Long-term degradation, damage and fracture in deep rock tunnels: A review on the effect of excavation methods

A BSTRACT . Rocks are frequently host materials for underground structures, particularly for deep Tunnels. Their behavior plays a fundamental role in the overall stability of these structures. In fact, the erection of deep tunnels imposes rocks excavations around the defined routes. These excavations are generally carried out by various methods of which the most used are Drill-and-Blast (DB) and Tunnel Boring Machine (TBM). However, regardless of the tunnelling method used, the impacts such as the perturbation of the initial stress field in rocks and the release of the stored energy are always significant. The impacts produce damage, fractures and deformations which are generally time-dependent and influence the long-term stability of deep tunnels built in rocks. Thus, by considering the aforementioned excavation methods, this paper identifies, reviews and describes the relevant factors generated during and after rock excavations. Interestingly, such factors directly or indirectly influence the long-term stability and therefore the structural integrity of deep rock tunnels. In addition, some recommendations and proposals for future works are presented. This paper can provide useful references in understanding the degradations, damage and fractures generated by tunnelling methods and facilitate suitable actions to ensure long-term stability of deep underground structures.

INTRODUCTION espite many challenges generally encountered in underground excavations, engineering projects for the construction of deep tunnels continue to grow. Indeed, the underground space needs are necessary from day to day particularly in developed countries. It is thereby widely known that deep tunnels can be built for various purposes such as water conveyance, reservoirs emptying, hydropower stations, sanitary drainage, etc. As a result, the longterm stability of these tunnels must be guaranteed, so that they can perform their roles correctly throughout their period of operation. However, this long-term stability is influenced by many potential factors, including the methods used to excavate these tunnels. D It is quite obvious that, when excavating deep rocks tunnels, the virgin rock mass properties are modified [1,2], and there are disturbances or alteration of the existing stresses in the rock massifs [3,4]. There is then an imbalance in the stresses state of rocks which mainly surround the generated openings. For that, a new state of stress equilibrium is inevitable to prevent any early collapse of the excavations. It greatly depends on geological and geotechnical characteristics of the rocks and also on the excavation methods used. According to Kaiser et al. [5], stress changes strongly influence the stability of underground projects. Their impacts could be even more considerable when tunnels are constructed at great depth and when the lithology is variable. For instance, long deep tunnels generally cross heterogeneous rocky environments. So, one of the main challenges to be faced is the lack of information on geological structures of the areas to be excavated [6]. This would therefore entail a risk during tunnelling. The risks globally provoked by the tunnelling methods are numerous, both on the quality of rocks and on their properties. Rocks instability caused by excavations affect the overall safety and even sometimes the feasibility or the continuation of deep underground projects. Barton [7] has showed that the hybrid solution of TBM and DB methods can reduce risk in long deep tunnels. On their side, based on microseismic monitoring technology, Tang et al. [8] revealed that the speed balancing of TBM can attenuate the risk of surrounding rocks instability during excavations. But anyway, tunnelling by TBM and DB led to the creation of impacted zones in which there are generally permanent modifications of rocks properties in the vicinity of deep tunnels. The aim of this paper is to describe the stages reflecting the influences of excavation methods on the long-term stability of deep tunnels built in rocky mediums. In fact, many studies have been carried out on the stability of underground structures, but attention has not been drawn sufficiently to the influence of tunnelling methods on long-term stability of deep tunnels. To address this issue, this review is presented in order to highlight the relevant factors generated during and after tunnels excavations. Each of these factors influence directly or indirectly the lifetime of tunnels. Their adequate consideration could constitute a source of ideas that could lead to the development of solutions to minimize their impacts on long-term stability of tunnels. It should be noted that the excavation operations of deep long tunnels can take years. In addition, due to stress factors, the strength of natural rocks is time-dependent [9]. The longer the excavations last, the greater the risk of deterioration of rocks properties due to the time effect. Accordingly, the more the tunnels created are unstable. Thus, study the influence of excavation methods on long-term stability of deep rocks tunnels is of tremendous importance.

