Search

In the last decade, the design and construction technologies of subgrade in high-speed railway (HSR) developed significantly. This article reviews corresponding development in five aspects, including mechanical properties of fill materials, dynamic performance of subgrade, foundation treatment, retaining structure, and smart construction technologies. It shows that for unbonded granular materials, it is acceptable to use static strength for subgrade design, but for clayey soil it would be more appropriate to base on shakedown theory. The mechanism for lime modified clay has been thoroughly reviewed, and the effect of lime content, curing age, and curing conditions on the behavior of lime-treated clay is discussed. The dynamic response of subgrade, especially the long-term deformation and dynamic stability analysis are important to understanding the behavior of HSR subgrade. The effect of track types, operation speed, etc. on the dynamic response of subgrade are reviewed first. Then, the prediction methods, influencing factors, and corresponding issue for long-term deformation of subgrade are presented, followed by the methods used for dynamic stability analysis. Three types of foundation treatment methods, including geosynthetic-reinforced pile-supported (GRPS) embankment, pile-raft structure, and pile-plate structure are reviewed for the corresponding load transmission mechanism, and application scenario. The static and dynamic behavior of four types of retaining structures are presented, including cantilever retaining wall, geosynthetic reinforced soil retaining wall, anchored retaining structure, and retaining wall reinforced by soil nailing. Finally, a series of new technologies correlated to smart construction are introduced, relating to the survey, design, construction, detection, and management of subgrade.

In this paper, a new type of MSE wall facing, termed as hybrid facing, is introduced and studied, which is built using a combination of concrete modular blocks and cast-in-place concrete. Two shaking table tests were carried out to compare seismic performances of model reinforced soil retaining walls with full-height vs. hybrid facing configurations. Results of this study show that the stability and performance of the hybrid facing model were similar to those of the full-height panel wall for peak input acceleration magnitudes less than 0.40 g. The amplification factors along the height of the facing were more uniform and smaller in the hybrid facing model as compared to the full-height panel wall, especially at higher peak acceleration amplitudes. Dynamic increment of lateral earth loads acting on the facing in both cases were found to be only 20% of the values calculated using pseudo-static methods. Connection loads in the hybrid facing model were smaller than those in the full-height panel wall, which was attributed to its smaller facing displacements.

In order to study the instability development process of the slope reinforced by anti-slide piles under earthquake conditions, the dynamic response characteristics of the slope are usually taken as the main characteristics, and the model test and numerical simulation are the main research methods. In this paper, a shaking table model test is designed and completed to investigate the influence of anti-slide piles with different initial damage on the failure mode of high and steep slope under earthquake conditions. The changes in velocity, strain and natural frequency during slope vibration are tested in combination with cloud maps when sinusoidal waves of different accelerations with a peak value of 5 Hz are applied. Thus, the differences of slope failure development process and dynamic response characteristics are obtained. The experimental results show that the anti-slide pile with different initial damage has obvious influence on the slope instability process. Under the condition of good anti-slide pile quality, the failure development of the slope behind the pile is limited to soil sliding on top of the slope, slope sliding and overburden sliding; the front slope foot of pile mainly forms shear belt and local sliding. With the decrease in the initial mass of the anti-slide pile, the slope failure develops into topsoil sliding, slope sliding and deep integral sliding; analogously, the failure of the slope in front of the pile develops into a whole slip along the slip belt. The natural frequency cloud map can directly reflect the damage location of the slope, and the frequency change rate is positively correlated with the cumulative shear strain. It shows that the macro-failure characteristics of the model slope change well when the natural frequency is used as the sensitive index to measure the influence of vibration on the model slope. The threshold value of the natural frequency change rate can distinguish different development stages of the slope; 1% is the threshold value of stage II, and 1.5% is the threshold value of stage III.

