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Dynamic process of the thermal regime of a permafrost tunnel on Tibetan Plateau
Tunnelling and Underground Space Technology 71 (2018) 159–165
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Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Dynamic process of the thermal regime of a permafrost tunnel on Tibetan Plateau
MARK
⁎
Wenbing Yua,b, Yan Lua,c, Fenglei Hana,b, , Yongzhi Liua, Xuefu Zhangb a b c
State Key Laboratory of Frozen Soil Engineering, Northwest Institute of Eco-environment and Resources, CAS, Lanzhou 730000, China College of Civil Engineering, Chongqing Jiaotong University, Chongqing 400074, China University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Tunnel Cold regions Thermal regime Field observation
The thermal status of the rock surrounding tunnel will change during construction and operation periods in cold regions, thereby affecting the stability of tunnel. Field observations of thermal regime of a permafrost tunnel were conducted on Tibetan Plateau. The results show that during the past 12 years, the surrounding rock experienced a warming that subsequently returned to the natural thermal regime, and that was followed by a cooling process. The thermal disturbance distance of the surrounding rock was approximately 30.5 m after the construction. Compared with the air temperature outside the tunnel, heat source of the construction has a great influence on the temperature of the permafrost surrounding the tunnel. The temperature of the permafrost is sinusoidally varying, which achieves a steady state after twelve freeze–thaw cycles. The mean annual ground temperature of the permafrost, and the maximum and minimum temperature amplitudes decreased with time. The heat convection between the air in the tunnel and the surrounding rock during the operation of trains needs to be considered for the design of the permafrost tunnel that could gradually reduce the temperature of the permafrost. The permafrost tunnel could occur to freeze–thaw damage for the sections that lack laid insulation material.
1. Introduction The excavation of tunnels in permafrost regions could inevitably break the original thermal steady state. Then, a new boundary of convection ventilation forms. This change leads to the seasonal freezingthawing in the surrounding rock. The long-term effect of the freezingthawing cycles on the lining structure has a detrimental impact on the stability of the tunnel and its operation (Lai et al., 2009). The understanding of thermal regime of the surrounding rock of tunnels is the key to solve the frost-heave and thaw settlement problems of tunnel in cold regions. Thus far, many studies have been conducted on the thermal regime of the surrounding rock of tunnels by numerical methods. These studies predicted the temperature fields of the surrounding rock by considering heat conduction or convection or seepages or the coupled analyses of these factors (Lai et al., 1999, 2002; He et al., 1999; Zhang et al., 2002, 2004; Lai et al., 2005; Zhang et al., 2006; Tan et al., 2013; Zhou et al., 2016; Wang et al., 2016; Zeng et al., 2017). The stress and strain of surrounding rock are another key issue concerned by engineers. Frost-heave behind the lining because of water gather in winter is the main destructive pressure in cold regions (Lai et al., 1998, 2000; Lai et al., 2000; Gao et al., 2012; Feng et al., 2014). The popular way to ⁎
mitigate the frost heave or thaw settlement of permafrost is to install insulation material behind the lining of tunnels, which has been applied in real engineering and was studied by numerical method (Zhang et al., 2002; Tan et al., 2014; Feng et al., 2016; Li et al., 2017). Insulation gate, electric heat tracing and thermosyphon are also studied to mitigate the frost problems of tunnels in cold regions (Lai and Wu, 2003; Lai et al., 2016; Zhang et al., 2017). By now, there is no field data report to show the real response of the thermal regimes of surrounding rocks to the construction and operation of tunnels in permafrost regions. This paper presents some novel information interpreted from long-term field data obtained from a permafrost tunnel on Tibet Plateau. These findings are informative for tunnel designs in cold regions. 2. Site description and methodology The Fenghuo tunnel is located in the mileage from DK1159+000 to DK1160+338 of Qinghai-Tibet railway in China, whose total length is approximately 1338 m, and whose highest altitude is approximately 4996 m. The excavation of the tunnel began in October 2001, the cutthrough of the tunnel was implemented in October 19, 2002, and the
Corresponding author at: State Key Laboratory of Frozen Soil Engineering, Northwest Institute of Eco-environment and Resources, CAS, Lanzhou 730000, China. E-mail address: [email protected] (F. Han).
http://dx.doi.org/10.1016/j.tust.2017.08.021 Received 24 August 2016; Received in revised form 25 July 2017; Accepted 16 August 2017 0886-7798/ © 2017 Elsevier Ltd. All rights reserved.
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construction of the outer lining structure was completed in August 2003. Moreover, the installation of the railway track was finished in November 2003, and the official opening and operation of trains occurred in July 2006. The Fenghuo Mountain tunnel, whose maximum buried depth is 100 m, and whose design slope in the longitudinal section is 1.2%, consists of a single track and a composite lining structure.
the weathered bedrock layer. The bedrock of the tunnel trunk has low ice content and high ice content permafrost, and its permafrost table is approximately 1.5 m in depth. The ice-rich and ice-saturated permafrost is well developed among the weathered bedrock layer locally, and its thickness ranges between 1.0 and 2.5 m (Zhang et al., 2002, 2006).
