Subgrade Settlement Research Articles

Influence of Lateral Differential Settlement of Subgrade on Dynamic Performance of High-Speed Vehicle System

Influence of Lateral Differential Settlement of Subgrade on Dynamic Performance of High-Speed Vehicle System

Coupling Deformation between the Non-uniform Settlement of Track-structure and Subgrade of the High-speed Railway above the Mined-out Areas

Coupling Deformation between the Non-uniform Settlement of Track-structure and Subgrade of the High-speed Railway above the Mined-out Areas

Time Dependence of Neutral Plane Position in a PHC-Supported Soft Subgrade

This case study focuses on an International Circuit project in China where unmanned automobiles are to be tested in the future. The subgrade undergoes extensive settlement because of the widely distributed lacustrine deposits with high void ratio and compressibility. Prestressed high-strength concrete (PHC) pile-supported earth platforms are used for ground improvement in this area, but their effectiveness has not yet been confirmed. Associated with this adoption, the negative skin friction (NSF) of the piles and the related neutral plane (NP) position is of importance. Although NSF has been examined in detail, the NP position has not, especially with regard to its time dependence. This paper reports the settlement of the pile-supported soft soil subgrade as obtained from long-term observations of full-scale tests. The NP position is determined according to the layered settlement of the soil and the pile settlement, both of which are obtained from long-term field observations. The results reveal that the NP position has a clear time dependence that develops in stages. The NP is initially elevated with time, and then it gradually approaches a constant value. The existing models fail to predict this time dependence of the NP position in the studied sites. A exponential model is used, the fitting parameters of which have practical implications. The adopted model works well with the studied sites, especially in terms of reflecting the time dependence of the NP position and distinguishing the different stages of its development. This paper enhances the understanding of the NP and provides a reference data set for related engineering practices.

Experimental research on a novel spring-shaped fiber-optic displacement sensor for settlement monitoring

In this paper an innovative spring-shaped fiber-optic displacement sensor (SSFODS) based on optical time domain reflectometer (OTDR) technology with simple construction and cheap cost was proposed for subgrade settlement monitoring. The sensor incorporated a simple spring-shaped fiber bending modulation that increased its sensitivity in bending, light source and detector. The sensing principle between the measured external displacement and bending loss of optical fiber was theoretically explained and its expression was also derived. Sensor calibration was conducted, which showed that the SSFODS was characterized by a large measurement range of 90 mm, minimum displacement resolution of 0.173 mm, maximum hysteresis error of 2.81% and maximum repeatability error of 8.42%, respectively. A soil drainage model test with the SSFODS was performed to study its performance in determining compression deformation of the soil. At last, a simple field monitoring application with two SSFODSs connected in series was conducted and the progressive compaction behavior of the fill slope was accurately captured by two SSFODSs. Additionally, it is possible to connect these SSFODSs in series on one optical link to realize quasi-distributed sensing monitoring. This capability supports a promising application of the sensor in geotechnical engineering for real-time monitoring of the stability of fill slopes, foundation pits and so on.

Long-term cyclic behavior of soft clay under different variable confining pressures and partially drained conditions

Traffic loading is one of the essential causes of the excess settlement of soft subgrade. The stress path induced by traffic loading is more complex than a single dimensional path and coupling of both vertical and horizontal stresses. Also, under long-term cyclic loading, the pore water pressure (PWP) generated in soft clay eventually dissipates. Therefore, to study the cyclic behavior of soft clay under a long-term traffic loading, a series of long-term (50,000 cycles) cyclic triaxial tests involving variable confining pressure (VCP) on saturated remolded clay were conducted under partially drained conditions in this study. The result shows both the cyclic vertical stress and VCP have detrimental effects on the soft clay. Thus, a new index (CSRη) is proposed to synthesize these two factors. The peak PWP and final volumetric strain are all linearly related to CSRη. An empirical model involving CSRη is proposed to predict the final permanent axial strain. Compare to the empirical models in previous studies in which both the constant confining pressure (CCP) and VCP tests are necessary, only three CCP tests are needed to obtain the parameters in this model.

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.

