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Numerical analysis of ground motion characteristics in permafrost regions along the Qinghai-Tibet Railway
Cold Regions Science and Technology 148 (2018) 88–95
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Cold Regions Science and Technology journal homepage: www.elsevier.com/locate/coldregions
Numerical analysis of ground motion characteristics in permafrost regions along the Qinghai-Tibet Railway
T
⁎
Tuo Chena, , Wei Mab, Guoqing Zhoua a b
The State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Xuzhou 221116, China Key State laboratory of Frozen Soil Engineering, Northwest Institute of Eco-Environment and resources, CAS, Lanzhou 730000, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Qinghai-Tibet plateau Permafrost Seismic ground motion Dynamic response
Due to the vulnerability of permafrost environment to climate changes, permafrost in the Qinghai-Tibet Plateau (QTP) is experiencing thickening of the active layer and warming ground temperatures that together have a significant influence on the ground motion characteristics. A series of in situ wave velocity tests were carried out in permafrost regions along the Qinghai-Tibet Railway (QTR) and the characteristics of shear wave velocity of frozen soil were summarized. Then, the ground motion responses in permafrost regions were simulated, and the impact of permafrost change on the seismic site response was discussed. Moreover, the dynamic response of the traditional embankment at the Beiluhe site in permafrost regions was studied. The changes of ground motion parameters were significantly impacted and showed reproducible changes with different active layer thicknesses and permafrost thicknesses. The ground motion amplification coefficients increased with the increase of active layer thickness, and the amplification coefficients were greater in a warm season than that in a cold season. Furthermore, the active layer thickness had little influence on the characteristic site period, and the variation of the characteristic site period with the permafrost thickness showed the logarithmic variation characteristic. Compared with the soil profile away from the center of the railway subgrade, the acceleration under the embankment showed a significant increase due to the overlying subgrade embankment. The increase in amplitude was about 29%, which illustrated that the deformation of the soil under the embankment became greater and may be more prone to damage during the seismic loading.
1. Introduction The Qinghai-Tibet Plateau (QTP) is the highest and one of the most extensive plateaus in the world. With a high elevation, low latitude mountain environment, the QTP has the largest extent, about 1.3 × 106 km2, of mountain permafrost on earth. Permafrost covers about 53% of the total area of the plateau (Cheng, 1984; Jin et al., 2006). Moreover, the QTP is one of the most tectonically and seismically active regions, where a total of 33 Ms 6.0–6.9 earthquakes and 3 Ms 7.0–8.5 earthquakes have occurred since 1980 (Wang et al., 2009; Deng et al., 2014). Field investigations after these earthquakes revealed that earthquake ruptures, fractures, liquefaction, seismic subsidence, and collapses formed in areas underlain by permafrost. The Ms 8.1 Central Kunlun earthquake of 14 November 2001 produced a 400kilometer-long surface rupture zone, with as much as 16.3 m of leftlateral strike-slip along the active Kunlun fault in northern Tibet (Lin et al., 2002). These earthquakes produced widespread damages to the infrastructure and construction projects in permafrost regions. Therefore, it is critical to study earthquake disaster mitigation and prevention ⁎
in the permafrost regions on the QTP. The warm permafrost is quite sensitive to temperature changes, because the physical, chemical and engineering properties change at these temperatures. Moreover, these characteristics are also influenced by the ice content, which varies with temperature changes (Xu et al., 2001; Li et al., 2008). Due to the vulnerability of permafrost environment and ongoing climate changes, permafrost on the QTP is experiencing thickening of the active layer and warming ground temperatures. These changes have a significant influence on the ground motion characteristics (Mu et al., 2012; Li et al., 2006). At present, study of ground motion characteristics in permafrost regions is in the exploratory stage, and the prevention of seismic disaster is a great challenge, as well as the earthquake site zonation. It is, therefore, important to study the characteristics of ground motion and the influence of active layer and permafrost on the parameters that control ground motion in permafrost regions on the QTP. The study of the ground motion effects is an issue of growing recent interest. Many resources have been invested to understand these phenomena, especially the seismic ground motion characteristics. Firstly,
Corresponding author. E-mail address: [email protected] (T. Chen).
https://doi.org/10.1016/j.coldregions.2018.01.016 Received 30 November 2016; Received in revised form 18 January 2018; Accepted 25 January 2018 0165-232X/ © 2018 Elsevier B.V. All rights reserved.
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the seismic data was recorded to understand and estimate the local site effects. Ernesto et al. (1993) used data from low intensity 1989 and 1990 events to make a preliminary evaluation of site amplification. The site amplification characteristics have been identified by Kumar et al. (2006) from the frequency bands of significant amplification observed in the spectral ratios of the horizontal to the vertical component records. Furthermore, Hassani et al. (2011) used the generalized inversion of the S-wave amplitude spectra from the strong-motion network data in East-Central Iran to estimate simultaneously source parameters, site response and the S-wave attenuation. In permafrost areas of China, the ground motion effects observed after earthquakes has attracted the interest of the research community. There are some examples in the literature, such as Xu et al. (2003), who calculated the earthquake response spectra of the permafrost sites with different temperature and different overlay thicknesses. Wang et al. (2004) studied the influences of temperature on the seismic displacement, velocity, acceleration and response spectra of permafrost. Yan et al. (2005) analyzed the stochastic earthquake responses of the permafrost sites along the QinghaiTibet Railway, by applying the random vibration theory and the finite element method. Qi et al. (2006) discussed the influence of the seasonal frozen layer on the ground motion features in seasonally frozen regions. In previous researches on the characteristics of ground motion in permafrost regions, the change of the active layer and the influence of permafrost thickness have not considered. In this paper, field wave velocity tests were carried out in permafrost regions and the elementary characteristics of the wave velocities of the permafrost soils were obtained. Then using the results from dynamic triaxial tests, the characteristics of ground motion at permafrost sites were analyzed using the equivalent linearization method. The influence of both the active layer thickness and the permafrost thickness were determined. Finally, the two-dimensional non-linear dynamic time history analysis was used to validate the results of ground motion calculations, and to evaluate the influence of the embankment on seismic ground motion.
