrated “seismic isolation–damping–anti-seismic” system. For critical tunnel projects in high-seismicity regions, subsea tunnels, mountain tunnels traversing active fault zones, or structures proximal to significant vibration sources, seismic isolation is typically achieved by installing shock absorption layers—such as foam concrete, asphalt, or rubber layers—between the surrounding rock and tunnel lining (or between the primary and secondary linings). This strategic placement prolongs the structural natural period and impedes seismic energy transmission, thereby achieving “seismic isolation”; damping bearings are installed between tunnel segments to dissipate seismic wave energy transmitted into the structure, thereby achieving “damping”; the lining structural strength is enhanced through increased reinforcement ratios or the use of high-strength concrete and steel-fiber-reinforced concrete for tunnel lining construction, thereby achieving “anti-seismic” [5,6].

This paper investigates the current state of research on tunnel shock absorption layer materials. In recent years, relevant research on tunnel shock absorption layers has gradually intensified. Scholars have validated the shock absorption effectiveness of such layers through shaking table model tests and numerical simulations. Theoretical achievements in tunnel shock absorption have progressively accumulated, and an increasing variety of shock absorption layer materials have been proposed. Despite these advances, the existing literature still presents notable deficiencies. First, while research on shock absorption materials is extensive, investigations specifically targeting tunnel applications remain relatively limited and exhibit a certain degree of fragmentation; individual researchers often focus on single or specific material types, and comprehensive review articles encompassing mainstream materials remain absent, lacking a widely accepted classification framework. Second, most studies emphasize performance analysis of individual materials, with insufficient systematic, multi-dimensional comparative evaluation among different materials. This deficiency leaves researchers and engineers without clear guidance for material selection in research and practical applications, requiring substantial time and effort for information collation and analysis. Therefore, a comprehensive and systematic review and synthesis of existing research is necessary to establish a clear classification framework, conduct detailed analysis and comparative evaluation of various existing shock absorption layer materials, and thereby provide valuable references for subsequent research and engineering practice.

The literature scope referenced in this review mainly covers relevant research results published in international authoritative journals and academic conferences from 2000 to 2026, and also refers to some representative papers and technical reports to ensure the comprehensiveness and cutting-edge nature of the content. A systematic literature retrieval was conducted across seven authoritative academic databases: Web of Science, Scopus, CNKI, Engineering Village, ScienceDirect, SpringerLink, and CrossRef. The search strategy employed Boolean combinations of keywords organized into two thematic clusters. The first cluster encompassed tunnel engineering and seismic mitigation terminology, including tunnel, shock absorption layer, shock absorption, seismic resistance, seismic isolation, performance, review, mechanical properties, shock absorption performance, durability, constructability, recyclability, and economy. The second cluster comprised material-specific terms: materials, shock absorption materials, concrete, foamed concrete, rubberized concrete, polymer, rubber, polypropylene, asphalt, porous metal, and aluminum foam. Various permutations of these keywords were systematically combined to ensure comprehensive coverage. This rigorous screening process ultimately yielded a corpus of 165 references. Chronological analysis revealed that 51.5% of the selected literature was published between 2020 and 2026, while 86.1% fell within the timeframe of 2010–2026. The structure of this paper is arranged as follows: First, the shock absorption mechanisms of shock absorption layers are elucidated. Second, the design requirements for shock absorption layers are summarized. Third, classifying the existing materials for shock absorption layers and providing a detailed description of the composition of each material category, their performance in tunnel shock absorption, along with their advantages and disadvantages, as well as the optimization studies conducted to address material limitations. Fourth, a horizontal comparison of current shock absorption layer materials is conducted from multiple dimensions, and their applicable conditions are discussed. Finally, research conclusions are presented, current research limitations are summarized, and research prospects and future development directions are proposed.

2. Primary Mechanisms of Seismic Isolation by Shock Absorption Layers

The effectiveness of tunnel shock absorption layers in enhancing the seismic performance of tunnel structures can be attributed to three core mechanisms:

(1)

Wave-Impedance Mismatch and Energy Reflection

When seismic waves propagate through a medium, their energy transmission characteristics are closely related to the wave impedance (product of density and wave velocity) of the medium. When seismic waves propagate from the surrounding rock to the lining, the increase in wave impedance tends to cause energy accumulation at the lining interface, intensifying the dynamic response of the structure. Consequently, the dynamic stress concentration factor (DSCF) of the lining structure increases abruptly. Conversely, wave reflection occurs at the interface, mitigating dynamic stress concentration to some extent [7,8,9]. As demonstrated by the study of Lu et al. [10], assuming P-wave incidence, the wave impedance of the selected Class III surrounding rock (elastic modulus E = 11,000 MPa, density = 2500 kg/m³, Poisson’s ratio = 0.24) is approximately 3.33 MPa·s, while that of the lining (elastic modulus E = 35,000 MPa, density = 2500 kg/m³, Poisson’s ratio = 0.20) is approximately 6.04 MPa·s. The wave impedance of the foam concrete shock absorption layer (elastic modulus E = 10 MPa, density = 1500 kg/m³, Poisson’s ratio = 0.30) is approximately 0.076 MPa·s. The wave impedance ratio between the surrounding rock and the lining is 1.81, whereas that between the surrounding rock and the shock absorption layer is 43.8. The wave function expansion method was employed to investigate the dynamic response of a circular tunnel incorporating a shock absorption layer. The findings indicated that in the elastic half-space model, the peak DSCF at the lining arch crown reached 31.4 in the absence of a shock absorption layer. However, upon installing a shock absorption layer with an elastic modulus of 10 MPa and a thickness of 0.3 m, the peak DSCF was reduced to 3.3. In the infinite elastic space model, the peak DSCF decreased from 5.8 without the shock absorption layer to 1.7 after its installation. Shock absorption layer materials, such as foam concrete and rubber, usually exhibit low elastic modulus and density (wave impedance < 0.1 MPa·s), resulting in wave impedance significantly lower than that of concrete linings (wave impedance > 1 MPa·s). When seismic waves enter the shock absorption layer, part of the energy is reflected at the rock-layer interface due to impedance mismatch, thereby reducing the energy transmitted further into the lining [10,11,12].

(2)

Low Elastic Modulus and High Damping for Energy Dissipation

Another key attribute of shock absorption layer materials is their combination of low elastic modulus and high damping capacity. Taking foam concrete as an example, its elastic modulus generally ranges from 20 to 1000 MPa, with a damping ratio between 3% and 10%. As seismic-wave-borne energy passes through the layer, the material is forced to undergo cyclic deformation (compression, shear). Ideal shock absorption materials (such as rubber–based composites, polymer foams) exhibit pronounced viscoelastic behavior under dynamic loading; internal macromolecular chain segments or micro-pore structures generate intense internal friction during motion, irreversibly converting part of the kinetic energy into deformation and thermal energy [13,14,15,16]. This process can be regarded as “filtering” and “absorbing” seismic energy. Kim and Konagai [17] demonstrated theoretically that flexible coating materials covering tunnels can effectively dissipate energy through damping, reducing structural dynamic responses. Wan [5] reported similar findings in tunnel shock absorption experiments involving rubber–cement–based composite materials. At a burial depth of 300 m, comparative tests were performed on three shock absorption layer materials: M1 (elastic modulus = 28,000 MPa, damping ratio = 0.03), M2 (elastic modulus = 520 MPa, damping ratio = 0.035), and M3 (elastic modulus = 240 MPa, damping ratio = 0.04). The measured displacements at the crown measuring point were 26.919 mm, 17.106 mm, and 15.378 mm, respectively. These results demonstrate that reducing the material elastic modulus while increasing the damping ratio yields superior shock absorption performance in tunnel applications.

