Analysis of the influence of tunnel portal section construction on slope stability

Abstract High-speed railway tunnel entrance as the basis, combined with the actual engineering geological conditions, the establishment of a dynamic construction of three-dimensional model of tunnel entrance. With the analysis of soil excavated hole portion of the hole and the hole body during deformation movement of surface soil slope. The main conclusions are: (1) on the stepped portion of the hole excavation on the scope of the maximum slope excavation slope deformation rate in step fastest, over time, the deformation gradually converge. (2) along the longitudinal direction of the tunnel, the more monitoring points away from the hole excavation unit distance, the smaller the amount of deformation of the slope obtained its monitoring. By monitoring the cross-section point comparison, the spatial shape deformation monitoring data presented is rounded surface on the axis of the tunnel to tunnel excavation monitoring sites found greater impact, as an extension to both sides, affect the value gradually weakened. (3) by Yang slope settling cloud contrast slope under different cavity length of the inner body can be seen: With the entry of the mountain tunnel and increase the body, affecting Yang slope deformation area also increased to reflect the slope according to the monitoring point. Monitoring data, with the increase of the hole inside the mountain itself, the settlement value of the monitoring points are increased, and finally stabilized.


Introduction
In recent years, China has increased the construction of high-speed railway network. However, in the loess area of western china where is undulating terrain, gully aspect, high, and steep slopes all over. In order to ensure the smooth running of the high-speed railway and the shortening of the mileage, a large number of high-speed railway projects in the area need to build a large number of tunnel projects. Tunnel excavation will damage the original stress state of slope body, and when the stress redistribution can produce stress concentration phenomenon, so it will be a big challenge to slope stability, and it's directly related to the safety of the engineering construction (Liu, Zhang, & Wang, 2010). Up to now, many research workers who major in the tunnel project bring a large number of mathematical analysis methods into the analysis of the stability of the slope such as block limit equilibrium method imbalance thrust method, Principle of red flat projection analysis (Jin, Cui, & Lu, 2000;Xiao, 2001;Yang & Cheng, 2002). Along with the development of computer technology, the researchers also bring the simulation research into the field (Tang, Wang, & Li, 2011;Zhang, Lou, Huang, Liu, & Yang, 2011;Zhao, Shao, & Hang, 2009). A preliminary study of the engineering stability of the Shilou tunnel in the Hipparion red clay were studied by the establishing of finite element numerical model (Peng, Ma, Wang, & Xie, 2013;Wang, Gao, Quan, & Qin, 2014;Wang & Wang, 2009;Wang, Xia, & Gu, 2013;Xie & Wang, 2004). Through the research, it is found that the excavation of tunnel portal will cause the back slope of the slope to sink, and the front part of the tunnel will be moved to the outside of the hole. Therefore, tunnel excavation induced soil collapse accident near the tunnel entrance (Chen, 2015;Wang, 2013;Lu, 2011;Wang, Wang, et al., 2016).
In order to study the influence of excavation and shallow tunnel excavation on slope body, this article sets up a dynamic simulation model which based on the Lan-Shan tunnel entrance and combined with practical engineering geological conditions for the excavation of the tunnel portal. The research results provide suggestions for the engineering support measures of portal slope.

General situation of engineering
Lan-Shan tunnel located at Long-xi loess plateau hilly region which is undulating terrain (Figures 1 and 2). The altitude of the Lan-Shan Mountain ranges from 100 to 570 m and slope degrees varying between 20 and 45.

INWASCON OPEN ACCESS
Tunnel entrance is located in the "V" font gully, but narrow valley. Tunnel mouth exposed bedrock which has weak swelling property. Slope after excavation easily lead to slope instability and tunnel engineering geological conditions are poor. The upper outlet tunnel of Upper Pleistocene of Quaternary Aeolian Sandy Loess has collapsibility, as following.

Geotechnical test research
Strength parameters of rock and earth mass are an important test index, and it related to engineering design, for example tunnel excavation, retaining, and protecting. The mechanical properties of the tunnel surrounding rock and earth mass, and the basis for train tunnel engineering design and construction was obtained through laboratory test and in situ test.

Large direct shear test
Large direct shear test (Figure 3) is in State Key Laboratory of Continental Dynamics based on "Code for Soil Test of Railway Engineering TB10102-2010". The results of large direct shear test ( Figure 4) in laboratory show shear stress increases constantly when shear displacement increases, and before sandy loess samples destroying, shear stress fastly increases with the displacement increasing, but growth rate weakly reduces. After destroying, shear stress slowly increases and growth rate rapidly change, even it has negative growth. The more vertical stress, the more the peak shear stress with increasing shear displacement.

