Zhiwen Shen ,Yuanjun Jiang ,Zhiqiang Zhang

a Institute of Mountain Hazards and Environment,CAS,Sichuan,Chengdu,610041,China

b University of Chinese Academy of Sciences.Beijing,100049,China

c Southwest Jiaotong University.Sichuan,Chengdu,610031,China

Keywords:Tunnel Landslide Stability Interaction Safety distance

ABSTRACT At present,substantial scientific research achievements have been made in the research on landslide occurrence,movement mechanism,mitigation measures,and structural stability during tunnel excavation.However,the interaction mechanism of a tunnel under-traversing a slope body with potential landslides is still not well understood.Based on the field data provided by previous investigations in the study area,six sets of 1:100 laboratory experiment model tests were conducted to study the stability of the landslide-prone zone of the slope body with an under-traversing tunnel.The selected distances between the tunnel and the sliding surface are 1.5,3,and 5 times of the tunnel diameter,respectively.The experiment results show the interaction between the landslide-prone zone and the tunnel,elucidating the effect of potential landslides during the tunnel excavation process and the reaction of the landslide slip on the tunnel structure.Several conclusions are obtained:①During the process of tunnel excavation,the vertical displacement of the tunnel vault decreases with the increase of the buried depth.②The vertical displacement of the sliding surface increases with the increase of the buried depth of the tunnel.The horizontal displacement of sliding surface decreases with the increase of the buried depth.③After the occurrence of a rainfall-induced landslide,the vertical displacement of the tunnel vault in the 1.5-diameter-distance case is 57.29%greater than that in the 3.0-dismeter-distance case.④For a two-cave tunnel,it is suggested that the cave farther from the landslide toe should be firstly excavated since it may generate less structural deformation.

1.Introduction

Currently,a large number of researchers have made great achievements in the research on the interaction mechanism between the landslide-prone zone and tunnel excavation.Koji(2008)studied a slope slip caused by tunnel excavation,and summarized the main factors and geological conditions that may result in such disaster in the book"Landslides and Slope Collapse and Its Prevention".Ma(2003)conducted a series of surveys on the interaction of a tunnel-landslide system based on field tests,laboratory model tests and numerical simulation methods with respect to many operating tunnels in mountainous areas in China.The results showed different deformation characteristics of a tunnel in different positions of a landslide-prone slope.Tao et al.(2003,2004),Mao et al.(2002) and Zhang and Zhou (1999) investigated the interaction between the slope deformation and the corresponding tunnel by utilizing model tests with the concern of the soil softening and creeping in the sliding zone.Yamaguchi et al.(1998)analyzed the parameters of the finite element numerical model based on the field data of ground and tunnel deformation,and simulated the settlement and uplifting that occurred on the ground surface,and successfully estimated its displacement in each stage of tunnel construction.Causse et al.(2015) used numerical simulation to study the influence of tunnel construction on the reactivation of landslides and summarized the safety distance between the tunnel and the sliding surface under the slow unloading condition.Selecting a landslide caused by a large-span double-arch tunnel excavation,Zhang et al.(2015) analyzed the characteristics of excavation-induced landslides from the aspects including the different section of the tunnel,construction defects,surrounding rock stability,and landslide conditions.Dai et al.(1999)estimated the interaction between a slope and a newly constructed tunnel through theoretical derivation and numerical simulation,revealing that the distance between the tunnel vault and the sliding surfacemay exert a significant effect on the surrounding rock stress and strain.A large area of plastic zone could seriously affect the stability of a slope.Kim et al.(1989) studied the effects of the stiffness of the tunnel lining on the stability of clay slope based on model test review and finite element numerical simulation,and analyzed the changes in displacement and stress of the existing nearby parallel tunnel linings during shield construction.Jin et al.,(2019)selected Linchang tunnel and Goumawan tunnel as examples to establish the corresponding tunnel-landslide geological model,and studied the surrounding rock pressure and landslide thrust.Finally,the estimation formula of load induced by the landslide thrust on the tunnel was derived.Huang (2019) employed numerical simulation to explore the influence of the tunnel on slope stability with the consideration of the tunnel position,the sliding surface,and the sliding bed.Based on the slip line theory,Zhang et al.(2017a,b)obtained the relationship between the minimum safety distance and the friction angle of the surrounding rock and the slip zone inclination angle.The method analyzed the safety factor of the slope in the case of a tunnel passing through a slope with a minimum safety distance.

