Support System for Tunnelling in Squeezing Ground of Qingling-Daba Mountainous Area: A Case Study from Soft Rock Tunnels

Tunnelling or undertaking below-ground construction in squeezing ground can always present many engineering surprises, in which this complicated geology bring a series of tunnelling difficulties. Obviously, if the major affecting factors and mechanism of the structure damage in these complicated geological conditions are determined accurately, fewer problems will be faced during the tunnel excavation. For this study, reference is made to four tunnel cases located in the Qingling-Daba mountainous squeezing area that are dominated by a strong tectonic uplift and diversified geological structures. ,is paper establishes a strong support system suitable for a squeezing tunnel for the purpose of addressing problems exhibited in the extreme deformation of rock mass, structure crack, or even failure during excavation phase. ,is support system contains a number of temporary support measures used for ensuring the stability of tunnel face during tunnelling. ,e final support system was constructed, including some key techniques such as the employment of the foot reinforcement bolt (FRB), an overall strong support measure, and more reserved deformation. Results in this case study showed significant effectiveness of the support systems along with a safe and efficient construction process. ,e tunnel support system proposed in this paper can be helpful to support design and provide sufficient support and arrangement before tunnel construction in squeezing ground.


Introduction
e railway/highway tunnels are considered one of the most efficient and environment-friendly ways to improve the transportation infrastructures and have been developing rapidly all over the world in recent decades [1][2][3][4]. In reality, determining the viable support system is a key factor affecting the cost and safety of underground works. When a tunnel is excavated, a support system is installed to ensure the stability of the excavated underground caverns by controlling the displacement of the rock mass, so as to address the safety issues that may arise [5][6][7][8]. In recent times, an increasing number of the complex geological conditions have been confirmed in underground space development [9][10][11][12], this is especially so in the squeezing ground areas which have resulted in a series of tunnel construction problems such as extreme deformation, support structure failure or even collapse, and examples can be easily found in the Zhegushan and Laodongshan tunnels in China [13,14], tunnel 35 of the Ankara-Istanbul high-speed railway project in Turkey [15], and Kaligandaki tunnel in Nepal [16]. e continuous hazards yield brand-new challenges to tunnel construction in squeezing ground [17][18][19].
With regard to the uncertainties existing in rock mass properties, the design of tunnel support in squeezing ground has always been a sophisticated task [20,21]. Many scholars have done research on tunnel support structure where the achievements are mainly carried out from three aspects of empirical method, analytical method, and numerical method [22][23][24]. As for tunnel support design, although analytical and numerical methods are generally considered to be the most effective and scientific way, they also have obvious shortcomings and limitations in use. For example, most analytical methods are established based on the assumption that a tunnel is a circular cavern, and some necessary simplifications are made to simplify the calculation [25,26]. ese assumptions are sometimes distant from the realities of the engineering conditions in most cases, because the shape and faced stress conditions of the tunnel vary greatly from one construction site to another. e analytical method is widely used in tunnels regarding the characterization, support evaluation, and back analysis [27,28]. Gao et al. [29] optimized the lining structure used in a soft rock tunnel by employing a finite difference software of FLAC 3D, and based on a comparison with field test data, they finally proposed an economical and simplified construction support system. A yielding support system was proposed by Wu et al. [30] to cope with the extreme deformation problem for a soft rock tunnel excavated in squeezing ground. Kanik and Gurocak [31] conducted a deep study on empirical rock mass classification system and established an optimal support unit through a comparative numerical analysis. e analytical methods provide a lot of support for tunnel design and construction simulation; however, an accurate rock mass model is difficult to establish due to the discontinuity, anisotropy, heterogeneity and inelasticity of rock mass [32][33][34][35]. e complex nature and different strata make rock mass a difficult material for a simulation of the numerical method [36][37][38].
Of course, the research and discussion of the effective support system from engineering practices have never stopped. Aksoy et al. [39,40] recently have developed new approaches to identify the time-dependent deformation behaviors of rocks under different loads so as to guarantee the stability of constructions built in rock masses. Kong et al. [41] emphatically analyzed the mechanism of primary support failure in a deep tunnel regarding a sidewall collapse during construction phase. Dadashi et al. [42] analyzed and revised the support structure parameters after a soil mechanics property evaluation, and then a support system using shotcrete, steel mash, and lattice beam was proposed for a squeezing tunnel based on the parameters obtained from the back analysis. Oliveira and Diederichs [43] discussed the brittle failure simulation of the sandstone and influence of the high geostress on tunnel support design. Panthi and Nilsen [44] believes that the best way to deal with severe squeezing is to preestablish a strategy including planning and design phases to minimize the stability problem and optimize the support and stability measures. In this view, a reasonable structure system becomes very useful in designing tunnel support.
ere are many literatures reporting tunnel support design or optimization regarding rock mass properties and support structures [45,46]. However, the number of literatures reporting extremedeformation-induced support structure failure is limited, and thus, a comprehensive supporting system is required to relieve the damage of tunnel structure in squeezing ground.
In this paper, study area with full of surprises during underground construction activities due to frequently geological changes in Qingling-Daba mountainous area is introduced firstly (Figure 1). Tunnels in this area are being constructed in squeezing ground with high tectonic activity and a large number of jointed rocks. e maximum deformation in four tunnel cases selected for study had reached 100.8 cm during tunnel excavation, causing the support structure failure, which requires a reshaping work for tunnel support system. In the view of this, the main purpose of this paper is to explore features of the structure damages in a squeezing condition through on-site investigation. Based on the discussion of structure failure and field test results, we establish a strong support system suitable for a squeezing tunnel that includes the temporary support measures and some necessary key techniques. e effectiveness of the proposed tunnel support system was verified by an analysis on the recorded data of the rock mass deformation throughout the construction phase.