(3)

System Stiffness Reduction and Natural Period Elongation

The tunnel structure and the surrounding soil form a coupled vibration system. Reduction in tunnel system stiffness and the consequent prolongation of natural vibration period effectively mitigate the amplification effects of high-frequency seismic energy. Taking the study by Lu et al. [18] as an example, as lining stiffness decreases, the flexibility ratio F (defined as the ratio of lining stiffness to surrounding rock stiffness) increases from 0.15 to 25.85. Consequently, the natural vibration period of the tunnel system extends from 0.06016 s (16.6 Hz) to 0.07739 s (12.9 Hz). Under Chi-Chi earthquake excitation, the amplification factor of peak spectral acceleration at the ground surface decreases from 1.377 (F = 0.15) to 0.627 (F = 25.85), representing a reduction of 54.5%. These results demonstrate that reducing system stiffness and extending the natural vibration period effectively circumvent high-frequency amplification effects. The installation of a shock absorption layer effectively introduces a flexible element into the original surrounding rock-lining system, thereby reducing overall structural stiffness, prolonging the natural vibration period, circumventing high-frequency amplification effects, and consequently diminishing the acceleration and inertial forces imposed on the structure [3,6,19,20].

3. Design Requirements for Shock Absorption Layers

According to the design requirements for underground structure seismic resistance [21,22,23,24,25], combined with current numerical simulation and mechanism research on shock absorption layers, the shock absorption layer design should meet the following requirements:

(1)

Material performance requirements: The material must simultaneously satisfy compressive strength requirements under static conditions and exhibit excellent deformability and fatigue resistance, while remaining capable of accommodating large deformations and withstanding multiple reversed loading cycles during seismic events. The elastic modulus of shock absorption layer materials should preferably be 1/100 to 1/50 of the surrounding rock elastic modulus, and the damping ratio should not be less than 5% [26,27].

(2)

Thickness design requirements: The thickness of the shock absorption layer is an important parameter affecting the shock absorption effect. Studies have shown that the ratio of shock absorption layer thickness to tunnel diameter should preferably be controlled between 1/40 and 1/20. A shock absorption layer that is too thin is difficult to play a shock absorption role, while one that is too thick will increase engineering costs and construction difficulty [26,28,29].

(3)

Durability requirements: Considering the influence of various factors such as environmental chemical action, stray current, crack effects, concrete shrinkage, stress corrosion, and freeze–thaw damage, shock absorption layer materials should have good long-term durability and be able to maintain stable performance in underground humid environments [30]. For example, for metallic materials, corrosion protection should be considered; for polymer materials such as polypropylene, aging issues and aging contamination phenomena should also be considered [31,32,33,34,35].

(4)

Construction feasibility requirements: Shock absorption layer materials should be convenient for construction and able to combine well with lining structures and the surrounding rock. For cast-in-place concrete materials, construction processes and curing conditions should be considered; for prefabricated materials, joint treatment and connection methods should be considered [26,36].

4. Classification and Description of Tunnel Shock Absorption Layer Materials

Based on a systematic review of existing research findings, this paper categorizes tunnel shock absorption layer materials into five major types according to their material composition and technical characteristics: foam concrete, other types of concrete, polymer materials, asphalt materials, and porous metallic materials. It subsequently provides a detailed introduction to the physical properties, tunnel shock absorption performance, and advantages and disadvantages of each of these five material types, as well as the optimization studies conducted to address their respective material limitations.

4.1. Foamed Concrete

4.1.1. Material Composition and Physical Characteristics

Foamed concrete (as shown in Figure 2 [37]) constitutes a cellular structure developed by incorporating numerous microscopic air voids into a cementitious matrix via specialized foaming agents. Characterized by lightweight properties, thermal insulation capabilities, and energy absorption capacity, it has emerged as a prominent research direction for tunnel shock absorption layer materials [13]. The water-to-binder (W/B) ratio serves as a critical governing parameter for foamed concrete performance. In conventional mix proportions, the W/B ratio typically falls within the range of 0.4–0.6. While elevated W/B ratios promote foam stability and enhance flowability, they concurrently reduce mechanical strength and increase shrinkage [38,39]. The category and quality of foaming agents directly govern foam stability, uniformity, and the resulting pore structural characteristics of the final material. The most distinctive physical attribute of foamed concrete is its internal architecture comprising abundant, uniformly distributed microscopic pores, achieving porosity levels of 40–80%, which confers distinctive mechanical behavior upon the material [40,41].

The density of foamed concrete is predominantly regulated through control of foam dosage, typically spanning 400–1800 kg/m³ to accommodate diverse engineering application requirements [42,43]. According to damage constitutive models for foamed concrete [44,45], a representative stress–strain curve for foamed concrete with a density of 800 kg/m³ under quasi-static loading conditions is schematically illustrated in Figure 3. As depicted, foamed concrete exhibits substantial ultimate compressive strain and demonstrates a distinct plateau stress region.

4.1.2. Tunnel Shock Absorption Performance

As illustrated in Figure 3, foamed concrete exhibits characteristic three-stage deformation behavior under uniaxial compression: the initial elastic stage, the plastic plateau stage, and the densification stage. This deformation characteristic endows the material with excellent energy absorption capacity [46]. During the plastic plateau stage, internal voids of foam concrete are progressively compressed layer by layer, and the compression–collapse process dissipates the kinetic energy transmitted by seismic waves. The relatively extended and stable plateau stress region (with strain persisting from 0.05 to 0.5) constitutes the primary mechanism enabling foamed concrete to achieve effective shock absorption. Through comparative shaking table tests on tunnels without shock absorption layers and those with added foamed concrete shock absorption layers, Yang et al. [42] observed that at a peak ground acceleration (PGA) of 0.4 g, tensile strain at the crown of the left tunnel decreased by 60% and compressive strain by 32.8%; at the left haunch of the right tunnel, tensile strain decreased by 66.9% and compressive strain by 8.4%. These results indicate that foamed concrete shock absorption layers can significantly reduce strains in both the transverse and longitudinal directions. However, as PGA increases from 0.1 g to 0.6 g, this strain reduction effect diminishes substantially. Notably, at the shoulder of the left tunnel, increasing PGA from 0.2 g to 0.4 g conversely resulted in amplification of the minimum principal strain. Liu et al. [47] confirmed through Split Hopkinson Pressure Bar (SHPB) tests that under impact loading, the cell walls of foamed concrete undergo plastic deformation and collapse failure, absorbing substantial energy during this process through material damping and energy dissipation—this constitutes the core mechanism of its function as a shock absorption layer material. Zhao et al. [48] revealed the strain rate effect of foamed concrete, demonstrating that within the intermediate strain rate range (10⁻⁵ s⁻¹ to 10⁻³ s⁻¹), material strength increases exponentially with strain rate, with significant enhancement of post-peak residual stress. These characteristics enable foamed concrete to simultaneously perform dual functions of seismic isolation and energy dissipation under seismic motion: on one hand, reflecting a portion of seismic wave energy through its low wave impedance characteristics; on the other hand, dissipating transmitted energy through material damping and plastic deformation. Ma et al. [49] found that superior seismic isolation performance is achieved when foamed concrete possesses lower density, greater thickness, and smaller spacing. Cui [50] analyzed the shock absorption performance of steel fiber-reinforced foamed concrete primary support from the perspective of wave theory, establishing a mechanical interaction model for the surrounding rock–primary support–secondary lining system. The study revealed that for SV wave incidence, optimal shock absorption performance is achieved when the elastic modulus of the primary support ranges between 1.7 GPa and 11.2 GPa, with the optimal mitigation effect occurring at approximately 3 GPa.

4.1.3. Advantages and Limitations

Advantages: (i) the porous structure of foamed concrete endows it with excellent shock-absorbing performance; (ii) the material density can be adjusted over a wide range, enabling the design of corresponding mechanical properties according to different engineering requirements; (iii) it possesses good construction performance and can be applied by pumping, making it suitable for complex tunnel construction environments; (iv) it realizes the dual function of a shock-absorbing layer and primary support in one, avoiding the increase in construction procedures and excavation cross-section associated with adding an independent shock-absorbing layer.