The above investigations focused solely on the deformation of the tunnel structure or the stability of the potential landslide zone.Although the studies regarding the influence of tunnel construction on the stability of slope and the interaction mechanism of the "landslide-tunnel"system have also been conducted in recent years,most of which focused on small and medium scale experiments,leading to obvious limitations in the results.Therefore,in this research the loose scope of the surrounding rock,the stability of the slope,and the interaction between the tunnel and the potential landslide zone are studied through laboratory model tests.The experimental results show that a large area of plastic zone is formed in the process of tunnel excavation in a landslide-prone slope,which seriously increases the possibility of potential landslides.The distance between the tunnel vault and the sliding surface has a significant effect on the surrounding rock stress and strain..Thus,this study can provide a scientific reference for the choice of tunnel locations in a slope with a landslide-prone zone.

2.Model test of landslide-tunnel interaction

2.1.Model test device design

The model test can control the boundary conditions as well as monitoring the physical characteristics of the landslide-tunnel interaction in detail.This study uses an experimental model with a scale of 1:100.Based on the similarity theorem,the similarity of each physical parameter in the model test is obtained in Table 1.According to the similarity of physical parameters,the size of the model test tank is determined to be 5.55 m×0.90 m×3.0 m.The model tank is made oflarge section steel frames with high-strength glass on one side for easier observation.In addition,a 1 mm PTFE film is adhered to the inner surface of the entire tank for the purpose of reducing the friction of the tank boundary.The model diagram is shown in Fig.1.

Fig.1. Tunnel and potential landslide layout (a),and sliding surface covered with PTFE (b).

Table 1 Model test similarity.

2.2.Selection of model test materials

According to the prototype parameters of various surrounding rocks in the potential landslide zone,the surrounding rocks are made by mixing several kinds of materials,such as barite powder,quartz sand and waste oil after several times of tests.In addition,quartz sand,talcum powder,sliding agent and water are mixed to simulate the sliding zone soil.The percentage of different mixing materials is shown in Table 2.Table 3 and Table 4 display the prototype physical parameters and model physical parameters of the model test.

Table 2 Mixing ratio of similar materials of surrounding rock.

Table 3 Physical and mechanical parameters of the prototype surrounding rock.

Table 4 Physical and mechanical parameters of the similar materials of surrounding rock.

2.3.Layout of the model test monitoring system

The layout of the measurement points on the experimental tunnel model is shown in Fig.2.The layout of the displacement monitoring system on the surface and inside the landslide is shown in Fig.3.

Fig.2. Tunnel monitoring system.(a)Tunnel displacement monitoring;(b) Step excavation;(c) Layout of the cave circumference and supporting points.

Fig.3. Surface and inner displacement measurement layout.

In Fig.3,the monitoring points DB1-DB8are the surface displacement measurement,among which DB1,DB3,DB5and DB7are the surface horizontal displacement measuring points,while DB2,DB4,DB6and DB8are the surface vertical displacement measuring points.H1–H4are the inner vertical displacement measuring points of the landslide.S1and S2are the vertical displacement measuring points on the tunnel vault.

3.Implementation of the landslide-tunnel model test

3.1.Treatment of experimental boundary conditions

Boundary of landslide toe:Due to the fact that the calculated experimental length of the landslide-prone zone exceeds too much of the length of the experimental tank,only the main body of the landslide which has interaction with the tunnel model is simulated in the experiment.In the experiment,a retaining plate is set in front of the cut landslide toe,by pulling out the retaining plate,the landslide starts to move downward.TEEF film and lubricant are used to reduce the friction generated by the front and rear retaining wall.

3.2.Model test implementation plan

After the test device was installed,the model should be kept still for two days to eliminate the impact of soil settlement.After the experimental model was stabilized,the left and right tunnel caves were successively excavated using the step method,and the deformation data was recorded once per excavation step.During the excavation process,the horizontal and vertical displacements of sliding surface,the horizontal displacement inside the landslide-prone zone and the vertical displacement of the tunnel vault were measured for the cases of tunnel vault and sliding surface distance ranging from 1.5-D,3-D and 5-D (D is tunnel diameter).Consequently,the influence of tunnel construction on the stability of the landslide-prone zone with different vault-surface distance was studied.

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After excavation,the landslide-prone zone was softened by water injection.Meanwhile,the settlement of tunnel vault,the deformation around tunnel and the vertical settlement inside the landslide-prone zone were respectively measured,providing a robust basis for investigating the consequence of landslide disasters sliding with different distances above the tunnel.

4.Analysis of landslide-tunnel interaction results

4.1.Analysis of landslide-prone zone deformation during tunnel excavation

After the landslide-prone zone reached a stable state,the step method was used to excavate tunnels at different distances,1.5-D(Experiment 1),3-D (Experiment 2),and 5-D (Experiment 3).The horizontal displacement,the vertical displacement of the sliding surface and the horizontal displacement of the landslide-prone zone interior show the influence of tunnel excavation,as shown in Figs.4-9

As shown in Fig.4,with the increase of the distance between tunnel vault and sliding surface,the stress induced by soil weight on the tunnel vault increases,and the settlement of the tunnel vault increases gradually.Thus,the vertical displacement in the landslide-prone zone increases gradually.However,as the buried depth of the tunnel increases,the difference between the vertical displacement in the landslide-prone zone above the arch and that in the surrounding rock mass becomes larger and larger due to tunnel excavation.Therefore,we suggest that the effect of tunnel excavation on the stability of the landslide-prone zone decreases with the increase of the distance between the tunnel vault and the sliding surface.