2.1. Geological conditions, climate and permafrost
Combined with the Fenghuo Mountain tunnel of the Qinghai-Tibet railway, the observation site is located in the mountaintop at an altitude of approximately 4996 m in order to deeply investigate the long-term thermal regime response of the rock surrounding the permafrost tunnel (Fig. 2). The depth and drilling diameter of the thermometer hole are 100.4 m and 105 mm, respectively. The distance between the thermometer hole and the inlet of the tunnel is 799.9 m in the horizontal direction, while the distance in the vertical direction between the bottom of the thermometer hole and the innermost concrete lining structure is 0.5 m (Fig. 1). The ground and air temperature are respectively monitored by a thermometer cable and a thermometer with a 0.05 °C precision. The layout of the temperature sensors are listed in Table 1. The observation data utilizes an automatic collection system considering the adversely environmental conditions, which is regularly loaded and analyzed. The frequency of observation is once a day. This study adopts monitoring data of the ground temperature collected from December 21, 2002 to March 19, 2014.
2.2. Methodology
The main strata of the Fenghuo Mountain tunnel are the proluvial and colluvial silty clay of the Quaternary Holocene, basal to the tertiary mudstone and sandstone. The proluvial silty clay, whose thickness ranges from 2.1 to 4.0 m, was distributed in the piedmont gentle slope zone of the tunnel inlet and outlet, has a brownish red color, has vegetation coverage, and is partly filled with a silt thin layer. The colluvial silty clay, whose thickness ranges from 1.0 to 2.1 m, was mainly distributed in the hillside and intermountain gentle slope, has a brownish red color, has sparse vegetation, and is filled with a silt thin layer. The primary terrane surrounding the tunnel is sandstone with a mudstone layer, which is well developed for joint fissures. The characteristics of sandstone and mudstone layers are similar in their bedded structure, have a worse diagenesis, and full light weathering. The sandstone layer consists of amaranth, has a fine structure and calcium cementation, while the mudstone layer has a politic texture, an argillaceous cementation, and a brownish red. The depth of the weathered layer at the inlet and outlet sides of the tunnel is generally more than 20 m, while it is 10–15 m in massif. Based on laboratory test, the active layer is a completely weathering layer with σ0 = 200 kPa. σ0 is the uniaxial compressive strength. Correspondingly, the permafrost layer is a weathered layer with σ0 = 400 kPa, and the bedrock is a slightly weathered layer with σ0 = 800 kPa. The Fenghuo Mountain region resembles the climate type of the Tibetan Plateau with snow and ice, which is dry, changeable, with brief spring and autumn seasons, and with low air temperature and air pressure. Based on the meteorological observation data, its annual average air temperature is approximately −6.11 °C. The maximum and minimum average air temperatures in extreme cases are 23.2 °C and −37.7 °C, respectively. The mean annual rainfall and evaporation are 290.9 mm and 1316.9 mm, respectively. The relative air humidity and the maximum wind velocity are 57% and 31 m/s, respectively. The Fenghuo Mountain tunnel, whose entire span is in permafrost, is the plateau permafrost tunnel (Fig. 1). There is no poor permafrost layer surrounding the tunnel except from the ice-rich permafrost, the ice-saturated permafrost, and the ice layer with soil, in its inlet and exit. The permafrost table ranges between 1.2 and 1.5 m in depth in the inlet side. The thicknesses of the ice-rich and the ice-saturated permafrost range from 2.1 to 4.0 m, which has rich content in the silty clay layer and at the interface between the soil and rock. The thickness of the icesaturated permafrost ranges between 1.0 and 2.5 m in the weathered bedrock layer. The permafrost table ranges from 1.45 to 1.8 m in depth in the outlet side. The thickness of the ice-rich and ice saturated permafrost ranges between 1.75 and 4.0 m, which has rich content in
3. Results and analyses 3.1. The geotemperature of the surrounding rock Based on the observed in-situ data of the ground temperature (Fig. 3), the variation of the ground temperature exhibits the typical characteristics of the permafrost. According to the maximum seasonal thaw depth recorded from 2002 to 2012, the permafrost table varied from 2.4 m to 2.9 m. However, the range changed from 2.9 m to 3.4 m in 2013. This change could have resulted from an increase of the mean annual air temperature. The surrounding rock was in a state of complete freezing under a depth of 3.4 m. The mean annual ground temperature was −2.58 °C, which is usually derived from the ground temperature at the depth of 34.4 m, with annual amplitude of ground temperature of 0.1 °C. The temperature distributions of different periods intersect at the depth of 65.4 m, and disperse from 65.4 m to 100.4 m. The change of the ground temperature amplitude for the observation points is equal to, or greater than 0.1 °C, and it ranges between 75.4 m and 100.4 m in depth. The ground temperature is gradually reduced as time progresses and the fluctuation range of the ground temperature increases upon the decrease of the distance between the observation points and the lining structure within the period from 2002 to 2013. This indicates that the tunnel construction and operation can produce a thermal disturbance to the surrounding rock. Based on the field data, the temperature gradient of different time is calculated to illustrate the dynamics process of the thermal regimes of the tunnel’s surrounding rock. It can be divided into three processes
Fig. 1. The longitudinal profile of the Fenghuo tunnel.
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Fig. 2. The monitoring site of the Fenghuo tunnel (photos taken by Yu in 2015).
indicated with letters from A to C ranging from 65.4 m to 100.4 m. It is 0.031 °C/m, 0.014 °C/m, and 0.009 °C/m, respectively. The temperature gradient A is calculated from the curve marked with time 2003, which indicates a warming response of the surrounding rock to the construction of the tunnel. The temperature gradient B is calculated from the curve marked with time 2007, which is similar to the natural ground temperature gradient of the local area. The monitored natural ground temperature gradient at the depth range of 60–100 m is 0.013 °C/m in the Fenghuo Mountain region. The difference between the temperature gradient B and the natural ground temperature gradient is only 0.001 °C/m. The temperature gradient C is calculated from
Table 1 The layout of temperature sensors. Depth (m)
Interval (m)
Amount
0.9–5.4 6.4–10.4 12.4–30.4 34.4–50.4 55.4–100.4
0.5 1.0 2.0 4.0 5.0
10 5 10 5 10
Fig. 3. Temperature profiles of the monitoring site at different time (figure c is the enlarged temperature profiles from 65.4 m to 100.4 m).