Limitations: (i) due to its high water-to-binder ratio and absence of coarse aggregate, the material is prone to significant shrinkage deformation during hardening, and its internal porous structure results in high water absorption; (ii) the material strength is relatively low, especially the tensile strength, which necessitates improvement through techniques such as fiber reinforcement; (iii) the pore structure has a significant influence on material performance, and unreasonable pore-size distribution or bubble-wall defects can markedly reduce material strength; (iv) in dry environments, dehydration collapse may occur, affecting material performance; under long-term wet–dry cycling, performance degradation may occur, requiring further optimization of the material’s durability.

4.1.4. Strategies for Overcoming Material Limitations

To address the deficiencies of foamed concrete materials, researchers have conducted extensive experimental investigations to optimize their low compressive strength, weak tensile performance, poor water resistance, and susceptibility to shrinkage cracking, thereby enhancing their comprehensive shock absorption layer performance. For instance, by incorporating steel fibers (volume fraction of 0.9%) into foamed concrete, controlling the water-to-binder ratio at 0.35, density at 1550 kg/m³, and cement-to-silica fume mass ratio at 9:1, steel fiber-reinforced foamed concrete with an elastic modulus of 1.9 GPa was obtained. When employed as tunnel primary support, the maximum shock absorption rate reached 36.7% [50]. Zhu [51] established a constitutive model for steel fiber-reinforced foamed concrete. Numerical simulation results demonstrated that adopting steel fiber-reinforced foamed concrete for tunnel primary support exhibits superior shock absorption performance compared to conventional shock absorption layer schemes, with improvements in shock absorption rates for controlling bending moment, axial force, and acceleration reaching up to 8.1%. Lin [52] discovered through experimental studies that the synergistic utilization of polypropylene (PP) fibers and silica fume can significantly enhance the strength of foamed concrete. The significance sequence of factors influencing the compressive strength follows: silica fume dosage > fiber length > fiber dosage. Foamed concrete incorporating PP fibers of 9 mm length (content of 0.5%) and silica fume (content of 15%) achieved a compressive strength of 7.2 MPa. Hazlin et al. [53] investigated the microstructure of foamed concrete, observing the pore characteristics of foamed concrete with varying densities and the interfacial bonding between concrete and PP fibers. The addition of 0.05% PP fibers resulted in tensile strength improvements of 35.06% and 40.30% for foamed concrete with densities of 1600 kg/m³ and 1800 kg/m³, respectively. Freccy et al. [54] investigated the effects of PP fiber dosage (0%, 0.25%, 0.40%) and water-to-cement ratio (0.30–0.40) on the mechanical properties of lightweight foamed concrete (LFC). Experimental results indicated that the tensile strength of specimens ranged between 0.991 and 2.138 MPa. While PP incorporation effectively enhanced the tensile strength of foamed concrete (density 1500 ± 50 kg/m³), no further significant improvement was observed when PP dosage increased from 0.25% to 0.40%, suggesting the existence of an optimal threshold for fiber dosage. Yu [55] conducted research targeting the susceptibility of foamed concrete to shrinkage cracking and its high water absorption characteristics. The study revealed that a rubber dosage of 5% yielded the minimum water absorption rate of 13.8%, with the dynamic elastic modulus reaching 8.00 GPa. During wet-dry cycling, the pore saturation degrees of the two tested materials were maintained below 10.22% and 12.12%, respectively, corresponding to saturation levels below 23.42% and 18.48%. These findings indicate that aggregates can impede water infiltration into specimens and enhance the crack resistance of foamed concrete. Essam et al. [56] incorporated crumb rubber derived from waste automobile tires into foamed concrete. The investigation found that at a rubber dosage of 17% of the total concrete volume, foamed rubber concrete with compressive strength exceeding 10 MPa and density below 1600 kg/m³ could be obtained. However, for mixtures with rubber dosages of 0%, 8.47%, 17%, and 47.8%, the compressive strength decreased by 3.3%, 14.8%, 29.4%, and 40.4%, respectively. At rubber contents of 17% and 47.8%, the material damping ratio increased by 21.1% and 17.3%, respectively, compared to the mixture without rubber. Damiani et al. [57] reported that compared to sand-incorporated mixtures, rubber foamed concrete mixtures with a density of 0.40 g/cm³ exhibited significant improvements in mechanical properties, including compressive strength, plateau strength, and impact resistance. Furthermore, the addition of crumb rubber particles can mitigate foam degradation issues caused by particle incorporation in low-density mixtures. Additionally, numerous researchers have employed various modification methods for foamed concrete, including adjusting bubble dimensions, incorporating lightweight ceramic particles, fly ash, utilizing magnetized water for mixing, and adding composite materials, all of which have contributed to the enhancement of foamed concrete mechanical properties to varying degrees [58,59,60,61,62,63,64,65,66,67].

4.2. Other Types of Concrete

4.2.1. Material Composition and Physical Characteristics

Currently, other types of concrete employed for tunnel shock absorption mainly include rubber–cement–based concrete and polypropylene foamed concrete. Distinct from foamed concrete, which achieves porosity through the entrainment of air bubbles into the cementitious matrix, these materials are developed by incorporating various modifying constituents into conventional cement–based matrices to enhance toughness, deformation capacity, and energy absorption characteristics. This section primarily elaborates on rubber–cement–based concrete and polypropylene foamed concrete.

Rubber–cement–based concrete is a composite material prepared by incorporating waste rubber particles into a cement matrix. Figure 4 presents a finished rubber cement slab product [68]. The fundamental material composition comprises ordinary Portland cement, water, fine aggregates, and waste rubber particles. The particle size, morphology, and dosage of rubber particles constitute critical parameters governing material performance. The rubber particle dosage typically ranges from 5 to 20. The incorporation of rubber particles improves the dynamic performance of the material; excessive dosage significantly compromises static strength [69,70].

Polypropylene concrete is a lightweight composite material prepared by incorporating expanded polypropylene (EPP) particles into the cementitious matrix. EPP particles exhibit exceptional lightweight characteristics, high elasticity, and chemical stability, which effectively enhance the physical and mechanical properties of concrete. Research by Lu et al. [71] demonstrated that when the EPP volume fraction increases from 0 to 28%, the apparent density decreases from 2331 kg/m³ to 1451 kg/m³. Concurrently, the mechanical properties exhibit systematic variations: compressive strength decreases from 54.5 MPa to 11.2 MPa, tensile strength from 3.6 MPa to 1.6 MPa, and elastic modulus from 34.0 GPa to 16.0 GPa, whereas Poisson’s ratio increases from 0.19 to 0.28. This performance evolution pattern provides critical guidance for material design.

4.2.2. Tunnel Shock Absorption Performance

The shock absorption performance of rubber–cement concrete is primarily manifested in its excellent deformability and energy dissipation characteristics. The elastic modulus of rubber (on the order of magnitude of 10² MPa) particles is substantially lower than that of the cement matrix (on the order of magnitude of 10⁴ MPa). Under seismic loading, rubber particles absorb considerable energy through elastic deformation, thereby attenuating the structural seismic response. Simultaneously, interfacial sliding between rubber particles and the cement matrix dissipates additional energy, constituting a dual energy dissipation mechanism. The incorporation of rubber significantly enhances the material’s fracture toughness, impact toughness, and energy absorption capacity [72,73,74,75]. Deng Zhao [76] demonstrated through numerical simulations that rubber–cement concrete shock absorption layers effectively reduce tunnel lining displacement, acceleration, and stress responses. The shock absorption effect is optimized when the elastic modulus of rubber–cement concrete is 308 MPa. With increasing thickness of the shock absorption layer, lining displacement and acceleration exhibit slight increases, though the overall influence on displacement remains insignificant. However, when the thickness increases substantially (e.g., to 35 cm), the efficacy of stress reduction diminishes, potentially resulting in increased lining stress. Zhang Jianyu et al. [72] experimentally validated the shock absorption performance of modified rubber concrete tough layers. Their findings indicate that tough layers with thicknesses of 10 cm, 15 cm, and 20 cm all effectively improve the seismic performance of tunnel structures, with the 15 cm configuration exhibiting optimal shock absorption efficiency. Static and dynamic testing revealed that rubber incorporation significantly increases concrete damping, which proves advantageous for seismic resistance [77,78]. Mei et al. [77] conducted hammer impact tests on tunnel lining structures composed of rubber–cement–based concrete, employing ordinary concrete as a control group. The control group exhibited the following parameters: vibration duration of 0.29 s, acceleration amplitude of 0.19 m/s², composite loss factor of 0.12, damping ratio of 8.85%, and total vibration level of 85.61. The corresponding values for the test group were 0.04 s, 0.10 m/s², 0.30, 14.83%, and 79.23, respectively. These results indicate that lining structures fabricated from rubber–cement–based concrete possess superior damping performance compared to conventional concrete.