Static triaxial test
In order to study the static characteristics of the sandy loess in the tunnel entrance, the authors made a study on the static three axis undrained consolidation test of the soil samples with different water content in the tunnel section of the tunnel in the typical section of the tunnel. The water content of the test is: natural moisture content, plastic limit water content, plastic limit + 4 water content and saturated water content. The strength and elastic modulus of soil samples obtained from the test are shown in Figure. Figures 5-6 show that the strength of the sand loess is relatively high, the cohesion range of 13-27 kPa under different water content, the internal friction angle of 11-26.5 degrees. The cohesion of sand loess C and internal friction angle φ decreases with the increasing water content, when the water content is more than plastic limit, the strength index out of sand loess happened suddenly change and the amplitude of its decrease increases; when the water content reaches the plastic limit of + 4, reduce the amplitude fall; the DM1 section in the water content reaches the plastic limit when the strength index is not the reason is that the mutated DM1 section of sand loess contains a lot of calcium nodules. The shapes of tuberculosis inlaid in the soil and they are anchoring effect on the soil, so as to improve the strength of soil. The decreases and the strength parameters c, φ of sand soil, and mutation of sand soil showed that water is an important factor affecting the strength of sand loess.

Numerical simulation analysis
This article take Lan-Shan tunnel entrance section of the slope as the analysis object, using finite element geotechnical calculation software MIDAS GTS numerical analysis model is established. Through the numerical simulation results, the influence of the excavation of the tunnel on the slope deformation and the influence of the tunnel on the slope after entering the slope are analyzed.

Numerical simulation model
The research model is established according to the longitudinal section of the actual position of the tunnel portal. According to the field survey; the upper slope where the tunnel inside is sandy loess, lower mudstone, and silty clay in the toe of a pile. The material parameters used in the calculation are combined with the data provided by the geotechnical test, the design side, the regional experience, and the trial calculation, See Table 3. In the supporting system of the tunnel, the length of the anchor rod is 4 m, the diameter of 2.5 cm; the thickness of shotcrete concrete is 24 cm. According to the calculation principle of underground structure, the influence range of tunnel excavation is 3~5 times of the tunnel diameter (Cao & Lu, 2013). Therefore, this calculation model is long, wide, and high, respectively, 110, 100, 90 m; the left and right boundary of the model about the middle line of the tunnel is 50 m. Model in the horizontal to on the slope before and after the left and right on the other two direction constraints, calculation established upward slope model bottom boundary to take full fixed constraints, the model of soil surface for any treatment, in the free state. The calculation model is shown in Figure 10. The numerical simulation of the dynamic excavation of the entrance of the tunnel is carried out by three steps. The total length of the tunnel is 30 m. Every excavation's footage is 2 m, upper, middle, and lower step excavation selection interval for 4 m circular excavation, excavation timely initial support. The initial supporting unit of tunnel is shown in Figure 11. In order to record the deformation of surrounding rock and slope surface in the simulation process, the monitoring points are arranged on the slope surface and the surrounding rock of the tunnel. Slope monitoring points as shown in Figure 12, the 1 to 5 for the slope monitoring points, each monitoring point of the longitudinal interval 4 m.
In Figures 7 and 8, the range of elastic modulus of sand loess content is 57-87 M Pa, at the natural moisture content, it is 29-48 MPa at saturated water content. Under two moisture content, the elastic model of sand loess with confining pressure was positively, its curve is concave upward trend, the greater the confining pressure, the elastic modulus of the faster growth. Elastic modulus of saturated sand loess is decreased by 44%-50% under the two different kinds of moisture content, which also reflects the water sensitive characteristics of sand loess soil.

In-situ shear test
Shear area of in situ shear test sample is 500 mm × 400 mm, and based on geotechnical test specification, when the horizontal crustal stress is 200 kPa or 400 kPa or 600 kPa, the vertical load is estimated, respectively. And then according to estimated C and φ, shear load is also estimated through Mohr-Coulomb equation, respectively. These calculations are showed in Table 1.
Based on the result of in situ shear test (Figure 9), shear stress and shear displacement of samples are in a state of weakly hardening. At the beginning of exerting shear force, shear stress mushroom, but when the shear achieves a certain level and shear strength tends to stable, it shows soil structure strength of sample is larger. Homologous values of C, φ are got based on vertical load and shear capacity of every group.
The shear capacity of every group has more difference, it may be related to sample moisture content. Mudstone structure is dense and itself has more strength. And it has been corroded by groundwater for a long time, soluble salt of mudstone has been dissolved in water, so its strength drops. Based on the test results (Table 2), cohesive force is 65.49 kPa and internal friction angle is 29.7°.

