Analysis of the Cracking Methods to the Secondary Lining in Mountain Tunnel

Yu Li1, He -Bin2，Li -zhe1；Tang Xia1； Lin1 1: PhD candidate, Faculty of engineering, China University of Geosciences， ， ， ， Wuhan Hubei 430071 China [email protected] 2: Center for hydrogeology and environmental geology survey，CGS，， BaoDing 071051;3.Faculty of engineering, China University of Geosciences， Wuhan，Hubei 430071，China

ABSTRACT The secondary lining cracking of the composite lining’s tunnel is more common in mountain tunnels, which affects the stability of the tunnel structure, it has important practical significance of the research methodology. On the basis of studied highway’s 48 mountain tunnel in Hubei Provincial which had the secondary lining cracking, we summed up the research methods of secondary lining cracking .According to the investigation on the fracture site, exploration, design and construction of data collection at cracking section,and and on-site test analysis, we got the primary reason for cracking. In accordance with the Indoor theoretical calculation, numerical simulation and similarity model test method to determine the physical mechanics of the secondary lining cracking reasons from the quantitative aspect, which can provide reference for relevant engineering.

KEYWORDS: composite lining; the secondary lining; mountain tunnels; cracks

INTRODUCTION In recent years, with the rapid development of the transportation, our country completed many tunnels。For the use of NATM method at tunnel structure, preliminary support structure is mainly in the form of anchor and shotcrete support ,composite lining structure is used cast-in-place concrete for the secondary lining。It is a common phenomenon that the secondary lining has cracks during the tunnel construction。It is safety that Some cracks tend to be stable after the cracking, and it is dangerous that some cracks trend in development after the cracking, we will find the reasons of the dangerous cracks and timely treatment to avoid collapse and damage phenomena. Domestic and foreign scholars made some research for the secondary lining。Asakura and Kojima(2003) described the deformation mechanism of lining; Su Sheng(2008) studied on mechanism and crack resistance test of the secondary lining cracking for the highway tunnel; Zhen He (2010) studied on the secondary lining cracking mechanism using

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Vol. 19 [2014], Bund. Z3 10552 fracture mechanics; Wang Rui Feng (2013) optimized design for the secondary lining force structure using Ansys Simulation on the arch tunnel ;Zhang Yong and Jun sheng (2007) analyzed the reasons of the secondary lining on the bias of shallow arch tunnel， illustrated the first tunnel excavation in the mountain inside to prevent the secondary lining cracking; Zhang Lian Cheng and Xiao Bo(2011) analyzed the slope sliding and secondary lining cracking on shallow buried tunnel，illustrated the entrance section of the slope slip which had caused the tunnel’s secondary lining cracking; Cheng Zu Hui and Chen Tie Long(2012) analyzed the reasons of secondary lining cracking form the Construction technology and construction materials, and proposed the corresponding measures; Liu Xue and Gu Yin Cheng(2013) analyzed the process of secondary lining cracking using the finite element numerical simulation under the action of the external force, and got the result of the process and order of tunnel cracks。From the present stage about tunnel’s secondary lining cracks, there are not many papers which had research on them。But in the tunnel construction, it has great significance of providing the system of a new research method for the prevention and control of the secondary lining cracking. For the secondary lining cracking, we must carry on the reason analysis at first。under the guidance of qualitative analysis on the spot investigation，we could adopt necessary test, field monitoring and indoor calculation analysis to analyze the reasons and provide reference basis for prevention and control the secondary lining cracking.

OVERVIEW OF THE SECONDARY LINING CRACKING REASONS The secondary lining cracking of tunnel reason is due to the structure (lining) of load bearing capacity rather than their own. The cause of secondary lining cracking mainly has two aspects: one is the secondary lining structure itself defects lead to cannot bear too much load; and the second is wall-rock pressure bearing capacity of more than the secondary lining structure which is complete. The secondary lining structure itself defects caused by mainly construction materials and construction technology. The secondary lining cracking caused by too much pressure of wall-rock, mainly related to the engineering geological conditions. Based on the above two main reasons, the secondary lining cracking analysis method is also revolves around the two reasons.

ANALYSIS OF THE SECONDARY LINING CRACKING ANALYSIS METHOD Based on surveyed the secondary lining cracking of 48 GuZhu highway tunnels in HuBei province, we summed up out the following research methods of the secondary cracking in mountain tunnel ( fig.1), which can provide a reference for corresponding projects. Vol. 19 [2014], Bund. Z3 10553

The Investigation of the Fracture

(1) Plot the cracks on diagram according to the cracks' position on the secondary lining, analyze the reasons of the cracking and make a record. (2) Collect monitoring and drilling data of the cracks, as well as the data of survey, design, deformation monitoring for the tunnel and the location information of the disasters in construction. (3) Analyze and sum up the characteristics of the secondary lining cracks in this tunnel.