4.2.3. Advantages and Limitations

Advantages: (i) Diverse performance profiles enable tailored solutions for different engineering demands; (ii) rubber concrete exhibits large deformation capacity and high toughness, effectively absorbing seismic energy; (iii) low density imposes minimal increase in tunnel structural self-weight; (iv) utilization of waste tires provides environmental benefits and supports sustainable construction practices; (v) construction procedures are straightforward, allowing adoption of conventional concrete placement techniques.

Limitations: (i) Incorporation of rubber or EPP causes significant compressive and tensile strength reduction, and the composite still exhibits some brittleness with a limited post-cracking plateau; (ii) interfacial bonding between rubber or EPP and the cement matrix remains weak and requires further enhancement; (iii) uniform dispersion of incorporated particles is sensitive to mixing procedures and equipment, raising construction complexity and cost.

4.2.4. Strategies for Overcoming Material Limitations

To overcome material limitations, researchers have optimized rubber–cement–based concrete and polypropylene concrete through various experimental investigations, aiming to enhance the elastic modulus, toughness, or damping properties of these materials. Zhang [72] concluded from multi-factorial cross experiments that the optimal synergistic modification of rubber–cement–based concrete is achieved with a 1% NaOH concentration for rubber surface treatment, 6% polyethylene (PE) fiber dosage, and 10% metakaolin dosage, resulting in a Poisson’s ratio of 0.307, with fracture toughness increasing by 253.48 N/m and impact toughness by 96.8 kN/m². Mo et al. [78] improved rubberized concrete by incorporating fly ash (FA) and ground granulated blast furnace slag (GGBFS), finding that a 30% FA dosage significantly enhances its damping ratio (increasing from 2.88% to 3.20% in the undamaged state, with a peak value reaching 6.34%), while additional incorporation of rubber powder (4.5% by mass of cementitious materials) into polypropylene fiber concrete enhanced the elastic-stage damping ratio by approximately 14.4% compared to the unmodified control group (from 2.64 to 4.72% to 3.03–4.72%), though this caused a 16–25% reduction in cubic compressive strength and approximately 17% increase in peak strain, with the modified material maintaining a high damping ratio of 4.29–7.98% during damage evolution through the viscoelastic deformation mechanism of rubber powder combined with fiber bridging effects; however, its characteristic of accelerating flexural dynamic stiffness degradation (with stiffness decay rates increasing by approximately 30–50% compared to unmodified specimens after 20 mm displacement) must be considered in shock absorption layer design. Zhou et al. [79] incorporated air-entraining agent (AGA) and polypropylene (PP) fibers into recycled aggregate concrete, finding that a 100% recycled aggregate replacement ratio increased the concrete damping ratio by 35.00% compared to the reference group but resulted in compressive strength and dynamic elastic modulus reductions of 40.04% and 56.07%, respectively, whereas at a 50% replacement ratio, with compressive strength reduced by only 3.37%, the further addition of 0.10% PP enhanced the damping ratio by 3.18% and incorporating 0.02% AGA controlled strength loss within 3.54% while achieving significant damping enhancement, providing optimized mix proportion references for shock absorption layer materials that balance load-bearing capacity and energy dissipation characteristics. Wang et al. [80] improved concrete brittleness by adding steel fibers (SF) and polypropylene fibers (PPF) at volume fractions ranging from 0.968% to 2.338%, with experimental results indicating that at a total fiber volume content of 1.653%, the maximum hysteretic viscous damping ratio reached 0.568–0.729, representing a 5- to 6-fold increase compared to plain concrete (0.104–0.114), and the hysteretic viscous damping model based on residual strain established in this study (R² = 0.72–0.93) provides a quantitative prediction methodology for stiffness degradation and energy dissipation mechanisms of shock absorption layer materials under seismic cyclic loading.

4.3. Polymeric Materials

4.3.1. Material Composition and Physical Characteristics

Polymeric materials are engineering materials based on macromolecular compounds, prepared through specific synthesis or modification processes, and exhibit excellent elasticity and deformation capacity. In the field of tunnel shock absorption, rubber and polyurethane represent the two most widely utilized classes of polymeric materials [81,82,83,84,85].

Rubber materials are primarily based on natural rubber (NR) or synthetic rubber, forming three-dimensional network structures through vulcanization cross-linking reactions. Rubber molecular chains contain unsaturated double-bond structures, which undergo cross-linking reactions with vulcanizing agents during the vulcanization process to form network structures, endowing rubber with excellent damping (5–30%), recoverability, and wear resistance. These characteristics constitute the primary attributes of rubber materials. The density of rubber materials typically ranges from 900 to 1600 kg/m³, and the elastic modulus can be tailored over a wide span, from soft rubbers with moduli as low as 1 MPa to hard rubbers reaching several hundred MPa [81,82,83,86,87,88].

Polyurethane materials are a class of polymer materials synthesized from isocyanates and polyols through polymerization reactions. By adjusting the types and ratios of isocyanates and polyols, as well as molecular weights, various polyurethane materials ranging from soft elastomers to rigid plastics with different densities can be prepared. Polyurethane materials employed for tunnel shock absorption typically exhibit low densities (100–600 kg/m³), with damping ratios reaching 6–10%. The mechanical properties of polyurethane materials are significantly influenced by the ratio of hard segments to soft segments: hard segments provide strength and stiffness, whereas soft segments provide elasticity and flexibility. Tailored material design can be achieved by regulating the proportion of these two segments [81,84,89,90].

4.3.2. Tunnel Shock Absorption Performance

Researchers have found that the application of polymeric shock absorption layers can effectively alleviate the acceleration amplification effect and reduce the peak stress and peak strain of the lining. This is primarily attributed to the significant internal friction and hysteretic effects generated by the molecular chain segments of the material under dynamic shear action, which effectively dissipate seismic energy and reduce structural seismic response. Additionally, the low stiffness characteristic of the material can effectively prolong the natural vibration period of the structure, avoiding the predominant period of ground motion. Figure 5 presents the field setup of tunnel seismic model tests employing rubber as a shock absorption layer [91]. Jiang et al. [91] investigated the shock absorption performance of rubber shock absorption layers through large-scale shaking table tests. The results indicate that rubber shock absorption layers can significantly reduce the peak acceleration response of tunnel linings. Under the action of El Centro and Kobe waves, rubber shock absorption layers demonstrate significant reduction effects on both horizontal and vertical accelerations, with horizontal acceleration reduction coefficients ranging from 0.24 to 0.47 and vertical acceleration reduction coefficients from 0.20 to 0.48. The maximum principal strain reduction coefficients range from 0.13 to 0.32, while the minimum principal strain reduction coefficients range from 0.20 to 0.33. Zhao et al. [92,93] demonstrated through experimental studies that rubber shock absorption layers cannot alter the overall acceleration response trend of tunnels under seismic action, but can mitigate partial dynamic effects of the surrounding rock on the tunnel structure through energy absorption and shock absorption functions, thereby enhancing the seismic resistance of tunnel structures. Xie et al. [94] reported that under 0.05 g seismic waves, optimal tunnel shock absorption performance is achieved when utilizing rubber shock absorption layers with an elastic modulus of 1 MPa. Ma et al. [95] measured the dynamic shear modulus (approximately 40.5 MPa), dynamic elastic modulus (approximately 89 MPa), and damping ratio (0.06–0.07) of non-water-reactive two-component polyurethane polymer with densities of 0.22–0.223 g/cm³ through resonant column tests and free-free resonant tests. They established seventeen infinite boundary finite element models to analyze the shock absorption performance of shield tunnels externally wrapped with this material. Numerical results indicate that when the density of the polymeric shock absorption layer decreases from 0.54 g/cm³ to 0.17 g/cm³, the peak tensile stresses of the external wrapping layer and lining decrease by 82.8% and 72.2% on average, respectively. When the thickness increases from 5 cm to 20 cm, the peak tensile stress of the tunnel lining can be reduced by approximately 80%. Considering both shock absorption effectiveness and engineering economy, the optimal density for this polymeric shock absorption layer is recommended as 0.22–0.33 g/cm³ with a thickness of 20 cm.