GENERAL ROCKS EXCAVATIONS METHODS OVERVIEW
he main methods for rocks excavations are Drill-and-Blast (DB), and Tunnel Boring Machine (TBM) [10,11]. The process of rocks removal is very different in the two mentioned tunnelling methods. In DB method, there are holes creation using long drill, and blasting the holes previously filled with explosives; while in TBM, the work is done thanks to the digging force of the machine equipped with rolling cutters operating at high pressure [12]. Tabs. 1 and 2 summarize the relevant advantages and disadvantages of the main tunnelling methods.  Rock mass excavation, as pointed out by Bao et al. [18], affects strongly the safety and the stability of tunnels. Referring to Liu et al. [19], it is assumed that tunnels shall be severely damaged when deformations induced by the excavations exceed established tolerable limits. Hence, for ensuring the long-term stability of deep tunnels, proper measurements of deformations caused by excavations are extremely important.

DEGRADATION OF PROPERTIES OF SURROUNDING ROCKS
unnel excavation is regarded as an unloading process that can cause degradation of the main properties of surrounding rocks. More broadly, Qiu et al. [20] and Niu et al. [21] have reported that the surrounding rocks endure a complex unloading-loading process during deep tunnelling. This process affects the main properties of rocks, and generates instability around tunnels. Cai and Kaiser [22] related that there is alteration of mechanical, hydraulic and geochemical properties of surrounding rocks when excavating underground openings. In their research, based on unloading effect, Luo et al. [23] studied the deterioration of the mechanical properties of surrounding rocks for deep tunnels and have found that the main mechanical properties (cohesion, internal friction angle, elastic modulus, and Poisson's ratio) exhibit nonlinear degradation features. Rocks strength around tunnels is reduced due to the degradation of their properties. The degradation of rocks properties is an unfavourable indicator for the long-term stability of tunnels. Any excavation method (DB, TBM) generates degradations in the rocks surrounding the openings. Although the degree of rocks degradation varies depending on the excavation method used, in all cases the rock properties remain altered in the vicinity of the tunnels. The extent of degradations generated by tunnelling on relevant rocks properties needs to be deeply investigated. This would help to elucidate in depth the influence of excavation methods on long-term stability of deep tunnels constructed in rocky environments.

EXCAVATION LOOSE ZONE AND EXCAVATION DISTURBED ZONE
he zones that directly influence the service life of deep tunnels are mainly the Excavation Loose Zone (ELZ), also called Excavation Damaged Zone (EDZ), and the Excavation Disturbed Zone (EdZ) (see Figs. 1 & 2). They play a key role in the understanding of the long-term stability of these structures. In the ELZ (EDZ), the physical, T T mechanical, hydraulic and geochemical properties of rocks are considerably degraded [24,25,26]. Consequently, in these zones, there is irreversibility of deformations. The ELZ or EDZ could also be called failed zone, referring to Eberhardt et al. [27], where slabbing failure can occur. It should also be noted that in EDZ, according to Verma et al. [28], not only there is degradation of the main properties of rocks, but there is also an increase in transferability properties. That is, in EDZ, transmissibility or transferability properties of rocks are augmented due to the degradations that could engender cracks and fractures. The evolution of EDZ (ELZ) strongly influence the stability of surrounding rocks around deep tunnels. Factors favorable to the formation and growth of EDZ must be identified and controlled. According to Yang et al. [29,30], three main aspects may contribute to the formation of EDZ namely in-situ stress or excavation process, time-dependent behavior of rocks, and rock mass quality or evolution of fractured zones in rock mass. In fact, the EDZ is particularly formed when concentrated compressive or tensile in-situ stress is higher than the rock strength [31,32,33]. Under geostress environment, referring to Yang et al. [30], EDZ can provoke at the same time cracks growth and stress redistribution around tunnels. It also facilitate the flow of groundwater inside deep tunnels, increasing the risk of instability of said structures [34]. The extent of EDZ depends on several factors, but mainly on the excavation method used. It could be thicker in Drill-and-Blast tunnelling than TBM [35,36]. The EdZ is the expansion of stress redistribution at some regions near the openings. In these zones, rocks are weakened [27]. In the EdZ, also called Excavation Influence Zone (EIZ), the hydromechanical and geochemical properties of rocks are not change considerably [35,26]. For circular tunnels, according to Du et al. [37], the extent of EdZ depends on the tunnels radius and generally limited to 3 times of their radius (≤3R). It is important to note that, as said by Perras and Diederichs [38], in in-situ measurements, the distinction between EIZ and EDZ is not obvious, and the transition between them is gradual. Their impacts on long-term stability of deep tunnels are significant. The extent evaluation of these zones must be taken into account since they can change with time [39,40,41,18]. In addition, they influence considerably the stress field in roof region of tunnels [42]. The view of stress-strain curve (Fig. 3) can show the irreversibility of deformations in the EDZ by triaxial compression test. As revealed by Kwon et al. [2], the characteristics of the EDZ are seriously influenced by the excavation method used. In fact, additional damage is generated by any excavation method. However, there are more supplemental EDZ provoked by the DB method than that of TBM method. Regarding the EdZ or EIZ, the effects of excavations are habitually minor, as outlined by Satici and Topal [45], but exist nonetheless. Depending on the magnitude of the stress disturbances beyond the EDZ, the amplitude of EdZ may vary and become significant, and its effects may therefore become considerable. It should thus be recognized that both TBM and DB excavations increase the magnitude of EDZ and EdZ in the surrounding rocks of tunnels.