Strength and settlement of subgrade soil in southern part of Kangeyam block

Subgrade must typically be capable of supporting weights transmitted from the pavement or building structure. The relative compaction, optimum moisture content, and type of soil everything have an influence on bearing capacity. A good subgrade can withstand a large amount of loading without an excessive deformation. When subjected to heavy moisture or freezing temperatures, most soils experience some volume change. In case of pavement, highway pavements eventually rest on native soil, the subgrade serves as the foundation material for the pavement construction (subgrade). As a result, the features of subgrade have an impact on the performance of pavement. It is therefore critical for an engineer to comprehend the precise nature of the relationship between subgrade strength and settlement characteristics due tomoisture fluctuation. This paper represents that the soil sample taken in and around southern part of Kangeyam block (Kangeyam, Ganapathipalayam, Alangadu, Sampanthanpalayam, Sivanmalai, Periyailliyam, Padiyur, Veeranaampalayam, Vattamalai, Aratholuvu) was tested and experimented for its index and engineering properties. The index properties test includes specific gravity, sieve analysis, OMC (optimum moisture content) and MDD (maximum dry density) was conducted for classification of soil. Since the soil samples collected was predominantly identified sand of various gradation, the engineering properties test such as direct shear test was conducted to evaluate their shear strength properties. The subgrade strength of the soil samples was estimated by conducting CBR (California Bearing Ratio) test. In addition, the settlement of soil samples was evaluated as flexible pavement. The results of the tests data were entered in GIS (Geographical Information System) to describe the data as spatial representation mapping for the zone. This will ensure that the numerous subgrade qualities found throughout the zone are evaluated for safety and referred for future approaches.

Gravel impact compaction piers as a method of soil improvement

The purpose of this study was to evaluate the performance of gravel impact compaction piers system (GICPs) in improving a 3.5m thick loose silty sand in a multilayer coastal soil system located in Bushehr, Iran. The liquefiable sandy soil layer was layered on clay layers with moderate to very stiff consistency and below the engineering embankment layer with a thickness of 1.8 m. Implementation of gravel impact compaction piers is a new generation of aggregate piers. Gravel impact compaction piers were used to improve the liquefiable soil layers and to increase the bearing capacity and reduce subgrade settlement to withstand surface infrastructures. The process of making gravel impact compaction piers in a triangular or square-patterned grid was created using three types of special mandrels and feeding and compacting the gravels in the cavity in several stages without removing the soil from the cavities. The experience gained in this case study showed that artificial liquefiable was created immediately after the construction of these piers in a limited area and the soil became unstable. After about 11-14 days, the soil stabilized rapidly. The results of the standard penetration test in the matrix soil around the piers showed that the amount of (N1)60 in compacted soils was in the range of 21-30 and on average 15 times the amount of (1-3) in the initial soil. Also, the relative density of the initial soil was increased from 25% to 75% after soil improvement. Accordingly, by relying merely on the compaction properties of the piers and without relying on other primary soil remediation factors, such as piers drainage and soil texture change, the safety factor of the improved soil is 1.7-1.95 times the minimum required according to the two risk levels in the design.

Evaluation and Analysis of the Effect of Lignin Amelioration on Loess Collapsibility

The road subgrade and road surface in collapsible loess area are prone to many engineering diseases such as uneven subgrade settlement, insufficient bearing capacity of soaked foundation, collapse and instability of subgrade side slope due to the special properties of loess. As an environment-friendly, low-cost soil modifier with good adhesion and chelation properties, lignin has been considered to be used in highway subgrade construction. In order to explore the effect of lignin on loess, the compressive and collapsible properties of modified loess with different lignin contents were analyzed based on consolidation compression test. The improvement mechanism of lignin on loess collapsibility was studied by means of infiltration test and SEM test. The results show that lignin fibers can promote the agglomeration of loose particles and form a network structure in the soil particle pores, enhance the cementation strength between particles and soil skeleton, and reduce the permeability of loess. With the increase of lignin fiber content, the improvement degree of loess collapsibility shows a trend of first increasing and then decreasing. When the lignin fiber content is 2%, the effect is the best, and the improved loess collapsibility is eliminated.

Effects of rainwater infiltration in low impact development facilities on adjacent municipal roads in collapsible loess

The effective and reasonable construction of the low impact development (LID) facilities in loess area depends on the functionality of typical LID facilities and the safety of surrounding structures in areas. A full-scale field test on rainwater-concentrated infiltration of bioretentions in a collapsible loess site was conducted in this study. The water content and deformation law of the site were analyzed, and the water movement law of the rainwater-concentrated infiltration at bioretention facilities in the loess site was determined. The site settlements were calculated as per the wetting deformation curve and infiltration depths were calculated on an improved infiltration depth model tailored to the loess area. The rainwater infiltration rules of different bioretention structural forms are different in the collapsible loess field. The diffusion rate of the retaining wall type in loess decreases over time, while that on a sloping type does not. Within the same infiltration time, the retaining wall has a stronger influence on the site than the sloping type. When the water is concentrated in the site, its influence on the subgrade settlement is small (generally less than 1.5 mm) enough to satisfy the relevant engineering requirements. Facilities water infiltration laws in the site can be predicted using the fractional unsaturated infiltration model and a modified Green-Ampt model based on assumed loess saturated–unsaturated stratification. The adverse effects of water infiltration related to stagnant bioretentions can be mitigated by adjusting the initial water content and saturated water content at the loess site.