4.3.3. Advantages and Limitations

Advantages: (i) The lightweight nature of these materials results in minimal impact on the self-weight of tunnel structures. (ii) Their elastic modulus is adjustable, and they can retain elastic recovery over a wide range of deformations. (iii) The long molecular chains within the material generate substantial internal friction and hysteretic effects under dynamic shear action, effectively dissipating seismic energy and attenuating structural seismic response. (iv) Furthermore, material properties can be tailored through formulation design and process parameter adjustment to accommodate diverse engineering requirements.

Limitations: (i) These materials exhibit relatively low mechanical strength, particularly in terms of compressive strength—with elastic modulus values potentially as low as 1 MPa—which renders them inadequate for meeting basic load-bearing requirements. (ii) Their temperature sensitivity requires improvement, as certain polymer materials tend to decompose or exhibit degraded mechanical properties at elevated temperatures. (iii) Their durability warrants further investigation, especially concerning aging behavior under complex environmental conditions.

4.3.4. Strategies for Overcoming Material Limitations

To overcome material limitations, researchers have employed various methodologies to enhance the compressive and tensile strength and thermal resistance of polymeric materials. Zhang et al. [96] synergistically modified natural rubber (NR) using hindered phenol AO-80, styrene-isoprene block copolymer (SIS), and epoxidized natural rubber (ENR) at a mass ratio of m(NR):m(AO-80):m(SIS):m(ENR) = 100:15:16:15. The resulting composite exhibited a loss factor peak of 1.42 in the low-temperature region (representing a 49.5% improvement compared to the NR/AO-80/SIS system), with the effective damping temperature range broadened from 25.17 °C to 137.04 °C, and achieved a tensile strength of 25.8 MPa and an elongation at break of 785.3%. Zhong et al. [97] surface-modified styrene-butadiene rubber (SBR) using graphene (G-MB), resulting in a 120% increase in thermal conductivity compared to neat SBR. Following thermo-oxidative aging at 100 °C for 240 h, the SBR/G-MB composite maintained an elongation retention rate of 55%, demonstrating excellent long-term stability. Wang et al. [98] blended highly epoxidized Eucommia Ulmoides Gum (EEUG, 32.0 mol% epoxidation degree) with nitrile rubber (NBR), observing that the loss factor (tanδ) peak of the composite increased from 1.166 to 1.855 (a 59% enhancement), with optimal damping performance approaching room temperature conditions. Simultaneously, the elongation at break increased significantly from 271% to 493% (an 81.9% improvement), resilience improved from 18.5% to 21.5%, and stable damping characteristics were maintained across a broad frequency range of 0.1–100 Hz (tanδ within 0.176–0.236). This provides a feasible material design strategy for tunnel shock absorption layers to achieve long-term stable energy dissipation across wide temperature and frequency domains. Bleszynski et al. [99] incorporated 3 wt% TiO₂ microparticles (approximately 40 μm particle size) into rubber, revealing that after aging in electrolytic saline solution (3% NaCl) for 8 h, the composite retained a contact angle of 84.6° and a hardness of 29 HA (representing only a 14% decrease), whereas the unmodified material completely lost hydrophobicity and exhibited a hardness reduction exceeding 40%, indicating that such modification strategies can effectively enhance the durability of rubber materials in highly oxidative and saline environments. Babar and Verma [100] modified polyurethane using Cloisite 20A nanoclay (1–3 wt%), significantly improving the material’s thermomechanical properties. Dynamic mechanical analysis indicated that the glass transition temperature (Tg) increased from 13.38 °C for neat PU to a maximum of 30.70 °C (an increase of 17.32 °C), while the storage modulus remained stable in contrast to the sharp decay observed in neat PU. Nanoindentation testing revealed that the elastic modulus could be regulated within the range of 0.006–0.080 GPa, with hardness values of 0.0001–0.0055 GPa. Zheng et al. [101] employed a carbon coating strategy to surface-modify hollow glass microspheres (HGMs) and incorporated them into a polyurethane (PU) matrix. The dissipation energy coefficient (DE) of this material increased significantly from 23.95% for neat PU to 35.13%, while the compressive strain under 4 MPa stress decreased from 36.25% to 24.06%. Furthermore, the material density decreased from 1.057 g/cm³ to 0.918 g/cm³ (a 13.2% reduction), achieving a synergistic enhancement of mechanical and damping properties while maintaining lightweight characteristics. Additionally, numerous scholars have conducted modification studies on rubber and polyurethane from various perspectives to improve material performance in multiple aspects [88,90,102,103].

4.4. Asphalt Materials

4.4.1. Material Composition and Physical Characteristics

Asphalt materials, as illustrated in Figure 6 [104], have attracted considerable attention in tunnel shock absorption layer applications as traditional construction and waterproofing materials. Asphalt–based grouting materials are primarily composed of emulsified asphalt, cement, bentonite, and water-reducing agents. This multi-component composite system effectively combines the advantages of various constituents to form high-performance shock absorption materials [105,106]. Emulsified asphalt serves as the core constituent of asphalt–based materials, forming stable emulsions by dispersing asphalt in an aqueous phase through emulsification technology. Cement, acting as the primary binding material, ensures the strength and stability of the composite. The type, grade, and dosage of cement significantly influence the mechanical properties and durability of the material. Within asphalt–based systems, cement and emulsified asphalt form a composite binding system, developing network structures through hydration reactions and physical cross-linking. The incorporation of bentonite primarily aims to improve the suspensibility and stability of the material, whereas the utilization of water-reducing agents serves to optimize the rheological properties [105,107,108,109,110].

The physical characteristics of asphalt–based materials are manifested in the following aspects [111,112,113,114,115,116]: first, viscoelasticity—asphalt materials exhibit pronounced viscoelastic characteristics, demonstrating dual elastic and viscous behavior under loading; second, temperature sensitivity—material properties vary significantly with temperature, necessitating modification techniques to improve thermal stability; third, self-healing capability—asphalt materials possess inherent self-healing capacities, enabling the repair of micro-cracks to a certain extent; and fourth, waterproofing performance—asphalt materials exhibit excellent waterproofing properties, effectively preventing water infiltration.