Study Area
Main body of the Qingling-Daba mountainous area is located in southern Shaanxi province, in which its administrative division includes Hanzhong, Ankang, and Shangluo. e southern Shaanxi is mostly located in the two major mountain systems of Qinling and Bashan, except for a small number of basins, Hanzhong and Ankang regions are dominated by complex and diverse topography and landform as well as the intricate flatland and hill. e mountain area is 47.9 thousand square kilometers, accounting for 69.42% of the total area. e study area stretches across three comprehensive stratigraphic zones, which was named north China, Qinling, and Yangtze, respectively. e area is located at the region between the southern margin of the Zhongchao platform and the northern margin of the Yangtze platform. Due to the action of compressive stress in north-south direction, large, deep, and dense faults are widely developed in the platform margin and geosyncline-folded system. ere are significant signs of neotectonics in southern Shaanxi, and differential movement of fault blocks is obvious after inheriting the Himalayan orogeny at the end of the tertiary period. It has a profound impact on the formation and development of the landforms which can be reflected by the formation of mountainous regions with high mountain, deep valley caused by continuous rising, and wide and flat basins induced by continuing decline since Quaternary. e differential uplift and movement of the fault caused deep gullies, broken topography, and strongly dynamic geological action in the area [47]. A geological profile of the study area is presented in Figure 2.
Surface runoff in the area is mainly three river systems, Hanjiang river, Danjiang river, and Jialingjiang river, and the site is located in the upper reaches of the three river systems. Groundwater in the area is mainly recharged by precipitation and surface water infiltration, which is characterized by a very uneven distribution due to the variation of hydrogeological conditions and can be divided into four types based on their different kinds of water-bearing media [47].

Pore Water of Quaternary in Loose
Rocks. It is mainly distributed in intermountain basins and broad valley section along the river. e aquifer is composed of alluvial sand, gravel, and pebble layer in the Quaternary, and it is further divided as phreatic water and con ned water, the former is mainly distributed in intermountain basins and broad valley section along the river and the latter is mainly distributed in intermountain basins.

Fractured Pore Water in Clastic
Rocks. It is mainly distributed in the southern region of the Hanzhong and Machi-Ankang basin and is mainly stored in sandstones and conglomerates in the period from Mesozoic to Cenozoic. Rare ssures can be found in storing rock, which are often lled with mud and sand. e main source is atmospheric precipitation while there is surface water supplement in some valley areas.