ENERGY RELEASE DURING ROCKS EXCAVATIONS
uring rocks excavations, the stored energy is disturbed. These excavations can lead to the release of elastic strain energy suddenly [46,47]. The perturbations can induce serious damage and affect the stability of rocks located in the vicinity of deep tunnels. Based on the work of Yan et al. [10], Tab. 3 presents the characteristics of the relevant factors for the Energy release during DB and TBM tunnelling.

Relevant Factors DB TBM
Behavior of the stress paths process  D According to Yang et al. [29], the elastic stress energy depends on various factors such as principal stresses, number of elements contained in the rocks, and the mechanical properties of rocks. Rocks with more elements can release more strain energy.
The most significant damage is related to the higher releases of the elastic strain energy. The strain energy released in the rocks is extremely essential to understand the sudden structural failure of the rocks [48]. As shown in Tab. 3, the energy release by the unloading stress during DB excavations is transient and higher. The DB tunnelling may cause more perturbation and more degradation in the surrounding rocks of tunnels. Moreover, as reported by Fan et al. [49], the energy release during the transient unloading of the in-situ stress is not only highest, but also fastest. So, DB tunnelling can deteriorate the surrounding rocks of tunnels faster than the TBM excavations. However, the effects of both tunnelling methods are permanent in the EDZ.

