Research Article Effects of High-Pretension Support System on Soft Rock Large Deformation of Perpendicularly Crossing Tunnels

Hindawi Advances in Civil Engineering Volume 2020, Article ID 6669120, 18 pages https://doi.org/10.1155/2020/6669120

Li Gan ,1 Ma Weibin ,2 Tian Siming ,3 and Zhou Wenhao 2

1State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China 2China Academy of Railway Sciences, Railway Construction Research Institute of China Academy of Railway Sciences Group Co., Ltd., Beijing 100084, China 3China Railway Economic and Planning Research Institute, Beijing 100084, China

Correspondence should be addressed to Ma Weibin; [email protected]

Received 22 November 2020; Revised 29 November 2020; Accepted 9 December 2020; Published 21 December 2020

Academic Editor: Hualei Zhang

Copyright © 2020 Li Gan et al. .is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

.e control mechanism of prestressed support system is studied by means of three-dimensional discrete element numerical simulation, theoretical analysis, and field measurement. .e actual project is selected to analyze, and the control effect is demonstrated. .e theoretical analysis results show that the prestressed support can improve the self-bearing capacity of surrounding rock, which is conducive to the stability of the surrounding rock. .e numerical calculation shows that, under the control of the long and short cables, the prestress can spread effectively and form two bearing arches. .e deformation is difficult to control, which is in the arch crown and the arch shoulder of the tunnel intersection. However, the deformation of the surrounding rock decreases with the increase of prestress. .rough the analysis of the measured results, it shows that the pretension support design scheme can effectively control the large deformation of the surrounding rock.