Fig. 1. The variations of shear wave velocity with depth of the typical boreholes.
2. Shear wave velocities and kinetic parameters of frozen soil Fig. 2. Ground temperature profiles of the typical boreholes in permafrost regions.
2.1. Shear wave velocities of frozen soil Different types of rocks and soils have different elastic wave propagation properties, and these directly influence the dynamic responses of a geological region. In an infinite elastic medium, the shear wave velocity Vs and the compression wave velocity Vp can be determined:
Vp =
E (1 − μ) ρ (1 + μ)(1 − 2μ)
Vs =
E = 2ρ (1 + μ)
G ρ
Table 1 Wave velocity in-situ tests in the Qinghai-Tibetan Plateau. Lithology
Depth (m)
Soil state
Vs/m/s
Silty clay
0–4
Fine sandy soil
4–10 0–4
Mudstone
4–10 0–4
Unfrozen Frozen Frozen Unfrozen Frozen Frozen Unfrozen Frozen Frozen
140–240 210–430 395–540 120–263 204–400 251–456 216–328 296–532 545–869
(1)
where E is Young modulus, G is shear modulus, ρ is the density and μ is the Poisson's ratio. The wave velocities, especially the shear wave velocity, have an important impact on calculating the seismic response. In permafrost regions, the wave velocity variations due to temperatures and ice contents at different locations were observed. Based on the in situ wave velocity tests, the characteristics of shear-wave velocity distribution of shallow ground in permafrost regions were obtained. Fig. 1 illustrated the variations of shear wave velocity with depth of the typical boreholes at the permafrost test sites. It was observed that shear wave velocity increased near the upper limit of the permafrost and the Vs of the frozen soil could reach 700 m/s at 12 m depth. Fig. 2 illustrated the ground temperature profiles of the typical boreholes in permafrost regions. It could be found that the temperature had an important influence on the shear wave velocity distribution. The active layer depths currently range from1 to 4 m along the QTR (Ma et al., 2008; Zhao et al., 2010). The freezing and thawing of active layer varied with the seasonal air temperature variation, while the stratigraphic layers beneath the active layer remaining frozen were less
4–10
affected by air temperature variation. Moreover, except for the measured data, the velocity data in this regions was collected and a statistical comparison with typical soils down to 10 m, i.e. silty clay, mudstone and fine sandy soil, was shown in Table 1. The velocities at different locations varied not only with the lithology but also with ground temperature. Ground with lower temperatures had higher velocities. 2.2. Kinetic parameters of frozen soils The kinetic parameters, including the shear modulus and damping ratios, change under different shear strain amplitude. The kinetic parameters of frozen soils were obtained using the dynamic triaxial 89
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Table 2 Kinetic parameters of soil seismic response analysis. Soil layer
Active layer (gravelly sand)
State
Thawed Frozen
Ice-rich frozen soil (clay)
Frozen
Frozen soil with less ice (clay)
Frozen
Index
αG λ αG λ αG λ αG λ
Shear strain γd (×10−4) 0.05
0.1
0.5
1
5
10
50
100
0.9900 0.0040 0.9993 0.0060 0.9995 0.0541 0.9985 0.0824
0.9700 0.0060 0.9986 0.0088 0.9989 0.0645 0.9970 0.0985
0.9000 0.0190 0.9932 0.0214 0.9946 0.0966 0.9852 0.1489
0.8500 0.0300 0.9865 0.0313 0.9892 0.1149 0.9709 0.1774
0.7000 0.0750 0.9359 0.0743 0.9482 0.1707 0.8697 0.2614
0.55000 0.0900 0.8795 0.1056 0.9015 0.2007 0.7695 0.3029
0.3200 0.1100 0.5935 0.2075 0.6467 0.2769 0.4003 0.3878
0.2000 0.1200 0.4220 0.2520 0.4770 0.3056 0.2502 0.4108
were ascertained and the numerical calculation model was built using this information (Zhang et al., 2007). As indicated in Fig. 4, the soil profile was determined to be the active layer, an ice-rich layer and an ice-poor layer of permafrost at the site. Considering the variations in active layer thickness under the global warming, the active layer thickness ranging from 1 m to 4 m was considered. The shear wave velocity of the shallow layer (less than 10 m in depth) was determined using the in-situ test results, while the shear wave velocity of the deep soil layer (more than 10 m in depth) were selected from literature values (Table 1). The characteristics of the response spectra at permafrost sites with different active layer thicknesses and different thermal regimes were calculated, and the peak ground acceleration (PGA) and the characteristic site period were analyzed as well. The acceleration response spectra in permafrost regions under seismic ground motion with exceedance probability of 10% in 50 years were illustrated in Fig. 5. The response spectra for the permafrost sites with different active layer thicknesses and different thermal regimes have the same shape, except for the short period segment (0–0.2 s). This was because the kinetic parameters of the active layer changed with the seasons, while the underlying permafrost remains constant. Meanwhile, the changes of the kinetic parameters were not sufficient to alter the inherent characteristics of these sites. The difference in the response spectra reflected the influence of the annual active layer freeze-thaw cycles. The peak values of acceleration response spectra of the sites during the warm season were greater than that during the cold season and increased with the active layer thickness. Moreover, the active layer thickness influenced less when the active layer was frozen. Compared with the numerical analysis results of ground motion effect in non-permafrost sites (Wu et al., 2012; Chen et al., 2017), the peak values of acceleration response spectra in permafrost regions were smaller, the short and middle-period response spectra accounted for a larger proportion, and the characteristic site periods in permafrost sites were shorter. This conclusion was earlier proposed by Wu et al. (2007). The PGA of permafrost sites with different active layer thicknesses and different thermal regimes under seismic ground motion was studied in this paper. In order to quantify the amplification effect of the permafrost site, the amplification coefficient, defined as the ratio of the PGA to the input seismic acceleration, was introduced in this paper. The distribution of the PGA amplification coefficient was illustrated in Fig. 6. It was found that the input seismic motion intensity had a profound impact on the site amplification effects. The PGA amplification coefficients decreased with the increase of the input seismic motion intensity. According to the numerical results, the amplification coefficients increased with the active layer thickness, and the amplification coefficients in a warm season were greater than that in a cold season. Compared with the frozen soil, the unfrozen soil had the larger magnification of the seismic ground motions. The PGA amplification coefficients were within the range of 1.1–1.7 during the different seasons. The characteristic site period is an important index in seismic