DAMAGE AND FAILURE
ne manner to take account of damage in a material is to use the concept of net stresses. As noted by Hoxha et al. [50], the ''net stresses'' concept is used to describe how damage influences the mechanical response of a damaged material. Lemaitre and Chaboche [51] have shown that, for a solid under a constant uniaxial tensile stress ( ), the "net stress" ( * ), which depends on the damage factor ( D ), is given by Rocks damage can be associated with creep (see Fig. 4) which are both time-dependent phenomenon. According to Brantut et al. [52], during creep, the damage parameter can take values from 0 (intact) to 1 (broken) and can be expressed as follows: where the applied stress is  , the Young modulus is E , and the creep strain is  . The constant  is called damaged parameter. When rock is deformed by creep, the elastic modulus is decreased owing to damage [53,54]. By equating   D , the last equation is More broadly, rocks damage can also be expressed using the energy dissipation concept. Referring to Yang et al. [55], the global damage equation of rocks using the energy dissipation concept is as follows: U is the critical strain energy corresponding to the initial damage; U is the strain energy expressed by where   The relationship between Damage and Energy release can explain that greater Energy corresponds to greater Damage. In other words, the more energy the rocks release, the greater the damage is. In fact, great damage evolution lead to instability O of structures, and can affect the bearing capacity of rocks. Mu et al. [56] have reported that the instability of structure is the macroscopic result of damage evolution and strength degradation in microstructures. Damage and failure are closely linked. Any damage generates a form of structural instability which can lead to failure. Tang et al. [57] pointed out that the failure of rocks is caused by the damage evolution from initial defects. The latter, when they exist, are even more vulnerable during excavations. Accumulation or evolution of damage in structures always results in failure when they exceed their allowable limit. According to Jiang et al. [58], the time-dependent damage affects the major safety of surrounding rocks when it is severe. In fact, failure process in tunnels is composed by 3 stages (Fig. 5) including initiation, propagation and coalescence of cracks [59,20]. When it is completed, partial or total failure can occur in deep tunnels. .  Tunnels failure usually occurs by loss of rocks strength, or by excessive deformations or by serious damage of the support systems. Any type of failure mode can occur during and after excavations. When rocks are subjected to stress, fracture toughness and fracture energy are two important characteristics that must be taken into account. In fact, it should be noted that there is an energy consumption or dissipation during the initiation of cracks. The energy dissipation increases with the evolution of cracks [60]. Cracks and fractures evolution depends on fracture toughness of rocks. Fracture toughness of rocks is the resistance to cracks expansion [61,62]. Fracture energy of rocks is the energy release rate which could be related to the disturbances generated by the excavation methods. More precisely, fracture energy refers to the energy required to cause fractures in rocks [62]. As related by Dong et al. [60], when the fracture energy is considerable up to its critical value, cracks can propagate in the surrounding rocks of tunnels. Toughness and fracture energy values can indicate how resistant rocks or structures are to the propagation of cracks or fractures. The most stable structures are those built with materials that have the highest fracture energies. Tunnels built in rocky environments with high fracture energies may have higher degree of integrity during and after excavations by TBM or DB. In plane strain condition, fracture energy and fracture toughness ( IC K ) and can be linked as follows [60]:   They demonstrated that the deterioration of these properties increases with the evolution of damage in rocks. It is important de remember that, tunnel excavations generate dynamic perturbations in the surrounding rocks [20]. It should therefore be understood that the greater the damage generated by the excavations, the greater the risk of degradation of rock properties. Thereby, more cracks and fractures can be spread in the surrounding rocks of tunnels. This increasingly increases the degree of damage to rocks and the risk of tunnel failure.