1. Introduction strategies of high-strength initial support system, multilevel layered support, yieldable support technology, and active With the implementation of the western development support system. At present, the main methods to increase strategy in China and the rapid development of tunnel initial support system are as follows: improved concrete engineering, the proportion of long and deep tunnels is strength [3–6]; encrypted steel arch [7, 8]; and new arch increasing. .e increase of tunnel construction depth and structures [9, 10]. Multistage layered support mainly in- length will inevitably face more complex geological envi- cludes advance pipe shed support [11, 12] and double-layer ronment. Especially, under the action of high ground stress, initial support [13]. .e above method improves the bearing high seepage pressure, and excavation disturbance, the capacity of the supporting structure, effectively controls the plastic deformation characteristics of soft rock are obvious. development of the plastic area, and alleviates the large It will lead to a series of problems, such as sudden and deformation of the soft rock tunnel. .e above strategy does serious malignant accidents and increased construction cost. not enhance the strength of the surrounding rock. Under the In the 1950s, Austrian scholars proposed a new Austrian action of high ground stress, the surrounding rock is prone method for underground engineering support, whose core is to continuous and slow deformation, which eventually leads to maintain and utilize the self-bearing capacity of the to the failure of support. surrounding rock [1, 2]. Considering the complex geological .e methods of yieldable support technology mainly conditions and construction environment of the soft rock include Grille support [14, 15], Foam concrete [16–18], Bolt tunnel, scholars have carried out much work on the stability and anchorage cable’s coupling yieldable support technology of bearing body under the joint action of support and [19,20], and Yielding arch [21, 22]. .e support strategy surrounding rocks. .e research focuses on the support allows a certain deformation of the surrounding rock to 2 Advances in Civil Engineering release the accumulated energy of the surrounding rock, occurred and the arch change rate reached 25%, as shown in while the support structure is always stable. According to the Figure 1. .erefore, it is necessary to explore a new support support method with rockbolt net spray, u-shaped shelf, and strategy to deal with the large cross-section area of inclined grouting, the surrounding rock of the tunnel is the main shaft and main tunnel. bearing structure, and the support body and surrounding rock are considered as the composites. .e final intention is that the surrounding rock can show maximum self-bearing 2. Engineering Geology capacity [23]. .e main method includes increasing the support density, grouting in the surrounding rock, in- .e Mu Zhai-ling tunnel on Lanzhou-Hainan motorway is creasing the length of anchor (cable), and increasing bolt located in Dingxi city in China’s Gansu province (cable) pretightening force. (Figure 2(a)). .e project mainly includes two main tunnels In the 1970s, the United States combined the expanding and three inclined shafts (Figure 2(b)). .e length of the Mu shell anchor with resin anchor and increased the diameter Zhai-ling left lane tunnel is 15,200 m. .e right of the Mu and strength of the anchor. At present, the prestress of bolt Zhai-ling left lane tunnel is 15,150 m. .e starting and support in American coal mine roadway is generally about ending points of the tunnel are located at AZK210 + 625 and 100 kN, which has achieved good support effect. In En- AZK225 + 825, respectively. gland, the anchoring capacity of anchor bolt used in coal .e tunnel is located in the boundary zone of multiple mine roadway support is about 500 kN. .e roadway of plates, and the maximum buried depth of the tunnel is about Australian coal mine is supported by 60 ∼ 70 kN prestressed 629.1 m. .e tunnel passes through three synclines and three anchor bolt, and the support control is effectiveness. In anticlines. .e crossing fault is mainly Meiwu–Xinsi fault. .e 2005, China puts forward high-prestressed bolt and anchor strike of the fault is 100–115° and north dipping and inclination cable support technology for deep coal mine roadway is 30°–70°, with the characteristics of compression and torsion. support, which greatly improved the support safety of coal .ere are 12 secondary faults caused by the Meiwu–Xinsi fault, mine roadway. However, in the field of deep tunnel sup- which are, respectively, f1 ∼ f12 (Figure 3). .e fault is mainly port, there are few research studies in this field. Kang and distributed in northwest direction. Both inclined shaft and Hongpu [24–26] studied the mechanical properties of high- main tunnel used the drilling-and-blasting method and three- prestressed anchor rod and cable under the action of resin step construction technique for tunneling. .e cross-section anchorage agent. .e theory of high prestress and strong shape of the inclined shaft and main tunnel is the same, the support is put forward for the deep difficult roadway of coal design height of cross-section is 11.127 m, and the width is mine. .ey further developed a series of products, such as 12.82 m. .e plane intersection angle between the inclined strong anchor (cable) and strong steel strip. .e field ap- shaft and main tunnel is 90° (Figure 4). plication shows that the strategy can effectively control the .e method of hydraulic fracturing is used to measure deformation of the surrounding rock. Zhang et al. [27] put the in situ stress. According to the relevant test data, three forward prestressed control methods such as high-per- main stress interaction relations are obtained. .e in situ formance pretension bolt, steel strand pretension truss, and stress is mainly tectonic stress, and the relationship between M-type steel strip, aiming at the high-prestressed bolt in situ stress and burial depth is fitted, as shown in Figure 5. support technology of coal mine roadway. Lin et al. [28] .e maximum horizontal principal stress directions mea- developed a large-scale test device for stress field of the sured by hydraulic fracturing method are near NE39.6° and anchor cable support and analyzed the distribution char- NE34.1°. .e direction of the maximum horizontal principal acteristics of the prestress with a single prestressed anchor stress is nearly parallel to the direction of the main tunnel cable. .e experimental results show that the distribution excavation. of prestress field in space is similar to “pomegranate.” Two .e entrance and exit sections of the tunnel are shallow compressive stress concentration zones are formed at both buried sections. It is mainly composed of slope loess soil, gravel ends of the free section, and one tensile stress concen- soil, paleogene conglomerate, and permian strongly weathered tration zone is formed near the anchorage section. .e carbonaceous slate and sandstone. .e stratum of this section is distribution rule of the stress along the axis of bolt is related complex. .e joints and fissures are developed, and the in- to the distance between the cable and the monitoring tegrity of the surrounding rock is poor, and it is easy to be position. .e macrodistribution characteristics of the stress weathered and destroyed when encountering air or water after field are basically the same with different pretensions, but excavation. .e tunnel intersection of the research object is the magnitude of the prestressing force at the same location located in the middle of the line with a buried depth of 510 m. will change. .e main lithology is thin-layer carbonaceous slate. According .is paper relies on the Mu Zhai-ling tunnel as the to the relevant GSI value standard, combined with the actual research object of the engineering experiment. .e original engineering geological conditions, the approximate value of support methods of the Mu Zhai-ling tunnel are as follows: GSI is 35. As shown in Figure 6, it is the surrounding rock Φ42 advance grouting small conduit 4500 mm, spacing condition of the tunnel face. According to the field investi- 400 mm; I18 steel arch frame, spacing 500 mm; and C25 gation, the dip angle of the surrounding rock changes greatly shotcrete 250 mm. However, according to the above support and the dominant joint angle is between 30° and 85°..e measures, during the advancing process of 2 # inclined shaft surrounding rock of the tunnel is in thin bed structure, and the of the Mu Zhai-ling tunnel, large-scale initial support failure thickness of slate is 5 cm–30 cm. Advances in Civil Engineering 3

Figure 1: Splitting of primary lining and exceeding clearance limit.

1# inclined sha Mu Zhai-ling tunnel

Mu Zhai-ling right lane tunnel

2# inclined sha

3# inclined sha

Mu Zhai-ling le lane tunnel

East China North China Central China Northeast China South China Northwest China Southwest China Figure 2: (a) Location of the Mu Zhai-ling tunnel, Gansu, China. (b) Plane view of the tunnel.