tests conducted at the State Key Laboratory of Frozen Soil Engineering, Chinese Academy of Sciences (Zhu, 2009). The kinetic parameters of typical frozen soil were illustrated in Table 2. The dynamic shear modulus ratio decreased and the damping ratios increased with the increase of shear strain. Moreover, the kinetic parameters of the active layer in winter were significantly larger than that in summer. 3. The seismic ground motion analysis in permafrost regions Ground motion accelerations and acceleration response spectra due to earthquakes in permafrost regions were determined using seismic site response analysis. In this paper, the one-dimensional equivalent linear method was used for numerical analysis of ground motion effects in permafrost regions. Equivalent linear analysis (Idriss and Bolton, 1968; Li, 1989) accounted for the soil nonlinearity using an iterative procedure to obtain values for shear modulus and damping ratio that were compatible with the equivalent uniform strain induced in each sublayer. The analysis was conducted using a set of properties (shear modulus or shear wave velocity, damping, thickness and total unit weight of layers) and the equivalent uniform shear strain induced in each sublayer was calculated. The shear modulus and the damping ratio for each sublayer were then modified according to the equivalent uniform shear strain based on the applicable relationship relating these two properties to shear strain. The analysis was repeated until strain compatible modulus and damping values were arrived at. The basic feature of this approach was that the equivalent shear modulus and damping ratios were used to describe the complex changes of the soil. Then the iterative method was employed in the dynamic response calculation, and thus the nonlinear problem was changed into a linear problem. 3.1. Input seismic motions The ground characteristics must be considered during the site response to seismic loading in permafrost regions. In this paper, the intensity of seismic ground motion, historical earthquake damages along with site conditions of the QTP were considered. Then the artificial seismic waves with exceedance probabilities of 63%, 10% and 2%, respectively, in 50 years (Fig. 3), were used as input to the bedrock (Lu and Li, 2001). Fig. 3 illustrated the maximum amplitudes of the seismic motion were 26 cm·s−2, 167 cm·s−2 and 326 cm·s−2, respectively. 3.2. Effects of active layer thickness The effects of the active layer thickness on seismic ground motion were investigated by applying the one-dimensional equivalent linear method to determine the seismic response spectra characteristics and the characteristic site periods. The typical permafrost site at Beiluhe segment along the Qinghai-Tibet Railway (K1129 + 410) was selected for the field research. Based on the boreholes, the stratigraphic layers 90
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a
the exceedance probabilities of 63.5% in 50 years
b
the exceedance probabilities of 10.0% in 50 years
c
the exceedance probabilities of 2.0% in 50 years
Fig. 3. Artificial acceleration time histories and spectra of ground motion (a) the exceedance probabilities of 63.5% in 50 years; (b) the exceedance probabilities of 10.0% in 50 years; (c) the exceedance probabilities of 2.0% in 50 years. Fig. 4. One-dimensional nonlinear calculation model considering the active layer thickness.
3.3. Effects of permafrost thickness
zonation and seismic resistance design. In seismic response spectrum theory, the characteristic design period is closely related with characteristic site period which is comprehensively covering site parameters (site categories, site shear wave velocity, site thickness, etc.). The distribution of characteristic site period of the response spectra with different active layer thicknesses during different seasons, under seismic ground motion with exceedance probability of 10% in 50 years, was presented in Fig. 7. It was found the characteristic site period became longer with the increase of active layer thickness. It was also indicated that the influence of the active layer thickness was not significant. The maximum value of the characteristic period in cold season was 0.32 s while the value could reach up to 0.38 s in a warm season.
Apart from the active layer thickness, another significant factor influencing the seismic ground motion in permafrost regions is the permafrost thickness. On the basis of the one-dimensional calculation results, the effects of permafrost thickness were conducted. The numerical calculation model considering the frozen soil layer thickness was illustrated in Fig. 8. In the calculation model, the thickness of the active layer was assumed to be a constant of 2 m, while the permafrost thickness ranged from 10 m to 60 m. Then the dynamic response of the permafrost sites with different permafrost thicknesses was obtained. The acceleration response spectra of the permafrost sites under seismic ground motion with exceedance probability of 10% in 50 years 91
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response to the active layer thickness. Fig. 10 indicated the distribution of the PGA amplification coefficient of the sites with the different permafrost thicknesses. The PGA amplification coefficients had an obvious decreasing trend with the increase of the permafrost thickness, and this phenomenon was named the inhibitory effects in this paper. The inhibitory effects were more obvious with higher input ground motion intensity. Under the action of artificial seismic load with the maximum amplitude of 26 cm·s−2, the influence of the permafrost thickness on PGA showed an amplification effect. The PGA amplification coefficients decreased with the increase of the thickness, and the value reached 1.17 when the permafrost thickness was 60 m. However, under the action of artificial seismic load with the maximum amplitude of 326 cm·s−2, the PGA amplification coefficients were less than 1 when the permafrost thickness was greater than 40 m. The results indicated that the permafrost thickness had an important influence on the site effect and the sites with large permafrost thickness had no amplification effects on the ground motion. The reason was thought to be that the dynamic shear moduli and damping ratios of frozen soils were greater compared with the unfrozen soil. Material damping, which was caused by the energy dissipation within the soil skeletal structure, reflected the degree of energy dissipation under seismic action. The characteristic site period distribution with different permafrost thicknesses was shown in Fig. 11. It was observed that the characteristic site period gradually increased with the permafrost thickness. The characteristic site period was 0.28 when the permafrost thickness was 10 m, while the value increased to 0.48 when the permafrost thickness reached 60 m. The variation of the characteristic site period with the permafrost thickness, using logarithmic function, could be fitted by Eq. (2) and the R-square was 0.9765.
