Roadbed Surface Research Articles

NUMERICAL METHODS OF HYDRO-DYNAMIC COUPLING IN SATURATED SOIL AND ITS APPLICATION TO RAILWAY ENGINEERING

Recent field case study shows that the roadbed of ballastless high-speed railway experienced water-induced defect such as excessive fines pumping and even local subgrade-track contact loss affecting the normal operation of highspeed train due to water immersion through gaps of waterproof materials in expansion joints between the concrete base, particularly in rainy seasons. However, the study about the dynamic behavior of high-speed railway subgrade involving water is currently rare. Based on the theory of fluid dynamics in porous medium and the vehicle-track coupling vibration theory, a numerical method of hydraulic-dynamic coupling was established to evaluate the dynamic responses of saturated roadbed surface layer under the high-speed train loading with the validation by comparing the calculated values and field data. The temporal and spatial characteristics of dynamic behaviors (stress, pore water pressure, seepage velocity, displacement) of saturated roadbed surface layer are fully discussed. Also, the effects of train velocity, permeability, on aforementioned dynamic responses of the saturated roadbed surface layer are evaluated. The study shows that improving the drainage of ballastless track roadbed has a significant effect on minimizing the mud pumping of ballastless track, and the influence zone of hydraulic-mechanical coupling is mainly within 0.1 m of the roadbed.

In Situ Investigation of the Dynamic Response and Settlement in the Expressway Culvert–Subgrade Transition Section Using a Vibration Exciter

During the operational phase of the expressway, a significant challenge arises concerning substantial differential settlement in the transition zone connecting the culvert and the general subgrade, affecting its smoothness. In order to address the issue of abrupt stiffness variations within the transition section and to mitigate the occurrence of differential settlement, a gradient pile–reinforced-concrete slab composite foundation was implemented for the first time within an expressway culvert–subgrade transition section. At the same time, an in situ vibration test was conducted through the SBZ30 vibration exciter to comprehensively understand the vertical dynamic responses in the culvert–subgrade transition section under various axle loads and speed conditions. Furthermore, continuous monitoring was conducted to track the long-term settlement of the roadbed. The findings indicate that the utilization of gradient pile–reinforced-concrete slab composite foundations can significantly mitigate the amplitude of the dynamic response parameters. Moreover, dynamic parameters and attenuation coefficients exhibit a gradual reduction as the depth increases. Dynamic stresses, acceleration, and displacements on the roadbed surface exhibited positive correlations with both the axle weight and vehicle speed. However, at deeper depths, the load weight exerted a more pronounced influence. As the speed rose, acceleration decayed faster, affecting a shallower depth. Conversely, the increased load slowed the acceleration decay. The cumulative deformation of the roadbed and the number of excitations followed exponential function characteristics. Settlement values progressively increased while the settlement rate gradually diminished, eventually reaching a stable state, ultimately stabilizing within 4.7 mm. These research outcomes offer valuable guidance and serve as a reference for the implementation of gradient pile–reinforced-concrete slab composite foundations within the culvert–subgrade transition section.

Numerical investigation into hydrodynamic behavior of graded aggregates of high-speed ballastless track considering the degree of saturation

The ballastless track inevitably suffers from some functional defects, such as expansion joint cracking, due to the long term combined effect of dynamic train loads and environmental factors. As a result, water can be somehow retained in the graded aggregates of the roadbed surface layer under ballastless track, which is a key structural component subject to high-speed train loads. Accordingly, it is of significance to study the hydrodynamic behavior of roadbed surface layer that can be prone to depend on variation of the degree of saturation (Sr) in graded aggregates. A coupled hydrodynamic model of roadbed surface layer under ballastless track subject to high speed train loads was established based on homogenization theory and elastic wave theory involving Sr of graded aggregates. The proposed model is validated by degrading to Biot's dynamic model in saturated porous medium and analytical method of unsaturated soil. The effect of different Sr on the pore fluid pressure, spatial seepage velocity of graded aggregates is investigated based on this model with the practical seepage boundaries in ballastless track considered. On this basis, the influence of different spatial locations on the dynamic seepage index is discussed, and the differences in the stress paths in the roadbed surface layer under different Sr at different depths are analysed. The numerical results show that the increase of Sr has a spatial and critical effect on the increase of pore water pressure and seepage velocity in the roadbed surface layer. When the expansion joint of ballastless track cracks, the hydrodynamic erosion in the surface layer of ballastless track roadbed has been accumulating continuously. The erosion is more serious within the depth of 15 cm under the surface of roadbed. For Sr above 0.7, extra attention should be given to prevent internal erosion diseases in the roadbed surface layer during maintenance of ballastless track.