3. Countermeasures for Damage limit equilibrium, as shown in formula (1)–(5). .e high- Mitigation with High-Pretension Cable pretension anchor cable can effectively improve the stress state of the surrounding rock in the pressure arch, 3.1. Mechanical Model of Tunnel Surrounding Rock Structure. as well as the cohesion and internal friction angle of the After the excavation of the tunnel, the original rock stress surrounding rock in the pressure arch: state is destroyed, the surrounding rock has uneven �� J − αI − k � 0, (1) displacement, and the internal stress state of the rock 2 1 changes, forming a secondary stress field. When the 2 sin φ stress is redistributed and stabilized, there will be a α � √� , (2) tangential compression area similar to the arch structure 3(3 − sin φ) above the excavation chamber, which is called pressure 6C cos φ arch. .e rock mass in the pressure arch bears the load of k � √� , (3) itself and the upper rock mass. .erefore, in order to 3(3 − sin φ) effectively restrain the large deformation of the surrounding rock, the bearing capacity of the surrounding I1 � σ1 + σ2 + σ3, (4) rock bearing arch should be strengthened. .erefore, �� build the model, as shown in Figure 7. In this paper, it is 1 2 2 2 J � σ − σ � +σ − σ � +σ − σ � . (5) considered that the surrounding rock obeys the D-P 2 6 1 2 2 3 3 1 [29–31] (Drucker–Prager) criterion under the action of 4 Advances in Civil Engineering

Geostress test location

1# inclined sha 2# inclined sha 3# inclined sha

Figure 3: Geostress test location and geological cross-section along the tunnel route.

Mu Zhai-ling right lane stress-arch side wall is aﬀected by the pretightening force, tunnel and the pretightening force of the anchor cable exerts the Large section stress on the surrounding rock is σ . .e stress parallel to the crossing tunnels 3 tunnel direction is σ2, so the maximum stress of the pressure arch is σ1. ° Assuming σ2 � 1, C � 0.6, and φ � 25 , when σ3 increases from 0 MPa to 7 MPa, the change of the maximum bearing capacity σ1 is shown in Figure 8. Assuming σ3 � 3, σ2 � 1, and C � 0.6, when φis increased from 5° to 45°, the change of the 2# Inclined sha maximum bearing capacity σ1 is shown in Figure 9. As- ° suming σ3 � 3, σ2 � 1, and φ � 25 , when C increases from 0 MPa to 6 MPa, the change of the maximum bearing ca- Figure 4: Layout of No. 2 inclined shaft of the Mu Zhai-ling tunnel. pacity σ1 is shown in Figure 10. .e results show that, with the increase of preload, internal friction angle, and cohesion, the bearing capacity of the pressure arch is signiﬁcantly Geostress (MPa) improved. 6 8 10 12 14 16 18 20 200

3.2. Pretension Cable-Mesh-Shotcrete + Steel Arch Combined 250 Support Technology. According to the above analysis and the three-step construction process, a combined support scheme of “high-pretension cable + mesh + shotcrete + steel arch” is 300 y = 22.599x + 3.53 proposed, within 20 m of the intersection point, as shown in R2 = 0.8133 Figure 11. Long anchor cable: the pretightening force is 350 kN,

Depth (m) 350 the diameter of steel strand is 21.8 mm, the length is 10300 mm, the strength grade is 1860 MPa, the drilling diameter is 32 mm, and the row spacing between anchor 400 cables is 1000 mm × 1200 mm. Short anchor cable: the pretightening force is 35 t, the diameter of steel strand is 21.8 mm, the length is 5300 mm, and the row distance 450 y = 27.125x + 5.17 between anchor cables is 1000 mm × 1200 mm. W-type R2 = 0.837 steel strip: 280 mm wide, 2.8 mm thick, and Q235 mate- rial. As shown in Figures 12–14, it is the support scheme of Horizontal maximum principal stress the inclined shaft, main tunnel, and intersection, and the Horizontal minimum principal stress support mode of the inclined shaft and main tunnel is the Vertical principal stress same. Figure 5: Map of principal stress variation with depth in the Mu (1) “Pretension cable-mesh-shotcrete” initiative sup- Zhai-ling tunnel area. port: the minimum pretightening force of the cable is no less than 350 kN. .e cable is designed to be where φ and C are the angle of internal friction and co- installed with a 3000 mm steel strip and mesh hesion, σ1 is the maximum principal stress, σ2 is the in- polymer. As shown in Figure 15, termediate principal stress, and σ3 is the minimum principal Φ21.8 mm × 10300 mm high-strength cables are in- stress. stalled with row spacing and column spacing of When the stress state of any point of the surrounding 1200 mm × 1000 mm. Φ21.8 mm × 5300 mm high- rock in the pressure arch meets formula (10), it will enter the strength cables are installed with row spacing and plastic failure state. After the excavation of the tunnel, the column spacing of 1200 mm × 1000 mm. Advances in Civil Engineering 5

Figure 6: surrounding rock of the tunnel face.

Load of surrounding 14 Load-bearing arch rock

12 Cable Pretightening force of cable 10

Ground resisting force Schematic diagram of Mechanical model of 4

anchor cable support Load-carrying Arch (MPa) stress principal maximum Ultimate Figure 7: Mechanical model of bearing structure under the action 2 of the prestressed cable. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Internal friction angle (radians)

Figure 9: .e change of the maximum bearing capacity with the 8 internal friction angle.