Karst Fissure Water in Carbonate Rocks.
Karst ssure water is the main form of groundwater, which is in great quantities, but not a large distribution area, accounting for more than half of the total reserves. It is mainly distributed in Ningqiang county, Zhenba county, and the eastern region of Xunyangba. It is stored in limestone, siliceous limestone, dolomite, and shale with its main source being atmospheric precipitation.

Bedrock Fissure
Water. It is dominated by a small distribution area found in metamorphic muddy clastic strata of the southern region of the Qinling mountain area and Hanzhoung city. e water is often found in schist, phyllite, shale, and various magmatic rocks and is mainly supplied by atmospheric precipitation. However, undeveloped rock ssures coupled with steep terrain cause most part of the precipitation lost in the form of the surface runo , which is unfavourable for groundwater recharge.

Mingyazi Tunnel.
e Mingyazi tunnel has a maximum depth of 320 m that is dominated by deep gully around the site where it has a gentle ridge. e tunnel sites are all exposed to bedrock except for some residual soils of Holocene accumulated in gentle slope and gully. e bedrock is Devonian sandstone with limestone, Cambrian limestone with siliceous rock, chlorite schist, aky carbonaceous shale, phyllite, and fault breccia ( Figure 3). e tunnel excavated in the area where ve deep and large fault zones developed. e fault plane is of steep upper wall and at lower wall, forming a shovel shape while the zones are lled by mud, breccia, broken stone, etc. e wide fault zones cause extensive fragmentation and even mud-gravel occurrence of the rocks. e groundwater in tunnel site is bedrock ssure water and Quaternary loose layer pore water [48].

Xiangshan Tunnel.
e Xiangshan tunnel is a singlehole two-way road tunnel and is located in a structural erosion mountain area. e strata in the area are slope gravel soil, pebble soil, silty clay, schist weathered from slightly to strongly, and fault crushed rocks. e tectonic system is a strong compression zone consisting of compressive faults with compact folds stretching in an east-west direction, and its axis is obliquely intersected with tunnel line at a large angle. A regional reverse fault that has a large width was detected in this area; it has a tendency to dip at angles of 15°C  ey are all shear joints and extend in a far distance in which are lled by a small amount of calcite. Surface water is not developed and mainly supplied by meteoric water, spring water, bedrock ssure water, and pore water of Quaternary loose accumulation layer. Groundwater in tunnel site is mainly composed of bedrock ssure water, which is directly  recharged by atmospheric precipitation and drained into valleys by seepage.

Yingfeng Tunnel.
e geomorphology of the Yingfeng area where the tunnel is situated is a low-hilly landscape formed by the movement action of the tectonic denudation and shallow cutting. e site surface is covered with Quaternary residual silty clay and gravel soil, and exposed bedrock is Silurian slate with di erent weathering degrees that embedded in Meiziya formation ( Figure 5). e tunnel site is dominated by the northern Dabashan Caledonian fold, which mainly consists of anticlinorium and synclinorium.
e tunnel had to pass through two fault zones and intensely jointed zones. F1 and F2 are the fault zones, and J1 and J2 are the intensely jointed zones. For the J1 and J2 jointed zones, the spacing of each joint set is 15-20 cm. e surface water in the tunnel area is mainly gulley water and stream water, which is recharged by precipitation. e surface ow changes obviously depending on the season. e groundwater in tunnel area consists of Quaternary pore water and ssure water and its sources mainly are the surface runo , evaporation, and spring drainage [49].

Yezhuping Tunnel.
e passage way for this tunnel runs through the mountainous landscape that is formed by structural erosion and water cutting. e rock formation in tunnel is mainly quartz schist and Devonian slate, and its overlying layer consists of Quaternary diluvium. e tunnel location is in the Shanyang-Fengzhen fault zone, which has three active faults named F1, F2, and T1. Figure 6 presents a geological section pro le along the Yezhuping tunnel. ere are three well developed sets of jointed zones with severe occurrence changes in tunnel site. e spacing of three joint sets are 0.15-0.2 m, 0.5-0.6 m, and 0.45-0.5 m, respectively [50].