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depth. At the depth of 70.4 m, the rate of ground temperature with time was almost 0 °C/d at a state of thermal stability. Therefore, the thermal disturbance distance for the surrounding rock of the permafrost tunnel was 30.5 m, which needed to be considered in the design of the thermal insulation layer. This was consistent with the change range of the temperature gradient. The maximum temperature of the surrounding rock at the depth of 80.4 m is −1.74 °C in June 2004, with an indicative shape change to a parabolic shape from 2002 to 2005. The temperature of the surrounding rock within the range of 15 m around the inner lining has a wavy parabolic variation. Correspondingly, the maximum and minimum values were −1.03 °C and −1.86 °C, respectively. The construction of the outer concrete lining in the tunnel occurred from October 2002 to August 2003. The observation point temperatures attained their maximum values in August 2003. The temperatures increased rapidly from Oct. 2002 to Apr. 2003 and then remained quasisteady from Apr. 2003 to Aug. 2003. The rapid increase stage was caused by the concrete hydration warmer boundary conditions and the stable stage was mainly caused by the insulation material. The specific values are −1.71 °C, −1.53 °C, −1.36 °C, and −1.28 °C, at the depths of 85.4 m, 90.4 m, 95.4 m, and 100.4 m, respectively. The deeper the location of observation points is, the higher the warming rate of the surrounding rock. The maximum warming amplitude is 0.43 °C. Meanwhile, the warming rate of the surrounding rock from April to August 2003 is shown to be lower compared to that recorded from October 2002 to April 2003, with the maximum warming amplitudes of 0.07 °C and 0.36 °C, respectively. Therefore, the temperature of the surrounding rock is mainly influenced by the hydration heat of the castin-situ concrete lining. At the same time, when the atmospheric temperature in October 2002 to November 2003 is in the first freezing–thawing cycle, the monitoring data indicates that the temperature of the surrounding rock is increased. The surrounding rock response could be fast to the heat source of construction compared to the air temperature outside the tunnel. After the construction in the tunnel is completed, the thermal insulation material could partly prevent the heat transfer of the construction source that produces the response lag of the surrounding rock temperature. It is necessary that the control of the environmental temperature in the tunnel and the selection of the thickness of the thermal insulation material are both considered for the design and maintenance of the permafrost tunnel. The construction of railway track was from August and November 2003, and the tunnel was ready for operation during the period between November 2003 and October 2005. The temperature distribution of the surrounding rock firstly exhibited a parabolic variation followed by a subsequent linear reduction. Maximum temperature and warming amplitudes were −1.03 °C and 0.26 °C, respectively. The increase of the air temperature in the tunnel caused by mechanical and human activities produced the warming response of the surrounding rock by means
curve marked with time 2013. It is smaller than the undisturbed natural ground temperature gradient. This temperature gradient indicates an overcooling phenomenon of the tunnel’s surrounding rock due to the air convection after operation. The final temperature of the surrounding rock is cooler than the normal natural ground temperature. The curve marked with date Oct. 21th, 2002, which is three days later after the cut-through of the tunnel, has a temperature gradient 0.016 °C/m. The data of this date indicated a slight warm disturbance of the thermal status of the surrounding rocks after about half a year’s construction. According to the temperature curve of the borehole along the Qinghai-Tibet highway in the permafrost regions, the ground temperature variation through the permafrost base has the characteristic of a polygonal line, whose slope decreases as depth increases (Xu et al., 2001). From Fig. 3(a), the temperature gradient gradually decreases with time, and the surrounding rock continuously cools after the tunnel construction and operation. From Fig. 3(b), the ground temperature increases linearly at a rate of 0.0131 °C/m at the depth range of 34.4–75.4 m, and at a rate of 0.0148 °C/m at the depth range of 75.4–100.4 m. Therefore, results show that the ground temperature of the surrounding rock increases during the construction, recover after cutthrough and eventually decreases during the operation. The thermal insulation layer paved between the inner and outer linings incompletely prevents heat transfer, and the ground temperature of the surrounding rock follows a process of dynamic reduction upon operation with the running of trains. From Fig. 4, the mean annual air temperature of the 799.9 m section distance from the tunnel inlet is −3.10 °C. The homologous ground temperature of the surrounding rock at the depth 100.4 m is −1.67 °C. The opening of the tunnel accelerates heat convection between the surrounding rock and the air, which gradually increases the accumulation of the cooling energy with time. That is the main reason for the observed reduction of the temperature gradient and the ground temperature from 2002 to 2013. 3.2. The temperature distribution of the surrounding rock in construction Fig. 5 indicates that the surrounding rock within a distance of 30.5 m around the inner lining is mainly less than −1 °C in a completely frozen state. Owing to the excavation of the perimeter blasting, the hydration heat of the cast-in-situ concrete lining, and other artificial activities, the frozen surrounding rock could be thawed (Huang and Su, 2005). From this figure, it can be seen that the thawed range around the inner lining could be absolutely refrozen. However, the change of the air temperature in the tunnel has a significant impact on the temperature distribution of the surrounding rock. The extent of the thermal disturbance of the surrounding rock caused by the construction of the tunnel gradually strengthened with an increase of the measuring point
Fig. 4. In-situ measured mean annual air temperatures at different sections (Zhang et al., 2006).