7 30 6 28 26 5 24 22 20 4 18 16 3 14

Ultimate maximum principal stress (MPa) stress principal maximum Ultimate 12 2 10 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8 Prestress of anchor cable (MPa) 6 4

Figure 8: .e change of the maximum bearing capacity with the (MPa) stress principal maximum Ultimate 2 pretension of the anchor cable. 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Cohesion of rock mass (MPa) (2) “Strengthened support with 18a I-steel and shotcrete”: according to the measured position, each Figure 10: .e change of the maximum bearing capacity with section of steel frame is connected by bolts. In order cohesion of rock mass. 6 Advances in Civil Engineering

surrounding rock under three-dimensional stress state, the D-P (Drucker–Prager) constitutive model is selected. .e Coulomb friction law can realize the sliding or opening of joints, which is very suitable for simulating the failure process of the surrounding rock. 20m In this paper, the method of equivalent ubiquitous joint 20m with dominant joint is proposed. .e lithology of the surrounding rock is mainly carbonaceous slate, and under the ′ A A long geological structure, the rock mass is distributed with complex and variable joints. In the process of numerical simulation, it is obviously impossible to consider all joints. 20m 2# inclined sha As shown in Figure 16, the distribution of surrounding rock

Mu Zhai-ling right lane tunnel lane right Zhai-ling Mu joints is generalized, and it is obvious that there are dominant joint groups and other joints. Other joints (except the dominant joints) can be regarded as isotropic macroscop- ically, and the method of uniform joint arrangement is used for continuity medium equivalence. After the joints are Figure 11: Tunnel intersection plan. evenly arranged, the dominant joints are divided. .e equivalent ubiquitous joint model with the dominant joint group is ﬁnally obtained. 1000mm As shown in Figure 17, the tunnel-intersection numerical 1000mm model is established, which includes the tunnel section, an- φ 21.8 × 10300mm chor cable, initial support and second-lining concrete, and High stress anchor uniform joint and dominant joint arrangements. .e buried cable (35t) depth at the intersection is 510 m. According to the in situ

R8342, 56 stress test results, the vertical in situ stress is 12.5 MPa, the

φ 21.8 × 5300mm R6390 maximum horizontal stress is 21 MPa (parallel to the main High stress anchor tunnel direction), and the minimum horizontal stress is cable (35t) R8342, 56

11672 12.5 MPa (perpendicular to the main tunnel direction). In R6390 order to fully consider the boundary eﬀect, the model length × width × height � 120 m × 80 m × 50 m, containing 246,400 blocks.

Figure 12: Inclined shaft and main tunnel support scheme layout. 4.2. Block Yield Model. Under the action of the high- to ensure that each section of the steel frame is placed prestressed support system, this paper believes that the on a stable foundation before the full-ring closure, rock in the range of influence of the support is under the virtual slag and sundries at the bottom of each stress in three directions. .e failure of the rock is related section of the steel frame shall be removed before to the three principal stresses and their ratios, so the installation. At the same time, four Φ42 lock foot Drucker–Prager elastoplastic constitutive model is se- anchor pipes are installed on each side to lock it, and lected to achieve the failure of the block. .e failure the back of the steel frame is filled with shotcrete. .e criterion is represented in the plane (σ, τ) as illustrated in longitudinal connection of the steel frame is con- Figure 18 (as shown in equations (10)–(14)). .e failure nected with steel bars, with a circumferential spacing envelope is defined by the Drucker–Prager yield function of 0.6 m. from point A to B, and the tension yield function from B to C. As shown in Figure 15, the construction process mainly includes tunnel excavation, anchor cable drilling, installation of the cable, pretension tension, mesh and steel strip 4.3. Contact Model. In 3DEC discrete element numerical installation, steel arch, and shotcrete installation. After the simulation, Coulomb slip constitutive model [32, 33] is deformation of the surrounding rock is stable, the second widely used in geotechnical engineering. .e joint model can lining construction shall be carried out. fully represent the shear failure, tensile failure, and compression deformation of jointed rock mass. .e tensile re- 4. Numerical Model Establishment sistance of joints is 4.1. Establishment of Numerical Calculation Model. A nu- Tmax � − TAc. (6) merical model of the tunnel intersection is created by 3DEC (3 Dimension Distinct Element Code) software. In where T is the input parameter that represents the joint order to consider the failure characteristics of the tensile strength and Acrepresents the area of the subcontact. Advances in Civil Engineering 7

1000mm φ 21.8 × 10300mm High stress anchor 1000mm cable (35t)

φ 21.8 × 5300mm High stress anchor Pilot tunnel cable (35t) The upper bench

The middle bench

The lower bench

Figure 13: Cross-section of the intersection support scheme layout (A-A′).