As shown in Fig.5 (a) and (b),the variation trend of horizontal displacement and vertical displacement of the sliding surface caused by tunnel excavation is consistent with that of vertical displacement in the landslide-prone zone.However,the maximum horizontal displacement of the sliding surface is at the point right above the right tunnel vault,while the maximum vertical displacement of the sliding surface is at the point above the left tunnel vault.The reason for that is the horizontal displacement of the sliding surface is accumulated due to the first excavation of the left tunnel and the second excavation of the right tunnel.The vertical displacement of the sliding surface is mainly caused by the excavation after the right tunnel,which further reduces the rigidity of surrounding rock of the left tunnel,resulting in deformation of the left tunnel and the accumulation of displacements.Consequently,the landslide mass is deformed.Therefore,in practical engineering,the tunnel farther from the landslide toe should be excavated before the tunnel closer to the landslide toe.In this way,large deformation of the landslideprone zone could be avoided,and thus the construction safety could be ensured.

In the tunnel excavation process,one tunnel cave was excavated by the length of two times of the tunnel diameter(2-D)in each step.For each cave 4 steps were carried out,and for the two-cave tunnel 8 steps were carried out to complete the excavation.The development of the vertical displacement of the sliding surface and the vertical displacement of the tunnel vault is shown in Fig.6 (a);Fig.6 suggests that when the tunnel vault is 1.5-D below the sliding surface,the vertical displacement of the surface increases with the excavation process and does not converge.Furthermore,for the case that the tunnel vault is 3-D below the sliding surface,when the right tunnel cave was excavated to the length of four times of the tunnel diameter,the vertical displacement of the surface is close to convergence.The final displacement of the 3-D case is about 1/3 of that of the 1.5-D case.When the tunnel vault is 5-D below the sliding surface,after the tunnel excavation is completed,the vertical displacement of the surface is almost zero,indicating that the tunnel excavation has exerted little effect on the stability of the landslide-prone zone.

The monitoring data of surface displacement caused by tunnel excavation is shown in Table 3.

As shown in Table 5,the displacements of the middle and the crest of a landslide-prone zone caused by tunnel excavation are greater than that of the landslide toe.With the increase of tunnel depth,the displacement of a landslide-prone zone decreases.For all the cases (1.5-D,3-D and 5-D),the affected area of the landslide-prone zone is mainly in the middle and crest,as shown in Figs.7–9.When the sliding surface is 1.5-D away from the tunnel vault,the tunnel excavation has caused the entire landslide mass to slide;when the sliding surface is 3-D away from the tunnel vault,the tunnel excavation mainly caused the landslide body that locates directly above the tunnel vault to slide;when the sliding surface is 5-D away from the tunnel vault,the tunnel excavation has almost no impact on the landslide mass.

4.2.Landslide development under rainfall conditions

After the tunnel excavation was completed for two days,a rainfall process was simulated by injecting water in the landslide-prone zone for five times,which consequently led to weakened soil parameters.As shown in Fig.10,the first water injection has led to different displacements of the toe,middle and crest.To be specific,the toe displacement is greater than that of the middle and the crest.Such phenomenon also occurred after the second water injection.After the third water injection,the toe displacement increased rapidly,while the displacements of the middle and the crest only showed small changes.The fourth water injection has suddenly increased the displacement of each measuring point,resulting in the entire collapse of the landslide-prone zone,and the displacement tends to converge after 3 h.After the fifth water injection,the displacement of each measuring point also increased rapidly.Table 4 suggests that the maximum displacement caused by tunnel excavation is 8.4 mm which is much smaller than the displacement associated with the rainfall process(Fig.10).In particular,the deformations of landslide toe are distinctly different in the cases of tunnel excavation and rainfall,corresponding to the smallest and largest deformation,respectively.

Table 5 Partial displacement table of the landslide-prone zone.

Fig.4. Vertical displacement of landslide-prone zone interior during tunnel excavation.

4.3.Impact of landslide sliding on tunnel stability

In the experiment,after the landslide-prone zone was injected with water,the landslide occurred.In this process the earth pressure produced by the landslide on the tunnel and the stability tunnel in terms of bending moment of lining were recorded and displayed in Figs.11 and 12.