This study aimed to illustrate the seismic response and instability process of a double-row anti-slide pile-reinforced bridge foundation and gravel landslide. Selecting the gravel slope of Jiuzhai Valley along the under-construction Chengdu-Lanzhou high-speed railway as the site prototype, large-scale shaking table tests were first conducted at a similitude ratio of 1:70, with sine waves and El Centro waves as the seismic wave inputs. The amplitudes of the input seismic waves were increased, while the acceleration, dynamic earth pressure, and strain distribution were monitored in the gravel landslide. The dynamic response patterns of anti-slide pile-reinforced bridge foundations in gravel landslides were illustrated. The front-row and back-row anti-slide piles should be a reasonable distance from the bridge foundation. The response acceleration manifested an elevation amplification effect with an increasing elevation in the slope behind the anti-slide piles. Back-row anti-slide piles reinforcing the bridge foundation can reduce the effect of landslide thrust on the bridge foundation and maintain a uniform distribution of earth pressure behind the bridge foundation, mitigating seismic effects. The dynamic earth pressure peaked at the top of the bridge foundation and then decreased along the depth. The back-row anti-slide piles displayed greater resonance coupling and unloading effects before and after reaching the load-carrying limit, respectively. In seismic strengthening design involving bridge foundations and gravel landslides, when the earthquake-induced resonance coupling effect on inclined, loosely packed land masses is fully considered, pre-reinforcement measures (e.g., high-pressure grouting and anchor spraying) should be carried out on the gravel slopes.

A good estimation of additional stress of a bridge approach foundation overlying embankment is of great importance in order to accurately calculate the differential settlements of foundation due to the axisymmetric distribution of the additional stress. Current design method commonly adopted in road and railway engineering in China is derived from the classical elastic theory and simply considers the embankment load by its self-weight (γh) without taking into account the influence of the width-to-height ratio. This may lead to an overestimation of additional stress and consequently an uneconomical overdesign. This study attempts to mitigate the gap by developing a new analytical solution. Based on the Boussinesq’s elastic theory, the proposed solution considers a constrained boundary condition associated with bridge abutment and incorporates a new concept of ground reaction, which takes into account the embankment load including the influence of the embankment width-to-height ratio. The proposed solution was finally validated by numerical analysis and shows a better performance in terms of additional stress estimation over the traditional calculation method. Overall, the proposed method proved to be a better alternative tool for the design of bridge approach embankment.

The paper reports a 1 g shaking table test that was carried out on a reinforced soil wall with an objective to study the acceleration amplification in the backfill, and phase differences between dynamic responses of the reinforced and retained zones. Results of the study show that including the observed larger acceleration amplification in the upper half of the wall, and the phase difference between maximum lateral earth pressure and inertial load in the backfill in the analysis would lead to more accurate predictions of: (1) the wall response relative to predicted reinforcement load, (2) elevations of line of action for both the inertial and lateral earth forces in the backfill, and (3) wall deformations, as compared to pseudo-static methods of analysis.

This study aimed to illustrate the seismic performances of anti-slide pile-reinforced bridge foundations in landslides. Based on the anti-slide reinforcement project at Yousuotun along the Chengdu-Lanzhou high-speed railway under construction, shaking table tests were performed on a double-row anti-slide pile-reinforced bridge foundation and landslide model with a 1:40 similitude ratio. Given that the similitude law was satisfied, seismic waves with different frequencies and acceleration amplitudes were input as base excitations to monitor the responses of dynamic parameters: slope acceleration, earth pressure, and pile strain. The amplification effect of peak ground acceleration (PGA) was analyzed. The landslide thrust distribution characteristics and slope response processes were further studied to verify the seismic design of such bridge foundations in landslide-prone areas. The response acceleration of the slope subjected to seismic loads showed a nonlinear amplification “elevation effect”, “surface effect”, and “geological structure effect”. When the distance between the bridge foundation and the back-row anti-slide pile was small, the pier maintained a relatively large PGA amplification factor and was subjected to strong seismic loads. The landslide thrust was mainly borne by the upper part of the back-row anti-slide piles as its distribution changed from spoon-shaped to bow-shaped. Under high-intensity earthquake events, the load-bearing section of the bridge foundation should be strengthened at the sliding surface. Anti-slide piles can effectively limit the dynamic response of the landslide and weaken the seismic response of the slope. The results of testing this reinforcement are the first results proving that the seismic design of reinforced bridge foundations with anti-slide piles can be reliable.