An Analysis of Dynamics of Retaining Wall Supported Embankments: Towards More Sustainable Railway Designs

Retaining walls are structures used to retain earth materials on a slope. Typically, they are designed for static loads, but for highway and railway infrastructures, vehicle-induced dynamic responses are also relevant. Therefore, retaining wall structures are often designed with a factor of safety that is higher than necessary, because it can be challenging to quantify the magnitude of expected dynamic stresses during the design stage. This unnecessary increase in material usage reduces the sustainability of the infrastructures. To improve railway retaining wall sustainability, this paper presents the results from a field monitoring campaign on a heavy-haul rail line with a retaining wall, studying the dynamics induced in response to 30-ton axle load trains running at speeds of between 5 km/h and 100 km/h. The site comprises an earth embankment supported by a gravity retaining wall, with accelerometers on the sleepers, roadbed surface, and retaining wall, velocity sensors on the roadbed, and strain gauges on the rail web to record wheel–rail forces. The vibration intensities collected from various locations are processed to explore the peak particle velocities, maximum transient vibration values, and one-third octave band spectrums. Two transfer functions define the vibration transmission characteristics and attenuation of vibration amplitude along the propagation path. The long-term dynamic stability of the track formation is studied using dynamic shear strain derived from the effective velocity. The peaks of observed contact forces and vibrations are statistically analyzed to assess the impact of train speed on the dynamic behavior of the infrastructure system. Next, a 3D numerical model expresses the maximum stress and displacements on the roadbed surface as a function of train speed. The model evaluates the earth pressures at rest and vehicle-induced additional earth pressures and horizontal wall movement. The investigation provides new insights into the behavior of railway track retaining walls under train loading, and the field data are freely available for other researchers to download. The findings could facilitate the design of more sustainable retaining walls in the future.

Field Measurements and Numerical Simulation of Snowdrift on Railway Subgrade

Abstract Snowdrift is a major natural hazard, endangering basic installation and human life in cold regions. Drifting snow can cause snow grains to accumulate on the road, blocking transportation, reducing visibility, and even leading to fatal traffic accidents. Field measurements and numerical simulations of roadbed snow characteristics were carried out along the railway in snowy weather to study the effect of snowdrift on railway subgrade. The influence of a typical roadbed structure is considered, and the basic model of a typical subgrade structure under the action of snowdrift is proposed. Snow particles that accumulate on the embankment present a U-shaped distribution along the inflow direction. Within the increase of embankment height, wind velocity on the roadbed surface is accelerated, and snow cover is reduced, thus mitigating the snow drift disaster of the embankment. Snow covers at embankment heights of 1 and 2 m are considerably greater than those at 3 and 5 m, and identical with those at roadbed heights of 3 and 5 m. Along the inflow direction, the snow cover on the cutting presents an M-shaped distribution. As cutting depth is reduced, wind velocity inside the cutting is decelerated, and snow cover is increased, which aggravates the snow drift disaster of the cutting. The snow depths of roadbed parts at 1-, 2-, and 3-m depths of the cutting are nearly the same, and that at a 5-m depth of the cutting is relatively small but still larger than that on the embankment. Numerically simulating the snow-driving wind via computational fluid dynamics, which can consider the motion characteristics of snow particles and their influence on flow field, is feasible. Simulation results agree with the measured results. Thus, this research provides a path for the further prediction of snow cover pattern.

Mud pumping in high-speed railway: in-situ soil core test and full-scale model testing

Mud pumping induced by moving train loads on rainwater-intruded roadbed causes intensive track vibrations and threatens safety of high-speed trains. In this paper, a vehicle–track–subgrade finite element model was established to analyze the dynamic responses of a ballastless track, and results showed that the concrete base and roadbed were detached because of the whipping effect arising from the rainwater intrusion channel. An in-situ soil core test showed that the intruded rainwater accumulated in roadbed to form standing water and saturated the roadbed. The flapping action of the concrete base caused by the whipping effect led to mud formation mixed with fine particles and rainwater, which migrated upward under the pore-water pressure (PWP) gradient. Mud pumping resulted from continuous particle migration in the saturated roadbed under moving train loads: under normal roadbed condition, coarse and fine particles were uniformly distributed in the roadbed; in early period of mud pumping, fine particles migrated downward to bottom of the roadbed because of the rainwater infiltration flow; in middle stage of mud pumping, fine particles migrated upward and gathered at the roadbed surface under PWP gradient; in later period of mud pumping, fine particles were entrained and removed with the dissipation of excess PWP. Moreover, a full-scale physical model was established to reproduce mud pumping, and polyurethane injection remediation against mud pumping was validated on this physical model. The remediation method was applied to an in-situ mud pumping. The deviation of the vertical track profile reduced remarkably and remained at a low level within half a year, showing a good long-term service performance of the polyurethane remediated roadbed.