Application of a Geotextile in the Treatment of Post-Subsidence in Karst Areas

The use of a geotextile to treat subgrade subsidence after subsidence has occurred is investigated in this paper. To optimize the anchorage length and buried depth of the geotextile and evaluate the influences of the two factors on subgrade subsidence treatment, finite element analysis is performed and validated with existing model tests. The soil pressure, displacement, tensile force and deformation of the geotextile are studied. The results showed that the geotextile prevented an upward development of subsidence and stabilized the upper soil. The increase of the anchorage length of the geotextile transferred greater soil pressure from the subsidence to a stable area, induced a greater tensile force in the geotextile, and resulted in less soil displacement. As the anchorage length of the geotextile increased from 375 mm to 1500 mm, the surface settlement was effectively reduced from 1.05% to 34.18% when comparing to the situation without a geotextile. As the buried depth of the geotextile increased from 2 m to 6 m, the percentage of surface settlement was effectively reduced from 29.14% to 65.91% when comparing with the settlement corresponding to a buried depth of 2 m. It is suggested that the anchorage length of a geotextile should be the length of the subsidence with respect to width and that the buried depth of the geotextile should be 3–4 m for subsidence treatment. This provides insight into the treatment of sinkholes using geosynthetic approaches in karst areas.

Analysis of the Embankment Settlement on Soft Soil Subgrade with a Cement Mixed Pile

The settlement of the widening of soft soil subgrade highways is typically associated with different treatment positions of cement mixed piles. In order to overcome this, in the current paper we employ the finite element method to simulate and analyze the influence of piles under an existing road slope and under an existing subgrade and new embankment on the settlement characteristics of the subgrade and foundation. In particular, we focus on the influence of the pile length and pile spacing on the subgrade and foundation settlements based on a northern high-speed reconstruction and expansion project. The subgrade and foundation soils in the finite element analysis are considered to be homogeneous, continuous, and isotropic elastoplastic materials. The Mohr–Coulomb ideal elastoplastic constitutive model is implemented as the constitutive soil model. The impact of piles under an existing subgrade and new embankment on the settlement is observed to be more significant than that of piles under the existing road slope. Moreover, the subgrade and foundation settlements increase with the pile spacing under the existing road slope and under the existing subgrade and new embankment. More specifically, an increase of the pile spacing from 200% to 400% of the pile diameter is associated with an increase in the maximum settlement of the foundation surface from 1.76 to 1.85 cm (existing road slope) and from 1.44 to 1.96 cm (existing subgrade and new embankment). In addition, the subgrade and foundation settlements decrease for increasing pile lengths under the existing road slope and under the existing subgrade and new embankment, the pile length increases from 4.7 to 9.2 m, and the maximum foundation surface settlement is reduced from 6.2 to 5.52 cm and from 9.73 to 5.43 cm, respectively. The results can provide reference for future subgrade widening projects.

Retaining Technology for Deep Foundation Pit Excavation Adjacent to High-Speed Railways Based on Deformation Control

Considering the deep foundation pit of a car dumper room close to the Beijing–Baotou railway, the countermeasures of the foundation pit design are analyzed according to the surrounding environment and stratum conditions. Through Midas GTS software as well as field measured results, the construction effect on the adjacent railway subgrade is discussed. Therefore, the deformation characteristics of the foundation pit as well as the retaining structure, including the railway subgrade, caused by the excavation process, are revealed. The results show that the five-sided water-stop structure formed by cement piles around and at the bottom of the pit avoids the consolidation settlement of the adjacent railway subgrade caused by deep dewatering and also reduces the lateral displacement (i.e., in a horizontal direction) and the heave of the pit bottom. As a result, the lateral displacement near the railway side is larger than that of the retaining structure on the other side due to the subgrade on the pit side. The cross-lot bracing across the foundation pit will transfer the bias pressure of the subgrade to the retaining structure far away from the railway, while the transfer effect of the knee bracing is not obvious. The deformation of the railway subgrade and its evolution rate caused by the removal of internal bracing is significantly greater than the subgrade deformation and the change rate of deformation caused by excavation. The research results provide useful guidance for the deep foundation pit design.

