Experimental investigation on the train-induced subsidence prediction model of Beiluhe permafrost subgrade along the Qinghai-Tibet Railway in China

Based on dynamic triaxial test at low temperature of the frozen clay from the Beiluhe permafrost subgrade along the Qinghai-Tibet Railway (QTR), residual deformation laws and dynamic subsidence prediction model of permafrost subgrade under train traffic were studied. First, time history curves of residual strain of frozen soil are obtained and analyzed under different temperatures, frequency, confining pressure and moisture content. And conclusions can be drawn that the axial strain rate is greatly affected by the amplitude of dynamic stress, as well as increases with dynamic to static stress ratio and temperature rising, while it decreases with the raise of frequency and moisture content. Hereby, the power functions were adopted to fit the relationships of axial dynamic strain rate vs. stress ratio, temperature, frequency, and moisture content, respectively. Simultaneously, the permafrost dynamic subsidence mechanism was interpreted rationally and the variation of fitting parameters was discussed. Furthermore, the long-term traffic loading subsidence model was established through observing the axis residual strain time histories of frozen specimens under the longtime cyclic loading and adopting the amendment of vibratory number of times. The model can comprehensively consider the effects of stress state, temperature, moisture content, and confining pressure of frozen soil, as well as the vibratory frequency and vibration number of longtime cyclic loading. Consequently, the model can be applied to the train-induced subsidence forecast investigation of permafrost subgrade. The paper has an important significance for rational safety evaluation on long-term operation of permafrost regions railway such as QTR. Meanwhile, the investigation provides basic data for the further research on dynamic damage constitutive model of frozen soil under train traffic and the gradual improvement of railroading criterion in cold regions.     

Characteristics of roadbed settlement in embankment-bridge transition section along the Qinghai-Tibet Railway in permafrost regions

The Qinghai-Tibet Railway (QTR) project was finished on July 1, 2006, and has served for over 3 years. Judging from the present situation, the roadbed is stable and train speed in permafrost regions achieves 100 km/h as expected during the designing. However, as half part of the roadbed was constructed over the permafrost characterized by high ground temperature and high ice content, slight changes of the permafrost might lead to roadbed problems, of which the settlement in embankment-bridge transition section is an obvious and special one. Investigated results of 164 bridges and accounting to 656 positions from the Xidatan Basin to the Chiqu Valley along the QTR in 2009 showed that the settlement was influenced by factors including bridge orientation, embankment slope direction, embankment height, ground temperature, ground ice content of permafrost and local subgrade soil type. For the average value of the settlement, it was greater at the northern end of a bridge than that at the southern end, and was greater in sunny-slope than that in shady-slope. It was greater in high ice permafrost regions than that in low ice regions, and was greater in high-temperature permafrost regions than that in low-temperature regions. Additionally, it increased logarithmically with the height of the embankment. In regions where the subgrade soils were dominated by silt, silty clay or fine sand, the settlement amount was higher than that in bedrock regions. Correlation analysis results showed that there were good relationships between the settlement and the slope direction, embankment height, ground temperature and ice contents when some of the later items were quantified. The correlation coefficients were 0.234, 0.213, −0.21 and 0.151 respectively, when the factors were quantified.     

Characteristics and mechanisms of embankment deformation along the Qinghai-Tibet Railway in permafrost regions

Based on field monitoring datasets, characteristics of embankment deformation were summarized along the Qinghai-Tibet Railway in four permafrost regions with different mean annual ground temperatures (MAGTs). Then, further analyses were carried out at some typical monitoring profiles to discuss mechanisms of these embankment deformations with consideration of detailed information of thermal and subsurface conditions. The results indicated that in regions with MAGT <− 1.5 °C, embankments only experienced seasonal frost heaves, and of which the magnitudes were not significant. So, the embankments in the regions performed satisfactorily. Whereas in regions with MAGT ≥− 1.5 °C, both traditional embankment and crushed rock embankment experienced settlements, but characteristics and mechanisms of the settlements were different for the two kinds of embankment. For crushed rock embankment, the magnitudes of settlement and differential settlement between right and left embankment shoulders were not significant and increased slowly. In respect that upwards movements of permafrost tables and better thermal stability of permafrost beneath embankment, mechanism of settlements on the embankment was inferred as creep of warm and ice-rich layer often present near permafrost table. While for traditional embankment, particularly in warm and ice-rich permafrost regions, the magnitudes of settlement and differential settlement between right and left embankment shoulders were significant and still increased quickly. Considering underneath permafrost table movements and permafrost temperature rises, mechanisms of settlements on the embankment included not only creep but also thawing consolidation of underlying permafrost. Therefore, some strengthened measures were needed to ensure long-term stability of these traditional embankments, and special attention should be paid on temperature, ice content and applied load within the layer immediately beneath permafrost table since warming and thawing of the layer could give rise to considerable settlement.     

