Subgrade Model Research Articles

Comparative study on in-situ resilient modulus of subgrade estimated using in-situ modulus detector

This study presents a comprehensive evaluation of the resilient modulus of unsaturated subgrade soils using a novel in-situ modulus detector (IMD) along with various laboratory and field-testing methods. The current study employs several testing devices, including bender elements, standard dynamic cone penetrometer, and crosshole-type dynamic cone penetrometer (CDP), to assess the reliability of the IMD in estimating the resilient modulus profiles of small-scale homogeneous subgrade models in unsaturated conditions. The index properties, compaction and strength characteristics of the compacted soils, are evaluated. The resilient modulus values obtained from the IMD are compared with those obtained from repeated load triaxial tests under various stress states. Experimental results show a decrease in the penetration depth per blow with increased dry unit weight. In addition, elastic modulus and shear wave velocity increase with soil density. The resilient modulus estimated from the IMD increase with both depth and soil density. A robust correlation has been established between the resilient modulus values estimated from the IMD and those estimated from the CDP or bender element tests. Furthermore, the resilient moduli estimated from the universal model and the IMD demonstrate linear relationships with a coefficient of determination greater than 0.8. Thus, the IMD may be useful for the in-situ evaluation of resilient modulus in unsaturated subgrade soils, considering influencing factors such as soil density and confining pressure.

Experimental investigation of influence of amide polymer on loess for subgrade

The effects of moisture and drying shrinkage can lead to uneven settlement, cracking, and other diseases in loess subgrade. The objective of this study was to investigate the effects of amide polymer (AP) on the permeability, mechanical properties and crack resistance of loess by orthogonal experiments. The basic properties of AP and the permeability, mechanical properties, and dry–wet variation properties of polymer-modified loess were tested, and a scale model verification and simulation analysis were conducted. In this paper, water migration in subgrade is regulated by improving the water sensitivity of loess. By reducing the variation range of subgrade water content, the stress accumulation in subgrade caused by water is weakened. The results show that the curing time and mechanical properties of AP are directly affected by the oxidant and reducing agent, and the mechanical properties of AP are compatible with the characteristics of loess. AP filled the grain gap and reduced the permeability of loess by 34.05–280.83%. The ductility of polymer-modified loess is significantly increased, and the strain of peak strength is increased by 17.21–126.36%. AP can regulate moisture change, reduce the surface tension between particles, and reduce stress concentration. The strength loss rate was reduced by 19.98–51.21% by enhancing the cracking resistance and weakening the strength loss caused by dry and wet cycling. The increase of upper layer moisture content in the scale model of polymer-modified loess subgrade is reduced by 31.38–36.11%.

Experimental study on the punching failure model of soft red rock subgrade by BPT-BVM methods and it’s assessment of the load bearing capacity

The recommended bearing capacity of medium weathering mudstone foundation is less than the capacity of the rock structure to withstand loads in Southwest China. A comprehensive failure characterization of medium weathering mudstone in Chengdu has been performed including bearing plate test (BPT), binocular vision measurement (BVM) test, uniaxial compressive strength test, trial trench test of shallow rock surface and 3D imaging in this paper. Failure behavior of rock has been modeled with 3D imaging algorithm that utilizes Zhang’s calibration method in BVM system combination with trial trench test of shallow rock surface. The bearing capacity of medium weathering mudstone foundation were extracted from uniaxial experiments and BPT-BVM test by fitting relevant material properties to the data. The results revealed that: Bearing capacity of medium weathering mudstone of layered isotropic in Chengdu is undervalued. Specifically, the characteristic load carrying value is in the range 1500–2500 kP, that is 50% higher than in the local standard system. Failure process is different from Hoek–Brown Failure Criterion, presenting a wave peak transfer phenomenon of the increment displacement into the distance. Thus, it can be reduced to that of punching failures for thin bedded structures of Moudstone foundations. Compressive strength of soft rock proves to be main factor limiting the bearing capacity, a clear correlation between the uniaxial compressive strength reduction coefficient and the bearing capacity has been used to establish, leading to the proposal of a load bearing capacity prediction model.