Analysis of displacement and deformation of the excavation slope at the entrance of the tunnel
As shown in Figure 13, the entrance to the tunnel in different excavation stage slope horizontal displacement nephogram (Liu, 2013;Zhang & Kazerooni, 2016). According to the vertical horizontal displacement of the slope, the horizontal displacement of the slope surface is increasing with the excavation of the tunnel entrance. By the contour of the displacement contour, the excavation of the upper stage of the tunnel entrance is very big. After the excavation of the different steps of the entrance, the soil mass is raised by the extrusion of the surrounding soil mass at the bottom of the step. Hole excavation slope horizontal displacement data can be seen in Figure 14, the longitudinal horizontal displacement of each monitoring point of the excavation of the cave increases, but horizontal displacement tend to convergence with time after the step excavation. According to the monitoring point and the location of the excavation, the farther the excavation of the distance, the lesser the horizontal displacement is. From the hole of the nearest 1 monitoring points, the displacement is 1.35 mm; and the displacement of 5 monitoring points is only .45 mm. As shown in Figure 15, tunnel excavation slope vertical settlement Cloud View, with the process of tunnel excavation, vertical settlement nephogram of roughly consistent with the morphology, vertical settlement is mainly concentrated at the top of the tunnel, and steps of the tunnel after excavation, soil at the bottom of the steps, due to the surrounding soil pressure, the uplift phenomenon. Comparison of different excavation stages of the cloud images can be found, in the upper stage of excavation, the slope of the surface of the soil disturbance is the most obvious (Liu, 2010). The data can be seen from the vertical settlement of the slope of the entrance of the tunnel in Figure 16. The distance between the excavation of the tunnel and the excavation of the tunnel is farther, the value of the vertical settlement of the slope is smaller. The vertical settlement of the one monitoring point is 3.19 mm, and the vertical settlement of the 5 monitoring points is only .21 mm. With the passage of time, the horizontal displacement of the longitudinal horizontal displacement tends to converge after the step of the stepped down. In summary, when the slope of the tunnel excavation, the first step into the hillside excavation on slope surface disturbance is very    slope shape is steep, corrosion rear slope shape, when the slope sliding, due to front slope is too steep, no plays the role of slope anti slide, and the rear soil extrusion, so the position of longitudinal horizontal displacement value difference with the other parts. As shown in Figure 19, each monitoring cross-section vertical settlement data distribution diagram can be seen: in each monitoring section settlement data are broadly consistent with the distribution of, into the lower concave arc, settlement along the midline of the tunnel to both sides of the decline. According to the comparison of the settlement of the different parts of the excavation, the settlement of the middle step excavation is increased in all the sections. After the low-step excavation, the settlement rate is reduced, and the settlement value has the trend of convergence. By the arc of the lower concave point of view: with the increase of the excavation section, the greater the arc. The greater the arc, the smaller the influence of the slope surface, and from the other point of view, it also shows that the surface soil mass of the slope is greatly affected by the initial excavation of the tunnel entrance. As shown in Figure 20, different monitoring cross-section vertical settlement data comparison chart reflects: slope surface soil from the mouth of the cave excavation distance closer, the greater amount of settlement, in the center of tunnel monitoring vertical settlement, also shows that with the increase of deep tunnel depth, tunnel excavation on the position of surface soil of the disturbance is small.