ROCKBURSTS
ockbursts are considered as frequent form of damage or disaster that can occur during excavations of deep tunnels. They are caused by excavation when unloading the in-situ stress [77,78]. Their risk is considerable during and after underground excavations. It increases when there is more elastic energy stored in the elastic zone, and less dissipated in the plastic zone [79,80]. In fact, rockbursts are related to the strain energy that exists in the surrounding rocks of tunnels [81]. They can thus crop up when there is the highest release of that energy. In deep excavations, rockbursts are dangerous phenomenon mainly generated by the brittle failure of rocks related with induced seismic events [79,82]. Although it remains a challenge particularly at great depth, rockbursts control and assessment are extremely important in the analysis of the longterm stability for deep tunnels. Indeed, Wu et al. [83]  ). This classification corresponds respectively to minor rockburst, moderate rockburst, intensive rockburst and extreme rockburst, according to Xie et al. [85]. Fractures or damage of surrounding rocks during and after excavation could be closely related to the intensity of generated rockbursts. It is obvious that heavy rockbursts produce more damage and more fractures. Many studies have been conducted about rockbursts prediction. Li et al. [86] have realised a study permitting to predict rockbursts based on seismic techniques. One of their findings is that the excavation rate influence the frequency of rockbursts. This frequency is an important factor in the stability of surrounding rocks. Some previous studies explained that rockbursts frequency increase as the excavation depth increases. They established empirical formulas [87] permitting to calculate a critical depth from which rockbursts frequency can be estimated. Rockbursts can occur frequently when the excavation depth is greater than the established critical depth. On their side, based on Acoustic Emission and on field measurements, Cheng et al. [88] have found that rockbursts signals are closely related to the adjustment of rock stresses. Xue et al. [89] established a hierarchical evaluation system for rockbursts prediction based on a set of index and criterion. Recently, He et al. [90] explained that one of the Rockbursts method prevention, is to avoid high stress concentration areas as early as possible when selecting the tunnel route. Indeed, rockbursts prediction could be used for a better control of the rockburst-induced damage during excavations. Tab. 6 presents the key factors and conditions favorable for rockbursts occurrence.   [96], and thus mainly depends on the excavation methods. When considering, for instance, the readjustment of stress or the unloading of the in-situ stress, they are also factors on which the occurrence of rockburst depends. Nevertheless, such factors depend on rock excavation methods. In fact, in deep underground spaces, the significance of the stress adjustment varies according to the rocks excavation methods. On another hand, the geological characteristics and conditions of rocks can also play a role in the rockburst occurrence. Here again, it's the excavation method used that can make this occurrence possible to any extent. When excavating deep rocks, the sudden and delayed occurrence of rockbursts is governed by the excavation methods [97]. Prompt rockbursts are typically generated during DB excavation method, while delayed rockbursts are mostly provoked by TBM excavations. Indeed, the in-situ stress is unloaded differently in the two excavation methods, as shown in Tab. 3. Taking as example the diversion tunnel of Jinping II hydropower station located at the southwestern of China, Xie et al. [85] demonstrated that in both tunnelling methods, minor and moderate rockburst can occur during excavations. Nevertheless, moderate rockbursts are predominantly frequent in DB excavations. Likewise, as also revealed by par Yan et al. [10], Intensive rockbursts occur mostly in DB tunnelling (Fig. 6). It is important to emphasise that under the effects of tunnelling methods, geological structures also influence the intensity and the frequency of rockbursts, as reported by Fan et al. [97]. Basically, the control of the energy release process in surrounding rocks during excavations is the best way to control rockbursts [98]. The greater the energy released, the larger the rockbursts and the greater the damage recorded in surrounding rocks. Thereby tunnels built in hard and brittle rocks executed by DB methods can generate a lot of rockbursts. This can increase damage and deformations of surrounding rocks and influence the long-term stability of deep Tunnels.

CONSIDERATION ON ADVERSE GEOLOGICAL CONDITIONS
dverse geological conditions can affect the whole parameters of surrounding rocks during and after excavations, particularly when tunnelling by TBM. Therefore, a good knowledge of the geological conditions is strongly recommended prior to excavations, in order to foresee the necessary precautions. For ensuring adequate excavations, the main adverse geological should be known. Among these adverse geological conditions, we can mention: cavities, fractured and faults areas [99], high water inflow, spalling, squeezing and swelling ground [100]. They strongly A influence the tunnelling and the long-term stability of deep tunnels. Huang et al. [14] reported that instability and collapse of the surrounding rocks are frequent when TBM excavations are done in zones with high fractures and faults. Active faults can cause partial or total collapse of tunnels. More specifically, some parts of tunnels can be collapsed when deformations produced by TBM excavation are enormous in faults areas [101]. As shown in Tabs. 1 and 2, one of the advantages of DB excavations is their great flexibility and adaptability in many geological conditions, while TBM are disadvantageous for unfavorable geological conditions. Such disadvantages of TBM must be seen both from a safety and an economic point of view. In fact, it is always reported by many Scholars and Researchers that there is high entrapment risk of TBM during excavations in adverse geological conditions. TBM entrapment generally cause serious excavations delays, and increase considerably the instability risk of tunnels and economic losses. Referring to Zhang et al. [6], due to unfavorable geological conditions, the TBM excavating the Yacambú-Quíbor deep tunnel in Venezuela was blocked and it took several years to remove its remains. In Taiwan, it took 15 years to complete the Syueshan tunnel. These examples can illustrate the complexity associated with tunnelling in adverse geological conditions. In fact, under these conditions, the surrounding rocks parameters become even more weakened by any excavation method. Nonetheless, DB excavations could produce less instability in the tunnels under such conditions.

CONSIDERATION ON HIGH GROUNDWATER PRESSURES AND FLOW
t great depth, high groundwater pressures can infiltrate into the excavated areas and attack the stability of surrounding rocks. According to Hoek [102] and Stille and Palmström [1], groundwater pressures generated overall instability and reduced rock strength and shear. Likewise, Liu et al. [103] reported that the unexpected groundwater inflow at tunnel head generate uncontrollable effects like for example mechanical instability and environmental impacts.