Stability Analysis of Subgrade under Dynamic Loading of Single and Double High-Speed Railways in Karst Areas

With the increase in the use of high-speed trains, karst collapse disasters have become increasingly prominent during the construction and operation of railway projects. To study the propagation characteristics of vibration wave caused by single and double railways in karst area, as well as the influence of sinkholes on the stability of the roadbed, a coupling model of vehicle-track-subgrade (including underlying sinkhole) considering track irregularity was established based on the geological engineering characteristics of a karst site. The results show that the subgrade stability is controlled by multiple factors related to the upper load, the stability of the caves, and the soil state; increasing the upper load of the track further develops the sinkhole and has a negative effect on the subgrade stability. The amplitude of vertical dynamic displacement on the subgrade surface can reach 1.31 mm under the superimposed effect of the vibration of double railways, which is approximately 50% higher than that of a single railway. Under the long-term cumulative effect of train vibrations, the sinkholes grow faster in dynamic displacement amplitude than the roadbed surface, and will lead to abrupt displacement of subgrade surface.

Dynamic performance evaluation of ballastless track in high-speed railways under subgrade differential settlement

Ballastless tracks are widely used to provide high rail smoothness in high-speed railways. However, the differential settlement inevitably develops in the subgrade soil, which poses a great threat to the track performance and riding quality via the train-rail dynamic interaction. Since the ballastless tracks are extremely difficult to repair during the maintenance window, it is quite necessary to carry out preventive maintenance to keep the track in good conditions. In order to relate the railway performance with the profiles of subgrade differential settlement, a three-dimensional train-ballastless track-subgrade model was developed incorporating 64 combinations of settlement wavelengths and amplitudes at the roadbed surface. The numerical results were first verified with the measured velocity responses at the concrete base in the Beijing-Tianjin high-speed railway. Then the dynamic responses of the train-track system caused by subgrade differential settlement were analyzed, including the dynamic displacement of train wheels and track structure, wheel-rail interaction forces and car body accelerations. Railway performance was further evaluated as track degradation, lower riding comfort and risk of train safety based on these indicators at different settlement profiles. Results show that increasing settlement amplitudes result in stronger dynamic wheel-rail interaction and vibration of car body. However, critical settlement wavelengths of 10 m and 10–20 m are found to exist where the wheel-rail interaction forces and accelerations of the car body reach their peak values, respectively. It also reveals that the current design limits on the subgrade settlement of high-speed railways are infeasible for infrastructure managers to evaluate the railway status or organize the maintenance works. Moreover, the wheel-rail interaction forces are more credible to determine the railway status than the car body acceleration.

In-situ investigation on mud pumping in ballastless high-speed railway and development of remediation method

Mud pumping induced by intensive precipitation and traffic loading is a typical distress in high-speed railway either for ballasted or ballastless tracks. In this study, in-situ investigations, numerical simulation and laboratory tests were conducted to reveal the formation of mud pumping and to explore an appropriate method to remediate mud pumping in ballastless railway. Field investigations and numerical simulation indicated that there was a whipping effect in the dynamic responses of the slab track due to train passages, which induced detachments at the end of concrete base from the roadbed surface. This detachment provided infiltration routes for rainwater into the roadbed layer and initiated the mud pumping. An evaluation method was proposed to analyze particles stability of roadbed materials taken from the investigating site, and the results indicated that instable finer particles promoted an increasing instability of coarse particles. Based on the characteristics of particle stability of roadbed materials and the formation process of mud pumping, an optimized polyurethane chemical injection (PCI) method was proposed to remediate mud pumping and tentatively applied to a mud pumping reproduced ballastless slab track in the full-scale physical model test. Both static and dynamic performances of the remediated ballastless track were restored to their initial values before the occurrence of mud pumping. The accumulative track settlement was less than 2.2 mm even under more than one million train loads, indicating that the PCI method is effective in mud pumping remediation of ballastless slab track.