Mu Zhai-ling right lane tunnel φ 21.8 × 10300mm High stress anchor cable (35t)

Steel strip

Mesh φ 2# inclined sha 21.8 × 5300mm High stress anchor cable (35t)

Steel strip

1200mm 1000mm 1200mm 1000mm

Mu Zhai-ling right lane tunnel Figure 14: Plane expansion diagram of the tunnel support.

.e expression of the maximum shear stress that the analyze the mechanical properties of carbonaceous slate and joint can bear is provide parameter basis for theoretical analysis and nu- s n merical calculation, the mechanical characteristics of cy- F � cA + F tan ϕ. (7) max c lindrical specimens under different confining pressures are where c and ϕ represent joint cohesion and internal friction analyzed by using rock mechanics test system. Using the angles, respectively, and Fnrepresents normal stress acting method of numerical simulation, the development law of on the joint. stress-strain relationship is analyzed. Compared with the Under the action of normal stress, joints will produce experimental data, the reliability of the constitutive model in compression deformation, which is related to the normal numerical simulation is verified. .e size of the sample is stiffness of joints. In this paper, the compression failure of 50 mm × 100 mm cylinder. In this loading process, the the block obeys the D-P criterion. When the joint is dam- displacement control mode is adopted, and the displacement aged, the internal friction angle and tensile strength of the rate is 0.001 mm/s. .e confining pressure range is joint surface will be classified as 0. 0–30 MPa. .e total stress-strain curves under different confining pressures are shown in Figure 19. .e experimental results show that, with the increase of confining 4.4. Physical and Numerical Experimental Analysis of Con- pressure, the overall trend of compressive strength and the ventional Triaxial Compression. In order to accurately plastic deformation after the peak value are increased. 8 Advances in Civil Engineering

Drilling of cable Install the cable Pretightening force of the cable

Control console of tensioning Applied mesh and steel strip Cable installation completed machine Figure 15: Pretension cable-mesh installation process.

(a) (b)

Figure 16: Modeling method of equivalent joints with dominant joints: (a) characteristics of the surrounding rock, (b) generalization of joint distribution, (c) dominant joints, and (d) equivalent ubiquitous joints.

As shown in Figure 20, a numerical model of rock triaxial Table 1. .e numerical results show that the bearing capacity of experiment is established in this paper, which includes loading the specimen is positively related to the surrounding rock plate; surrounding rock area, and rock sample bin. .e size of pressure. .e simulation results are in good agreement with the the rock sample in the similar simulation is the same as that in test results, as shown in Figure 21. the conventional physical experiment, and the joint with the length of 0.5 mm is evenly distributed. .e block constitutive model is the D-P model, and the joint constitutive model is the 4.5. Engineering Rock Mass Parameter Value. .e selection Coulomb slip model. .rough rock mechanics experiments of engineering rock mass mechanical parameters just and repeated attempts, the parameter values are shown in relies on the experimental data and cannot meet the Advances in Civil Engineering 9

80m

Second lining concrete

50m

120m Excavation area Initial support

Surrounding rock

Anchor cable

Figure 17: Numerical calculation model.

A τ E 1 − D/2 rm �� . + �, ( ) f s = 0 0 02 (60+15 D− GSI)/11 8 Ei 1 + e

m + s − a m − s �� m as− 1 t b 4 b 8 b f = 0 σ � σci � � , (9) B cmass 2(1 + a)(2 + a) 4 + s kϕ GSI − 100 m � m exp� �, (10) b i − D C σ 28 14 t σ GSI − 100 s � exp� �, (11) 9 − 3 D kϕ/qϕ 1 1 Figure 18: .e Drucker–Prager failure criterion. a � + �e− (GSI/15) − e− (20/3)�, (12) 2 6

In the formula, Erm is the elastic modulus of rock mass, requirements. .e selection of parameters in numerical σcmass is the uniaxial compressive strength of rock mass, calculation plays an important role in the calculation GSI is the geological strength index,Ei is the deformation results. .e selection of parameters not only needs to modulus of rock, miis Hoek–Brown strength parameter of consider the solid properties, such as rock strength, but the intact rock block, σci is the uniaxial compressive also needs to consider the occurrence environment and strength of intact rock, and D is the damage coeﬃcient of structural characteristics of rock mass. Hoek established rock mass. a method of rock mass strength conversion based on GSI According to the principle of joint parameter selection geological classiﬁcation index, which was converted by in the literature “Itasca Consulting Group, Inc. 3DEC the following formulas (8)–(12) [34]: User Manual and Itasca Consulting Group, Inc.: 10 Advances in Civil Engineering

Axial strain (MPa) (MPa) 1

1 40 σ

Transverse strain σ 20 Axial strain Stress Stress Transverse strain 20 10 10MPa 0MPa

0 0 –2.0 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 ε –3 ε –3 Axial strain 1/10 Axial strain 1/10 (a) (b) 100 120

100 80

Transverse strain Transverse strain 80 Axial strain 60 Axial strain (MPa) (MPa) 1 1 60 σ σ 40 Stress Stress 40

20 20 20MPa 30MPa

0 0 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 2.5 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 2.5 ε –3 ε –3 Axial strain 1/10 Axial strain 1/10 (c) (d)

Figure 19: Stress-strain curves of slate on triaxial compression tests under diﬀerent conﬁning pressures: (a) 0 MPa, (b) 10 MPa, (c) 20 MPa, and (d) 30 MPa.

