Model Embankment Research Articles

Runoff and infiltration characteristics of rainfall and snowmelt water on a model embankment slope composed of sandy soil simulating vegetation works.

Runoff and infiltration characteristics of rainfall and snowmelt water on a model embankment slope composed of sandy soil simulating vegetation works.

Reinforcement load in geosynthetic-reinforced pile-supported model embankments

The stress conditions of geosynthetic reinforcements (GRs) are crucial in achieving the accurate serviceability design of geosynthetic-reinforced pile-supported (GRPS) embankments. However, the sensitivity of load distribution to the settlement process has been reported in geosynthetic-reinforced embankment overlying cavities. In this study, a three-dimensional model embankment was used to perform experiments and evaluate the load acting on the GR. A flexible pressure-mapping sensor was introduced to investigate the pressure distribution for two types of supporting conditions: partitioned displacement by multiple movable trapdoors and even trapdoor settlement underneath different subsoil materials. The results showed that the load on the GR was concentrated on the strip areas between adjacent pile heads along with the settlement. The measured load on the GR strip area was related to the settlement process and finally exhibited a U-shaped distribution after detachment from the support underneath. The soil arch height in the subgrade continuously increased with the settlement; meanwhile, the pile head load increased rapidly at first and then decreased slightly or remained stable depending on the foundation support stiffness. For both types of settlement behaviours, soil arching exhibited stress history-related characteristics that influence the load transfer in GRPS embankments.

Influence of Fines Content on the Stability of Volcanic Embankments under Rainfall and Earthquake

This study aims to investigate the effects of fine content on the mechanical behavior of embankments constructed from volcanic soil subjected to rainfall and earthquake. To accomplish this purpose, a series of 1 g model experiments on slopes using Komaoka volcanic coarse-grained soils as materials was conducted with a spray nozzle and shaking table. In the experiments, shear strain, acceleration, pore water pressure, and saturation degree were monitored and measured to provide an understanding of the failure mechanism of the model embankment with different fine particle contents during post-rainfall earthquakes. The results show that the increase in the fines content of the volcanic soil reduces the permeability of the volcanic embankment but has no significant effect on rainfall-induced slope failure until the shear strain is less than 6%. Moreover, the seismic resistance of volcanic slopes subjected to previous rainfall increases when the fine particle content increases to a certain threshold of about 27%.

Centrifuge modelling of small earth dams subjected to the combined effects of rainfall and earthquakes

In this study, the combined effects of rainfall and earthquakes on earth dams were modelled using a 1/40 scale model embankment, spraying device, and shaking table. Two scenarios were tested: one where the embankment was subjected to seismic motion after rainfall, and another where the embankment was subjected to rainfall after seismic motion. When the rain occurred first, causing the phreatic line to rise, the excess pore-water pressure at the toe of the embankment was higher than if there had been no rainfall, and the seismic stability of the embankment deteriorated. When the shaking occurred first, cracks due to seismic motion promoted subsequent rainfall seepage, increasing the amount of eroded soil in the embankment. Thus, rainfall damage and seismic damage influence and exacerbate each other.

植生工を模擬した砂質土からなる模型盛土斜面の降雨浸透特性

植生工を模擬した砂質土からなる模型盛土斜面の降雨浸透特性

Centrifugal and numerical modeling of stiffened deep mixed column-supported embankment with slab over soft clay

A stiffened deep mixed (SDM) column can significantly increase the bearing capacity, reduce settlement, and enhance the slope stability of soft clays as compared with a conventional deep mixed (DM) column. This technique involves inserting plain concrete, reinforced concrete or a steel pile into the center of the DM column after the DM column is constructed. In this paper, a series of centrifugal modeling tests were conducted to investigate the performance of an SDM column-supported embankment over soft clay. A model embankment supported only by DM columns was constructed for comparison. Two ideal numerical models of column-reinforced soil under equal stress and equal strain conditions were established to explore the role the column played in accelerating soil consolidation. A parametric study was conducted to investigate the influence factors of the length of the core pile, column spacing, thickness of the underlying soil, modulus and thickness of the cushion, and modulus of the slab on the load transfer of the system, and some recommendations were proposed for its application. The load-transfer mechanism of an SDM column-supported embankment system with a slab was established based on the development of the volumetric strains and the principal stresses in the numerical models.