Characterizing the permanent deformation Response-Behavior of subgrade material under cyclic loading based on the shakedown theory

Coarse-grained soil is widely used as a filling material in the railway subgrade to provide foundational support to the long-term effects of dynamic (cyclic) train loads transmitted by the track structure. Despite this wide usage, there is limited literature on its permanent deformation behavior characterization under different magnitudes of dynamic stress. The GDS (Geotechnical Digital System) dynamic triaxial tests were presented in this paper to characterize and quantify the dynamic response-behavior of coarse-grained railway subgrade soils when subjected to dynamic loading. Based on the shakedown concept, the patterns of accumulated axial plastic strains for the coarse-grained soil under different magnitudes of dynamic stress were classified into three categories, namely plastic shakedown, plastic creep, and incremental collapse. As a result, the critical dynamic stresses for plastic shakedown and plastic creep state could easily be determined. The experimental test results indicated that whilst the critical dynamic stress of the coarse-grained soil increased with an increase in the effective confining pressure, it correspondingly decreased as the loading frequency increased from 1.0 to 3.6 Hz, respectively. From the regression fitting-analysis of the experimental test results, an empirical model is proposed in this paper, which denotes the critical dynamic stress of coarse-grained subgrade soils as a function of the effective confining pressure. As a contribution to the literature and potential engineering applications, the proposed models may provide theoretical significance for the subgrade state evaluation and settlement control.

Predicting method on settlement of soft subgrade soil caused by traffic loading involving principal stress rotation and loading frequency

Since the alarming settlement of transport infrastructures on soft subgrade caused by long-term traffic loading, a method for predicting settlement is of considerable significance in the engineering design. In this study, we proposed a practical and simple predicting method on the settlement of soft subgrade considering traffic loading effects. The model used in this method involves the principal stress rotation and loading frequency, which needs only four parameters with clear meaning obtained by several cyclic triaxial tests. The applicability of the model has been verified by previous results from other researchers. Moreover, the engineering practicality of this method was validated by the field data of a highway. The settlements under low-frequency and high-frequency are the upper limit and lower limit of the traffic load-induced settlements, respectively. With the increasing embankment thickness, the settlements decrease because the cyclic stress ratio decreases and the detrimental effect of the principal stress rotation on the settlement reduces. The vehicle load is a more influential factor in the settlement than the traffic volume.

Study on Engineering Properties of Deep Marine Soft Clay in Guangdong-Hong Kong-Macao Greater Bay Area

A large number of Marine soft clay are distributed in the Pearl River Delta region of China. The soft clay features mainly on high water content, high sensitivity, high compressibility, low density, low strength and low perviousness, which dramatically restricts local project construction. These features often cause engineering problems such as subgrade settlement, pavement cracking, house cracking, tilting and insufficient bearing capacity of the foundation. Therefore, analyzing the soft clay engineering properties in this area is of great significance for guiding engineering design and construction. Based on the statistic analysis of a large amount of in-situ exploration and experimental data in the western central urban area of Zhuhai, this paper focuses on the distribution and engineering properties of the deep Marine soft clay. The results suggests that the thickness of soft clay is at a range of 10m~ 55.1m. The average water content of soft clay is as high as 68.05%, the plasticity index is 28.16, the fast shear cohesion is only 4.93 kPa, and the internal friction angle is 3.26°. Statistical analysis of layered soft clay shows that the parameters of soft clay vary greatly in depth. The preconsolidation pressure and unconfined compressive strength increases with depth. The water content, liquid limit, plastic limit and plasticity index increase first and then decrease with depth, and the average values reach the maximum values at the depth of 10m ~ 25m. However, the direct shear strength index is on the contrary, and shows a track of decreasing first and then increasing. The minimum values of cohesion and internal friction angle come out at the depth of 10m-25m, with the values of 4.83kPa and 3.10° respectively. The results of in-situ test can better reflect the true mechanical properties of soft clay.