Fig. 5. Acceleration spectra of ground motion with exceedance probabilities of 10.0% in 50 years.
y = 0.1105 ln(x) + 0.0276
(2)
In the formula mentioned above (Eq. (2)), x was the permafrost thickness, and y referred to the characteristic period. 4. The seismic ground motion analysis of embankment Based on the study of one-dimensional seismic ground motion analysis (1-D), two-dimensional non-linear dynamic time history analysis (2-D) was conducted. The numerical simulation of dynamic response of the traditional sand gravel embankment at the Beiluhe segment in permafrost regions was performed to establish the influence of the embankment on seismic ground motion. Comparing the results of applying different methods of seismic ground motion simulation, the influence of embankment on seismic ground motion was evaluated, and the distribution characteristic of the seismic acceleration beneath the embankment was presented. The numerical model illustrated in Fig. 12, was established using the actual soil layer determined by drilling and the railway subgrade structure. The soil layer distribution in the 2-D numerical model was the same as the 1-D numerical model. The simulation was carried out using the plane strain assumption and the influence of boundary conditions diminished when using the infinite element boundary. The soil was regarded as an elastic-plastic material and the Mohr-Coulomb yield criterion was adopted, and the ballast was assumed to be a linear elastic material as well (Che et al., 2006). The artificial seismic waves with exceedance probabilities of 10% in 50 years were used as the seismic loading for the numerical calculation. Based on the results of dynamic triaxial tests at low temperature and the in situ wave velocities, the soil kinetic parameters in finite element numerical calculation were determined. The soil parameters of the numerical analysis were presented in Table 3. In view of the research purposes and significance, two monitoring profiles were arranged in the numerical model (Fig. 12). The monitoring profile AA was arranged along the centerline of the embankment, while the monitoring profile BB was 20 m away from the toe of the roadbed slope.
Fig. 6. Distribution of the amplification coefficient with the different active layer thicknesses.
Fig. 7. The distribution of characteristic site period with the different active layer thicknesses.
were illustrated in Fig. 9. The acceleration response spectra of the sites with different permafrost thicknesses had a great difference during short response periods. The response spectra had the obvious movement toward the direction of long-period segment with the increase of the permafrost thickness. Moreover, the peak values of acceleration response spectra decreased with the increase of the permafrost thickness, which reflected a significant difference compared with the 92
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Fig. 8. One-dimensional nonlinear calculation model considering the permafrost thickness.
Fig. 11. The characteristic site period distribution curves with different permafrost thicknesses.
beneath the embankment showed a significant increase compared with the profile BB away from the subgrade center. The acceleration difference between these two profiles increased with the increase of active layer thickness. The maximum horizontal acceleration of the profile BB was 245 cm·s−2 when the active layer thickness was 4 m, while the value of the profile AA could reach up to 316 cm·s−2 under the same condition. Compared with the profile away from the subgrade center, the acceleration of the profile under the embankment was larger and the amplitude increased about 29%, which illustrated that the deformation of the soil under the embankment was greater and more prone to damage under the seismic loading.
Fig. 9. Acceleration spectra of ground motion with exceedance probabilities of 10.0% in 50 years.
5. Conclusion A series of wave velocity tests were carried out in permafrost regions in the Qinghai-Tibet Plateau, and the seismic ground motion simulation and analysis in permafrost regions were processed. Moreover, the dynamic responses of the traditional sand gravel embankment at the Beiluhe segment in the permafrost regions were studied. The computational analyses resulted in the following conclusions: Fig. 10. Distribution of the amplification coefficient with the different permafrost thicknesses.
(1) Velocities at different locations varied not only with the lithologies but also with ground temperature. The soil layers with lower temperature had higher velocities. (2) The influence of the active layer thickness and permafrost thickness on ground motion was significant. The ground motion intensity had an important impact on the PGA and site amplification effect. (3) The ground motion parameters responded to changes in active layer and permafrost thickness. The ground motion amplification coefficients increased with the increase of active layer thickness, and the values were greater in warm seasons than that in cold seasons. The PGA amplification coefficients had an obvious decreasing trend with the increase of the permafrost thickness. Moreover, the active layer thickness had little influence on the characteristic site period, and the variation of the characteristic site period with the permafrost thickness followed a logarithmic variation response. (4) The acceleration in the monitoring profile under the embankment
Fig. 13 indicated the horizontal acceleration contour of embankment under the seismic loading at the peak acceleration time of the seismic loading. It was found that the horizontal acceleration magnification effect increased with height. Due to the influence of the subgrade, the energy accumulation effect was observed under the seismic loading, and the maximum acceleration of the model appeared at the top of embankment. Moreover, the acceleration of the underlying active layer under the embankment was greater compared with the soil layer away from the embankment. Fig. 14 indicated the distribution of the PGA of profile AA and profile BB, with different thawed active layer thicknesses. It was obvious that the distribution of the PGA calculated by both 1-D equivalent linear method and 2-D finite-element method coincided. Due to the influence of the subgrade, the acceleration of monitoring profile AA 93
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Fig. 12. The numerical model of the embankment dynamic response analysis.
Table 3 Parameters of the numerical model. Lithology
Soil type
Density kN/m3
Dynamic shear modulus/kPa
Poisson ratio
Cohesion force/kPa
Internal frictional angle/°
Ballast Upper roadbed layer roadbed layer Roadbed fill Active layer
Unfrozen Unfrozen Unfrozen Unfrozen Frozen Frozen Frozen
20.0 19.5 19.0 18.0 18.0 16.0 16.0
2.86E5 2.26E5 2.26E5 1.36E5 2.76E5 3.20E5 2.69E5
0.3 0.3 0.31 0.35 0.31 0.35 0.35
/ 26 16 12 24 82 110
/ 25 25 20 18 24 29
Ice-rich frozen soil Frozen soil with less ice
Fig. 13. The horizontal acceleration contour map.
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Fig. 14. The distribution of the peak ground acceleration.
indicated a significant increase compared with the profile away from the subgrade center. The increase amplitude reached 29% which illustrated that the soil under the embankment could easily enter a plastic yield state under the seismic loading.
Acknowledgements Financial support for this project is from the Fundamental Research Funds for the Central Universities (Grant No. 2015QNB17). We would like to express our sincere thanks to the anonymous reviewers for their valuable comments and suggestions.