Field Test

(1) Drilling test -- observe the integrity of the secondary lining.If it's intact, It shows that the cracks is mainly caused by the pressure of surrounding rock. (2) Acoustic test -- test the depth of the secondary lining cracks. (3) Crack width monitoring -- plot the cracks width changes on figures, analyse the trend of the fractures. (4) Surrounding rock pressure monitoring -- analyze of the portion of the pressure supported by the secondary lining and its force condition.

Laboratory Test and Calculation

(1) Theoretical calculation: calculate the pressure of surrounding rock and the ultimate bearing capacity of the secondary lining, and make a judgment on the safety of secondary lining. (2) Numerical simulation: observe the stress situation of the secondary lining. Verify the crack position and analyze the destruction process. (3) Similar physical model test: observe the emergence and development of the cracks indoor and analyze the causes.

Vol. 19 [2014], Bund. Z3 10554

1.Crack position

Investigation 2.Collect exploration,design and the cracks construction data of the crack section

3.Summary of fracture characteristics 1.Drilling test The preliminary 2.Acoustic test Analysis of the Field test causes of crack 3.crack width monitor 4.wall-rock pressure monitor

1.Theoretical

Further analysis 2.Numerical Laboratory of its causes

3.The similar material model test

Summarize the reason of cracks, and puts forward the corresponding treatment

Figure 1: The secondary lining cracking’s research methods in mountain tunnel

APPLICATION INSTANCE

Tunnel profile

Ji Jia-Wan tunnel’s left mileage pile number is ZK84 + 975 ~ ZK85 + 670,total length of 695 m,the maximum depth of about 123.1 m. Right mileage pile number is YK84 + 975 ~ YK85 + 655, total length of 680 m, maximum buried depth is about 128.1 m, this tunnel is a separated tunnel.Tunnel site area formation is mainly the Quaternary’s silty clay and the Silurian System’s schist.The tunnel site is not the development of surface water.Supporting form is the composite lining.

Crack investigation

Our group investigated the secondary lining cracking of Ji Jia-Wan tunnel on April 18, 2013,the cracking section pile is ZK84 + 975 ~ ZK85 + 975 (the secondary lining supporting began to crack a year later).Level of wall-rock is grade IV, level of surrounding the hole is grade V ,the secondary supporting type was all concrete lining.A typical of Cracking was the section（ZK85 + 086 ~ ZK85 + 131) ，as shown in Figure 2. There were more obvious 9 cracksin this segment ,extending from the wall to vault oblique.Cracks⑥ and ⑧, ⑦and ⑨ Has a tendency to link up. Vol. 19 [2014], Bund. Z3 10555

Figure 2: tunnel lining cracking site record expansion plan（ ZK85+086~ZK85+131） Cracks①~⑨ statistics were shown in table 1.Crack width was greater than 5 mm, all need for processing, and the development of the crack length in 4 ~ 10 m, estimated depth was greater than 5 cm, and cracks ⑤to⑨ had a development trend. Preliminary analysis of the reason: The secondary lining cracking is caused by the pressure of wall-rock. Cracks Extended from the wall to the vault , crack sidewall got the pressure from the external load at first, its bearing capacity can't resist the external load and produced damage. Table 1: expansion plan cracks characteristics statistics Crack width(m) Length estimated depth(m) Stable or not s (m) ① 0.006 8~9 0.06 Stable ② 0.007 6~7 0.07 Stable ③ 0.006 6~7 0.06 Stable ④ 0.005 6~8 0.05 Stable ⑤ 0.006 8~10 0.06 Not Stable ⑥ 0.005 6 0.05 Not Stable ⑦ 0.008 8 0.08 Not Stable ⑧ 0.006 4 0.06 Not Stable ⑨ 0.009 8 0.09 Not Stable

Field tests （1）Drilling test Analysis of causes of cracking in the secondary lining, we must eliminate cracks in view of the leakage and frost damage at first, because these tunnels had no these problems (located in hubei province, there is no freezing injury; and the secondary lining has no leakage phenomenon). We mainly thought about the integrity of the secondary lining structure. Based on this, we made some drilling near the secondary lining cracking , in order to verify the integrity of the secondary lining structure, as shown in figure 3. From the scene of the experimental ,we know that the secondary lining structure was complete, consistent with the Vol. 19 [2014], Bund. Z3 10556 size of the design. As a result, the tunnel secondary lining cracking should be considered the outside load.

Crack

Figure 3: second lining crack drilling position (2) Crack width monitoring Construction units monitored the crack tendency (figure 4), the monitoring picture of crack ⑨was seen below, we can know that, the first six months of the growth of crack width was smaller, crack growth was obvious after 6 months. According to the scene, at the time of 6 months, The above construction tunnel appeared cracks , and with the arrival of the rainy season, mountain cracks widened, which had great influence on secondary lining cracks widened.