For bridge engineering on steep slope, where the deformation of piled bridge foundation is strictly controlled, stabilizing piles are sometimes installed to minimize the soil shear deformation around the bridge foundation. A limited number of work has been carried out about the effects of stabilizing pile on the behavior of piled bridge foundation on the slope. In this study, model test and numerical simulations were carried out aiming at clarifying the lateral behavior of piled bridge foundation and stabilizing piles on steep slope. Evolution of bending moment in piles, soil pressure around piles, and lateral displacement of piles and soils were monitored throughout the model test. Model test suggested that back stabilizing piles subjected to much larger bending moment along the pile and force that acts on piles above the sliding surface than those of bridge piles. Numerical analyses were carried out to optimize the location of stabilizing piles. Front stabilizing piles should be located close to the bridge foundation. The optimum position for back stabilizing piles was 3 to 5 times of stabilizing piles width away from the bridge foundation.

Although weathered red mudstone (WRM) is widely distributed in the southwest of China, its suitability as a fill material for the subgrade bed of a high-speed railway (HSR) has not been comprehensively investigated. This paper presents the results of a field full-scale model testing of the cyclic loading and response of a railway track-subgrade system for the Dazhou-Chengdu Railway, where some WRM has been used in a newly designed subgrade structure. A control section of the HSR was also built using a traditional subgrade bed (Group A&B material) to compare the performances of the different sections. To better understand the dynamic characteristics and cumulative deformation of the two types of subgrade, the dynamic actions of different axle loads and different train speeds were simulated using specifically designed track-cyclic-loading equipment. The transverse and vertical distributions of the dynamic stress, dynamic displacement and acceleration of the track-subgrade system were measured and evaluated. The influence of the wheel axle load on the growth factor of the dynamic parameters, the vertical attenuation coefficient of the dynamic stress, and the effect of using WRM in the subgrade on the post-construction settlements were investigated. The tests enabled the development of cumulative settlement laws with railway loading for the two types of subgrade. Although the dynamic parameters and cumulative settlement of the WRM subgrade are always greater than those of better-quality material (Group A&B subgrade), they comply with HSR regulations. In conclusion, the results demonstrated that weathered red mudstone can be used as a filling material in the newly designed subgrade structure for HSRs.

This paper presents a comprehensive field investigation of the swelling-shrinkage behavior of an expansive soil ground under high-speed railway embankment loads. In this study, a test site close to the Kunming-Nanning high-speed railway (KNHR) was chosen for the construction of four full-scale field test facilities for artificially soaking the expansive soil ground. Three of the facilities consist of embankments of three different heights, while the fourth facility is for a series of plate load swelling tests. All the test embankments were fully instrumented to monitor the ground deformation and the changes in volumetric water content profiles of the foundations. The full-scale field tests were complemented by a detailed site investigation comprised of cone penetration tests (CPTs), standard penetration tests (SPTs) and a comprehensive laboratory characterization of intact expansive soil samples retrieved from the test site. The results obtained from the laboratory and field tests show that the swelling behavior of the expansive soil ground mainly depends on the embankment load. By properly designing the embankment height and considering the maximum swelling pressure the expansive ground could induce, the heave of the embankment could be controlled efficiently. The measured displacements at the ground surface are well correlated with the evolution of measured volumetric water contents within a ground depth of around 4.5 m. The majority of these displacements occurred when the ground was approaching saturation along both wetting and drying paths. Finally, a simple method based on one-dimensional test results was proposed, and a good performance was shown in predicting the heave or settlement of embankments over an expansive soil ground upon wetting and drying.