As shown in Fig.11,when the landslide sliding surface is 1.5-D away from the tunnel vault,the tunnel earth pressure has increased 100%–200% compared to the situation without landslide;when the landslide sliding surface is 3-D away from the tunnel vault,the tunnel earth pressure has increased by 50%–100%;when the landslide sliding surface is 5-D away from the tunnel vault,the tunnel earth pressure has increased 10%–50%.Therefore,during the rainy season,the earth pressure variations should be observed around the tunnel.In the experiment,the buried depth of the left cave is larger than that of the right cave,and the variation of the earth pressure in the left cave is slightly larger than that in the right cave.In addition,due to the bias effect caused by the slope,the pressure increase on the right side of the tunnel cave is slightly larger than that on the left side.For a real tunnel traversing a landslide-prone slope,the greater the contact pressure between the surrounding rock and the lining,the greater the squeezing load generated by the landslide.Therefore,during the tunnel excavation,in order to ensure the safety of the tunnel,the landslide-prone zone should be monitored in terms of surface displacement and contact pressure on tunnel lining.

As shown in Fig.12,when the sliding surface is 1.5-D away from the tunnel vault,the tunnel bending moment caused by the landslide is the largest and the most complex.Moreover,the bending moment on the right side is significantly greater than that on the left side,which indicates that the landslide can amplify the bias effect.When the sliding surface is 3-D away from the tunnel vault,the bending moment is 40%of that of the 1.5-D case,and the maximum bending moment is on the vault.When the sliding surface is 5-D away from the tunnel vault,the bending moment has almost no change.The landslide produces the largest earth pressure at the vault of the tunnel,and the overall earth pressure decreases as the buried depth of the tunnel increases.

5.Conclusion

When a tunnel traverses a landslide-prone slope orthogonally,the tunnel excavation could break the original stress balance of the potential landslide zone,generating different degrees of displacements and even sliding.The sliding of the landslide will cause further deformation of the tunnel structure.Under such circumstances,the tunnel excavation may lead to the instability of the landslide-prone zone and the failure of tunnel structure.Based on the investigation of geological characteristics in the study area,this study carried out 6 sets of laboratory model tests,analyzed the interaction between the landslide-prone zone and tunnel,studied the effect of tunnel excavation on the landslide-prone zone and the reaction of landslide occurrence on tunnel safety.The main conclusions are as follows:

(1) During tunnel excavation,the vertical displacement of the landslide-prone zone interior and the vertical displacement of the sliding surface increase as the distance between tunnel vault and the sliding surface increases,while the horizontal displacement of the sliding surface decreases with the increase of the distance between tunnel vault and the sliding surface.Therefore,from the perspective of monitoring and measurement,the vertical displacement of the sliding surface should be the key monitoring object during tunnel excavation.

Fig.5. Surface displacement of the landslide-prone zone during tunnel excavation

Fig.6. Diagrams showing the Change of vertical displacement of the sliding surface with tunnel vault displacement.

Fig.7. The scope of the influence of tunnel excavation when 1.5-D distance between tunnel vault and sliding surface is selected.

Fig.8. The scope of the influence of tunnel excavation when 3-D distance between tunnel vault and sliding surface is selected.

Fig.9. The scope of the influence of tunnel excavation when 5-D distance between tunnel vault and sliding surface is selected.

Fig.10. Variation of each displacement point after water injection into the landslide-prone zone.

(2) The part of the landslide-prone zone directly above the tunnel vault is mostly affected by tunnel excavation,and the horizontal displacement of the sliding surface of this part is the most significant.Furthermore,the closer the tunnel to the sliding surface,the greater the horizontal displacement is.Therefore,in the selection of tunnel location in the construction process,the increase of the buried depth of tunnel should be considered.(3)When the tunnel vault is 1.5-D below the sliding surface,the tunnel excavation process has finally caused the failure of the entire landslide-prone zone;when the tunnel vault is 3-D below the sliding surface,the tunnel excavation only resulted in the part of landslide directly above tunnel vault to deform.As the buried depth increases,the vertical displacement of the sliding surface decreases without losing stability.In the case of 5-D,tunnel excavation has only affected the settlement of the tunnel vault and exerted almost no effect on the landslide-prone zone.

Fig.11. The influence of landslide on the earth pressure exerted on the tunnel.

Fig.12. Diagram of tunnel lining bending moment change during a landslide (a) 1.5-D distance between the tunnel vault and the sliding surface;(b) 3-D distance between the tunnel vault and the sliding surface;(c) 5-D distance between the tunnel vault and the sliding surface.

(4) When the tunnel vault is 3-D away from the sliding surface,the tunnel is still in the influence zone of the potential landslide but within the acceptable range.Thus,for a tunnel under-traversing a landslide-prone slope the distance between the tunnel vault and the sliding surface should not be less than 3-D.Additionally,if the slope contains two tunnels,it is suggested that the tunnel farther from the landslide toe should be firstly excavated since it will generate less deformation of the landslide-prone zone.

Acknowledgement

This project is sponsored by the funding of CAS Pioneer Hundred Talents Program.