Figure 4: the width of crack pet-name ruby along with the development of the time (3) wall-rock pressure monitoring The burying status of pressure cells between tunnel rock and primary supporting, between primary supporting and secondary lining was shown in fig.5 and each kind of pressure monitoring has five pressure cells, and these pressure cells were arranged symmetrically along the center line of supporting structures, one at vault, two at hunch, one at left side wall and one at right side wall. Vol. 19 [2014], Bund. Z3 10557

Figure 5: the schematic plot of arrangement of pressure cells Through wall rock pressure testing between the tunnel primary support and secondary lining in the field, we obtained the secondary lining pressure share a higher proportion of. The secondary arch rock pressure waist and side wall lining shared approximately 60%~70% of wall rock pressure.

Rock pressure calculation (1) Mechanical calculation According to small clear distance of weak rock tunnel is deduced formula (Li Tao 2014) can be calculated, the result is as flow: Table 2: the calculation result of wall rock pressure coefficient and pressure

lateral pressure Horizontal lateral Vertical pressure coefficient pressure（KPa） （KPa） 0.329 188.964 232.102 0.217 109.101 353.133 0.095 42.548 233.460 0.259 97.776 200.257 Left Cave vertical pressure on the left side of the top is ，inside is 。wall rock pressure empathy right tunnel at the top can also be obtained by the same method to calculate

，the upper and lower are q3 and q4; shows the side of the tunnel corresponding lateral pressure coefficient. The above data shows the left side of the hole left which bear the larger wall rock pressure (bias), which may lead to crack. （2）Numerical Simulation We selected a typical cross-section of the tunnel entrance section k85 +110 for example, the minimum depth of the section is approximately 18m, to build the plane strain model is shown in Figure 7, in order to facilitate the modeling and research, the horseshoe-shaped tunnel has become simplified circular tunnel, the selected model horizontal length of 92m, 50m of vertical range,production-like rock is 215°∠30°, the top surface of a simplified model. Vol. 19 [2014], Bund. Z3 10558

Because of shallow, vertical stress applied to rock gravity (  g-rock unit weight, H-depth),

    the level of stress h 1   (  - Poisson's ratio). The model was applied to both sides of the horizontal displacement constraint, the constraint was applied vertically at the bottom, and the surface was a free surface.

Table 3: Grade V SLATE physical parameters

Modulus of elasticity weight Cohesion Angle of internal Poisson's ratio GPa (kN·m-3) MPa friction/(0)

Rock 1.5 0.40 18 0.15 43

Schistosity 0.06 0.4 -- 0.15 36

Table 4: tunnel lining supporting parameters

Modulus of elasticity weight Cross-sectional area Moment of inertia/(0) Supporting way GPa (kN·m-3) MPa

Anchor system 210.0 78 0.00038 --

the preliminary lining 25.0 23 0.10000 --

The secondary lining 30.0 27 0.3000 0.00225

We used wall rock as elastic-plastic material, Using UDEC to simulate，this Software met Mohr-Coulomb yield criterion, the use of beam elements to simulate the preliminary lining and secondary lining, Simulated bolt by lever unit . Rock parameters and support parameters were showed Table 3 and Table 4. Excavation sequence used for the first excavation hole left, conducted preliminary support and secondary lining when the excavation was completed, then excavated the right hole, we would see the stress state of left hole’s secondary lining.

Figure 6: Principal stress Sxy left hole excavation stress nephogram Overall, the secondary tunnel lining damage was the result of external force. According to the simulation results can be seen in the tunnel,the vault’s wall rock pressure of the left hole left was reached about 0.2MPa, described that here was easier to crack. And it can explained that the main reasons of cracks were terrain bias and rock structure bias.

CONCLUSION According to the actual experience of the secondary lining cracking survey, we summed up a set of tunnel secondary lining cracking methods. According to the survey collection, Vol. 19 [2014], Bund. Z3 10559 design, construction, preliminary data ,we can preliminary analysis reasons of the secondary lining cracking .Then according to the site’s drill test, acoustic testing laboratories, testing and monitoring of crack width and rock pressure monitoring tests, fully grasping the trend of the secondary lining cracks,and analyze the secondary suffered lining’s wall rock pressure changes to lay the foundation for further analysis of the secondary lining cracking reasons.Finally, according to theoretical calculations, numerical simulation , model test and other,we can study on the specific formation mechanism of the secondary lining cracking, for the prevention of tunnel secondary lining provide reference destruction. We selected the JI Jia-wan tunnel described as an example, although not all methods were used in the whole theoretical system, but with a few ways we could explain the reason for the secondary lining cracking that were the rock terrain bias and structural bias, it was recommended to take appropriate the measures to prevent bias load on the second lining,which explained that the set of theoretical methods had practical significance, it could provide a reference for more mountain tunnel.

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