A
The fight against heavy water pressure infiltration during tunnelling is frequent particularly for deep long tunnels, and requires much sustained attentions. In fact, as noted by Hwang and Lu [104] and Zhang et al. [6], one of the challenges to be faced in deep tunnels is the high groundwater pressure. The problems induced by these inflows are usually assigned to water-rich fracture zones, particularly when using TBM [105]. Thereby, prediction and identification of high groundwater pressures must be done. Then, an adequate treatment is required to ensure tunnels stability. Referring to Zhang et al. [6] and Zhang et al. [106], some techniques (pre-excavation, decompression, blocking, and radial grouting) are generally used in order to fight against high-groundwater pressure around deep long tunnels. But the impacts of these pressures and water flows are not negligible in the long-term. When studying the long-term stability of deep underground structures, emphasis should be placed on the creep behavior of their surrounding rocks. As pointed out by Zhou et al. [107], the long-term strength criterion can be considerably described by the creep failure of rocks. The time-dependent creep behavior of rocks is influenced by water and hydraulic pressures. Thus, taking into account the groundwater pressures and flows generated during excavations is important for a real evaluation of rocks creep strength. Fig. 7 illustrates uniaxial creep curves of red Sandstone samples studied by Tang et al. [108] in order to reveal the influence of water on creep behavior of this rock type. The red Sandstone samples was initially immersed in water at different durations like 2 days, 4 days, 6 days and 8 days. In the figure, the symbol "d" refers to days, the times are multiplied by 4 10 s , the creep strain rates are multiplied by 5 10 1 s . s refers to second. The literature reveals that the creep behavior of all types of rocks is significantly influenced by water. Tab. 7 presents the relevant effects of water on creep behavior for different types of rocks.  Indeed, groundwater pressures and flow generated by the tunnelling methods (DB and TBM) lead to an increase of water content in the surrounding rocks of tunnels. As shown in Fig. 7 and Tab. 7, the presence of water deteriorates the mechanical and creep properties of rocks. Accordingly, under the effect of excavation methods, water engenders long-term degradation in tunnels built in rocky environments.

CONVERGENCE DEFORMATION
s already mentioned, during and after excavations, the initial stress field always tries to stabilize by itself. As a result, rocks tend to closure around the openings. This closure is the deformation that called convergence of the rocks surrounding the tunnels. Convergence deformation is one of the severe effects of excavations which can be spectacular. It evolves as time increase owing to the time-dependent behavior of rocks [116,4]. Convergence is inevitable, as revealed by Kontogianni et al. [117], since it is part of tunnels deformations mechanisms. According to them, due to the natural rocks behavior, tunnels convergence can be manifested in 3 stages: an accumulation rate, an evolution, and finally a stabilization. Furthermore, tunnel convergence can be limited to a distance less than twice the diameter of the openings. In other words, when an underground opening is created, the convergence deformation is naturally observable owing to the rocks behavior. However, the extent of such a deformation is influenced by the degree of rocks damage during and after excavations. In addition to the properties of rocks, any excavation method has impact on the tendency and extent of convergence. Paraskevopoulou and Diederichs [116] have showed that both tunnelling methods (DB and TBM) influence the convergence of tunnels. By considering an elasto-visco-elastic rock mass and the CVISC (visco-elastic-plastic) model, they numerically demonstrated that DB and TBM excavations effects increase the total displacement of rocks surrounding the tunnels. However, under similar duration of the excavation sequences, when rock mass conditions are favorable and tunnels length is considerable, TBM excavations can generate fewer additional displacements than that of DB. Otherwise, in adverse geological conditions, the additional wall displacements of tunnels are greater under TBM tunnelling. Convergence is an essential factor in the study of tunnels stability. In fact, tunnel performance depends on it [118]. Consequently, tunnel convergence should be seriously controlled since the earlier stage of excavations, and throughout the lifetime of tunnels. As explained above, both tunneling methods influence the extent of tunnel convergence deformation by increasing it. The influence of excavation methods on tunnel convergence can be reduced if proper measures are taken as soon as possible. It is therefore extremely important to understand in depth such tunnel behavior and to adopt suitable solutions to limit it considerably. Tab. 8 presents an overview of the salient details in the study of convergence deformations for deep tunnels. Broken soft rocks