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Fig. 5. The temperature distribution of the monitoring points.
temperature amplitudes of the monitoring points. For a depth of 75.4 m, the ground temperature is approximately linearly reduced, and it is not easy to be affected by the ambient temperature. The difference between the maximum and minimum values for the different monitoring points with respect to depth decreases with time, and the attained maximum and minimum values are 0.20 °C in 2007 and 0.01 °C in 2013 for a freezing-thawing cycle, respectively. The maximum and minimum temperatures at the depth of 100.4 m is −1.53 °C in February 2007 and −1.93 °C in September 2013, respectively, and the difference between them is 0.4 °C. Compared to the construction in the tunnel, the heat transfer between the surrounding rock and air in the tunnel is a long-term process and slow in operation during the twelve freezingthawing cycles within the period from 2003 to 2014. The temperature of the surrounding rock is in a state of periodic cooling. The mean annual temperature of the surrounding rock for the monitoring points is linearly reduced as shown in Fig. 9. Its slope gradually increases at increasing depths. It shows an unobvious correction between the outside air temperature and the surrounding rock’s temperature. In 2009 and 2012, when the outside air temperature decreased obviously, the surrounding rock’s temperature dropped accordingly. And when the outside air temperature increased in 2013, the surrounding rock’s temperature increased accordingly. The air temperature rise from −8 °C to 0–2 °C under conditions of thermal insulation and upon door shutting, can demonstrate that air convection is a dominant factor deterministic of the environmental temperature (Lai et al., 2005). Although the monitoring section is far from the inlet of the permafrost tunnel, the operation of the train enhances the convective heat transfer between the surrounding rock and the air in the tunnel.
of the heat convection. The temperature and the slope of the curve for the surrounding rock in regard to the track installation are lower compared to the lining construction. This is manifested by the hysteresis effect during the period of these three months. Fig. 9 shows a parabolic variation from Aug. 2003 to Sep. 2004, a slow linear reduction till Feb. 2005, a rapid linear reduction till Sep. 2005 and a stable stage after that. The corresponding cooling rates are 0.0011 °C/d, 0.0002 °C/d, and 0.0010 °C/d, respectively. The temperature increasing after August is mainly because of the hysteresis effect of surrounding rock’s thermal response to the construction. The cooling and stable process can be explained by the existing of the large permafrost geologic body. Therefore, the thermal response of the surrounding rock caused by the tunnel construction led to a gradual refreezing as a result of the cooling capacity transfer of the permafrost itself. Thus, it can be seen that the unfavorable influence of construction in the tunnel could be recovered with time. 3.3. The temperature distribution of the surrounding rock in operation From Figs. 6 and 7 it can be seen that the temperature of the surrounding rock in regard to the operation of the tunnel from July 2006 to March 2014 is a sine curve, with a cycle period of 14 months. The maximum and minimum values occur in February and September of each year, which is contrary to the change of the air temperature distribution outside the tunnel (Fig. 8). The maximum and minimum temperature amplitudes of the surrounding rock for the monitoring points gradually reduce in each of the freezing-thawing cycles. The closer the monitoring point is to the inner lining, the larger the range of
Fig. 6. The temperature distribution of monitoring points in operation.
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Fig. 7. The temperature distribution of the monitoring points in 2010.
Fig. 8. The air temperature curves of the inlet of the Fenghuo tunnel.
Fig. 9. The mean annual air temperature and the variation of the monitoring point temperatures.
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References
Furthermore, the freezing index is greater than the thawing index in the permafrost regions, which is owing to the reduction of the temperature of the surrounding rock throughout the year during the process of cyclical changes. In fact, the relationship between the surrounding rock’s temperature and the air temperature is affect by air convection speed, which is determined by running train.
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4. Conclusions The construction of the tunnel in the permafrost region inevitably affects the thermal regime of the surrounding rock. According to the long-term field monitoring data of the ground temperature in a deep borehole above the tunnel vault on Tibet Plateau, the analyses of the thermal regime of the surrounding rock leads to the following conclusions: (1) The thermal disturbance distance of the surrounding rock of the tunnel was approximately 30.5 m around the tunnel vault. The thermal response of the surrounding rock to the construction was temporary and more sensitive, while the thermal regime response to the ambient temperature outside the tunnel was long-term and slow. The formerly affected temperature distribution of the surrounding rock was more obvious than the latter. Therefore, the control of the air temperature in the tunnel should be considered for the construction of the tunnel. (2) The temperature distribution of the surrounding rock achieved a relative steady state after at least twelve freezing-thawing cycles during the operating, in accordance to the change of a temporally varying periodic sine curve. The maximum and minimum temperature amplitudes gradually decreased with time, and the temperature fluctuation strengthened with depth increasing. (3) The heat inputted into the surrounding rock during the construction was cooled by the permafrost and the heat convection between the surrounding rock and the air in the tunnel. (4) The surrounding rock of the tunnel experienced a warming process during its construction. It returned to the undisturbed status after the operation, and then the cooling process continued after the natural status was reached during the past 10 years. The findings of the overcooling phenomena of the surrounding rock are informative for the design of frost heave mitigation of the tunnel in cold regions. The insulation plate, the running speed controlling of vehicle in tunnel, the insulation gate and electric heating methods should be considered. Acknowledgement This study was supported by the National Natural Science Fund (41571070), the Fund of SKLFS (52Y652H21), the Fund of the National Key Basic Research and Development Program (2012CB026102), the Funds of Key Research Program of Frontier Sciences of CAS (QYZDYSSW-DQC015, QYZDY-SSW-DQC011,) and fund HHS-TSS-STS-1502. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tust.2017.08.021.