A Prediction Method for Postconstruction Settlement of Pile-Soil Composite Subgrade Based on Fuzzy Comprehensive Evaluation

This paper presents a postconstruction settlement prediction method for pile-soil composite subgrade based on the multilevel fuzzy comprehensive evaluation principle. In this method, the variation range of postconstruction settlement can be obtained from a simple calculation based on the basic data of actual engineering. Firstly, according to the characteristics of influencing factors in the construction of soft soil subgrade, the evaluation index set and two-level factor index sets were selected. The grading standards of the evaluation index and factor index were determined according to the allowable value of the standard and the numerical simulation results. Secondly, each factor index was standardized, and the normal distribution function in the form of exponential was used to construct the standard membership function for the first and second factor indexes. Finally, the comprehensive evaluation matrix of postconstruction settlement of composite subgrade was constructed based on the entropy weight method. The variation range of postconstruction settlement was predicted by the principle of maximum membership. The example analysis shows that the predicted results of the prediction method and the field measurement method are in good agreement, indicating that the proposed method can realize the postconstruction settlement prediction of composite subgrade, and the results are more accurate and more instructive.

Numerical Analysis of Additional Stresses in Railway Track Elements Due to Subgrade Settlement Using FEM Simulation

The prediction of additional stresses in ballasted track due to subgrade deformation is the main objective of the present paper. In this context, a 2D finite element model of ballasted railway track was built using the ANSYS Workbench program. Based on this model, an investigation of stresses and deformation values of track elements was conducted in three cases with different contact types. It was found that the case introducing the status of a new track, which has frictional contacts between sleepers and ballast with bonded contacts between other elements, has lower stresses in most of the track elements. Moreover, this case was applied for studying the effect of the settlement on track elements. It was found that stresses increased with increasing the settlement value. The average percentages of increased stresses are 4.18%, 5.85%, and 7.21% in railhead, tie plate, and sleeper, respectively, due to a 1 mm increase in the settlement. Finally, a second-degree polynomial equation was derived to predict the additional stresses in each element due to track settlement. It is expected that this study would help to decrease the maintenance costs and extend the service life of the track elements by predicting the additional stresses in them.

Influence of Lightweight Foamed Concrete as Backfill Material on Stress and Deformation of Buttressed Earth-Retaining Wall

Controlling settlement and earth pressure behind retaining wall in soft soil area are ongoing practical problems for the construction and operation of highway, which are mainly caused by the poor nature of soft soil. To reduce the pushing force on retaining wall and subgrade settlement, the authors propose the use of lightweight foamed concrete as subgrade filler behind the buttressed earth-retaining wall. However, the mechanical properties and deformation behavior of the buttressed earth-retaining wall remain unknown when lightweight foamed concrete is used as a backfill behind the wall. To solve this problem, a scale model of the subgrade filled with lightweight foamed concrete behind the buttressed earth-retaining wall is established to determine its stress and deformation characteristics under different factors. Lateral earth pressures and wall displacements at different elevations of the retaining wall model were monitored during the tests. Then, a series of orthogonal experiments are conducted to analyse and compare the effects of overload, density, and replacement thickness of lightweight foamed concrete on the earth pressure and displacement of this retaining wall. The results show that the size of earth pressures at the same position of retaining wall is affected by overload, density, and replacement thickness of lightweight foamed concrete, but its change of distribution form is only related to the replacement thickness of this backfill. Additionally, the primary-secondary relations of different factors’ influence extent on the forces and deformation of the buttressed earth-retaining wall filled with lightweight foamed concrete as backfill are obtained by using range analysis method.

A novel time-power based grey model for nonlinear time series forecasting

To deal with various nonlinear issues in real applications, a novel time-power based grey model is put forward. However, in the original form of this model, the time-power parameter α normally equals to an integer, and then the analytical expression of the time response function will be obtained. Otherwise, if the parameter α equals to a non-integer, one cannot obtain the concrete time response function for future estimations. This situation may significantly restrict the applications of this grey model. To address such drawbacks, an optimized version is designed in this work. In the proposed model, a simplified solution to the differential equation is derived by using the definite integral technique. Furthermore, for improving accuracy, the time-power parameter α is optimized by utilizing the Particle Swarm Optimization algorithm based on the model parameter packages. Subsequently, the efficacy and practicality of this simplified function have been verified by numerical simulations and experimental studies. Moreover, the method of probability density prediction is employed for verifying the reliability and stability of the proposed model for the first time when predicting the settlement of the soft-clay subgrade on an expressway. The demonstration cases illustrate that the quantitative improvements over forecasts of the proposed model are even more pronounced with a level accuracy of 2.29% and 1.19% MAPE values in the fitted and predicted periods, respectively, which can significantly increase the predicting accuracy by more than 10% with respect to the other benchmarks. Therefore, the new proposed model not only has greater application fields and prospects but also achieves higher and more reliable predicting accuracy with the optimal α under the support of the Particle Swarm Optimization algorithm, compared with the competing models.