Minneapolis, MN, USA, 2016,” the joint uniform distri- Ks � 0.4Kn. (16) bution modeling is carried out, such as formulas (13)–(16), and the characteristic element is selected According to the characteristics of laboratory test and Δz � 0.8 m: field engineering, the GSI value is 35, the Hoek–Brown strength parameter mi of the intact rock mass is 10, the E K � , (13) uniaxial compressive strength of the rock block is 40 MPa, 3(1 − 2μ) the elastic modulus of the rock block is 33 GPa, Poisson’s ratio is 0.25, and the damage coefficient of the rock is 0.5. E According to the above calculation formula and the re- G � , (14) 2(1 + μ) sults of relevant parameters, the mechanical parameters in the numerical simulation are obtained, as shown in Ta- K + 3/4G ble 2. After multiple inversion analysis, combined with the Kn � 10� �, (15) field-measured deformation data of the surrounding rock, Δzmin the normal stiffness of the dominant joint is 2 GPa, the Advances in Civil Engineering 11

Vertical stress loading plate

Conﬁning pressure Rock sample loading

Vertical stress loading plate

Figure 20: Numerical experiment model.

Table 1: Parameters for the rock. Block properties Contact properties 3 b b b n s j j j Density (kg/m ) K (GPa) G (GPa) C (MPa) φ σ t (MPa) K (GPa k (GPa) C (MPa) φ σ t (MPa) 2420 12.3 8.1 13.6 32 2.30 87.31 33.11 1.55 31 1.34

120

100

60 Stress (MPa) Stress 40

0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 ε –3 Axial strain 1/10

σ σ 3 = 0MPa 3 = 20MPa σ σ 3 = 10MPa 3 = 30MPa Figure 21: Stress-strain curves of slate. shear stiffness is 1 GPa, the cohesion is 0.9 MPa, the in- rock and eliminate the interference of in situ stress, only the ternal friction angle is 25°, and the tensile strength is application of prestress is considered. .e 5 m anchor cable 0.5 MPa. embedment length is 1 m. .e 10 m anchor cable embedment length is 2 m. In the process of prestress application, the tensile 5. Numerical Analysis of Therapeutic stress of the anchor cable is transferred from the end to the Effects under Different Pretensions depth and finally reaches the equilibrium state. As shown in Figure 22, the stress characteristics of the anchor cable near the 5.1. Diffusion Characteristics of Prestress in the Surrounding anchorage section under different preloads. .e axial stress of Rock. In this part of the study, in order to better understand the anchor cable shows that, with the increase of pretension, the the diffusion characteristics of prestress in the surrounding tensile stress increases linearly in the free segment part. .e 12 Advances in Civil Engineering

Table 2: . Mechanical parameters of carbonaceous slate. Rock mass parameters Homogeneous joint parameters Lithology 3 b b b n s j j j Density (kg/m ) K (GPa) G (GPa) C (MPa φ σ t (MPa) K (GPa) k (GPa) C (MPa) φ σ t (MPa) Carbonaceous slate 2500 0.56 0.27 1.68 23 0.6 9.53 0.38 3.75 29 3.45

400 Embedment length Free segment 350

300

250

200

150

100 Force of the prestress cable (kN) cable the prestress of Force 50

0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 Distance from anchor starting point (m)

50kN preload 250kN preload 150kN preload 350kN preload Figure 22: .e stress distribution curve of the prestressed cable.

Section A Section B

Anchorage position of 10m anchor cable

Inclined Control range of 5m sha advance anchor cable support direction

Main tunnel advance direction Figure 23: Compressive stress distribution contour with the pretension cable. tensile stress of the anchor cable decreases gradually along the range of 10 m, and the effective stresses produced by the long direction of hole bottom in the embedment part. Under the and short anchor cables are superimposed on each other, action of 350 kN preload, the influence area of the embedment forming the prestressed expansion area of the double-arch section is 0.5 m, so the length of the anchorage section must not prestressing force. For the low-arch formed by 5 m prestressed be less than 0.5 m in the construction, and the safety factor anchor cable, the prestress diffusion in the arch is relatively needs to be considered. uniform, and the prestress value is about 3.3 MPa. Compared As shown in Figures 23 and 24, it is the prestressed dif- with the low-arch, the high-arch formed by 10 m prestressed fusion effect under the combined action of 5 m and 10 m anchor cable has poor uniformity of prestress diffusion and the anchor cables. .e prestress of the anchor cable is 350 kN. .e prestress value near the anchorage position is 2–3 MPa. anchor cables are evenly distributed (the circumferential spacing is 1 m and the longitudinal spacing is 1.2 m). Because of the combination of long and short anchor cables, the support 5.2. Deformation Characteristics of the Surrounding Rock. resistance in the support area is significantly increased. Under As shown in Figures 25 and 26, they represent the settlement the action of high pretension, long and short anchor cables can of the surrounding rock and the horizontal convergence of produce better active control effect on the surrounding rock. the tunnel under the support of the prestressed anchor cable. .e effective prestressing force is effectively diffused in the .is paper focuses on the deformation of the surrounding Advances in Civil Engineering 13