Tunnel hole slope deformation release rate
Because of the deformation of the slope, the tunnel buried depth also has a great relationship. Slope type orthogonal tunnel buried depth with tunnel into the hole after gradually increased, when the tunnel in shallow zone, the excavation of the tunnel body will also have an impact on the tunnel entrance slope. So, in this section, the change of the slope of the entrance to the mountain is studied with the tunnel which is excavated in the mountain. As shown in Figure 21, with the hole into the mountain slope after the settlement large, so in practical engineering due to strictly follow the principle of "early into the night".
A monitoring point is set up every 2.5 m on the surface of the slope. Cross-section monitoring span is 20 m. As shown in Figure 17, the longitudinal horizontal displacement of each monitoring section can be seen: the distribution of the monitoring data of each section is generally consistent. At the top of the tunnel, the horizontal displacement of the slope at the top of the slope is larger than that of the other parts of the cross section, and the displacement value of the horizontal displacement is reduced when the horizontal displacement is expanded. Data reflected in the shape of the image are arched. With the increase of the excavation section, the curvature of the arch is increasing, which is caused by the influence of the excavation site on the slope body. The smaller the influence range, the greater the curvature of the cross-section displacement value, which also shows that the influence of the slope is larger when the initial excavation is carried out from the side. As shown in Figure 18, different monitoring cross section of longitudinal horizontal displacement comparison chart can be seen, with longitudinal extension, monitoring pitch tunnel excavation of more far, excavation of the effect is small, arch curvature smaller and smaller. Figure 18 section of monitoring point 1 and 2 monitoring section of longitudinal horizontal displacement value of contrast, found with other sections of the numerical slightly different, which is due to No. 2 arrangement of monitoring points in the uphill slope rate change parts, the anterior  to be stable. According to the monitoring point of tunnel distance, as the distance increases, the settlement data of the monitoring points are reduced. The settlement at the top of the tunnel arch is the largest, and 7.22 mm is achieved. The settlement of the 5 monitoring points, which is the farthest from the tunnel body, is only 2.9 mm, which is 40% of the crown settlement. According to the settlement curve trend can be seen, the beginning of a sedimentation rate increase stage, settlement rate reaches a maximum value, the subsidence rate began to decrease, the final settlement amount of regional stability, as shown in Figure  of the settlement can be seen: with the increase in the tunnel into the mountain, the impact of the deformation area of the slope also increased. According to the cloud pictures, the settlement of the vault is delayed, and the settlement of the soil will increase with the increase of time, and the settlement will be stable. Due to the extrusion of the surrounding soil, the soil at the bottom of the step is raised. As shown in Figure 22, the vertical settlement monitoring data of each monitoring point under different length of the tunnel can be found: with the increase of the internal hole of the mountain, the settlement value of the monitoring points will increase and finally tend  soil subsidence map, as shown in Figure 24. During the continuous period of one-month monitoring period, the settlement of the roof soil reached 8.2 mm, according to the settlement observation data of the following 3 months, the final settlement of the arch crown was converged to 8.4 mm. Compared with the numerical value of the field monitoring and numerical calculation, the settlement of the calculation is slightly less than the field monitoring value of 7.22 mm, and the analysis of the reason is as follows: (1) in the process of calculation and of the surrounding rock stress release coefficient cannot fully with the actual situation match; (2) numerical model establishment process and can't take into account with the passage of time, settlement tends to be stable. To No. 4 monitoring points, for example: 4 monitoring dot tunnel vault vertical distance of 16 m, when a tunnel is excavated hole 14 m, No. 5 monitoring points due to the influence of soil excavation, sedimentation rate began to rise significantly, with the excavation of tunnel body of the time, sedimentation rate first increase after a period of time, and reach the peak, the subsidence rate began to decline (Figure 23 No. 4 monitoring points settlement rate is shown).

Field monitoring results validation
On 16 June 2015, began monitoring of Lan-Shan tunnel entrance slope surface, for the comparison of the numerical calculation results, select opening the vault upper

Conclusions
(1) By comparing the different parts of the excavation hole, the scope of the horizontal displacement monitoring point of slope data, cloud and vertical settlement data, cloud, stepped on the excavation hole slope produced the largest deformation occurs in the biggest stage Sidestep excavation, over time, gradually slowing the rate of deformation.
(2) By comparing data from different monitoring sections slope monitoring points, found along the longitudinal direction of the tunnel, the more monitoring points away from the hole excavation unit distance, the smaller the amount of deformation of the slope obtained its monitoring. By monitoring the cross-section point comparison, the spatial shape deformation monitoring data presented are rounded surface on the axis of the tunnel to tunnel excavation monitoring sites found greater impact, as an extension to both sides, affect the value gradually weakened. (3) According to monitoring data reflect the slope monitoring point of view, with the increase of the hole inside the mountain itself, the settlement value of the monitoring points is increased, and finally stabilized. According to the monitoring body from the tunnel and pitch    of view, as the distance increases, monitoring settlement data points are also decreased. When the tunneling portion pushed close monitoring position, sedimentation rate will increase, after the passage of time, sedimentation rate gradually slowed. In this paper, the results are used in the design of the high-speed railway slope reinforcement, and obtained good effect.