Comparative Analysis of Settlement and Pore Water Pressure of Road Embankment on Yan soft soil Treated with PVDs

The application of prefabricated vertical drains (PVDs) in the road embankment construction has been successfully performed in many projects throughout the country. The simulation of finite element method (FEM) can assist engineers in modelling very complex structures and foundations. This paper presents a plane–strain numerical analysis that was performed to verify the effectiveness of the model embankment stabilised with PVD using Plaxis 2D version 8. This study employed the smear effect of permeability ratio (kr) of 3 in the PVD modelling. The data of settlement and pore water pressure in the left and right sides of road embankment were monitored for 177 days, then the data were collected and compared by a numerical simulation. The coefficient of determination (R2) was used to assess the performance of the comparative analysis. The results of numerical simulation on settlement and pore water pressure obtained a coefficient of determination of greater than 0.9 which has reached a good agreement with those of the field measurement. On other the hand, there was no significant difference in the performance between both sides of the embankment. The smear effect parameter (kr = 3) is recommended for PVD designs and can provide accurate FEM prediction.

Evaluation of Bearing Capacity of Fly-Ash Highway Subgrade Based on Model Test

Abstract Based on the mechanical properties of fly ash (FA) and the similarity theory, a small-scale geomechanical model was virtually built for evaluating the actual bearing capacity of FA embankments. The embankment model was divided into two sides: the right was covered by a concrete surface and the left was uncovered. The monitoring results of stress and displacement in different depths of the model embankment during the loading process were both presented. In the process of testing on the covered right side, both the additional stress and displacement of soil in different depths increased with an increasing load. There was a significant increase of displacement under large loading in different depths of the covered side, which suggested that plastic failure may happen and would gradually develop from bottom to top. The limit-bearing capacity of the FA embankment covered by concrete surface yield from the model test can be adopted as 419 kN that was assumed to fulfil the requirements of traffic load. As to the uncovered left side, plastic failure was very likely to occur in the soil beneath the bearing plate with a certain applied load, which caused a decrease of additional stress. Plastic failure gradually develops from top to bottom of the uncovered side. The limit-bearing capacity of the FA embankment without the concrete surface can be taken as 193 kN that may not meet the requirement for the overloading of vehicles.

COMPARISON OF DIRECT AND INDIRECT MEASURED SOIL-WATER CHARACTERISTIC CURVES FOR A SILTY SAND

It is time consuming and needs sophisticated testing apparatus to determine unsaturated soil properties such as hydraulic conductivity and shear strength. Therefore, engineers hesitate to use unsaturated soil properties in economical geotechnical problem solving. To promote the use of unsaturated soil properties in geotechnical engineering designs, numbers of methods have been developed to estimate/predict unsaturated soil properties. The Soil-water characteristic curve (SWCC) defines the relationship between the soil suction and water content. During past few decades, different measuring and estimation/prediction methods have been developed by researchers to ascertain SWCC of soils. Among them, direct and indirect methods are widely used to measure the SWCC of soil. Indirect methods such as axis-translation technique are commonly used in the laboratory to determine SWCC. It is important to understand how well the indirectly measured SWCC is related to actual soil-water retention properties of soil during drying and wetting process. Bridging that research gap, in this study, Soil water characteristic curve (SWCC) of sandy soil was measured using both indirect (Axis translation method using Tempe Pressure Cell) and direct methods. The direct measured SWCCs were obtained by subjecting the instrumented soil column and model embankment to wetting and drying cycles. When comparing SWCCs measured by direct and indirect methods, it was found that the indirect method provides a very close agreement with the outcomes of direct methods. This ensures that the SWCCs measured in the laboratory by using indirect methods can be used in Geotechnical Engineering practice.