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Cold Regions Science and Technology journal homepage: www.elsevier.com/locate/coldregions
Numerical analysis of ground motion characteristics in permafrost regions along the Qinghai-Tibet Railway
T
⁎
Tuo Chena, , Wei Mab, Guoqing Zhoua a b
The State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Xuzhou 221116, China Key State laboratory of Frozen Soil Engineering, Northwest Institute of Eco-Environment and resources, CAS, Lanzhou 730000, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Qinghai-Tibet plateau Permafrost Seismic ground motion Dynamic response
Due to the vulnerability of permafrost environment to climate changes, permafrost in the Qinghai-Tibet Plateau (QTP) is experiencing thickening of the active layer and warming ground temperatures that together have a significant influence on the ground motion characteristics. A series of in situ wave velocity tests were carried out in permafrost regions along the Qinghai-Tibet Railway (QTR) and the characteristics of shear wave velocity of frozen soil were summarized. Then, the ground motion responses in permafrost regions were simulated, and the impact of permafrost change on the seismic site response was discussed. Moreover, the dynamic response of the traditional embankment at the Beiluhe site in permafrost regions was studied. The changes of ground motion parameters were significantly impacted and showed reproducible changes with different active layer thicknesses and permafrost thicknesses. The ground motion amplification coefficients increased with the increase of active layer thickness, and the amplification coefficients were greater in a warm season than that in a cold season. Furthermore, the active layer thickness had little influence on the characteristic site period, and the variation of the characteristic site period with the permafrost thickness showed the logarithmic variation characteristic. Compared with the soil profile away from the center of the railway subgrade, the acceleration under the embankment showed a significant increase due to the overlying subgrade embankment. The increase in amplitude was about 29%, which illustrated that the deformation of the soil under the embankment became greater and may be more prone to damage during the seismic loading.
1. Introduction The Qinghai-Tibet Plateau (QTP) is the highest and one of the most extensive plateaus in the world. With a high elevation, low latitude mountain environment, the QTP has the largest extent, about 1.3 × 106 km2, of mountain permafrost on earth. Permafrost covers about 53% of the total area of the plateau (Cheng, 1984; Jin et al., 2006). Moreover, the QTP is one of the most tectonically and seismically active regions, where a total of 33 Ms 6.0–6.9 earthquakes and 3 Ms 7.0–8.5 earthquakes have occurred since 1980 (Wang et al., 2009; Deng et al., 2014). Field investigations after these earthquakes revealed that earthquake ruptures, fractures, liquefaction, seismic subsidence, and collapses formed in areas underlain by permafrost. The Ms 8.1 Central Kunlun earthquake of 14 November 2001 produced a 400kilometer-long surface rupture zone, with as much as 16.3 m of leftlateral strike-slip along the active Kunlun fault in northern Tibet (Lin et al., 2002). These earthquakes produced widespread damages to the infrastructure and construction projects in permafrost regions. Therefore, it is critical to study earthquake disaster mitigation and prevention ⁎
in the permafrost regions on the QTP. The warm permafrost is quite sensitive to temperature changes, because the physical, chemical and engineering properties change at these temperatures. Moreover, these characteristics are also influenced by the ice content, which varies with temperature changes (Xu et al., 2001; Li et al., 2008). Due to the vulnerability of permafrost environment and ongoing climate changes, permafrost on the QTP is experiencing thickening of the active layer and warming ground temperatures. These changes have a significant influence on the ground motion characteristics (Mu et al., 2012; Li et al., 2006). At present, study of ground motion characteristics in permafrost regions is in the exploratory stage, and the prevention of seismic disaster is a great challenge, as well as the earthquake site zonation. It is, therefore, important to study the characteristics of ground motion and the influence of active layer and permafrost on the parameters that control ground motion in permafrost regions on the QTP. The study of the ground motion effects is an issue of growing recent interest. Many resources have been invested to understand these phenomena, especially the seismic ground motion characteristics. Firstly,
Corresponding author. E-mail address: [email protected] (T. Chen).
https://doi.org/10.1016/j.coldregions.2018.01.016 Received 30 November 2016; Received in revised form 18 January 2018; Accepted 25 January 2018 0165-232X/ © 2018 Elsevier B.V. All rights reserved.
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the seismic data was recorded to understand and estimate the local site effects. Ernesto et al. (1993) used data from low intensity 1989 and 1990 events to make a preliminary evaluation of site amplification. The site amplification characteristics have been identified by Kumar et al. (2006) from the frequency bands of significant amplification observed in the spectral ratios of the horizontal to the vertical component records. Furthermore, Hassani et al. (2011) used the generalized inversion of the S-wave amplitude spectra from the strong-motion network data in East-Central Iran to estimate simultaneously source parameters, site response and the S-wave attenuation. In permafrost areas of China, the ground motion effects observed after earthquakes has attracted the interest of the research community. There are some examples in the literature, such as Xu et al. (2003), who calculated the earthquake response spectra of the permafrost sites with different temperature and different overlay thicknesses. Wang et al. (2004) studied the influences of temperature on the seismic displacement, velocity, acceleration and response spectra of permafrost. Yan et al. (2005) analyzed the stochastic earthquake responses of the permafrost sites along the QinghaiTibet Railway, by applying the random vibration theory and the finite element method. Qi et al. (2006) discussed the influence of the seasonal frozen layer on the ground motion features in seasonally frozen regions. In previous researches on the characteristics of ground motion in permafrost regions, the change of the active layer and the influence of permafrost thickness have not considered. In this paper, field wave velocity tests were carried out in permafrost regions and the elementary characteristics of the wave velocities of the permafrost soils were obtained. Then using the results from dynamic triaxial tests, the characteristics of ground motion at permafrost sites were analyzed using the equivalent linearization method. The influence of both the active layer thickness and the permafrost thickness were determined. Finally, the two-dimensional non-linear dynamic time history analysis was used to validate the results of ground motion calculations, and to evaluate the influence of the embankment on seismic ground motion.
Fig. 1. The variations of shear wave velocity with depth of the typical boreholes.