Investigation of the discharge performance in infiltration wells based on several soil subgrade models

Heavy precipitation would result in hydrometeorological disasters such as landslides and floods. In general, disasters cause hazardous situations and property loss. Infiltration wells are water and soil conservation techniques that can reduce surface runoff and increase infiltration to mitigate the effects of hydrometeorological disasters. However, evaluations of geotechnical properties have not yet jointly or actively considered these environmental developments. This paper aims to analyse the wide range of soil types against several diameters and depths of the holes. The variation in soil varied from type 1 to 6, containing different properties and consistencies. The holes diameter used are 0.4m, 0.5m, 0.6m, 0.7m, and 0.8m, associated with several depths; 0.5m, 0.75m, 1m, 1.25, and 1.5m. The analysis was conducted with Geostudio SEEP-W in different model variations to obtain the well’s discharge value. The result shows that all the depth depicts similar behaviour of the discharge well. The sand model has the highest value of all categories, with the second place being the sand-clay layer model. In contrast, the clay simulation generates the lowest value of this research. This output offers a strong recommendation between infiltration well structure and soil parameters, which show sand is the most required property to increase the performance of hole infiltration.

Analysis of settlement thresholds for a new ballastless track-concrete box subgrade structure

In this paper, the effects of settlement on force, track deformation, and train dynamics to determine the settlement thresholds of ballastless track-concrete box subgrade (BTCBS) are studied. Firstly, this study introduces four settlement types, namely V-shape, box rolling, box faulting, and trapezoid settlement. The nonlinear finite element model (FEM) of BTCBS and the train-ballastless track-concrete box subgrade (TBTCBS) model considering nonlinear contact between track slabs and base slabs are established. Then, the effects of settlement on fastener force, track deformation, and vehicle dynamics are studied. According to the calculated results and China’s codes, the settlement limit values of BTCBS can be obtained. The results indicate that the settlement thresholds with box rolling and faulting are controlled by vertical force of fasteners, and the settlement limit values with V-shape and trapezoid settlement are controlled by 10 m chord versine of vertical irregularity (CVVI). The suggested settlement limit values of BTCBS for box faulting, box rolling, trapezoid, and V-shape settlement are respectively 2.48 mm, 2.55 mm, 17.43 mm, and 8.79 mm.

Liquefaction test and seismic load simulation of saturated sand subgrade with different salt contents

On the basis of the highway subgrade construction of North Hobson Salt Lake mining area in Qinghai Province, the soil samples of high saline saturated sand adjacent to the salt lake were remolded, and four types of saturated sand containing 0%, 10%, 20%, and 30% salt were configured respectively, so as to explore the liquefaction effect of saline saturated sand under seismic load. After the indoor geotechnical test, the soil samples were reshaped, The liquefaction test of saline saturated sand was carried out by using the GDS dynamic triaxial device to simulate seismic load to investigate the liquefaction mechanism of saturated sand under different salt contents and verify the liquefaction discrimination standard, then based on the data of dynamic elastic modulus, dynamic shear modulus, damping ratio coefficient, and soil density obtained from previous soil tests and dynamic triaxial tests, four subgrade models with different salt contents were constructed using ANSYS simulation software according to actual engineering, and the settlement and deformation of the subgrade were analyzed by inputting EL-concrete seismic waves. The results show that the maximum value of axial stress of saturated sand decreases with the increase of salt content; the load vibration times of saline saturated sand reaching the initial liquefaction state are more than that of common saturated sand; the higher the salt content, the weaker the liquefaction resistance of saturated sand; when the pore pressure ratio coefficient reaches 1, the axial double amplitude strain εd of the soil also reaches 5%, and the soil reaches the initial liquefaction state; the area of shear stress and strain hysteresis curve increases with vibration load, and the elastic modulus decreases gradually; the shear dilation and compaction effects occur in each hysteresis cycle, and the former is greater than the latter, the research results have guiding significance for similar studies in highly saline saturated sand areas.

Reliability of depleted cement–ground slab with waste granodiorite dust admixture on semi-saturated substrate

There are many types of pavements for roads. For technical roads, one of the most effective and economically advantageous pavement types is chemically stabilized soil pavement. When designing a pavement made of soil admixed with binders, it is necessary to meet the load-bearing conditions while considering the effects of environmental factors. The aims of this study were to optimize the strength parameters of pavement, minimize negative environmental effects, minimize the carbon footprint, and simultaneously dispose of mineral waste materials from the production of asphalt mixes for roads. After testing the strengths of samples admixed with cement or with reduced cement content and the addition of dust, questions arose as to how both materials affect the reliability of the pavement structure. Samples of soil doped with cement and dust were subjected to reliability analyses. Abaqus software with a three-dimensional subgrade model was used for a comparative analysis. The substrates under soil stabilization slabs were subjected to varying saturation states, which affected their vulnerability. In this study, the changes in the saturation characteristics of the subsoils associated with different types of climatic interactions were simulated. The Darcy–Buckingham law was used to describe the flow of liquids through the subsoil. Pavement reliability calculations were performed for a multidimensional space of random variables. The results served as an interesting starting point for recommendations to decrease the use of energy-intensive materials, increase the reuse of waste, and reduce the effects of the strengthening process on the environment.