4.4.2. Tunnel Shock Absorption Performance

Asphalt materials exhibit favorable shock absorption performance due to their excellent damping properties and energy dissipation capacity. Specifically, the primary mechanisms include the following aspects [117,118,119,120]: first, viscous energy dissipation—under dynamic loading, friction and motion between molecular chains effectively dissipate energy; second, interface slippage—microscopic sliding occurs at the interface between asphalt and aggregates under cyclic loading, thereby consuming energy; third, plastic deformation—asphalt materials can undergo plastic deformation under large deformations to absorb impact energy. Collectively, the shock absorption performance of asphalt materials primarily originates from their distinctive viscoelastic characteristics. Under dynamic loading, asphalt materials dissipate energy through molecular chain motion and rearrangement, exhibiting favorable damping properties [121,122,123]. Zhao [105] investigated the dynamic response characteristics of tunnels utilizing asphalt–based grouting materials as shock absorption layers through numerical simulation methods. The critical damping ratio was set at 5%, with static boundaries applied at the model base and free-field boundaries along the model periphery. The model dimensions were 150 m in the longitudinal direction, 80 m in the transverse direction, and 75 m in the vertical direction, with a tunnel burial depth of 20 m. The study demonstrated that the application of asphalt shock absorption layers can reduce the peak tensile stress of the lining by 15% to 45% and the peak compressive stress by 15% to 30%, while significantly narrowing the stress variation range. The smaller the stiffness of the shock absorption layer, the more pronounced the reduction in lining stress peaks, with the tensile stress reduction rate of the lining reaching over 60%. Taking the first principal stress at the crown as an example, when the shock absorption layer thicknesses were 5 cm, 15 cm, and 30 cm, the corresponding shock absorption rates were 46.3%, 50.4%, and 58.2%, respectively, indicating that the shock absorption rate increases with thickness, though the increment is limited. After the shock absorption layer thickness exceeds a certain value, the distribution of lining relative displacement along the tunnel longitudinal direction can be improved, yet a continued increase in thickness does not enhance the reduction rate of relative displacement. Furthermore, by adjusting the material mix proportions and modifier types, the stiffness, strength, and damping characteristics of the material can be regulated to satisfy shock absorption requirements under various engineering conditions. This design flexibility constitutes a significant advantage of asphalt materials.

4.4.3. Advantages and Limitations

Advantages: (i) Widespread availability of raw materials, mature production technologies, and relatively low costs; (ii) well-established construction techniques enabling diverse application methods including grouting and spraying, demonstrating high adaptability; (iii) favorable viscoelastic properties enabling effective dissipation of seismic energy; (iv) excellent waterproofing performance allowing these materials to simultaneously serve dual functions as waterproofing and shock absorption layers; (v) certain self-healing capabilities enabling partial repair of damage.

Limitations: (i) Pronounced temperature sensitivity resulting in significant performance variations with temperature, potentially compromising material behavior under high- or low-temperature conditions; (ii) durability concerns arising from aging during long-term service, leading to progressive performance degradation; (iii) relatively low mechanical strength, particularly weak tensile strength, necessitating enhancement through modification techniques; (iv) environmental friendliness requiring improvement, as certain asphalt materials may exert environmental impacts during production and application.

4.4.4. Strategies for Overcoming Material Limitations

Scholars have conducted modification studies on asphalt materials using different methods to overcome their inherent deficiencies. Sun et al. [124] prepared polyurethane (PU) modified asphalt using a disk-toothed mixer. Under conditions of 4.1 wt% PU dosage and an NCO-to-[H] molar ratio of 10:1, the resulting modified asphalt exhibited a dynamic viscosity at 60 °C reaching 5 times that of SBS (Styrene-Butadiene-Styrene) modified asphalt, with the dynamic modulus at 15 °C increased by 96% and at 60 °C increased by 222% compared to SBS modified asphalt. However, the low-temperature performance of this modified asphalt was somewhat compromised, exhibiting brittle fracture at 5 °C ductility, with the maximum flexural tensile strain only 78% of that of SBS modified asphalt. Yang et al. [125] chemically modified base asphalt using synthesized composite polyurethane. The resulting modified asphalt exhibited an unrecoverable creep compliance (Jnr) of 3.7889 kPa⁻¹ under 3.2 kPa shear stress, with an elastic recovery rate (R) of 22.3145%. At a low temperature of −24 °C, the flexural creep stiffness (S) of the modified asphalt was 567.3 MPa, significantly lower than the 697 MPa of base asphalt; simultaneously, the creep rate (m) increased from 0.175 for base asphalt to 0.272, indicating significant improvement in low-temperature crack resistance. Ibrahim et al. [126] reviewed the modification mechanisms of crumb rubber on asphalt rheological properties, noting that rubber particles can absorb light components in asphalt and swell to 3–5 times their original size. When the dosage increased from 0% to 21%, the viscosity of modified asphalt at 176 °C increased from 60 cP to 6000 cP, the softening point increased from 50 °C to 72 °C, and the resilience at 25 °C increased from 0 to 47%. Jeong et al. [127] demonstrated that a 20% crumb rubber dosage can increase asphalt viscosity by 550%, reduce low-temperature creep stiffness, and enhance low-temperature crack resistance. A 15–20% crumb rubber dosage can effectively inhibit the growth of macromolecular size during the aging process, slow down the hardening rate, and improve aging resistance.

4.5. Porous Metallic Materials

4.5.1. Material Composition and Physical Characteristics

Porous metallic materials represent a novel class of lightweight shock-absorbing materials fabricated from metallic bases through specialized processes, possessing substantial porosity. These materials combine the strength of metals with the lightweight characteristics of porous structures, exhibiting excellent mechanical properties and functionality. Currently, research on various porous metallic materials employed as tunnel shock absorption layers remains relatively scarce, with closed-cell aluminum foam representing the sole material documented in the literature.

Aluminum foams are categorized into open-cell and closed-cell configurations, as shown in Figure 7. Aluminum foam constitutes a porous metallic material produced from aluminum or aluminum alloys through foaming processes. Primary fabrication methodologies include melt foaming, powder metallurgy foaming, and deposition techniques. Melt foaming represents the most commonly employed approach, wherein foaming agents are introduced into molten aluminum, decomposing at specific temperatures to release gases and form uniform cellular structures. The porosity and pore size distribution can be regulated by controlling foaming agent type, particle size, addition dosage, as well as foaming temperature and duration [128,129,130,131,132,133,134].

The physical characteristics of aluminum foam are primarily manifested in the following aspects [135,136,137,138]: First, low apparent density, typically ranging from 200 to 800 kg/m³, equivalent to merely 1/10–1/3 that of solid aluminum; second, high specific strength—owing to the low density, aluminum foam exhibits elevated specific strength (the ratio of strength to density); third, excellent energy absorption capacity—aluminum foam absorbs substantial energy through pore collapse during compression; furthermore, aluminum foam exhibits an extended plateau stress region (with strains reaching 0.5 and above), demonstrating favorable energy absorption efficiency.

4.5.2. Tunnel Shock Absorption Performance

The shock absorption performance of aluminum foam benefits from its excellent energy absorption density. Under dynamic loading, aluminum foam can exert shock absorption effects through multiple mechanisms. First, pore collapse energy dissipation—during compression, the porous structure undergoes progressive collapse, effectively absorbing and dissipating energy. The pore collapse process encompasses three stages: elastic deformation, plastic yielding, and densification, each contributing to energy dissipation. Second, the damping characteristics of the metallic matrix—aluminum and its alloys inherently possess certain damping characteristics, dissipating energy through lattice vibration and dislocation motion during oscillation. The porous structure of aluminum foam further amplifies this damping effect. Synthesizing findings from various literature sources, the energy absorption density of aluminum foam ranges approximately from 1 to 20 J/g (referring specifically to closed-cell aluminum foam with corresponding density ranges of 200–800 kg/m³), significantly surpassing that of other shock absorption layer materials [139,140,141]. Su et al. [142] conducted shock absorption model tests on closed-cell aluminum foam lining structures for high-speed railway tunnels under two test conditions: with aluminum foam shock absorption layers and without shock absorption layers. The results indicated that under PGA conditions of 0.2 g, 0.4 g, 0.5 g, 0.6 g, 0.7 g, and 0.9 g, both the acceleration attenuation coefficient (Ka) and strain attenuation coefficient (Ks) were generally less than 1, validating the effectiveness of aluminum foam in tunnel shock absorption applications.

4.5.3. Advantages and Limitations

Advantages: (i) Exceptional mechanical properties—aluminum foam exhibits high specific strength and specific stiffness, maintaining structural stability and withstanding substantial loads; (ii) superior energy absorption capacity—aluminum foam absorbs considerable energy during compression, delivering significant shock absorption performance; (iii) excellent fire resistance—aluminum foam satisfies the fire protection requirements for tunnel applications; (iv) material recyclability—aluminum foam can be recycled and reused, conforming to sustainability requirements.