Structure Damage and Failure Identi cation
3.2.1. Occurrence Mode of Structure Damages. As mentioned above, the tunnels are mainly constructed in strong compression zones with active tectonic activities, causing various adverse conditions a ecting tunnelling such as development of folding and fault and shearing action. e severe geological deformation and weathering degree of rock mass as well as unreasonable excavation methods all breed structural damage or total failure. e support structure deformation degree of the case tunnel can vary and mainly depend on ground stress property and rock mass quality, where it is summarized into four modes for support structure damage from a collection of construction experience in case tunnels, i.e., drop block, structure crack, structure failure, and invert uplift. Some on-site photos are illustrated in Figure 7, followed by detailed characterization in Table 1. Table 2, the abovementioned problems occur in tunnel support structure; it not only results in the reshaping of the tunnel but also requires an adjustment in the excavation method and support parameters to ensure safety during construction. is challenge negatively impacted the project scope by increasing both time and construction cost. To develop e cient measures for support system usage, it is necessary to explore main features of structure problems in terms of extreme rock mass deformation, to collect useful data to analyze in an e ort to support design and construction.

Features Associated with Extreme Deformation. As shown in
(1) Large Deformation. e excavated tunnel in soft rock of squeezing ground, especially in fault fracture zone, is mainly dominated by extreme rock mass plastic deformation; displacement around tunnel can be from a few millimeters up to decimeters, as this was noted at the Mingyazi tunnel where the maximum movement recorded was 100.8 cm.
(2) Sharp Displacement Rate. e rock mass movement of the excavated tunnel cases always produces an amazing displacement speed. In the case of the Xiangshan tunnel, the average displacement rate was estimated cm per day with the highest movement recorded being 5.4 cm and still maintained a considerable rate after completion of primary support. e rate of the Yezhuping tunnel in the fault fracture zone is as large as 2-3 cm/d in the early stage.
(3) Long Duration. Soft rocks in squeezing ground are always characterized by rheological and low-strength properties, which extend the time required for rock mass to reach stability. For example, after 120 consecutive days of  Altitude (m) Figure 5: Geological section pro le along the Yingfeng tunnel [49].

Shotcrete crack
Drop block monitoring Yingfeng tunnel, it was deemed unstable based on it exhibiting extreme deformation characteristics. A vault settlement rate of more than 10 mm/d at ZK1 + 589 section of Yezhuping tunnel lasted for 15 days after tunnel excavation, which was characterized with obvious creep deformation characteristics.
(4) Unsymmetric Deformation. Due to sensitivity difference in different parts submitted to disturbance, the aggregated displacement in different parts varies greatly, in which the deformation value of a certain part is several times than that of the other part. On the other hand, the sensitivity of different parts is different.

Safety and Harm Assessment.
Structural support problems can cause significant cost overruns, project delivery delays, and even safety issues for working personnel. Some of these unfavourable issues were recorded in studied tunnels. For example, the Yingfeng tunnel had experienced the most reshaping works among the four case tunnels as a result of the rock mass collapse and failure of the support structures. Moreover, huge extra economic investment was sacrificed for these works, taking one of them as an example, about a total of 190,000 dollars was required to successfully complete the reshaping work at the zone between the YK10 + 680 and YK10 + 660. For the Yezhuping tunnel, the excavation was forced to stop for 40 days after the primary support was failure due to water inrush on February 13, 2017. A more serious case was the consecutive collapse events occurred in July, which caused two 20-day shutdowns. Rock mass in some excavation sections of Xiangshan tunnel was extremely broken, resulting in large-scale collapse and sidewall instability during tunnel excavation, and ultimately these problems were only addressed after the implementation of enhanced advance support measures. Once these difficulties occur, they will bring about considerable concerns for designers and engineers. Of course, if such issues are left unchecked or uncorrected, they have the potential to create hazardous conditions in the future for both operations and maintenance of the plant. Extreme-deformation-induced ground surface cracks or even collapse in shallow tunnels may destroy natural environment and threaten surface buildings around region.