2. Shear wave velocities and kinetic parameters of frozen soil Fig. 2. Ground temperature profiles of the typical boreholes in permafrost regions.
2.1. Shear wave velocities of frozen soil Different types of rocks and soils have different elastic wave propagation properties, and these directly influence the dynamic responses of a geological region. In an infinite elastic medium, the shear wave velocity Vs and the compression wave velocity Vp can be determined:
Vp =
E (1 − μ) ρ (1 + μ)(1 − 2μ)
Vs =
E = 2ρ (1 + μ)
G ρ
Table 1 Wave velocity in-situ tests in the Qinghai-Tibetan Plateau. Lithology
Depth (m)
Soil state
Vs/m/s
Silty clay
0–4
Fine sandy soil
4–10 0–4
Mudstone
4–10 0–4
Unfrozen Frozen Frozen Unfrozen Frozen Frozen Unfrozen Frozen Frozen
140–240 210–430 395–540 120–263 204–400 251–456 216–328 296–532 545–869
(1)
where E is Young modulus, G is shear modulus, ρ is the density and μ is the Poisson's ratio. The wave velocities, especially the shear wave velocity, have an important impact on calculating the seismic response. In permafrost regions, the wave velocity variations due to temperatures and ice contents at different locations were observed. Based on the in situ wave velocity tests, the characteristics of shear-wave velocity distribution of shallow ground in permafrost regions were obtained. Fig. 1 illustrated the variations of shear wave velocity with depth of the typical boreholes at the permafrost test sites. It was observed that shear wave velocity increased near the upper limit of the permafrost and the Vs of the frozen soil could reach 700 m/s at 12 m depth. Fig. 2 illustrated the ground temperature profiles of the typical boreholes in permafrost regions. It could be found that the temperature had an important influence on the shear wave velocity distribution. The active layer depths currently range from1 to 4 m along the QTR (Ma et al., 2008; Zhao et al., 2010). The freezing and thawing of active layer varied with the seasonal air temperature variation, while the stratigraphic layers beneath the active layer remaining frozen were less
4–10
affected by air temperature variation. Moreover, except for the measured data, the velocity data in this regions was collected and a statistical comparison with typical soils down to 10 m, i.e. silty clay, mudstone and fine sandy soil, was shown in Table 1. The velocities at different locations varied not only with the lithology but also with ground temperature. Ground with lower temperatures had higher velocities. 2.2. Kinetic parameters of frozen soils The kinetic parameters, including the shear modulus and damping ratios, change under different shear strain amplitude. The kinetic parameters of frozen soils were obtained using the dynamic triaxial 89
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Table 2 Kinetic parameters of soil seismic response analysis. Soil layer
Active layer (gravelly sand)
State
Thawed Frozen
Ice-rich frozen soil (clay)
Frozen
Frozen soil with less ice (clay)
Frozen
Index
αG λ αG λ αG λ αG λ
Shear strain γd (×10−4) 0.05
0.1
0.5
1
5
10
50
100
0.9900 0.0040 0.9993 0.0060 0.9995 0.0541 0.9985 0.0824
0.9700 0.0060 0.9986 0.0088 0.9989 0.0645 0.9970 0.0985
0.9000 0.0190 0.9932 0.0214 0.9946 0.0966 0.9852 0.1489
0.8500 0.0300 0.9865 0.0313 0.9892 0.1149 0.9709 0.1774
0.7000 0.0750 0.9359 0.0743 0.9482 0.1707 0.8697 0.2614
0.55000 0.0900 0.8795 0.1056 0.9015 0.2007 0.7695 0.3029
0.3200 0.1100 0.5935 0.2075 0.6467 0.2769 0.4003 0.3878
0.2000 0.1200 0.4220 0.2520 0.4770 0.3056 0.2502 0.4108
were ascertained and the numerical calculation model was built using this information (Zhang et al., 2007). As indicated in Fig. 4, the soil profile was determined to be the active layer, an ice-rich layer and an ice-poor layer of permafrost at the site. Considering the variations in active layer thickness under the global warming, the active layer thickness ranging from 1 m to 4 m was considered. The shear wave velocity of the shallow layer (less than 10 m in depth) was determined using the in-situ test results, while the shear wave velocity of the deep soil layer (more than 10 m in depth) were selected from literature values (Table 1). The characteristics of the response spectra at permafrost sites with different active layer thicknesses and different thermal regimes were calculated, and the peak ground acceleration (PGA) and the characteristic site period were analyzed as well. The acceleration response spectra in permafrost regions under seismic ground motion with exceedance probability of 10% in 50 years were illustrated in Fig. 5. The response spectra for the permafrost sites with different active layer thicknesses and different thermal regimes have the same shape, except for the short period segment (0–0.2 s). This was because the kinetic parameters of the active layer changed with the seasons, while the underlying permafrost remains constant. Meanwhile, the changes of the kinetic parameters were not sufficient to alter the inherent characteristics of these sites. The difference in the response spectra reflected the influence of the annual active layer freeze-thaw cycles. The peak values of acceleration response spectra of the sites during the warm season were greater than that during the cold season and increased with the active layer thickness. Moreover, the active layer thickness influenced less when the active layer was frozen. Compared with the numerical analysis results of ground motion effect in non-permafrost sites (Wu et al., 2012; Chen et al., 2017), the peak values of acceleration response spectra in permafrost regions were smaller, the short and middle-period response spectra accounted for a larger proportion, and the characteristic site periods in permafrost sites were shorter. This conclusion was earlier proposed by Wu et al. (2007). The PGA of permafrost sites with different active layer thicknesses and different thermal regimes under seismic ground motion was studied in this paper. In order to quantify the amplification effect of the permafrost site, the amplification coefficient, defined as the ratio of the PGA to the input seismic acceleration, was introduced in this paper. The distribution of the PGA amplification coefficient was illustrated in Fig. 6. It was found that the input seismic motion intensity had a profound impact on the site amplification effects. The PGA amplification coefficients decreased with the increase of the input seismic motion intensity. According to the numerical results, the amplification coefficients increased with the active layer thickness, and the amplification coefficients in a warm season were greater than that in a cold season. Compared with the frozen soil, the unfrozen soil had the larger magnification of the seismic ground motions. The PGA amplification coefficients were within the range of 1.1–1.7 during the different seasons. The characteristic site period is an important index in seismic