Experimental investigation of the dynamic behavior of frozen clay from the Beiluhe subgrade along the QTR

The dynamic behavior of frozen clay obtained from the Beiluhe permafrost subgrade along the Qinghai-Tibet Railway (QTR) was investigated through cryo-dynamic triaxial experiments. The effects of several key factors, including temperature, moisture content, frequency and confining pressure on the dynamic behavior, were analyzed. It was found that a hyperbolic model could describe the dynamic behavior of the frozen clay well. The maximum dynamic shear modulus of the frozen soil decreases as the temperature rises and as the confining pressure decreases; the reference shear strain magnitude decreases as the temperature and confining pressure increase and as the moisture content decreases; the maximum damping ratio increases as the temperature, frequency and confining pressure increase and as the moisture content decreases. There exists a critical moisture content (around 18%) that maximizes the dynamic shear modulus; at the critical frequency (about 6 Hz), the dynamic shear modulus is at its maximum, and the reference shear strain magnitude and the damping ratio attenuation exponent are at their respective minima. Moreover, functions were proposed to quantitatively describe the relationship between frozen clay dynamic parameters and various influencing factors.     

In-situ test study on the cooling effect of two-phase closed thermosyphon in marshy permafrost regions along the Chaidaer-Muli Railway, Qinghai Province, China

More than half of the recently built 142-km long Chaidaer-Muli Railway (CMR) in northern Qinghai Province, China travels across warm (≥−1 °C), ice-rich permafrost in wetlands on the southern flank of the Qilian Mountains. In comparison with the Qinghai-Tibet Railway from Golmud to Lhasa, the CMR traverses mostly across wetlands underlain by more ice-rich permafrost. Warm and ice-rich permafrost is sensitive to human activities and environmental changes, which can result in changes in the active layer thickness and permafrost temperatures, inducing instability and failure of infrastructures in permafrost regions. Thermosyphons were adopted in a quarter of the whole CMR route. For studying the cooling effect of the thermosyphon technique, two monitoring sites with different mean annual ground temperatures were installed since 2007. According to analysis of the ground temperature monitoring results from 2007 to 2010, the thermosyphon technique cooled the underlying permafrost and raised the permafrost table. The CMR has been put in operation since February 2010. The deformation monitoring data from 2008 to 2010 showed that the maximum accumulated settlement was 0.08 m and the minimum was 0.01 m. The settlements mainly happened in the initial months after the embankment construction was finished. In-situ monitoring results indicate that the thermosyphon technique has effect on cooling down the underlying permafrost and keeping the thermal stability of embankment in the unstable, marshy and ice-rich cold regions.     

The temperature field induced by the dynamic stresses of a transverse crack in periodically layered composites

The temperature field induced by the dynamic application of a far-field mechanical loading on a periodically layered material with an embedded transverse crack is investigated. To this end, the thermoelastically coupled elastodynamic and energy (heat) equations are solved by combining two approaches. In the first one, the dynamic representative cell method is employed for the construction of the time-dependent Green’s functions generated by the displacement jumps along the crack line. This is performed in conjunction with the application of the double finite discrete Fourier transform on the thermomechanically coupled equations. Thus the original problem for the cracked periodic composite is reduced to the problem of a domain with a single period in the transform space. The second approach is based on wave propagation analysis in composites where full thermomechanical coupling in the constituents exists. This analysis is based on the coupled elastodynamic-energy continuum equations where the transformed time-dependent displacement vector and temperature are expressed by second-order expansions, and the elastodynamic and energy equations and the various interfacial and boundary conditions are imposed in the average (integral) sense. The time-dependent thermomechanically coupled field at any observation point in the plane can be obtained by the application of the inverse transform. Results along the crack line as well as the full temperature field are given for cracks of various lengths for Mode I and Mode II deformations. In particular the temperature drops (cooling) at the vicinity of the crack’s tip and the heating zones at its surroundings are generated and discussed.