Support System Design Based on
Geotechnical Analysis e geological report should be a key element in the decision-making process, as this would help to better address the best methods for a cost efficient excavation, design of support system, and the overall construction. In this section, the shared adverse geological conditions intensified structure damage among the four case tunnels is discussed, and it can help to design the viable and efficient support system.

Rock Structure.
Dense fractures and poor integrity in rock mass easily produce sliding failure along weak structural planes, which significantly promotes rock mass deformation and structure damages. e existence of rock structural planes also brings about change of stress Table 1: Characterization for the structure damage modes identified in studied tunnels [1].

Damage modes
Damage characterization Announcement

Drop block
Rapid displacement of primary support after excavation, and a sharp vault subsidence and sidewall convergence leads to shotcrete cracking, spalling and drop block in a short time, but it will not develop further. Please note that these four structure damage modes were identified in four case tunnels, i.e., (i) Mingyazi (ii) Xiangshan (iii) Yingfeng (iv) Yezhuping Structure crack e steel arch is twisted and bent along the radial or longitudinal direction of the tunnel. An unsymmetric rock mass deformation and the occurrence of radial bulging and invasion of primary support, resulting in support structure crack.

Structure failure
Continuous displacement of rock mass and serious deformation in primary support cause structure failure and partial collapse in tunnel.
Invert uplift e high water pressure in rock mass and the excessive rock mass pressure cause break or uplift to tunnel invert. distribution in rock mass and weakens the overall rock mass. e geotechnical information among the four tunnels is summarized in Table 3. Figure 8 reveals broken rock mass and weak intercalated layer excavated in tunnel site. In fact, the stability of tunnel rock mass and its failure mode mainly depend on spatial combination among the abovementioned adverse factors. For example, the existence of multiple structural planes in Yezhuping tunnel results in an extremely broken rock mass in tunnel face. Also, another case could be found in the Yingfeng tunnel, where a weak intercalated layer reduced the structural properties of the rock mass, which resulted in rock mass undergoing both the material and structural deformation, and finally leading to a considerable cumulative deformation.

Rock Strength.
To a certain extent, rock strength determines degrees of rock mass deformation and structure damages. Engineering practice have already proved on rock mass in grade VI or V (six grades are determined, in which is varying from good to poor [51]) is more easily faced with extreme deformation problems than other rock mass in favourable characteristics during the tunnelling. e test results of the uniaxial compressive strengths of the rocks in dried and saturated conditions are presented in Table 4. Dried rock samples exhibited unsatisfactory strength values of less than 10 MPa, this strength got even weaker as the rocks were saturated with water. ese features may aggravate the rock mass's stability, which can eventually result in structural cracks, extreme deformation which may lead to total failure. For this study, the rock mass was evaluated by the classification system recommended in China Code for Design of Road Tunnel [51], for which is proposed on the basis of the combining qualitative and quantitative methods. e qualitative characteristics include the hardness degree and intactness index, as well as the quantitatively basic quality index BQ is supplemented. e BQ can be obtained by the following equation: where R C is the uniaxial compressive strength of the rock mass and K V is the intactness index of the rock mass.

Groundwater Conditions.
Some waterproofing techniques and drainage measures must be adopted during excavation to alleviate or totally avoid the water-induced adverse impacts on safety during construction. e groundwater discharge in studied tunnels is presented in Table 5. e precipitation infiltration method is herein adopted to evaluate the groundwater discharge during tunnel construction; it is shown in the following equation [52]: where a is the infiltration coefficient of precipitation, F is the catchment area, and P is the annual maximum precipitation.
Practically, some methods could fail if support structure was destroyed in some extreme unfavourable circumstances. A large amount of groundwater causes softening and disintegration on the rock mass leading to a significant weakening in the mechanical properties of rock mass. In the case of the Yezhuping tunnel, more time was taken than previously anticipated due to significant in flows of groundwater during excavation. e softening of rock mass reduces compressive strength and makes it more susceptible to disturbance; this can be noted in the case of the Yingfeng tunnel and is illustrated in Figure 9.