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Dynamic process of the thermal regime of a permafrost tunnel on Tibetan Plateau
MARK
⁎
Wenbing Yua,b, Yan Lua,c, Fenglei Hana,b, , Yongzhi Liua, Xuefu Zhangb a b c
State Key Laboratory of Frozen Soil Engineering, Northwest Institute of Eco-environment and Resources, CAS, Lanzhou 730000, China College of Civil Engineering, Chongqing Jiaotong University, Chongqing 400074, China University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Tunnel Cold regions Thermal regime Field observation
The thermal status of the rock surrounding tunnel will change during construction and operation periods in cold regions, thereby affecting the stability of tunnel. Field observations of thermal regime of a permafrost tunnel were conducted on Tibetan Plateau. The results show that during the past 12 years, the surrounding rock experienced a warming that subsequently returned to the natural thermal regime, and that was followed by a cooling process. The thermal disturbance distance of the surrounding rock was approximately 30.5 m after the construction. Compared with the air temperature outside the tunnel, heat source of the construction has a great influence on the temperature of the permafrost surrounding the tunnel. The temperature of the permafrost is sinusoidally varying, which achieves a steady state after twelve freeze–thaw cycles. The mean annual ground temperature of the permafrost, and the maximum and minimum temperature amplitudes decreased with time. The heat convection between the air in the tunnel and the surrounding rock during the operation of trains needs to be considered for the design of the permafrost tunnel that could gradually reduce the temperature of the permafrost. The permafrost tunnel could occur to freeze–thaw damage for the sections that lack laid insulation material.
1. Introduction The excavation of tunnels in permafrost regions could inevitably break the original thermal steady state. Then, a new boundary of convection ventilation forms. This change leads to the seasonal freezingthawing in the surrounding rock. The long-term effect of the freezingthawing cycles on the lining structure has a detrimental impact on the stability of the tunnel and its operation (Lai et al., 2009). The understanding of thermal regime of the surrounding rock of tunnels is the key to solve the frost-heave and thaw settlement problems of tunnel in cold regions. Thus far, many studies have been conducted on the thermal regime of the surrounding rock of tunnels by numerical methods. These studies predicted the temperature fields of the surrounding rock by considering heat conduction or convection or seepages or the coupled analyses of these factors (Lai et al., 1999, 2002; He et al., 1999; Zhang et al., 2002, 2004; Lai et al., 2005; Zhang et al., 2006; Tan et al., 2013; Zhou et al., 2016; Wang et al., 2016; Zeng et al., 2017). The stress and strain of surrounding rock are another key issue concerned by engineers. Frost-heave behind the lining because of water gather in winter is the main destructive pressure in cold regions (Lai et al., 1998, 2000; Lai et al., 2000; Gao et al., 2012; Feng et al., 2014). The popular way to ⁎
mitigate the frost heave or thaw settlement of permafrost is to install insulation material behind the lining of tunnels, which has been applied in real engineering and was studied by numerical method (Zhang et al., 2002; Tan et al., 2014; Feng et al., 2016; Li et al., 2017). Insulation gate, electric heat tracing and thermosyphon are also studied to mitigate the frost problems of tunnels in cold regions (Lai and Wu, 2003; Lai et al., 2016; Zhang et al., 2017). By now, there is no field data report to show the real response of the thermal regimes of surrounding rocks to the construction and operation of tunnels in permafrost regions. This paper presents some novel information interpreted from long-term field data obtained from a permafrost tunnel on Tibet Plateau. These findings are informative for tunnel designs in cold regions. 2. Site description and methodology The Fenghuo tunnel is located in the mileage from DK1159+000 to DK1160+338 of Qinghai-Tibet railway in China, whose total length is approximately 1338 m, and whose highest altitude is approximately 4996 m. The excavation of the tunnel began in October 2001, the cutthrough of the tunnel was implemented in October 19, 2002, and the
Corresponding author at: State Key Laboratory of Frozen Soil Engineering, Northwest Institute of Eco-environment and Resources, CAS, Lanzhou 730000, China. E-mail address: [email protected] (F. Han).
http://dx.doi.org/10.1016/j.tust.2017.08.021 Received 24 August 2016; Received in revised form 25 July 2017; Accepted 16 August 2017 0886-7798/ © 2017 Elsevier Ltd. All rights reserved.
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construction of the outer lining structure was completed in August 2003. Moreover, the installation of the railway track was finished in November 2003, and the official opening and operation of trains occurred in July 2006. The Fenghuo Mountain tunnel, whose maximum buried depth is 100 m, and whose design slope in the longitudinal section is 1.2%, consists of a single track and a composite lining structure.
the weathered bedrock layer. The bedrock of the tunnel trunk has low ice content and high ice content permafrost, and its permafrost table is approximately 1.5 m in depth. The ice-rich and ice-saturated permafrost is well developed among the weathered bedrock layer locally, and its thickness ranges between 1.0 and 2.5 m (Zhang et al., 2002, 2006).
2.1. Geological conditions, climate and permafrost
Combined with the Fenghuo Mountain tunnel of the Qinghai-Tibet railway, the observation site is located in the mountaintop at an altitude of approximately 4996 m in order to deeply investigate the long-term thermal regime response of the rock surrounding the permafrost tunnel (Fig. 2). The depth and drilling diameter of the thermometer hole are 100.4 m and 105 mm, respectively. The distance between the thermometer hole and the inlet of the tunnel is 799.9 m in the horizontal direction, while the distance in the vertical direction between the bottom of the thermometer hole and the innermost concrete lining structure is 0.5 m (Fig. 1). The ground and air temperature are respectively monitored by a thermometer cable and a thermometer with a 0.05 °C precision. The layout of the temperature sensors are listed in Table 1. The observation data utilizes an automatic collection system considering the adversely environmental conditions, which is regularly loaded and analyzed. The frequency of observation is once a day. This study adopts monitoring data of the ground temperature collected from December 21, 2002 to March 19, 2014.