Vibration Reduction of Layered Sandbags under Traffic Load

In this study, a new type of traffic load test device was adopted to investigate the vibration isolation of layered sandbags under train load. The upper part of this device is composed of five high performance hydraulic actuators, to simulate the traffic load produced by train movement. First, sandbags were stacked on the subgrade, which were used as an active vibration isolation barrier. The variations of the soil stress, acceleration and vertical deformation of the sandbag subgrade were analysed by simulating different train speeds, and the relationship between the surface acceleration and train load was established. The attenuation of surface acceleration with depth was further analysed. Then, expanded polystyrene (EPS) foam plates with different thicknesses (placed at a certain distance from the subgrade) were used as a passive vibration isolation barrier. The vibration isolation effect of the EPS foam plate was also analysed. Results show that the sandbag layer used as a subgrade exhibited good vibration isolation effect. At a train load of 15 kN, the acceleration at a depth of 25 cm is less than that at the roadbed surface. The faster the train speed, the higher the acceleration attenuation rate.

Исследование транспортно-эксплуатационного состояния автомобильной дороги общего пользования на примере улиц Гражданская–Производственная в г. Каменка

The article uses a practical example to consider changes in the transport and operational state of the public highway and road surface, with the possibility of justifying and taking into account these changes in the design, operation and repair.
 The methodological approach of the road and road surface survey provides for the first stage of visual inspection of the road and identification of areas with visible defects of the existing roadbed and road surface. At the second stage, the asphalt roadbed was opened by pits in the most visible areas of damage in order to take samples of road clothing and subsequent analysis. Defects and causes of deformations of asphalt concrete pavement and existing roadbed were identified. Recommendations for eliminating the causes of deformations and strengthening the coating are given.

Quality problems and preventive measures for municipal road and bridge construction

In the process of construction of municipal roads and bridges, it is especially important for the construction of waterproof roadbed. If the leakage of waterproof foundation surface will inevitably lead to the impact of the quality of roads and bridges, not only the economic and social benefits of the enterprise be lost but also threaten the safety of pedestrians' lives and property. Therefore, this paper analyzes the influencing factors in the waterproof roadbed surface of municipal road bridge construction, and proposes corresponding solutions.

Effects of suffosion-induced contact variation on dynamic responses of saturated roadbed considering hydro-mechanical coupling under high-speed train loading

Recent experience of high-speed railway in operation has indicated that the hydro-mechanical coupling effect under dynamic loads from high-speed train may lead to excessive pumping of fine particles in saturated roadbed surface layer. This can result in large variations of contact condition between track and subgrade structure layers while study on this issue remains scare. Based on the theory of fluid dynamics in porous medium and the vehicle-track-subgrade coupling vibration theory, a numerical model is established to evaluate the dynamic responses of saturated roadbed surface resulting from the contact condition variation under the high-speed train load considering the hydro-mechanical coupling effects. A concept of three contact types (fully continuous contact, vibration contact and contact loss) is proposed and three contact shape forms (rectangular, cosine and elliptical shapes) simplified for describing contact damaged zones are compared in terms of dynamic responses (stress, pore water pressure, seepage velocity) evaluated. Also, time-frequency analysis is performed to investigate the influence of contact variation on dynamic responses of roadbed surface in aforementioned contact types. Results show that this progressive development of contact damage can pose extra great stress and pore-water pressures in roadbed and a critical length of contact loss zone is suggested.

Investigation of Concrete Base‐Roadbed Surface Contact Variation‐Induced Vibration Characteristics of Vehicle‐Slab Track‐Subgrade System considering Fluid‐Solid Interaction

The excessive pumping of fines in saturated roadbed surface layer, which is induced by the fluid‐solid interaction under dynamic loads from high‐speed train, is a special form of high‐speed railway subgrade defect reported recently. This can deteriorate the interface between nonballasted track structure bottom layer and roadbed surface layer and therefore lead to associated contact variation with the moving of trains. According to the dynamic Biot’s equations known as u‐p formulation and the vehicle‐track coupling dynamics theory, a vertical vehicle‐slab track‐subgrade coupling vibration model is developed to investigate the aforementioned contact variation‐induced dynamic behavior of the whole system considering the fluid‐solid interaction. Dynamic measurements from a field case study are adopted to verify the computation model proposed. Based on the numerical model validated, the effects of three contact variation statuses (continuous contact, vibrating contact, and contact loss) on dynamic responses of track subsystem and subgrade subsystem, such as dynamic pore‐water pressure, vertical accelerations, and dynamic displacements both in time and frequency domains, are investigated. Also, a sensitivity analysis involving rail speeds and lengths of contact loss zone is performed, and the critical length of contact loss zone is suggested.