(Max. compression) stress 0.0000E + 00 –2.5000E + 05 –5.0000E + 05 –7.5000E + 05 –1.0000E + 06 –1.2500E + 06 –1.5000E + 06 –1.7500E + 06 –2.0000E + 06 –2.2500E + 06 –2.5000E + 06 –2.7500E + 06 –3.0000E + 06 –3.2500E + 06 –3.3000E + 06

Section A Section B

Figure 24: Stress of the surrounding rock under preloading 350 kN.

X-displacement X-displacement Plane: active on Plane: active on 6.5000E – 01 4.3000E – 01 6.0000E – 01 4.0000E – 01 5.0000E – 01 4.0000E – 01 3.0000E – 01 60.11cm 57.59cm 42.71cm 39.91cm 3.0000E – 01 2.0000E – 01 2.0000E – 01 1.0000E – 01 1.0000E – 01 0.0000E + 00 0.0000E + 00 –1.0000E – 01 –1.0000E – 01 –2.0000E – 01 –3.0000E – 01 –2.0000E – 01 –4.0000E – 01 –3.0000E – 01 –5.0000E – 01 –6.0000E – 01 –4.0000E – 01

Prestressed 50kN Prestressed 150kN X-displacement X-displacement Plane: active on Plane: active on 3.4000E – 01 3.0000E – 01

3.0000E – 01 2.0000E – 01

33.51cm 32.46cm 2.0000E – 01 27.74cm 27.03cm 1.0000E – 01 1.0000E – 01

0.0000E + 00 0.0000E + 00

–1.0000E – 01 –1.0000E – 01 –2.0000E – 01 –2.0000E – 01 –3.0000E – 01

–3.3000E – 01 –3.0000E – 01

Prestressed 250kN Prestressed 350kN

Figure 25: Horizontal displacement under diﬀerent prestressing (Section A). rock under diﬀerent prestress values. Compared with 50 kN average decrease of 10.9 cm. However, this trend is not prestressing force, every 100 kN of prestressing force in- linear. With the continuous increase of prestress, dome creased, the tendency of vault subsidence decreased, with an subsidence reduction and prestressed increment ratio 14 Advances in Civil Engineering

Z-displacement Z-displacement Plane: active on Plane: active on

0.0000E + 00 0.0000E + 00

–5.0000E – 02 –2.5000E – 02 –5.0000E – 02 –1.0000E – 01 –7.5000E – 02 –1.5000E – 01 –1.0000E – 01 –2.0000E – 01 31.37cm 52.1cm –1.2500E – 01

–2.5000E – 01 –1.5000E – 01

–3.0000E – 01 –1.7500E – 01 –2.0000E – 01 –3.5000E – 01 –2.2500E – 01 –4.0000E – 01 –2.5000E – 01 –4.5000E – 01 –2.7500E – 01

–5.0000E – 01 –3.0000E – 01

–5.2100E – 01 –3.2000E – 01

Prestressed 50kN Prestressed 150kN Z-displacement Z-displacement Plane: active on Plane: active on