2.2. Methodology
The main strata of the Fenghuo Mountain tunnel are the proluvial and colluvial silty clay of the Quaternary Holocene, basal to the tertiary mudstone and sandstone. The proluvial silty clay, whose thickness ranges from 2.1 to 4.0 m, was distributed in the piedmont gentle slope zone of the tunnel inlet and outlet, has a brownish red color, has vegetation coverage, and is partly filled with a silt thin layer. The colluvial silty clay, whose thickness ranges from 1.0 to 2.1 m, was mainly distributed in the hillside and intermountain gentle slope, has a brownish red color, has sparse vegetation, and is filled with a silt thin layer. The primary terrane surrounding the tunnel is sandstone with a mudstone layer, which is well developed for joint fissures. The characteristics of sandstone and mudstone layers are similar in their bedded structure, have a worse diagenesis, and full light weathering. The sandstone layer consists of amaranth, has a fine structure and calcium cementation, while the mudstone layer has a politic texture, an argillaceous cementation, and a brownish red. The depth of the weathered layer at the inlet and outlet sides of the tunnel is generally more than 20 m, while it is 10–15 m in massif. Based on laboratory test, the active layer is a completely weathering layer with σ0 = 200 kPa. σ0 is the uniaxial compressive strength. Correspondingly, the permafrost layer is a weathered layer with σ0 = 400 kPa, and the bedrock is a slightly weathered layer with σ0 = 800 kPa. The Fenghuo Mountain region resembles the climate type of the Tibetan Plateau with snow and ice, which is dry, changeable, with brief spring and autumn seasons, and with low air temperature and air pressure. Based on the meteorological observation data, its annual average air temperature is approximately −6.11 °C. The maximum and minimum average air temperatures in extreme cases are 23.2 °C and −37.7 °C, respectively. The mean annual rainfall and evaporation are 290.9 mm and 1316.9 mm, respectively. The relative air humidity and the maximum wind velocity are 57% and 31 m/s, respectively. The Fenghuo Mountain tunnel, whose entire span is in permafrost, is the plateau permafrost tunnel (Fig. 1). There is no poor permafrost layer surrounding the tunnel except from the ice-rich permafrost, the ice-saturated permafrost, and the ice layer with soil, in its inlet and exit. The permafrost table ranges between 1.2 and 1.5 m in depth in the inlet side. The thicknesses of the ice-rich and the ice-saturated permafrost range from 2.1 to 4.0 m, which has rich content in the silty clay layer and at the interface between the soil and rock. The thickness of the icesaturated permafrost ranges between 1.0 and 2.5 m in the weathered bedrock layer. The permafrost table ranges from 1.45 to 1.8 m in depth in the outlet side. The thickness of the ice-rich and ice saturated permafrost ranges between 1.75 and 4.0 m, which has rich content in
3. Results and analyses 3.1. The geotemperature of the surrounding rock Based on the observed in-situ data of the ground temperature (Fig. 3), the variation of the ground temperature exhibits the typical characteristics of the permafrost. According to the maximum seasonal thaw depth recorded from 2002 to 2012, the permafrost table varied from 2.4 m to 2.9 m. However, the range changed from 2.9 m to 3.4 m in 2013. This change could have resulted from an increase of the mean annual air temperature. The surrounding rock was in a state of complete freezing under a depth of 3.4 m. The mean annual ground temperature was −2.58 °C, which is usually derived from the ground temperature at the depth of 34.4 m, with annual amplitude of ground temperature of 0.1 °C. The temperature distributions of different periods intersect at the depth of 65.4 m, and disperse from 65.4 m to 100.4 m. The change of the ground temperature amplitude for the observation points is equal to, or greater than 0.1 °C, and it ranges between 75.4 m and 100.4 m in depth. The ground temperature is gradually reduced as time progresses and the fluctuation range of the ground temperature increases upon the decrease of the distance between the observation points and the lining structure within the period from 2002 to 2013. This indicates that the tunnel construction and operation can produce a thermal disturbance to the surrounding rock. Based on the field data, the temperature gradient of different time is calculated to illustrate the dynamics process of the thermal regimes of the tunnel’s surrounding rock. It can be divided into three processes
Fig. 1. The longitudinal profile of the Fenghuo tunnel.
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Fig. 2. The monitoring site of the Fenghuo tunnel (photos taken by Yu in 2015).
indicated with letters from A to C ranging from 65.4 m to 100.4 m. It is 0.031 °C/m, 0.014 °C/m, and 0.009 °C/m, respectively. The temperature gradient A is calculated from the curve marked with time 2003, which indicates a warming response of the surrounding rock to the construction of the tunnel. The temperature gradient B is calculated from the curve marked with time 2007, which is similar to the natural ground temperature gradient of the local area. The monitored natural ground temperature gradient at the depth range of 60–100 m is 0.013 °C/m in the Fenghuo Mountain region. The difference between the temperature gradient B and the natural ground temperature gradient is only 0.001 °C/m. The temperature gradient C is calculated from
Table 1 The layout of temperature sensors. Depth (m)
Interval (m)
Amount
0.9–5.4 6.4–10.4 12.4–30.4 34.4–50.4 55.4–100.4
0.5 1.0 2.0 4.0 5.0
10 5 10 5 10
Fig. 3. Temperature profiles of the monitoring site at different time (figure c is the enlarged temperature profiles from 65.4 m to 100.4 m).