Multi-point extensometers
Floor heave represents 65% of the total roof and floor deformation.
There is huge deformation with rapid speed, and unevenly distributed in the entire deep tunnel cross section.   Fig. 8 illustrates that the deformations wall-wall (which is assimilated to convergence) of tunnels increases with the increasing of time after the excavation period. This rheological behavior is strictly related to the rocks types and excavation methods. Huge rocks perturbations during excavations can involve huge convergences over time for deep tunnels.

DISCUSSION AND PROPOSITIONS
he analysis of papers highlights different potential factors of excavations methods which impact the stability of deep tunnels. These factors evolve over time, and their influence on the above structures stability will be for longterm. The extent of these influences depends firstly on the characteristics of the encountered rocks at the excavations depth and the geological conditions, and secondly on the excavations methods used. In fact, unfavourable geological conditions delay tunnelling [126], and consequently lead to additional impacts to the rocks around tunnels. For example, initially fractured rocks are generally more vulnerable to excavations and more damaged than that of unfractured. Zhang et al. [106], revealed that the damage generated by the fractures of rocks are frequent. When tunnelling in these rocks, damage would be added to the initial damage increasing the instability of deep underground structures. Adverse effects exist in all tunnelling method. In order to design adequate tunnels supports and reduce the excavations impacts, adequate control and measurement of damage or deformations should be done. In fact, the duration of the excavation operation is an important factor. Some tunnels collapse during the period of their excavation [127]. The excavation time depends on several parameters including the geotechnical characteristics of rocks, the tunnelling methods as well the capacity and speed of work. From an intact deep rock mass until the setup of support systems for deep tunnels, potential factors illustrating the influence of excavations methods are in the diagram hereinafter (Fig. 9). The lifespan of tunnels could be influenced by the manner in which the surrounding rocks have been disturbed, damaged and deformed during and after excavations. Indeed, the least durable tunnels could be those whose surrounding rocks were the most severely damaged and deformed during and after excavations. Considering a saturated viscoplastic rock mass by referring to Deleruyelle et al. [128], the life cycle of deep tunnels can be detailed in four stages such as Excavation, Tunnel unlined, Lining emplacement, and post-closure (Fig. 10). The post-closure stage is the total destruction of tunnels which mainly depends on the extent of convergence deformation accumulated in the first three stages, and over time. In order to mitigate the influence of excavation methods in deep tunnels, the following steps are proposed: Firstly, a good evaluation of the geological and hydrogeological properties of the areas to be excavated is crucial. Secondly, an evaluation of the tunnelling methods to be used should always be made. Indeed, especially for deep long tunnels, combined actions of two tunnelling methods could be beneficial at certain levels, and unfavourable in some points. So a good assessment of their impacts are required prior to tunnelling. Thirdly, a good selection of excavation sequences and direction is extremely important for the surrounding rocks stability. It also plays a great role in the stability of different components of tunnels. Fourthly, field monitoring and measurements are necessary throughout the tunnelling periods, in order to control damage fractures and rockbursts signals. All cracks and deformations generated during the excavations periods of deep tunnels should be well identified and assessed and all actions should be optimized during and after tunnelling. This will limit damage evolution by putting in place suitable support systems as quickly as possible in order that tunnels be stable during their service life. The aforementioned steps can be performed through different methodologies and instrumentations such as: fields monitoring and relevant surveys, relevant empirical and/or analytical formulas, adequate experimental tests and numerical methods. More precisely, steps 1 and 2 can be effectuated by in-situ measurements, experimental tests, as well as numerical methods. In fact, both experimental tests and numerical methods can be employed to determine the hydro-mechanical properties of rocks. To ensure reliability, in many cases, numerical results can be compared with experimental results. Referring to Wang et al. [129], we outline that Machine Learning Techniques can also be used to determine or predict mechanical properties of rocks. For the step 3, pertinent numerical methods can provide goods results regarding the excavations sequences and directions of deep rock tunnels. Step 4 can be performed using field measurements and relevant numerical methods. Note also that the support systems or linings can be calculated using appropriated empirical or analytical formulas and numerical methods. Fig. 11 illustrates the steps in more detail.