Key Techniques Used in Support System of the Tunnels
Extreme rock mass deformation must be controlled to prevent the support structure from significant displacement, damage, or even total failure. e control attempts can benefit from the reasonable selection of the tunnelling method, improvement of support system, and the implementation of necessary temporary supports. Apart from an ideal bench length in tunnelling with bench method, the stability of rock mass mainly depends on the applied support system in excavated soft rock tunnel, despite the fact that there is negligible selfstabilizing ability that could be used for supplement.

Preparation for CCM-Based Design.
Analysis of the tunnel stability is needed to better understand the behavior between rock mass and support system. e convergence confinement method (CCM) can be considered as the most commonly used means for support design; latterly, another important support philosophy, Non-Deformable Support System [15,53], has been introduced for this work. It can be not only successfully applied in urban tunnels, but also in squeezing and swelling rocks tunnels to address tunnelling problems [54]. However, many complex parameters in support design phase should be considered in order to obtain a very high-performance support system. In addition, time-dependent deformation features of the rock mass and right materials-failure models should be determined accurately. Practically, design and construction works notably benefit from the experience and engineering judgment. So, the in situ measurement-led CCM is suggested for preliminary investigation on support design, for which can provide more intuitive on-site information used for evaluation on stability of rock mass and structures. CCM is considered one of the most convenient tools to describe interaction between rock mass and support structure after tunnel excavation. Its principle has been well documented and can be found in literatures of this field and can be referenced in support of acceptable levels of deformation within the tunnel profile [55][56][57]. is method needs to establish three basic curves: (1) longitudinal deformation profile describing relationship between tunnel deformation and distance from tunnel face [58]; (2) ground reaction curve describing the internal pressure and radial displacement of the tunnel; and (3) support reaction curve describing stress-strain behavior of support system [59,60]. 8 Advances in Civil Engineering Longitudinal deformation profile in Figure 10 describes the rock mass deformation throughout tunnel excavation phase, it highlights some predeformation prior to the start of excavation, loss deformation, and measured deformation, in which the latter two parts accounting for a majority of the total radial displacement. Hence, the design of support system is very important in reducing or even eliminating structural damage that can be potentially caused by extreme rock mass deformation. Specifically, in severe cases, the predeformation in tunnel face can be controlled by Weak intercalated layer (b) Figure 8: On-site exposed rock mass after tunnel excavation. e left one is found in Yingfeng tunnel and the right is in Yezhuping tunnel [49,50].  Advances in Civil Engineering temporary or auxiliary methods, while the convergence deformation of an excavated tunnel requires a comprehensive support system. As for soft rock tunnel excavated in squeezing ground, the engineering criterion and successful experience from the similar cases provide reference for tunnel design and construction. Based upon the abovementioned geological survey, a preliminary CCM-based predesign of support system can be established, and then evaluated by usage of the numerical method to provide an elementary understanding on variations of stress and displacement of rock mass and support system. Furthermore, the designed support system can be modi ed based upon the construction site data to ensure the better e ectiveness.

Temporary Support Methods.
Temporary measures are usually used as auxiliary or special construction tools in the case of the conventional support means or partial excavation measures failed to function e ectively to ensure rock mass stability, especially for tunnels or underground projects with di culties in soft rock, fault fracture zone, and other unfavourable geological conditions. e purposes of the temporary measures are to reduce the predeformation shown in Figure 10 and ensure rock mass stability in tunnel face and safety of tunnel structures and surrounding environment. e tunnels considered are large with a span that exceeds 12 m. ere are a few regions with excellent conditions, but most of the tunnels studied are excavated under challenging geological conditions. ese tunnels were excavated using the bench method, and many special measures were adopted to stabilize the working face. ere are a number of temporary support methods commonly used in these tunnels including advance grouting pipe, advance pipe umbrella, face bolt reinforcement, temporary invert, etc, which are summarized in Table 6. Figure 11 illustrates some of the temporary measures utilized during construction in the Yezhuping tunnel, in addition to these two measures, other immediate shotcreting, and core retaining had been used for tunnel face stability.
e variations and universality of engineering