Dynamic responses of high-speed railway transition zone with various subgrade fillings

In order to explore more economic filling alternatives than typically used graded gravel + 5% cement in a high-speed railway transition zone, engineering properties of graded gravel (without cement) and A, B group filling (well graded coarse-grained soil with less than 30% of fine-grained soil) were tested. This is followed by the establishment of a 3D vertical coupling dynamic model of a tunnel-culvert-tunnel transition section based on the D'Alembert’s principle of energy weak variation and the Lagrange scheme. The model results have been validated against the in situ measurements. The analysis from the model show that both graded gravel and A, B group filling are well-graded with high strengths, and the dynamic responses of the roadbed supported by the two fillings are both less than allowable values at the speed of 350 km/h. However, the curves of vertical displacement along the longitudinal transition section are great like a shape of “W” with the A, B group filling in the transition zone. Therefore, the graded gravel is recommended to be more suitable than the A, B group filling for the studied tunnel-culvert-tunnel transition zone. This recommendation may be applicable to the case with a rock subgrade underneath to support the transition zone. Comparatively, for a soil subgrade under the transition zone, our results indicate that graded gravel + 5% cement is still the best filling material, while the other two less stiffer filling materials would result in considerable fluctuations to the roadbed surface.

Force and Deformation of a Shallow Soil-Buried Super-Long Box Culvert during Jacking Construction on a Highway

An example of a project on an oversized box culvert jacked into highway subgrade in Guangdong area was investigated to determine the mechanical behavior of shallow soil-covered super-long box culvert in the jacking construction process. On the basis of 3D finite element simulation, the law for stress-strain change and the effective construction area of the box were studied by considering different lengths and thicknesses and drag reduction versus non-drag reduction of overburden soil. Result shows that under the same working condition, the stress-strain of the roof and bottom plate exhibits an 'armchair' change shape with an increase in jacking length, whereas the side wall presents a 'trapezoid' shape in the box culvert jacking process. The thickness and characteristics of the covered soil (on top of the box culvert) exert considerable influences on the stress-strain of the box culvert structure. If the covered soil is thick and dense, then the stress-strain of the structure will be high. The pavement (on top of the box culvert) settlement ranges from 7.0-17.1 mm due to the effect of vibrations produced during box culvert jacking construction, and the scope of the disturbed roadbed and road surface is 23-30 m along the jacking direction. The drag reduction effect of the paraffin oil mixture is more effective than that of slurry drag reduction. The research results will play a significant guiding role in the design and construction of similar projects.

Fuel pool development in tunnel and drainage as a means to mitigate tunnel fire size

One of the most dangerous fire events that can happen in road tunnels is a fuel tanker fire, when flammable or combustible liquid fuel spills on the tunnel roadbed and catches fire. The fire develops extremely fast, with its size directly dependent on the size of the liquid pool on the roadbed surface. The tunnel roadbed is typically not flat and is usually sloped both longitudinally and transversely (cross slope). There have been some attempts to evaluate the fuel pool size on a flat or longitudinally sloped surface, whereas there is a lack of methodology for evaluation of fuel pool size, fire zone length and fire heat release rate on a sloped roadbed surface. While there is some controversy as to the exact heat release rate for a given fuel type and fuel pool depth, very little experimentation has been performed or documented on fuel flow across an unbounded sloped surface. This paper presents results of the scaled tests that measured the size and shape of water and fuel on a sloped road surface as well as attempts to verify CFD analysis. The estimated results from the measured data extrapolated to 12 l/s are compared against the CFD model and previously published papers. A 1.2 m × 1.2 m (4 ft × 4 ft) section of concrete surface representing a concrete road was built, and flow tests were performed on the sloped surface with water and gasoline at different flow rates, different surface slope and different surface roughness. The test data was used to determine the liquid pool area based on the flow rate. This data shows that the geometrical shape of the pool remains proportionate for the different parameters of slope, flow rate, and surface roughness that were tested. Since this is an unbounded experiment the test results can be directly applied to a fuel spill on a sloped surface (roadway) by adjusting the fuel flow rate. This flow pattern and flow dynamics were then used to calibrate a CFD model that emulated the flow pattern of the liquid fuel across an inclined concrete road surface. Using the calibrated CFD model and standard drainage calculations the drainage system can be designed to limit the size of the pool of liquid fuel. This paper addresses the pool size and the area of the pool on a sloped concrete surface.