tests conducted at the State Key Laboratory of Frozen Soil Engineering, Chinese Academy of Sciences (Zhu, 2009). The kinetic parameters of typical frozen soil were illustrated in Table 2. The dynamic shear modulus ratio decreased and the damping ratios increased with the increase of shear strain. Moreover, the kinetic parameters of the active layer in winter were significantly larger than that in summer. 3. The seismic ground motion analysis in permafrost regions Ground motion accelerations and acceleration response spectra due to earthquakes in permafrost regions were determined using seismic site response analysis. In this paper, the one-dimensional equivalent linear method was used for numerical analysis of ground motion effects in permafrost regions. Equivalent linear analysis (Idriss and Bolton, 1968; Li, 1989) accounted for the soil nonlinearity using an iterative procedure to obtain values for shear modulus and damping ratio that were compatible with the equivalent uniform strain induced in each sublayer. The analysis was conducted using a set of properties (shear modulus or shear wave velocity, damping, thickness and total unit weight of layers) and the equivalent uniform shear strain induced in each sublayer was calculated. The shear modulus and the damping ratio for each sublayer were then modified according to the equivalent uniform shear strain based on the applicable relationship relating these two properties to shear strain. The analysis was repeated until strain compatible modulus and damping values were arrived at. The basic feature of this approach was that the equivalent shear modulus and damping ratios were used to describe the complex changes of the soil. Then the iterative method was employed in the dynamic response calculation, and thus the nonlinear problem was changed into a linear problem. 3.1. Input seismic motions The ground characteristics must be considered during the site response to seismic loading in permafrost regions. In this paper, the intensity of seismic ground motion, historical earthquake damages along with site conditions of the QTP were considered. Then the artificial seismic waves with exceedance probabilities of 63%, 10% and 2%, respectively, in 50 years (Fig. 3), were used as input to the bedrock (Lu and Li, 2001). Fig. 3 illustrated the maximum amplitudes of the seismic motion were 26 cm·s−2, 167 cm·s−2 and 326 cm·s−2, respectively. 3.2. Effects of active layer thickness The effects of the active layer thickness on seismic ground motion were investigated by applying the one-dimensional equivalent linear method to determine the seismic response spectra characteristics and the characteristic site periods. The typical permafrost site at Beiluhe segment along the Qinghai-Tibet Railway (K1129 + 410) was selected for the field research. Based on the boreholes, the stratigraphic layers 90
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a
the exceedance probabilities of 63.5% in 50 years
b
the exceedance probabilities of 10.0% in 50 years
c
the exceedance probabilities of 2.0% in 50 years
Fig. 3. Artificial acceleration time histories and spectra of ground motion (a) the exceedance probabilities of 63.5% in 50 years; (b) the exceedance probabilities of 10.0% in 50 years; (c) the exceedance probabilities of 2.0% in 50 years. Fig. 4. One-dimensional nonlinear calculation model considering the active layer thickness.
3.3. Effects of permafrost thickness
zonation and seismic resistance design. In seismic response spectrum theory, the characteristic design period is closely related with characteristic site period which is comprehensively covering site parameters (site categories, site shear wave velocity, site thickness, etc.). The distribution of characteristic site period of the response spectra with different active layer thicknesses during different seasons, under seismic ground motion with exceedance probability of 10% in 50 years, was presented in Fig. 7. It was found the characteristic site period became longer with the increase of active layer thickness. It was also indicated that the influence of the active layer thickness was not significant. The maximum value of the characteristic period in cold season was 0.32 s while the value could reach up to 0.38 s in a warm season.
Apart from the active layer thickness, another significant factor influencing the seismic ground motion in permafrost regions is the permafrost thickness. On the basis of the one-dimensional calculation results, the effects of permafrost thickness were conducted. The numerical calculation model considering the frozen soil layer thickness was illustrated in Fig. 8. In the calculation model, the thickness of the active layer was assumed to be a constant of 2 m, while the permafrost thickness ranged from 10 m to 60 m. Then the dynamic response of the permafrost sites with different permafrost thicknesses was obtained. The acceleration response spectra of the permafrost sites under seismic ground motion with exceedance probability of 10% in 50 years 91
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response to the active layer thickness. Fig. 10 indicated the distribution of the PGA amplification coefficient of the sites with the different permafrost thicknesses. The PGA amplification coefficients had an obvious decreasing trend with the increase of the permafrost thickness, and this phenomenon was named the inhibitory effects in this paper. The inhibitory effects were more obvious with higher input ground motion intensity. Under the action of artificial seismic load with the maximum amplitude of 26 cm·s−2, the influence of the permafrost thickness on PGA showed an amplification effect. The PGA amplification coefficients decreased with the increase of the thickness, and the value reached 1.17 when the permafrost thickness was 60 m. However, under the action of artificial seismic load with the maximum amplitude of 326 cm·s−2, the PGA amplification coefficients were less than 1 when the permafrost thickness was greater than 40 m. The results indicated that the permafrost thickness had an important influence on the site effect and the sites with large permafrost thickness had no amplification effects on the ground motion. The reason was thought to be that the dynamic shear moduli and damping ratios of frozen soils were greater compared with the unfrozen soil. Material damping, which was caused by the energy dissipation within the soil skeletal structure, reflected the degree of energy dissipation under seismic action. The characteristic site period distribution with different permafrost thicknesses was shown in Fig. 11. It was observed that the characteristic site period gradually increased with the permafrost thickness. The characteristic site period was 0.28 when the permafrost thickness was 10 m, while the value increased to 0.48 when the permafrost thickness reached 60 m. The variation of the characteristic site period with the permafrost thickness, using logarithmic function, could be fitted by Eq. (2) and the R-square was 0.9765.