Numerical modelling and analysis for nonlinear stiffness of subgrade soil subject to 3-D vehicle-track interaction

In this paper, a subgrade model with nonlinear stiffness is established and integrated into a previously constructed 3-D vehicle-track-subgrade (VTS) dynamic model, in which the prediction and assessment of the vehicle-induced railroad vibrations are achieved with high accuracy and versatility. The system excitations are originated from the initial and time-dependent geometric irregularity of the system, which includes several internal excitations, namely conventional rail irregularity and the soil deformation-induced track irregularity. In the numerical analysis, the nonlinear behavior of the subgrade soil is revealed by considering shear modulus and traffic loads. The numerical experiment indicates that subgrade soil has a greater dynamic stiffness reduction if the vehicle enters the soil region with initial deformations. Besides, it is shown that the soil vibrations will be significantly amplified once the vehicle running speed is larger than the Rayleigh wave velocity of the subgrade soil.

Optical-Fiber-Embedded Beam for Subgrade Distributed Settlement Monitoring: Experiments and Numerical Modeling

In this study, Brillouin optical time domain analysis (BOTDA) sensing technology was utilized for monitoring settlement in a similarity model of a highway subgrade. As contact winding cannot be used for an optical fiber that is buried directly in the soil, uncoupling between the fiber and the soil can occur. Thus, an optical-fiber-embedded beam (OFEB) was developed, and a method for measuring and calculating the beam’s deformation was proposed. A calibration test and a test on a similarity model of a subgrade were carried out to investigate the applicability and monitoring accuracy of the OFEB. It was concluded that the OFEB can accurately measure beam deflection, and the maximum relative error between measurements by the optical fiber and a displacement transducer was approximately 5%. The OFEB was embedded directly into a similarity model of a subgrade to monitor settlement. The deflection deformation of the OFEB was found to be close to the subgrade settlement over a certain settlement range, with a relative error below 8.1%. Thus, the OFEB can be used to measure large-range distributed settlement in a subgrade. A numerical simulation was performed to identify appropriate beam dimensions and material design parameters, thereby extending the measurement range before decoupling of the OFEB and the soil occurs. The enhancement of the measurement range and the accuracy of the OFEB based on the preliminary experiments carried out in this study enables further investigation of settlement monitoring.

Study on mechanism and application of PFWD for subgrade quality detection: semi-analytical approach and experiment

The mechanism and possible application of Portable Falling Weight Deflectometer (PFWD) for subgrade quality control are investigated by different methods, including semi-analytical simulation, laboratory test, and field test. A layered elastic subgrade model is proposed to consider the stress dependency of subgrade soil under PFWD loads. The elastic modulus of each layer is determined referring to the distribution of stress state along with the depth and laboratory dynamic triaxial test data. It is found that the effective test depth of the PFWD device in the present study is 0.6 m using the analytical model. Furthermore, based on the same static homogeneous elastic half-space theory, the relationship between the test results of PFWD and the Benkelman Beam method is investigated through field tests. Using this relationship, the PFWD is then adopted to evaluate the compaction quality of two sections of the subgrade with different rolling times.

The Influence of Vegetation Cover on the Settlement Behavior of Permafrost Subgrade in the Greater Khingan Mountains Forest Region

The subgrade construction in the permafrost forest region will aggravate the degradation of frozen soil, which will lead to the settlement of the subgrade. Based on the road project of National Highway 332 in the Great Khingan Mountains, by means of field observation, experimental testing, and numerical simulation, a thermo-hydro-mechanical coupling numerical model of the permafrost subgrade considering vegetation cover was established to analyze the influence of vegetation cover on the settlement behavior of the permafrost subgrade. The study indicates that vegetation cover mainly influences the seasonal active layer temperature of permafrost, and its cooling effect on permafrost in the warm season is more significant compared with the warming effect in the cold season. The volumetric water content of the subgrade with vegetation cover is greater than that without vegetation cover in the cold season. The situation is just the opposite in the warm season. The damage to the subgrade is mainly reflected in the settlement caused by the thawing of frozen soil. The maximum settlement of the subgrade with and without vegetation cover is 8.3 mm and 9.5 mm at the foot of the subgrade slope. After construction, the settlement behavior of the permafrost subgrade will undergo a degradation period of 3 years, a restoration period of 2 years, and finally, tend to be stable.