FUTURE RESEARCH NEEDS
he long-term stability of deep rocks tunnels built in rocky mediums must always be sought. Despite that many previous studies are conducted, there are still several areas of research which deserve further studies in order to support accurate predictions of deep tunnel's long-term stability. Although it remains as challenge tasks, it is of tremendous importance to accurately estimate: T -The extent of degradations of the mechanical properties for surrounding rocks and its influence on long-term stability of deep tunnels. -The overall magnitude of EDZ and its influence on long-term life of deep rocks tunnels based on unloading effect.
-How excavations methods influence the rocks bearing capacity of deep tunnels based on time-dependent effect.
-The total damage and deformations generated in the surrounding rocks after TBM and/or Drill-and-Blast excavations based on creep effect. -Comparison of the convergence deformations magnitude between tunnels excavated by TBM and DB under same geological and hydrogeological conditions. -Long-term Stability Analysis of deep tunnels based on post-excavation evaluation.
-The degradation of the fracture toughness and fracture energy in deep rocks tunnels after TBM and Dill-and-Blast excavations. Figure 11: Proposed methodologies to control and mitigate the effects of excavations methods on the long-term stability of deep rocks tunnels.

CONCLUSIONS
he excavations methods have considerable influence on long-term stability of deep rock tunnels. The main conclusions related to this paper are summarized as follows: T a) The long-term stability of deep tunnels is influenced by the long-term stability of its surrounding rocks. Any excavation method has adverse effects on the properties of rocks, and impact the durability of deep rocks. Thereby it influences the long-term stability of deep tunnels owing to the degradation of the physical, mechanical, and hydraulic properties of the surrounding rocks. b) Owing to the Energy release during excavations, Rockbursts are inevitable, especially during hard and brittle rocks excavations. In engineering practice, the tunnelling rate must well be controlled to prevent intensive rockbursts. The latter cause generally more damage and weaken much more the surrounding rocks of tunnels. Consequently the life time of these tunnels can be more impacted. c) For deep long tunnels crossing complicated geological zones, when a combination of Drill-and-Blast and TBM methods is employed, high stress concentrations in the intersection areas will also influence the lifetime of deep tunnels. d) The proper design of the lining systems depends on the conditions of surrounding rocks after excavations. Thus, the excavation methods used govern on the long-term the support systems of deep rock tunnels. The greater the damage or deformation, the more resistant the support systems must be. e) After rocks excavations, deformations increase continuously and failure can occur over time due to the timedependent behavior of natural rocks. Hence, the total convergence deformation caused by the excavations should be predicted minutely considering all the relevant factors. Indeed, long-term monitoring is necessary during the lifetime of deep tunnels. f) All tunnelling methods impose additional deformations on surrounding rocks of deep tunnels. These deformations can be minimized if actions are optimized during the tunneling period. However in any case they influence the tunnels long-term stability. g) To ensure the long-term stability of deep rocks tunnels, a high degree of structural integrity is required.

DB:
Drill-and-Blast TBM: Tunnel Boring Machine ELZ: Excavation Loose Zone EDZ: Excavation Damaged Zone EIZ: Excavation Influence Zone EdZ: Excavation Disturbance Zone ERR: Energy Release Rate ERS: Energy Release Speed. SCB: Semi-circular bend SENRBB: Single edge-notched round bar in bending SENB: Single edge notch bend FBD: Flattened Brazilian Disc SECBD: Single Edge Cracked Brazilian Disk in diametric compression BDT: Brazilian Disk Test FEMDEM: Combined finite-discrete element method SR: Short rod DCB: Double Cantilever Beam CCCD: Center-Cracked Circular Disc CVISC: Visco-elastic-plastic or Burger Constitutive Viscoplastic.