Dynamic behavior of new cutting subgrade structure of expensive soil under train loads coupling with service environment

Expansive soil is sensitive to dry and wet environment change. And the volume deformation and inflation pressure of expansive soil may induce to cause the deformation failure of roadbed or many other adverse effects. Aimed at a high-speed railway engineering practice in the newly built Yun−Gui high-speed railway expansive soil section in China, indoor vibration test on a full-scaled new cutting subgrade model is carried out. Based on the established track-subgrade-foundation of expansive soil system dynamic model test platform, dynamic behavior of new cutting subgrade structure under train loads coupling with extreme service environment (dry, raining, and groundwater level rising) is analyzed comparatively. The results show that the subgrade dynamic response is significantly influenced by service conditions and the dynamic response of subgrade gradually becomes stable with the increasing vibration times under various service environment conditions. The vertical dynamic soil stress is related with the depth in an approximate exponential function, and the curves of vertical dynamic soil stress present a “Z” shape distribution along transverse distance. The peak value of dynamic soil stress appears below the rail, and it increases more obviously near the roadbed surface. However, the peak value of dynamic soil stress is little affected outside 5.0 m of center line. The vibration velocity and acceleration are in a quadratic curve with an increase in depth, and the raining and groundwater level rising increase both the vibration velocity and the acceleration. The vertical deformations at different depths are differently affected by service environment in roadbed. The deformation of roadbed increases sharply when the water gets in the foundation of expansive soil, and more than 60% of the total deformation of roadbed occurs in expansive soil foundation. The laid waterproofing and drainage structure layer, which weakens the dynamic stress and improves the track regularity, presents a positive effect on the control deformation of roadbed surface. An improved empirical formula is then proposed to predict the dynamic stress of ballasted tracks subgrade of expansive soil.

Roadbed temperature study based on earth-atmosphere coupled system in permafrost regions of the Qinghai-Tibet Plateau

Abstract In this work, a two-dimensional unsteady model for analyzing the roadbed temperature in permafrost regions of the Qinghai-Tibet Plateau (QTP) is developed and the heat transfer processes of comprehensive earth-atmosphere coupled system are simulated by the numerical method. In the computation, the surrounding air environment of ground surface is brought into the research model and the corresponding unsteady and non-uniform influence factors of earth-atmosphere coupled system are also considered to obtain reasonable simulation results. Meanwhile, the model is validated with monitored data at a depth of 0.5 m in different roadbed positions. Based on the above model, the spatial and temporal temperature distributions of the roadbed surface in Wudaoliang region are determined. Then the influences of the system parameters, such as the ambient temperature, wind speed, roadbed height and route trends, on the local temperature distribution are investigated. The results show that, temperature of surface regions exhibits a periodic sinusoid trend and always higher than the ambient air temperature. For different surface regions, the temperature of asphalt pavement keeps the maximum value and the natural ground surface has an abrupt step change in temperature at beginning of April and end of October. The variation of ambient air temperature has little influence on annual average temperature of permafrost roadbed surface. The deviations between the surfaces' annual average temperature increase and their average values are less than 1%. Annual average temperature of various surfaces decreases as the wind speed and the roadbed height increase. Furthermore, the annual average temperature of leeward slope (also sunny side) along EW direction is the highest one among all three route directions and such temperature decreased systematically from SW45° direction to SN direction. However, the annual temperature of upwind slope (also shady side) is just contrary to the leeward slope.

Cooling effect of ballast revetment on the roadbed of Qinghai-Tibetan Railway

The data obtained through the roadbed surface thermal regime experiment (ROBSTREX), which was carried out at Beiluhe test section of Qinghai-Tibetan Railway from October to December in 2002, were used to estimate the cooling effect of ballast revetment on the road bed. The results show that both riprap rock ballast revetment and crushed stone ballast revetment can reduce the temperature of the roadbed. But the cooling effect of riprap rock ballast revetment is better than that of crushed stone ballast revetment when the temperature of roadbed is higher. The cooling effect of crushed stone ballast revetment is better than that of riprap rock ballast revetment when the temperature of the roadbed is lower, especially at deeper roadbed layers. In the frozen season, the heat release from the roadbed also shows that the cooling effect of ballast revetment on the roadbed is obvious, and the cooling effect of crushed stone ballast revetment on the roadbed is much evident than that of riprap rock ballast revetment.