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depth. At the depth of 70.4 m, the rate of ground temperature with time was almost 0 °C/d at a state of thermal stability. Therefore, the thermal disturbance distance for the surrounding rock of the permafrost tunnel was 30.5 m, which needed to be considered in the design of the thermal insulation layer. This was consistent with the change range of the temperature gradient. The maximum temperature of the surrounding rock at the depth of 80.4 m is −1.74 °C in June 2004, with an indicative shape change to a parabolic shape from 2002 to 2005. The temperature of the surrounding rock within the range of 15 m around the inner lining has a wavy parabolic variation. Correspondingly, the maximum and minimum values were −1.03 °C and −1.86 °C, respectively. The construction of the outer concrete lining in the tunnel occurred from October 2002 to August 2003. The observation point temperatures attained their maximum values in August 2003. The temperatures increased rapidly from Oct. 2002 to Apr. 2003 and then remained quasisteady from Apr. 2003 to Aug. 2003. The rapid increase stage was caused by the concrete hydration warmer boundary conditions and the stable stage was mainly caused by the insulation material. The specific values are −1.71 °C, −1.53 °C, −1.36 °C, and −1.28 °C, at the depths of 85.4 m, 90.4 m, 95.4 m, and 100.4 m, respectively. The deeper the location of observation points is, the higher the warming rate of the surrounding rock. The maximum warming amplitude is 0.43 °C. Meanwhile, the warming rate of the surrounding rock from April to August 2003 is shown to be lower compared to that recorded from October 2002 to April 2003, with the maximum warming amplitudes of 0.07 °C and 0.36 °C, respectively. Therefore, the temperature of the surrounding rock is mainly influenced by the hydration heat of the castin-situ concrete lining. At the same time, when the atmospheric temperature in October 2002 to November 2003 is in the first freezing–thawing cycle, the monitoring data indicates that the temperature of the surrounding rock is increased. The surrounding rock response could be fast to the heat source of construction compared to the air temperature outside the tunnel. After the construction in the tunnel is completed, the thermal insulation material could partly prevent the heat transfer of the construction source that produces the response lag of the surrounding rock temperature. It is necessary that the control of the environmental temperature in the tunnel and the selection of the thickness of the thermal insulation material are both considered for the design and maintenance of the permafrost tunnel. The construction of railway track was from August and November 2003, and the tunnel was ready for operation during the period between November 2003 and October 2005. The temperature distribution of the surrounding rock firstly exhibited a parabolic variation followed by a subsequent linear reduction. Maximum temperature and warming amplitudes were −1.03 °C and 0.26 °C, respectively. The increase of the air temperature in the tunnel caused by mechanical and human activities produced the warming response of the surrounding rock by means
curve marked with time 2013. It is smaller than the undisturbed natural ground temperature gradient. This temperature gradient indicates an overcooling phenomenon of the tunnel’s surrounding rock due to the air convection after operation. The final temperature of the surrounding rock is cooler than the normal natural ground temperature. The curve marked with date Oct. 21th, 2002, which is three days later after the cut-through of the tunnel, has a temperature gradient 0.016 °C/m. The data of this date indicated a slight warm disturbance of the thermal status of the surrounding rocks after about half a year’s construction. According to the temperature curve of the borehole along the Qinghai-Tibet highway in the permafrost regions, the ground temperature variation through the permafrost base has the characteristic of a polygonal line, whose slope decreases as depth increases (Xu et al., 2001). From Fig. 3(a), the temperature gradient gradually decreases with time, and the surrounding rock continuously cools after the tunnel construction and operation. From Fig. 3(b), the ground temperature increases linearly at a rate of 0.0131 °C/m at the depth range of 34.4–75.4 m, and at a rate of 0.0148 °C/m at the depth range of 75.4–100.4 m. Therefore, results show that the ground temperature of the surrounding rock increases during the construction, recover after cutthrough and eventually decreases during the operation. The thermal insulation layer paved between the inner and outer linings incompletely prevents heat transfer, and the ground temperature of the surrounding rock follows a process of dynamic reduction upon operation with the running of trains. From Fig. 4, the mean annual air temperature of the 799.9 m section distance from the tunnel inlet is −3.10 °C. The homologous ground temperature of the surrounding rock at the depth 100.4 m is −1.67 °C. The opening of the tunnel accelerates heat convection between the surrounding rock and the air, which gradually increases the accumulation of the cooling energy with time. That is the main reason for the observed reduction of the temperature gradient and the ground temperature from 2002 to 2013. 3.2. The temperature distribution of the surrounding rock in construction Fig. 5 indicates that the surrounding rock within a distance of 30.5 m around the inner lining is mainly less than −1 °C in a completely frozen state. Owing to the excavation of the perimeter blasting, the hydration heat of the cast-in-situ concrete lining, and other artificial activities, the frozen surrounding rock could be thawed (Huang and Su, 2005). From this figure, it can be seen that the thawed range around the inner lining could be absolutely refrozen. However, the change of the air temperature in the tunnel has a significant impact on the temperature distribution of the surrounding rock. The extent of the thermal disturbance of the surrounding rock caused by the construction of the tunnel gradually strengthened with an increase of the measuring point
Fig. 4. In-situ measured mean annual air temperatures at different sections (Zhang et al., 2006).