Development of OF based intelligent geotextile and its case study in high-speed railway subgrade

Geotextile is one of the important reinforcement materials for high-speed railway subgrade, and its service performance is closely related to subgrade safety. In order to grasp the mechanical properties of geotextile, the OF (optical fiber) based intelligent geotextile by braiding FRP (fiber reinforced polymer) packaged OF sensor into geotextile is developed, and applied for the settlement monitoring of karst cave expansion process in one subgrade model and one high-speed railway subgrade construction. The test and field monitoring data show that the strain data measured by the optical fiber sensor is basically consistent with that measured by the settlement plate which indicates that the developed intelligent geotextile not only has good reinforcement function, but also has good sensing function.

Fracture characteristics of a cement concrete pavement plate considering subgrade modulus decay based on a meshless finite block method

The decrease in the subgrade modulus immersed in rainwater can significantly increase the fracture risk of a cement concrete pavement plate. The aim of this study was to develop a meshless finite block method (MFBM) to reveal the failure mechanism of a cement concrete pavement due to the weakening of the subgrade modulus. A normal distribution function was adopted in this study to represent the distribution of the subgrade modulus at the bottom of the cement concrete pavement plate. The settlement results show that the progressive softening model of soil subgrade is more suitable to represent subgrade modulus decay. The maximum stress of the cement concrete pavement mainly concentrates at 1.05–1.15 m of the plate edge. The weak fracture position is influenced by the subgrade modulus reduction, the size of the immersion range, and the pavement and subgrade thickness. When improving the subgrade modulus, adding plate thickness appropriately can effectively control the cracking of the cement concrete pavement. Compared with the finite element model, it is proved that the proposed MFBM has an advantage in the solution of pavement fracture with high accuracy and less computation time. In addition, findings in this study may provide evidence for understanding the effect of the subgrade modulus on the durable pavement design.

Lateral Static Response of Piles Based on a Full-Fledged Two-Parameter Foundation Model

A rigorous analytical continuum subgrade model translated into an equivalent two-parameter mechanical model of the Pasternak type is used to study the static response of piles to lateral loads. The model fully accounts for the soil shear missing in the conventional Winkler representation. The two parameters of the model are derived in advance. After conducting static analyses of a wide range of cases of the pile–soil system using both the proposed analytical model and finite-element models, closed-form relations are obtained for the adjustment factor left open in the model parameters that are found to mainly depend on the soil’s Poisson’s ratio. Back substitution of this factor showed that the model parameters take simple forms that depend only on a few factors such as the pile–soil stiffness ratio and the pile-end conditions. The model is validated by means of results of finite-element models. Closed-form pile head stiffness coefficients for various pile-end conditions are provided for both infinite- and finite-length piles. Expressions for a newly defined pile critical length are proposed for several combinations of pile-end conditions. It is demonstrated that the analytical approach employed is convenient and avoids the high degree of empiricity inherent in the conventional Winkler-based backward approach. The model has also the unique advantage of being convenient to conduct parametric studies, including the influence of soil shear resistance.

Extended UH model and deformation prediction of high-speed railway subgrade

For the rapidity of trains in high-speed railway, it is crucial to guarantee the reliable and safe operation. Strict controls are needed to prevent excessive subgrade deformation. In order to perform more accurate subgrade deformation calculation, the constitutive model used in simulation should demonstrate the small-strain stiffness for uneven settlement calculation, and illustrate time-induced deformation for post-construction deformation. An E-UH model is established based on the original UH model. In the E-UH model, the elastic hysteresis relation is added to describe the initial high stiffness and nonlinear stiffness decay. In addition, the E-UH model also couples the equivalent formula between overconsolidated characteristics and time-induced deformation. Then the effectiveness of this new model is testified by element test prediction. By means of the second development interface of a finite element software, the E-UH model is developed as a subroutine and applied to numerical simulation of subgrade deformation. The simulation represents that the E-UH model is capable of performing deformation prediction during the subgrade construction and post-construction phases, and indicates that considering small-strain stiffness can make the uneven settlement more prominent. This research provides a practical and efficient constitutive model for numerical simulation of the high-speed railway subgrade deformation.

Settlement on asphalt concrete pavement in permafrost regions under the dynamic load of aircraft.

The settlement in permafrost regions has a significant effect on the safety of the aircraft. Therefore, a numerical model of asphalt concrete pavement and subgrade in permafrost regions is proposed and verified by comparing with previous studies. Numerical models under the dynamic load of aircraft are used to analyze the settlement nephogram, time-dependent curve, and settlement curve. Results show that the influences of different parameters on settlements decrease slowly at the depth of 0-1.45m, then decrease quickly at the depth of 1.45-2m, finally tend to be stable. The peaks of settlements increase with the growth of international roughness index, taxiing speed, and aircraft's weight. The settlement increases little when the international roughness index is more than 3. The settlement varies significantly when the taxiing speed is from 30m/s to 70m/s. The study provides a theoretical basis for the construction and maintenance of asphalt concrete pavement and subgrade in permafrost.