The objective of this study is to verify the feasibility of using centrifugal model tests to characterize the ground reaction response of a high-speed railway embankment to predict its ground settlement. To do so, a series of Centrifugal Model Tests (CMTs) of the embankment were performed to investigate the variation of ground reaction and corresponding derived modulus at different embankment ratios. The ground reaction determined from the CMTs was compared with those obtained from corresponding full-scale field tests of instrumented embankment. The results show that the ground reaction at the embankment centreline gradually approaches the embankment weight (γH) with increasing embankment ratio. Considering the embankment filling process with different embankment ratios, it seems inappropriate to employ the value of γH as the ground reaction without any correction in settlement prediction. Finally, the predictive capability of the proposed approach for embankment settlement estimation was assessed against the corresponding field measurements and those determined based on the calculation methods suggested in the current design standard in China. The results show that the proposed simple approach satisfactorily estimates the ground settlement of HSR embankments and its deviation of predicted settlements is below 11.9% compared with the data obtained from the field full-scale tests. However, the deviation of the result calculated by the current recommend design standard is 17.7%-148.3%.

The construction of a high-speed railway (HSR) in Southwest China is being hindered by a severe shortage of high-quality subgrade materials. However, red mudstone is widely distributed in the Sichuan Basin of China. The ability to use weathered red mudstone (WRM) to fill subgrade beds by controlling its critical stress and cumulative strain would enable substantial savings in project investments and mitigate damage to the ecological environment. To better understand the dynamic behaviour of WRM, both monotonic and cyclic triaxial tests were performed. The evolution of the cumulative strain vs. increased loading cycles was measured. The influences of confining pressure and loading cycles on the dynamic modulus, damping ratio, critical cyclic stress ratio (CSR), and dynamic stress level (DSL) were investigated. The relationship between the CSR and loading cycles under different failure strain criteria (0.1%-1.0%) was analysed. The prediction model of cumulative strain was also evaluated. The results indicated that the shear strength of WRM sufficiently meets the static strength requirements of subgrade. The critical dynamic stress of WRM can thus satisfy the dynamic stress-bearing requirement of the HSR subgrade. The critical CSR decreases and displays a power function with increasing confining pressure. As the confining pressure increases, the DSL remains relatively stable, ranging between 0.153 and 0.163. Furthermore, the relationship between the dynamic strength and loading cycles required to cause failure was established. Finally, a newly developed model for determining cumulative strain was established. A prediction exercise showed that the model is in good agreement with the experimental data.

Quaternary silt is widely distributed in China and easily liquefies during earthquakes. To identify the influence of the dry density on the liquefaction behaviour of Quaternary silt, 40 cyclic triaxial liquefaction tests were performed on loose silt (dry density ρd=1.460 g/cm3) and dense silt (ρd=1.586 g/cm3) under different cyclic stress ratios (CSRs) to obtain liquefaction assessment criteria, determine the liquefaction resistance, improve the excess pore water pressure (EPWP) growth model and clarify the relationship between the shear modulus and damping ratio. The results indicate that the initial liquefaction assessment criteria for the loose and dense silts are a double-amplitude axial strain of 5% and an EPWP ratio of 1. The increase in the anti-liquefaction ability for the dense silt is more significant under lower confining pressures. The CSR of loose silt falls well within the results of the sandy silt and Fraser River silt, and the dense silt exhibits a higher liquefaction resistance than the sand-silt mixture. The relationships between the CSR and loading cycles were obtained at a failure strain of 1%. The EPWP development in the dense and loose silts complies with the “fast-stable” and “fast-gentle-sharp” growth modes, respectively. The power function model can effectively describe the EPWP growth characteristics of the dense silt. Finally, based on the liquefaction behaviour of silt, a suggestion for reinforcing silt slopes or foundations is proposed.

Considering the potential use of cementstabilized loess (CSL) as a construction material for structures that are subjected to frequent loess landslides, this paper explores the stabilization and improvement of geotechnical characteristics of loess achieved by the addition of 0%-9% cement by dry weight. Laboratory evaluations investigated the consistency limits, compaction, compressibility, California bearing ratio (CBR), direct shear strength, and unconfined compression strength (UCS) of CSL for different curing stages. A durability index was quantified to estimate the influence of wetting-drying (w-d) cycles on CSL strength, and an optimum cement dosage was also identified. The results reveal that the cohesion of CSL is substantially more sensitive to structure than its friction angle and that cohesion is responsible for shear strength increase after remoulding. The cement proportions have an effective role in the enhancement of compressibility. The development of UCS can be categorized into the early stage ( 14 days). The increase in strength primarily occurred in the first 14 days. The w-d cycles have a significant influence on the decrease in compression strength. The CBR value increases with increments of additional proportions and compaction times. The relationships of UCS versus the compressibility modulus and UCS versus CBR are established to facilitate the mix design for strength. A rational predictive exponential equation is proposed to predict the durability index for different w-d cycles.