Limitations: Porous metallic materials also present certain technical challenges. (i) High cost—the complex fabrication processes for aluminum foam result in relatively high production costs; (ii) corrosion susceptibility—aluminum products are prone to corrosion in soil and humid environments, compromising material performance; (iii) poor recoverability under cyclic loading; (iv) limited waterproofing capability—the abundant and interconnected pores in aluminum foam are detrimental to tunnel waterproofing.

4.5.4. Strategies for Overcoming Material Limitations

To address the aforementioned material deficiencies, researchers have pursued performance improvements through various approaches. Wang et al. [143] fabricated a multilayer composite shock absorption layer comprising aluminum foam and polyurethane (AF/PU), and conducted shaking table model tests to validate its shock absorption performance. This composite material integrates the metallic characteristics of aluminum foam with the polymeric properties of polyurethane, enhancing waterproofing, corrosion resistance, and recoverability while maintaining excellent shock absorption performance; schematic diagrams and physical specimens of this composite are presented in Figure 8 and Figure 9, respectively. Research indicates that under seismic action with PGA = 0.6 g, the maximum dynamic earth pressure on the tunnel lining with a 1 cm-thick AF/PU shock absorption layer was 0.57 kPa, representing an 88.6% reduction compared to the case without a shock absorption layer (5.01 kPa), and significantly outperforming the rubber shock absorption layer of equivalent thickness (0.73 kPa); at PGA = 1.2 g, structural damage occurred in the lining with the rubber shock absorption layer, whereas the AF/PU configuration maintained structural integrity. Numerical simulations demonstrate that AF/PU shock absorption layers with thicknesses of 10 cm, 20 cm, and 30 cm achieved shock absorption efficiencies of 22.3%, 29.03%, and 31.41%, respectively, with average reductions in the first principal stress and strain of the lining reaching 22.3% and 21.5%, respectively, both significantly exceeding those of the rubber shock absorption layer (5.50% and 6.3%, respectively). Bao and Li [144] prepared AF-PU composite materials by coating closed-cell aluminum foam (AF, density 0.53 ± 0.03 g/cm³) with high-elasticity polyurethane (PU), finding that as the composite density increased from 0.743 ± 0.011 g/cm³ (AF-PU (0 mm)) to 0.774 ± 0.013 g/cm³ (AF-PU (6 mm)), the self-recovery factor (γ) of AF-PU (6 mm) remained above 0.79 under 15% strain amplitude, substantially higher than the 0.25–0.30 observed for AF-PU (0 mm). Regarding energy dissipation, the loss factor (η) of AF-PU (6 mm) ranged from 0.105 to 0.254, with a tangent modulus between 86.7 and 1045.6 MPa; the reversible energy dissipation factor stabilized at approximately 0.5 under 15% strain (compared to approximately 0.15 for AF-PU (3 mm)), indicating that this material combines the high stiffness of metallic foam with the high damping characteristics of polymers, and cyclic loading tests demonstrated that AF-PU (6 mm) maintained stable hysteretic energy dissipation capacity after 50 loading cycles. Xia et al. [145] fabricated manganese-doped closed-cell aluminum foam using the melt foaming method, discovering that at a manganese content of 1.0 wt%, the yield strength of the aluminum foam reached a maximum value of 6.55 MPa, representing an increase of over three times compared to pure aluminum foam (approximately 1.5 MPa), with the plateau region compressive stress approximately three times that of pure aluminum foam; at a strain of 0.5, the energy absorption density reached 3.74 MJ/m³, approximately 5.3 times higher than that of pure aluminum foam (0.70 MJ/m³).

5. Material Performance Comparison and Evaluation

To comprehensively evaluate the performance characteristics of various shock absorption layer materials, this study compiles available data from relevant research on tunnel shock absorption layer materials to present the approximate parameter ranges for each material category. A comprehensive comparative analysis is subsequently conducted across six dimensions: mechanical properties, durability, seismic performance, constructability, recyclability, and economic viability. The comparative results are presented in

Table 1. Comparison of performance of five types of tunnel shock absorption layer materials.

Material Type	Mechanical Properties				Durability			Seismic Resistance	Constructability	Economics	Recyclability	Optimal Application Scenarios	References
	Density (kg/m3)	Elastic Modulus (MPa)	Damping Ratio (%)	Energy Absorption Density (J/g)	Waterproofness	Corrosion Resistance	Endurance
Foam concrete	300–1600	20–1000	3–10	0.1–1	Poor	Moderate	Excellent	Good	Excellent	Excellent	No	Economically constrained tunnel projects with shock absorption requirements under conditions of non-permanent water immersion or without frequent freeze–thaw cycles.	[13,42,47,48,146,147,148,149,150,151]
Other concrete types (exemplified by rubber cement concrete)	1800–2200	12,000–25,000	5–12	0.05–0.8	Moderate	Moderate	Good	Moderate	Excellent	Excellent	No	Resource recycling-oriented tunnels with specific seismic resistance requirements and environmental grades	[71,72,73,74,76,77,78,152,153]
Polymeric materials (exemplified by rubber)	900–1600	1–300	5–30	0.1–2	Excellent	Good	Moderate	Good	Good	Good	No	Tunnels requiring high shock absorption performance with favorable corrosion resistance and waterproofing adaptability under ambient temperature conditions.	[81,82,83,84,85,86,87,91,92,93,94,96]
Asphalt	1000–1400	2000–20,000	10–20	0.02–0.3	Good	Good	Moderate	Moderate	Excellent	Excellent	Yes	Tunnel projects requiring combined functions of waterproofing, structural reinforcement, and shock absorption under normal temperature conditions.	[105,154,155,156]
Porous metallic (exemplified by closed-cell aluminum foam)	200–800	50–800	5–12	1–20	Good	Excellent (requires anticorrosion treatment)	Excellent (requires anticorrosion treatment)	Excellent	Good	Moderate	Yes	Critical tunnel infrastructure with specific seismic resistance grade requirements in non-corrosive environments.	[139,140,141,142,143,157,158,159,160,161,162,163,164,165]

.

As summarized in Table 1, the five categories of materials exhibit distinct advantages and limitations. Foamed concrete demonstrates inferior durability; however, it exhibits excellent shock absorption performance and constructability, coupled with favorable economic viability, rendering it the most extensively investigated and widely applied tunnel shock absorption material to date. Rubber–cement–based concrete offers advantages in mechanical properties and durability, though its seismic performance is somewhat compromised. Polymeric materials exhibit exceptional shock absorption characteristics, yet their mechanical parameters require stringent control. Asphalt materials present favorable economic viability and constructability; nevertheless, their temperature adaptability remains inadequate. Closed-cell aluminum foam possesses the highest energy absorption density and excellent shock absorption performance; however, it exhibits insufficient corrosion resistance in soil environments and relatively high costs.

For varying tunnel engineering conditions and shock absorption requirements, appropriate material selection should be conducted accordingly. For general tunnels with seismic requirements and economic constraints, foamed concrete represents the preferred material. For tunnels in high-seismicity regions or critical infrastructure, rubber concrete, porous metallic materials, or composite materials may be considered contingent upon geological conditions. For tunnels exposed to complex environmental conditions, polymeric materials constitute a superior alternative. For applications requiring both tunnel reinforcement and seismic mitigation, asphalt materials offer certain advantages owing to their construction convenience.

6. Research Limitations, Development Directions, and Future Trends

6.1. Current Research Limitations

Despite significant advances in research on tunnel shock absorption layer materials, several limitations persist:

(1)

Insufficient systematicity: Existing studies predominantly focus on individual materials, lacking systematic and comprehensive horizontal comparisons and integrated evaluations across different material categories. To date, comprehensive review articles presenting comparative analyses of tunnel shock absorption layer materials remain scarce.