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Fig. 5. The temperature distribution of the monitoring points.
temperature amplitudes of the monitoring points. For a depth of 75.4 m, the ground temperature is approximately linearly reduced, and it is not easy to be affected by the ambient temperature. The difference between the maximum and minimum values for the different monitoring points with respect to depth decreases with time, and the attained maximum and minimum values are 0.20 °C in 2007 and 0.01 °C in 2013 for a freezing-thawing cycle, respectively. The maximum and minimum temperatures at the depth of 100.4 m is −1.53 °C in February 2007 and −1.93 °C in September 2013, respectively, and the difference between them is 0.4 °C. Compared to the construction in the tunnel, the heat transfer between the surrounding rock and air in the tunnel is a long-term process and slow in operation during the twelve freezingthawing cycles within the period from 2003 to 2014. The temperature of the surrounding rock is in a state of periodic cooling. The mean annual temperature of the surrounding rock for the monitoring points is linearly reduced as shown in Fig. 9. Its slope gradually increases at increasing depths. It shows an unobvious correction between the outside air temperature and the surrounding rock’s temperature. In 2009 and 2012, when the outside air temperature decreased obviously, the surrounding rock’s temperature dropped accordingly. And when the outside air temperature increased in 2013, the surrounding rock’s temperature increased accordingly. The air temperature rise from −8 °C to 0–2 °C under conditions of thermal insulation and upon door shutting, can demonstrate that air convection is a dominant factor deterministic of the environmental temperature (Lai et al., 2005). Although the monitoring section is far from the inlet of the permafrost tunnel, the operation of the train enhances the convective heat transfer between the surrounding rock and the air in the tunnel.
of the heat convection. The temperature and the slope of the curve for the surrounding rock in regard to the track installation are lower compared to the lining construction. This is manifested by the hysteresis effect during the period of these three months. Fig. 9 shows a parabolic variation from Aug. 2003 to Sep. 2004, a slow linear reduction till Feb. 2005, a rapid linear reduction till Sep. 2005 and a stable stage after that. The corresponding cooling rates are 0.0011 °C/d, 0.0002 °C/d, and 0.0010 °C/d, respectively. The temperature increasing after August is mainly because of the hysteresis effect of surrounding rock’s thermal response to the construction. The cooling and stable process can be explained by the existing of the large permafrost geologic body. Therefore, the thermal response of the surrounding rock caused by the tunnel construction led to a gradual refreezing as a result of the cooling capacity transfer of the permafrost itself. Thus, it can be seen that the unfavorable influence of construction in the tunnel could be recovered with time. 3.3. The temperature distribution of the surrounding rock in operation From Figs. 6 and 7 it can be seen that the temperature of the surrounding rock in regard to the operation of the tunnel from July 2006 to March 2014 is a sine curve, with a cycle period of 14 months. The maximum and minimum values occur in February and September of each year, which is contrary to the change of the air temperature distribution outside the tunnel (Fig. 8). The maximum and minimum temperature amplitudes of the surrounding rock for the monitoring points gradually reduce in each of the freezing-thawing cycles. The closer the monitoring point is to the inner lining, the larger the range of
Fig. 6. The temperature distribution of monitoring points in operation.
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Fig. 7. The temperature distribution of the monitoring points in 2010.
Fig. 8. The air temperature curves of the inlet of the Fenghuo tunnel.
Fig. 9. The mean annual air temperature and the variation of the monitoring point temperatures.
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References
Furthermore, the freezing index is greater than the thawing index in the permafrost regions, which is owing to the reduction of the temperature of the surrounding rock throughout the year during the process of cyclical changes. In fact, the relationship between the surrounding rock’s temperature and the air temperature is affect by air convection speed, which is determined by running train.
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4. Conclusions The construction of the tunnel in the permafrost region inevitably affects the thermal regime of the surrounding rock. According to the long-term field monitoring data of the ground temperature in a deep borehole above the tunnel vault on Tibet Plateau, the analyses of the thermal regime of the surrounding rock leads to the following conclusions: (1) The thermal disturbance distance of the surrounding rock of the tunnel was approximately 30.5 m around the tunnel vault. The thermal response of the surrounding rock to the construction was temporary and more sensitive, while the thermal regime response to the ambient temperature outside the tunnel was long-term and slow. The formerly affected temperature distribution of the surrounding rock was more obvious than the latter. Therefore, the control of the air temperature in the tunnel should be considered for the construction of the tunnel. (2) The temperature distribution of the surrounding rock achieved a relative steady state after at least twelve freezing-thawing cycles during the operating, in accordance to the change of a temporally varying periodic sine curve. The maximum and minimum temperature amplitudes gradually decreased with time, and the temperature fluctuation strengthened with depth increasing. (3) The heat inputted into the surrounding rock during the construction was cooled by the permafrost and the heat convection between the surrounding rock and the air in the tunnel. (4) The surrounding rock of the tunnel experienced a warming process during its construction. It returned to the undisturbed status after the operation, and then the cooling process continued after the natural status was reached during the past 10 years. The findings of the overcooling phenomena of the surrounding rock are informative for the design of frost heave mitigation of the tunnel in cold regions. The insulation plate, the running speed controlling of vehicle in tunnel, the insulation gate and electric heating methods should be considered. Acknowledgement This study was supported by the National Natural Science Fund (41571070), the Fund of SKLFS (52Y652H21), the Fund of the National Key Basic Research and Development Program (2012CB026102), the Funds of Key Research Program of Frontier Sciences of CAS (QYZDYSSW-DQC015, QYZDY-SSW-DQC011,) and fund HHS-TSS-STS-1502. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tust.2017.08.021.
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