0.0000E + 00 0.0000E + 00

–2.5000E – 02 –2.0000E – 02

–5.0000E – 02 –4.0000E – 02

–7.5000E – 02 –6.0000E – 02

26.24cm –1.0000E – 01 –8.0000E – 02 19.33cm –1.2500E – 01 –1.0000E – 01

–1.5000E – 01 –1.2000E – 01

–1.7500E – 01 –1.4000E – 01

–2.0000E – 01 –1.6000E – 01

–2.2500E – 01 –1.8000E – 01

–2.5000E – 01 –2.0000E – 01

–2.7000E – 01 –2.1000E – 01

Prestressed 250kN Prestressed 350kN

Figure 26: Subsidence characteristics under different prestressing (Section B). decreased significantly. When the prestress is increased from right, and the deformation is uneven. As the prestress 50 kN to 150 kN, the amount of vault sinking is reduced by gradually increases, the tendency of asymmetric deforma- 20.73 cm. When the prestress is increased from 250 kN to tion weakens. At the same time, the horizontal convergence 350 kN, the amount of vault sinking is reduced by 6.91 cm. deformation is also reduced, and its slowing trend is similar .e maximum amount of vault subsidence occurs at the to that of vault sinking. position where the inclined shaft crosses the main hole so attention should be paid to this position during construction 6. Field Measurement and Analysis and monitoring. .is paper also has analyzed Section A to obtain the horizontal deformation characteristics of the .e complete field monitoring data can provide basic data tunnel surrounding rock under different prestresses. for the implementation of tunnel support, which is an According to the analysis, as the prestress value increases, important guarantee for the consolidation and develop- the horizontal displacement of the tunnel slows down, but ment of tunnel support engineering. Its main purpose is to the trend is not linear. Under the prestressed anchor cable analyze the monitoring data of the stability of the sur- support, due to its unique joint structure, the deformation of rounding rock, master the dynamic and regularity of the the surrounding rock on the left is greater than that on the surrounding rock of the tunnel, provide scientific basis for Advances in Civil Engineering 15

Main tunnel steel arch

Inclined sha

Inclined sha section Main tunnel

(a) (b)

D A B1 B2 E

45° 45°

C1 C2

Section B Section A Figure 27: Construction of intersection and monitoring points: (a) the inclined shaft into the main tunnel and (b) the main tunnel driving.

34 Middle and lower bench 32 excavation on main tunnel 30 28 26 Middle and lower bench 24 excavation on inclined sha 22 20 Upper bench excavation 18 on main tunnel 16 14 Upper bench excavation 12 on inclined sha 10 Deformation value (cm) value Deformation 8 6 4 2 0 0 10 20 30 40 50 60 70 80 Observation time (d)

A measuring point C1 measuring point B1 measuring point C2 measuring point B2 measuring point Figure 28: Deformation of Section A. the daily dynamic management of the tunnel support, test scientific basis through supervision. .e survey data can the rationality of the support structure, design parameters be used as a standard to judge the quality inspection and and construction technology, modify and optimize the acceptance of tunnel engineering. As shown in Figure 27, support parameters and reasonably determine the con- five displacement monitoring points are set near the last struction time of different support measures, and provide row of steel arch frame of the inclined shaft, i.e., Section A: 16 Advances in Civil Engineering

22 20 Middle and lower bench excavation on main tunnel 18 16 Middle and lower bench excavation on inclined sha 14 12 Upper bench excavation 10 on main tunnel 8

Deformation value (cm) value Deformation 6 4 2 0 0 10 20 30 40 50 60 Observation time (d)

D measuring point E measuring point Figure 29: Deformation of Section B.

Middle and lower bench 160 excavation on main tunnel 140 Middle and lower bench 120 excavation on inclined sha 100 Upper bench excavation 80 on main tunnel

60 Stress value (MPa) value Stress 40

0 0 10 20 30 40 50 60 70 80 Observation time (d)

Le arch foot of upper stage Right arch foot of upper stage e vault of upper steps Figure 30: Stress of the steel arch. arch crown, left-arch shoulder, right-arch shoulder, left- third stage of the section, after the completion of the arch waist, and right-arch waist. .e three stress moni- closed-loop, the deformation rate decreases obviously and toring points of steel arch are vault, left-arch shoulder, tends to be stable. However, it has a disturbing eﬀect on and right-arch shoulder. Two observation points of vault Section A, in the process of the lower part excavation of subsidence are arranged on Section B. the main tunnel. As shown in Figure 29, the deformation .e deformation convergence of the surrounding rock of the surrounding rock in Section B tends to be stable in Section A is shown in Figure 28. .e deformation of the after 45days. After the excavation of the upper bench, it surrounding rock tends to be stable at about 65days, and increases rapidly in 0–10days, and in this process, the the deformation of left-arch shoulder is the largest. After crown subsidence is 0.75 cm per day on average. During the upper bench excavation on Section A, the average the excavation of the middle and lower steps in the in- deformation rate of the left spandrel is 0.61 cm/d. When clined shaft and main tunnel, the deformation rate of the the inclined shaft enters the main tunnel, the upper stage surrounding rock increases. After the inverted arch is excavation stage of the main tunnel, the average defor- closed, the deformation of the surrounding rock is mation rate of point B1 of this section is 0.66 cm/d. In the gradually stable. Advances in Civil Engineering 17

.e stress of steel arch in Section A is shown in Figure 30. Data Availability .e stress of the steel frame at the left-arch shoulder is the largest, but it does not exceed the yield strength. After the .e data used to support the findings of this study are installation of the upper steel arch, the stress of the arch available from the corresponding author upon request. increases rapidly. After the excavation of the middle stage, because the steel arch of the upper stage is suspended, the Conflicts of Interest stress of the arch decreases and then increases slowly with the deformation of the surrounding rock. .e excavation of .e authors declare that they have no conflicts of interest. the main tunnel will increase the stress of the steel frame. When the inverted arch is excavated and closed completely, the stress of the steel arch is gradually stable. Acknowledgments .is work was supported in part by the Research on In- telligent Construction Method and Technical System of 7. 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