This study aims to verify the suitability and reinforcement behavior of newly designed bored piles with retaining walls (BPRWs) for cutting-slope stabilization in the Tibet Plateau. As the first BPRWs prevention works in the alpine and hypoxic areas, full-scale performance testing is conducted of the high cutting slopes along the Lhasa-Nyingchi Railway under construction. Amid the construction processes of slope excavation and BPRWs installation, the overall structural performance, including lateral displacement, bending moment and lateral earth pressure, was determined to evaluate the cutting-slope stability and retaining mechanisms of BPRWs. The monitoring parameter distribution rules were thoroughly analysed. The relationship between the lateral earth pressure and pile displacement was studied, and a soil resistance calculation model was proposed. Finally, the BPRWs suitability was assessed. The results revealed that the retaining behaviour of BPRWs is closely related to the construction process. The displacement rate over time is characterized by alternating peaks and valleys, and the maximum displacement rate and bending moment occurs at the retaining wall pouring stage. The bow-shaped bending moment distribution with the pile depth exhibits low values on both ends and a high value in the middle. The ground reaction coefficient is allometrically attenuated with increasing lateral displacement. The proposed soil resistance model can satisfactorily predict the lateral earth pressure. The BPRWs deformation satisfies the service requirements at the construction and service stages. This case study provides a reinforcement reference for BPRW in alpine and hypoxic areas.

Three shaking table tests were carried out to investigate the influence of loading frequency on seismic performance of MSE walls constructed with full-height panel facing. Results of this study provide quantitative results on how base excitation frequency in combination with base acceleration amplitude could influence the dynamic response of these MSE walls relative to their deformations, earth pressures, acceleration amplifications and connection loads. Results indicate that model walls responded primarily in tilting mode. However, base sliding could also occur as an additional deformation mode at larger base excitation amplitudes, and at frequencies that are closer to the fundamental frequency of the wall, resulting in significant connection loads and other response characteristics. Findings of the study are compared with results from other studies and recommendations in design guidelines, as applicable.

The suitability of lime-stabilized weathered red mudstone (LSWRM) as a subgrade filler material has not been comprehensively investigated. This study aims to determine the dynamic response and cumulative settlement characteristics of a newly designed LSWRM subgrade and evaluate the adaptability of LSWRM for use in high-speed railways (HSRs). Full-scale model testing of the LSWRM subgrade was performed using a cyclic loading apparatus. Under dynamic wheel loads and rainfall conditions, the distribution characteristics of the dynamic stress, dynamic displacement, and acceleration of the subgrade were monitored. The cumulative settlement law of the subgrade bed was analysed to determine the adaptability of LSWRM. Through a comparison with the standard Group A&B subgrade, the dynamic wheel load was identified as a main factor influencing the distributions of the dynamic parameters, whereas the influence of daily rainfall was limited. The dynamic parameters showed a saddle-shaped transverse distribution. Additionally, the appropriate thickness of the subgrade bed was evaluated based on the dynamic stress attenuation characteristics, and the cumulative settlement of the bottom layer accounted for 0.3% of the subgrade bed height. These dynamic parameters can meet the requirements of existing HSR specifications, and thus, filling the bottom layer with LSWRM is an appropriate option.