Research on Dynamic Stress–Strain Change Rules of Rubber-Particle-Mixed Sand

We conducted GDS dynamic triaxial tests to study the change rules of the hysteresis curve morphology, axial strain, dynamic elastic modulus, and damping ratio of waste tire rubber-mixed sand-based subgrade model samples with different rubber particle sizes, rubber mixing amounts, and loading times. The research revealed the developmental rule of the hysteresis curves of waste tire rubber-mixed-sand samples under cyclic loading. From the analyses and comparison of the dynamic stress–strain change rules of rubber-particle-mixed-sand samples under different test conditions, it was concluded that the dynamic elastic modulus and shear stiffness of rubber-mixed-sand samples were smaller than those of pure sand samples under cyclic loading while their damping ratios were greater than that of pure sand samples, promoting vibration resistance and reduction to a larger extent. Therefore, this conclusion is of guiding significance for engineering practice.

The modelling of railway subgrade strengthening foundation on weak soils

Abstract As a consequence of increased axle loads and speeds of trains on modernised railway lines, there may occur problems with bearing capacity and stability of the subgrade in some sections of the railway network. This is the situation we are dealing with right now on the Polish State Railways network. Therefore, as a case study, a fragment of an existing railway embankment based on a weak foundation was chosen for the analysis of train–track–subgrade interaction. A two-stage train–track–subgrade model has been developed. The model consists of the upper part (train–track) and the lower part (subgrade-foundation). The first part is modelled as a self-contained system of differential equations which are solved by means of finite difference method and yield the stress levels on the subgrade. These stresses are treated as a load for the lower system modelled using FEM. The model has been validated using experimental data from literature, authors’ measurements, and railway staff measurements of the track geometry. Several cases of strengthening methods were calculated and compared with measurements on the railway section under consideration. Good agreement between the prediction and the measurement was found. The novelty of the model is including the heterogeneity of the subgrade, the strengthening methods, and very deep layers of its foundation as well as adding the influence of vibration on the weakening of soils. It was found that this influence is noticeable and should be included in the prediction of railway subgrade behaviour

Wind Dynamic Characteristics and Wind Tunnel Simulation of Subgrade Sand Hazard in the Shannan Wide Valley of the Sichuan-Tibet Railway.

The Shannan wide valley section of the Sichuan–Tibet Railway is located in the middle reaches of the Yarlung Zangbo River, where sand hazard is severe. A wind tunnel simulation experiment was conducted by establishing a subgrade model and performing field observation to carry out research on the dynamic environment of blown sand and the sand hazard formation mechanism of subgrade in the Shannan wide valley. Observation results showed that the sand-moving wind of the Shannan wide valley was chiefly derived from the ENE direction, and the resultant sand transport direction was WSW. Wind speed, the frequency of sand-moving wind, the sand drift potential, and the maximum possible sand transport quantity were relatively high in the spring. Meanwhile, the dynamic of the wind-blown sand flow was further enhanced in the spring, particularly influenced by the flow action of the Yarlung Zangbo River. Thus, sand hazard mainly occurred in the spring. The Sichuan–Tibet Railway subgrade evidently changed the wind speed, the wind-blown sand flow field, and conditions of transport and accumulation. Within the distance of 5 times the model height in the windward direction and at the subgrade top center to 20 times the model height of the leeward direction was the wind speed deceleration zone, resulting in sand particle sediments. A wind speed acceleration zone appeared on the subgrade windward slope shoulder, resulting in wind-blown sand flow erosion. This study provides a scientific basis for sand hazard prevention and control in the Sichuan–Tibet Railway.

Calculation Method of Foundation-Subgrade Dynamic Interaction for Large Wind Turbine

Taking a 3MW large wind turbine foundation as the research object, the dynamic interaction model of wind turbine foundation and subgrade is established. By intercepting the subgrade in a limited range and setting spring-damper elements at the boundary of the subgrade, the semi-infinite space of the subgrade is simulated. In order to facilitate the use of structural designers, the calculation methods of the stiffness coefficient and damping coefficient of the spring-damper element are simplified. The simplified calculation formulas for the stiffness coefficient and damping coefficient of the spring-damper unit are obtained when the intercepted subgrade range is 5 times and 10 times the wind turbine foundation.