Fig. 5. Acceleration spectra of ground motion with exceedance probabilities of 10.0% in 50 years.
y = 0.1105 ln(x) + 0.0276
(2)
In the formula mentioned above (Eq. (2)), x was the permafrost thickness, and y referred to the characteristic period. 4. The seismic ground motion analysis of embankment Based on the study of one-dimensional seismic ground motion analysis (1-D), two-dimensional non-linear dynamic time history analysis (2-D) was conducted. The numerical simulation of dynamic response of the traditional sand gravel embankment at the Beiluhe segment in permafrost regions was performed to establish the influence of the embankment on seismic ground motion. Comparing the results of applying different methods of seismic ground motion simulation, the influence of embankment on seismic ground motion was evaluated, and the distribution characteristic of the seismic acceleration beneath the embankment was presented. The numerical model illustrated in Fig. 12, was established using the actual soil layer determined by drilling and the railway subgrade structure. The soil layer distribution in the 2-D numerical model was the same as the 1-D numerical model. The simulation was carried out using the plane strain assumption and the influence of boundary conditions diminished when using the infinite element boundary. The soil was regarded as an elastic-plastic material and the Mohr-Coulomb yield criterion was adopted, and the ballast was assumed to be a linear elastic material as well (Che et al., 2006). The artificial seismic waves with exceedance probabilities of 10% in 50 years were used as the seismic loading for the numerical calculation. Based on the results of dynamic triaxial tests at low temperature and the in situ wave velocities, the soil kinetic parameters in finite element numerical calculation were determined. The soil parameters of the numerical analysis were presented in Table 3. In view of the research purposes and significance, two monitoring profiles were arranged in the numerical model (Fig. 12). The monitoring profile AA was arranged along the centerline of the embankment, while the monitoring profile BB was 20 m away from the toe of the roadbed slope.
Fig. 6. Distribution of the amplification coefficient with the different active layer thicknesses.
Fig. 7. The distribution of characteristic site period with the different active layer thicknesses.
were illustrated in Fig. 9. The acceleration response spectra of the sites with different permafrost thicknesses had a great difference during short response periods. The response spectra had the obvious movement toward the direction of long-period segment with the increase of the permafrost thickness. Moreover, the peak values of acceleration response spectra decreased with the increase of the permafrost thickness, which reflected a significant difference compared with the 92
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Fig. 8. One-dimensional nonlinear calculation model considering the permafrost thickness.
Fig. 11. The characteristic site period distribution curves with different permafrost thicknesses.
beneath the embankment showed a significant increase compared with the profile BB away from the subgrade center. The acceleration difference between these two profiles increased with the increase of active layer thickness. The maximum horizontal acceleration of the profile BB was 245 cm·s−2 when the active layer thickness was 4 m, while the value of the profile AA could reach up to 316 cm·s−2 under the same condition. Compared with the profile away from the subgrade center, the acceleration of the profile under the embankment was larger and the amplitude increased about 29%, which illustrated that the deformation of the soil under the embankment was greater and more prone to damage under the seismic loading.
Fig. 9. Acceleration spectra of ground motion with exceedance probabilities of 10.0% in 50 years.
5. Conclusion A series of wave velocity tests were carried out in permafrost regions in the Qinghai-Tibet Plateau, and the seismic ground motion simulation and analysis in permafrost regions were processed. Moreover, the dynamic responses of the traditional sand gravel embankment at the Beiluhe segment in the permafrost regions were studied. The computational analyses resulted in the following conclusions: Fig. 10. Distribution of the amplification coefficient with the different permafrost thicknesses.
(1) Velocities at different locations varied not only with the lithologies but also with ground temperature. The soil layers with lower temperature had higher velocities. (2) The influence of the active layer thickness and permafrost thickness on ground motion was significant. The ground motion intensity had an important impact on the PGA and site amplification effect. (3) The ground motion parameters responded to changes in active layer and permafrost thickness. The ground motion amplification coefficients increased with the increase of active layer thickness, and the values were greater in warm seasons than that in cold seasons. The PGA amplification coefficients had an obvious decreasing trend with the increase of the permafrost thickness. Moreover, the active layer thickness had little influence on the characteristic site period, and the variation of the characteristic site period with the permafrost thickness followed a logarithmic variation response. (4) The acceleration in the monitoring profile under the embankment
Fig. 13 indicated the horizontal acceleration contour of embankment under the seismic loading at the peak acceleration time of the seismic loading. It was found that the horizontal acceleration magnification effect increased with height. Due to the influence of the subgrade, the energy accumulation effect was observed under the seismic loading, and the maximum acceleration of the model appeared at the top of embankment. Moreover, the acceleration of the underlying active layer under the embankment was greater compared with the soil layer away from the embankment. Fig. 14 indicated the distribution of the PGA of profile AA and profile BB, with different thawed active layer thicknesses. It was obvious that the distribution of the PGA calculated by both 1-D equivalent linear method and 2-D finite-element method coincided. Due to the influence of the subgrade, the acceleration of monitoring profile AA 93
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Fig. 12. The numerical model of the embankment dynamic response analysis.
Table 3 Parameters of the numerical model. Lithology
Soil type
Density kN/m3
Dynamic shear modulus/kPa
Poisson ratio
Cohesion force/kPa
Internal frictional angle/°
Ballast Upper roadbed layer roadbed layer Roadbed fill Active layer
Unfrozen Unfrozen Unfrozen Unfrozen Frozen Frozen Frozen
20.0 19.5 19.0 18.0 18.0 16.0 16.0
2.86E5 2.26E5 2.26E5 1.36E5 2.76E5 3.20E5 2.69E5
0.3 0.3 0.31 0.35 0.31 0.35 0.35
/ 26 16 12 24 82 110
/ 25 25 20 18 24 29
Ice-rich frozen soil Frozen soil with less ice
Fig. 13. The horizontal acceleration contour map.
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Fig. 14. The distribution of the peak ground acceleration.
indicated a significant increase compared with the profile away from the subgrade center. The increase amplitude reached 29% which illustrated that the soil under the embankment could easily enter a plastic yield state under the seismic loading.
Acknowledgements Financial support for this project is from the Fundamental Research Funds for the Central Universities (Grant No. 2015QNB17). We would like to express our sincere thanks to the anonymous reviewers for their valuable comments and suggestions.
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