(2)

Inadequate mechanistic understanding: Quantitative relationships between material microstructure and macroscopic shock absorption performance have not been thoroughly investigated, with a notable absence of theoretical models capable of guiding rational material design.

(3)

Scarcity of long-term performance data: The majority of existing research relies upon laboratory experiments and numerical simulations, with insufficient monitoring and data accumulation regarding long-term service performance of materials in actual engineering applications.

(4)

Absence of standardized systems: Unified standards for the design, construction, and quality acceptance of tunnel shock absorption layers have yet to be established, thereby constraining the standardized application and widespread adoption of these technologies.

6.2. Development Directions for Tunnel Shock Absorption Layer Materials

(1)

Synergistic Optimization of Material Properties

One of the future research priorities will be the synergistic optimization of key properties such as strength versus toughness and stiffness versus damping through multi-scale composite modification technologies. Single materials often struggle to simultaneously satisfy all performance requirements; therefore, compounding materials with complementary properties represents an effective pathway for achieving performance breakthroughs. For instance, composite materials combining high-toughness polymers with high-strength aluminum foam can be fabricated to possess both high load-bearing capacity and excellent energy absorption characteristics. Furthermore, regulating material microstructures through nanotechnology—such as introducing nanoparticles and nanofibers—can fundamentally improve mechanical properties and durability. Future research should focus intensively on intrinsic relationships between material microstructure and macroscopic performance, establishing multi-scale mechanical models to provide theoretical guidance for developing high-performance shock absorption materials.

(2)

Development of Multifunctional Integrated Materials

With increasing functional and safety requirements for tunnel engineering, developing novel composite materials integrating multiple functions—including shock absorption, waterproofing, thermal insulation, fire resistance, and sound insulation—will constitute an important future direction. For example, incorporating phase change materials into foamed concrete can enable thermal regulation capabilities alongside shock absorption; adding flame retardants to polymer foams can enhance fire resistance ratings; and waterproofing functionality can be achieved through surface coating or utilizing intrinsic material hydrophobicity. The application of multifunctional integrated materials can simplify tunnel structures, reduce construction procedures, lower engineering costs, and improve comprehensive performance and service life.

(3)

Intelligent Shock Absorption Materials and Technologies

Intelligent materials represent the frontier of materials science development. Introducing smart materials and technologies into tunnel shock absorption layers to develop adaptive shock absorption systems capable of real-time performance adjustment based on ground motion characteristics will be a hotspot for future research. For example, utilizing the superelasticity and self-centering capabilities of shape memory alloys (SMA) enables the development of self-centering shock absorption layers; exploiting the rheological properties of magnetorheological fluids (MRF) or electrorheological fluids (ERF) allows for the development of controllable dampers for active control of shock absorption forces. Additionally, embedding fiber Bragg grating sensors within shock absorption layers enables real-time monitoring of structural stress, strain, and temperature, providing data support for post-earthquake damage assessment and structural health monitoring.

(4)

Green Environmental Protection and Sustainable Development

Against the backdrop of “dual carbon” goals, green environmental protection and sustainable development have become essential criteria for materials R&D. Future tunnel shock absorption layer materials should prioritize environmental friendliness. On one hand, industrial wastes such as waste tires, waste plastics, fly ash, and slag should be extensively utilized to prepare green shock absorption materials, achieving resource recycling. On the other hand, biodegradable and recyclable shock absorption materials should be developed to minimize environmental impact. Furthermore, low-energy and low-emission processes should be adopted during material production and construction to minimize environmental damage.

(5)

Establishment of Technical Standards and Construction Specification Systems

Currently, unified technical standards and specifications for the design, construction, and quality acceptance of tunnel shock absorption layers are lacking, significantly constraining engineering application and promotion of these technologies. Therefore, establishing a scientific and comprehensive technical standard system is imperative. This includes: developing performance testing methods and evaluation standards for various shock absorption materials; proposing design methodologies for shock absorption layers under different geological conditions and seismic fortification requirements; compiling detailed construction technical regulations and quality control standards; and establishing long-term performance monitoring and evaluation systems for shock absorption layers. Through standardization efforts, market order can be regulated, engineering quality assured, and the healthy, rapid development of tunnel shock absorption layer technology promoted.

6.3. Future Research Trends

Based on the aforementioned analysis, future research trends in tunnel shock absorption technology are anticipated as follows:

(1)

Deepening mechanistic investigations: Strengthening fundamental research on the correlation between material microstructure and macroscopic performance, establishing multi-scale mechanical models.

(2)

Developing novel composite materials: Through multi-component and multi-scale hybridization approaches, developing new materials that combine excellent shock absorption performance with multifunctional characteristics.

(3)

Advancing intelligent applications: Exploring the integration of smart materials and technologies in tunnel shock absorption layers, achieving active and adaptive control of shock absorption performance.

(4)

Enhancing engineering application research: Conducting long-term field monitoring trials to accumulate service performance data of materials in actual engineering applications.

(5)

Perfecting standardization systems: Accelerating the formulation of relevant technical standards and specifications to provide scientific foundations for engineering design and construction, thereby promoting the industrialization of these technologies.

7. Conclusions

This paper systematically reviews the current state of research on tunnel shock absorption layer materials and prospects their future development trends. The main conclusions are as follows:

(1)

The shock absorption mechanisms of shock absorption layers are categorized into three aspects: (i) wave impedance mismatch and energy reflection, (ii) material damping and energy dissipation, and (iii) reduction in system stiffness and extension of natural vibration period, each of which is elaborated in detail.

(2)

Existing tunnel shock absorption layer materials are systematically classified into five categories: foamed concrete, alternative concrete variants, polymeric materials, asphalt materials, and porous metallic materials. For each category, the composition, physical characteristics, tunnel shock absorption performance, respective advantages and limitations, and pertinent optimization studies addressing material-specific deficiencies are elaborated.

(3)

Comparative analysis of the five categories of shock absorption layer materials reveals that foamed concrete and modified concrete exhibit relatively low costs and construction convenience, yet their waterproofing and durability require improvement; polymeric materials demonstrate excellent shock absorption performance, but face prominent issues regarding fire resistance and aging; asphalt materials possess waterproofing functionality and recyclability, but exhibit high temperature sensitivity; porous metallic materials display superior performance and recyclability, but entail high costs. Each material category exhibits distinctive performance characteristics, and no “universal” material exists; therefore, selection necessitates trade-offs based on specific engineering requirements.

(4)

This study summarizes the limitations of existing research and proposes future development directions and research trends for tunnel shock absorption layer materials. Currently, research on tunnel shock absorption layers still faces challenges, including insufficient systematicity, inadequate depth in mechanistic investigations, scarcity of long-term performance data, and absence of standardized systems. Regarding tunnel shock absorption layer materials, emphasis should be placed on synergistic optimization of material properties, development of multi-functional integrated materials, intelligent shock absorption materials and technologies, green environmental protection and sustainable development, and establishment of technical standards and construction specification systems. Future efforts should focus on deepening mechanistic research, developing novel composite materials, promoting intelligent applications, strengthening engineering applications, and improving the construction of standard systems.

(5)

Based on the systematic review presented herein, the authors’ research team maintains a strong ongoing interest in the future development directions for tunnel shock absorption layer materials outlined in Section 6. Specifically, ongoing and planned research includes: (i) development of novel multi-layer composite materials (such as polyurethane/closed-cell aluminum foam composites) possessing both high energy absorption capacity and excellent durability and waterproofing performance; (ii) establishment of multi-scale mechanical models to quantitatively characterize the correlation between material microstructure and macroscopic shock absorption performance; (iii) conduct of model tests and field monitoring studies to accumulate service performance data for novel materials in actual tunnel engineering applications; (iv) participation in the formulation of technical standards and construction specifications for tunnel shock absorption layers to promote the industrialization of this technology. Through these continued efforts, the authors hope to contribute to addressing current research limitations and advancing the practical application of high-performance tunnel shock absorption materials.