Reinforced soil retaining walls have been widely constructed all over the world. Although the limit equilibrium (LE) method is common practice in design, a growing body of experimental and analytical evidence has demonstrated that it could lead to overly conservative (and hence, uneconomical) design. In this paper, a kinematic limit analysis method, combined with a pseudo-dynamic method of analysis, is developed for rotational stability analysis of reinforced soil retaining walls based on the log-spiral failure mechanism. The proposed analysis method is validated against the results from different pseudo-static methods. However, the rotational stability and the corresponding yield acceleration coefficients from the proposed method are shown to vary with time, and with factors such as the backfill shear modulus and the frequency of ground motion. Parametric analyses are also conducted to study the influences that pseudo-dynamic analysis parameters, reinforcement properties, wall height, and soil strength have on rotational stability and failure geometry of reinforced soil retaining walls. Results indicate that optimum value of ultimate reinforcement strength is greater when the reinforcement is more widely spaced. Additionally, the cross-sectional area of the rotational failure mass is smaller for walls with lower-strength reinforcement and larger vertical spacing.

A new approach was proposed to describe settlement behavior of an unsaturated soil with subgrade filling for high-speed railway. Firstly, based on Terzaghi consolidation theory, equations considering the variation coefficient of consolidation with void ratio and saturation for consolidation of an unsaturated soil under stage continuous loading were derived, and according to analytical solutions of equations, a formula for settlement computation under stage continuous loading was obtained. Then, combined with the width-to-height ratio of subgrade to compute ground reaction, and by means of in-situ plate loading curves, a correctional approach was presented for the analysis of nonlinear settlement of foundation. Also, the comparison between calculated and measured load-settlement behavior for an unsaturated soil in Qingdao-Ji’nan high-speed railway was given to demonstrate the effectiveness and accuracy of the proposed approach. It can be noted that the presented solution can be used to predict the settlement of an unsaturated soil foundation under stage continuous loading in engineering design.

This paper presents a special case study on the performance of geosynthetics-reinforced and pile-supported embankments (GRPS) over silty clay of medium compressibility. A full-scale high-speed railway embankment with four instrumented subsections over medium silty clay was constructed in three stages. Three of the sections were reinforced by geosynthetics and cement-fly ash-gravel (CFG) piles with different configurations, while the other section was only reinforced by geosynthetics, which was used as a reference. The discussion mainly focused on the deformation behaviors of test embankments. The results show that (1) both the vertical settlements and the lateral displacements slightly increased until a certain fill height. Afterwards, they increased rapidly due to the transformation from the over-consolidated state to partly or entirely normally consolidated state of the foundation soil. (2) CFG pile reinforcement performed well in terms of reducing settlements and lateral displacements. (3) The lateral displacement is a useful indicator for evaluating external stability of GRPS embankments. Moreover, the comparison of soil stiffness obtained from in situ and laboratory suggests that an appropriate equivalent modulus may be obtained based on the stiffness-strain curve from a triaxial test with local strain measurements while considering the correct strain level obtained from field measurements of vertical settlements.

In order to study the characteristics of dynamic stress in subgrade bed, a large-scale ballastless track dynamic model test was made based on the Suining-Chongqing high-speed railway. The results indicate that the dynamic stress has an uneven distribution along subgrade cross-section, and the unevenness is more and more obvious as the dynamic loading increases. Along the depth direction, the dynamic stress in the surface layer of subgrade bed decays more rapidly than in the base layer. The dynamic stress can be calculated by Odemark and Elasticity theory as its distribution under foundation slab of ballastless track is simplified uniform along lateral direction, and triangular along longitudinal direction. On the premise that dynamic stress was known, this paper assumed each soil layer of subgrade to be one-dimensional compression model and determined the limited thickness of the compression layer. Based on this, an approach of conversion from multi-layer system to the equivalent Winkler foundation was explored, and the coefficient of subgrade reaction in different working conditions can be acquired.

As a special material, red mudstone shows different engineering mechanical properties in practical application compared with the general subgrade fillers. Combined with a test section of subgrade in one passenger dedicated line, this contribution studied the settlement property of red mudstone. First, this contribution grasped some physical and mechanical properties of red mudstone and its improvement through laboratory experiments. Secondly, in order to obtain the settlement data of subgrade filled with this special filler, this contribution conducted centrifugal model tests, and also measured the field settlement on the test section. To further research this topic, the two types of tests mentioned above were simulated separately by applying finite element analysis software. Based on the results of all tests, this contribution analyzed the settlement property of subgrade in different stages, and concluded that both red mudstone and its improvement can meet the demands of ballastless track subgrade.