Tunnel support
Face extrusion

Convergence Preconvergence
Advance support Figure 10: e radial displacement along the tunnel [56,57]. geological behaviors have led to the variable scopes of applicability of temporary support measures. Guided by the recorded tunnel construction data, there are many concepts and principles for temporary support measures that can be used for tunnels constructed in various rock mass types. erefore, this work requires detailed design and analysis of support system in order to adapt to the site conditions and particularities of each project.
For example, the 42 mm diameter advance pipes were constructed in Mingyazi tunnel to reinforce face before tunnelling in broken rock mass section while the 25 mm diameter grouting rock bolts were used in better section to ensure safety excavation. As for the Yezhuping tunnel, it was found to have large deposits of groundwater; therefore, a water stopping wall was firstly erected at a certain distance from the face to prevent the water from gushing, and then the curtain grouting was used to seal face of the excavated part. As shown in Figure 12, after the completion of the curtain grouting construction, the 100 mm diameter drainage holes were set along the inner tunnel annulus to reduce water pressure acting on primary support. e depth of the hole is not less than 5 m, and the holes were filled with hemp materials to prevent the holes from being blocked as water-flow-carrying sediment accumulation.

Detailed Support System.
After achieving stability of the tunnel face, it is imperative to establish the final support Grouting slurry fully fills and saturates the cracks and improves strength and stiffness of rock mass, thereby improving its overall bearing capacity. e reinforcing ring formed by grouting slurry plays a "bearing arch" role to support weight of the upper strata.
✓ ✓ 2 Advance grouting pipe It improves structural and mechanical properties of soft and broken rock mass, fills joint fissures and cracks ahead the tunnel face, and forms an improved reinforcing belt with strong bearing capacity at the outer tunnel annulus.

Face bolt reinforcement
Reinforcement bolts are inserted into core soil to anchor the tunnel face where bolt materials usually employ the glass fibre-reinforced plastics (GFRP) for easy cutting off.

Temporary invert
Regarding the tunnelling with bench method, the steel sets and temporary invert at the bottom of the excavated bench are connected timely and followed by shotcrete construction to form a load-bearing ring, so as to improve the overall stability and bearing capacity.

Full-face grouting reinforcement
Full-face grouting is most used in tunnel face with extremely soft and broken rock mass, improving overall stability of tunnel face by usage of a large-scale grouting in working face.

✓ ✓
Note. RMI and WSU refer to rock mass improvement and water sealing up, respectively.

Advance grouting rockbolt (a)
Advance pipe umbrella (b) Figure 11: Photos of temporary measures construction in site of Yezhuping tunnel [50].
Advances in Civil Engineering system consisting of the shotcrete layer, rock bolt, steel sets, and other elements in various combinations to ensure the stability of the excavated section. e main idea of successfully controlling the extreme deformation of soft rock tunnel was to adopt construction methods and support measures suitable for the geological environment of the case tunnels. As shown in Table 7, the final support system was completed together with the construction method of controlling the rock mass deformation.
A key support system for V grade rock mass shown in Figure 13 was employed in a case tunnel and the other supports of the case tunnels were presented in Table 8. During tunnelling, the design strategy combining big reserved deformation and strong support was widely used in the support system, which obtained a very good effect. e basic principle of these two combinations is maintaining the stability of rock mass with a certain support resistance, while allowing the support system to produce a certain displacement, so as to give full play to the combined function of supporting, yielding pressure, and load discharge simultaneously. e complete support system of the tunnel brings an adequate structural support strength. Specially, the usage of advance grouting pipes in Xiangshan tunnel can resist the rock slippage of the vault under the action of concentrated shear stress. e risk reduction of bending failure and layer separation of rock mass in Yezhuping tunnel benefited from its reasonable selection of bolt insertion angle. e use of grouting bolt can effectively fill rock mass cracks, improving the stress release and ensuring the stability of deep rock mass.
e mentioned measures form the support system with a considerable rigidness, which can constrain the loosing zone expansion of the rock mass.
In addition to the adoption of strong support measures, a large reserved deformation should be considered in some cases in order to deal with complicated geological conditions. It mainly works by the way of allowing a larger rock mass deformation and prevents tunnel from suffering risks of clearance interfering. e specific implementation of temporary measures and support techniques should be adjusted and modified based upon the site geology of the tunnel. It should be emphasized that in most cases, the recoded results had been validated by interpretation of convergence measurements and observation of support performance of support system installed in case tunnels.

Discussion
e design of tunnels requires a comprehensive strategy, which can be beneficial for saving time and cost and ensuring safety construction especially in squeezing ground; this idea is particularly important for extreme deformation control. e strategy for extreme deformation prevention and control in squeezing ground should be established on a dynamic basis, in which it emphasizes on dynamic information exchange and adjustment among deformation prediction, design ideas of support, field geological survey before tunnelling, and advance geological prediction during tunnelling. Recorded rock mass deformation, mapping geological information, laboratory testing of rock mechanical properties, and CCM can be used for evaluating in situ rock mass parameters and predesigning corresponding support system.
e key techniques, together with the abovementioned temporary support measures, have achieved an ideal deformation control effect and avoided structure damage of tunnel support system, which can be confirmed by in situ monitoring data from case tunnels shown in Figure 14. Compared with the aforementioned rock mass deformation hazards, the implementation of the proposed support system reduces the displacement rate of rock mass, the stability time for final deformation completion, and the total deformation amount.

Usage of FRB.
e studied tunnels constructed in squeezing ground were excavated via using the bench excavation method; the separate installation of the steel sets for the top excavated section cannot form a closed support loop to deal with deformation problem. An illustration for the FRB is presented in Figure 15. e FRBs can be used to connect with steel sets bottom, so as to address the steel sets deviation caused by the insufficient load-bearing capacity of the arch springing or existence of the excavated space. It not only prevents the arch springing from shrinking and dropping, but also plays a role of advance or temporary support for the next bench excavation. e monitoring data of the field tests conducted in Xiangshan tunnel indicated that a significant effect was achieved by using the FRB for controlling primary support structure deformation. e setting of FRBs could reduce the displacement rate by more than a half. After extreme deformation and support structure failure occurred in some sections of the Yingfeng tunnel, the FRB technique was used to protect tunnel system suffering from such structure damages and ensure safety construction. An ideal effectiveness was confirmed by the implementation feedback regarding the rock mass displacement shown in Figure 16.

Usage of Strong Support System.
e usage of double steel sets was an effective way to prevent rock mass  e second steel sets are constructed when there is 2/3 of the amount of the reserved deformation retained, and the short steel bar and wooden wedge are used to transfer the rock mass pressure between the rst and second support steel sets.    14 Advances in Civil Engineering

More Reserved Deformation.
e original designed reserved deformation varying from 10 to 15 cm failed to satisfy the requirement of the rock mass deformation, leading to structure crack, failure, and even partial collapse. For example, based upon the in-situ monitoring and eld test, more reserved deformation was used for support system redesign in unfavourable sections of the Yingfeng and Xiangshan tunnels, which e ectively addresses the deformation problems.

Conclusions
e wide distribution of soft rock and active geological structure in the Qingling-Daba mountainous area brings challenges and problems to the design and construction of tunnels in this region. Four case tunnels constructed in squeezing ground are studied in this paper, and the following conclusions can be drawn: (1) e structure damage modes and a ecting factors in four squeezing tunnels were studied for the purpose of improving stability of a tunnel excavated in rock masses by developing a support system.
(2) Four categories of damage for support structure are established from the previous experiences of the four case tunnels, i.e., drop block, structure crack, structure failure, and invert uplift. (3) Main factors a ecting structure damage are identi ed as rock structure, rock strength, and groundwater conditions, which is a key component for the design of a viable and e cient support system. (4) For the proposed support system, temporary support measures are necessary for a tunnel excavated in squeezing ground; also the usage of key techniques such as strong support measures, FRB, and large reserved deformation provides helps for reducing damage risks of support structure for a tunnel in squeezing ground. (5) e e ectiveness of the proposed support system had been veri ed by in situ feedbacks and eld test results presented in the paper, which will provide useful information and guidance for similar projects.

Advances in Civil Engineering
Data Availability e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare no conflicts of interest.