COMPARISON OF DIRECT AND INDIRECT MEASURED SOIL-WATER CHARACTERISTIC CURVES FOR A SILTY SAND

It is time consuming and needs sophisticated testing apparatus to determine unsaturated soil properties such as hydraulic conductivity and shear strength. Therefore, engineers hesitate to use unsaturated soil properties in economical geotechnical problem solving. To promote the use of unsaturated soil properties in geotechnical engineering designs, numbers of methods have been developed to estimate/predict unsaturated soil properties. The Soil-water characteristic curve (SWCC) defines the relationship between the soil suction and water content. During past few decades, different measuring and estimation/prediction methods have been developed by researchers to ascertain SWCC of soils. Among them, direct and indirect methods are widely used to measure the SWCC of soil. Indirect methods such as axis-translation technique are commonly used in the laboratory to determine SWCC. It is important to understand how well the indirectly measured SWCC is related to actual soil-water retention properties of soil during drying and wetting process. Bridging that research gap, in this study, Soil water characteristic curve (SWCC) of sandy soil was measured using both indirect (Axis translation method using Tempe Pressure Cell) and direct methods. The direct measured SWCCs were obtained by subjecting the instrumented soil column and model embankment to wetting and drying cycles. When comparing SWCCs measured by direct and indirect methods, it was found that the indirect method provides a very close agreement with the outcomes of direct methods. This ensures that the SWCCs measured in the laboratory by using indirect methods can be used in Geotechnical Engineering practice.

Centrifuge modelling of railway embankments under static and cyclic loading

With past embankment failures on the heavy haul coal export railway line between Ermelo and Richards Bay in South Africa, a study was conducted focusing on the modelling of embankments in a geotechnical centrifuge. This was done in order to investigate the influence of static and cyclic loading on settlement, and to evaluate the potential failure of the embankment under the loading conditions. The effect that moisture has on the slope stability was also investigated. A suitable loading system was developed that could be used to apply static and cyclic loading to a model embankment in the centrifuge. Embankment models representing a standard slope and using one material type with varying moisture content were built and tested in the centrifuge. Clayey sand material was sampled from one of the sites where a failure occurred on the coal line and was used for the tests. The loading system simulated the loading created by a heavy haul coal train with a 26 t per axle load along the length of an embankment. The results from the tests conducted in the geotechnical centrifuge indicated that settlement resulting from the cyclic loading was on average 67% higher than that of the static loading when considering loaded time. It was observed that the tests with increased moisture content exhibited significantly reduced stability. A completely developed slip surface failure was not observed. However, cracks formed at the crest of the embankment along its length, which indicated a downward shift of material. The research therefore concluded that cyclic loading on railway embankments increases permanent vertical settlement of the embankment compared to static loading. As the moisture content was increased for the different tests, there was a clear increase in crack development at the crest of the embankment along its length.

Comparative Study of Measured Suction in Fine-Grained Soil Using Different In-Situ and Laboratory Techniques

This paper presents a comparison between soil suction test results for a lean clay (CL) using four different measurement techniques, which included thermal conductivity sensors, in-situ psychrometers, a dew-point potentiometer and filter paper. The primary objective of these tests was to evaluate and compare the performance of these techniques and to demonstrate the sensitivity of the measured suction data to the quality of contact between the soil and the sensor, and measurement time and procedure, which could lead to orders of magnitude difference between the anticipated and measured results. The tests using a dew-point potentiometer and thermal conductivity sensors were carried out at different gravimetric water content values (referred to as moisture content in this paper) within the ranges of 10–23% and 8–24%, respectively, which were significantly above and below the optimum moisture content (OMC) of 18%. However, those using in-situ psychrometers and filter paper were carried out for moisture contents between OMC − 2% and OMC + 2%, as part of model embankment tests that were constructed at those moisture contents. Results of the study indicate that the in-situ psychrometer data were consistent with the soil water characteristic curve established using the dew-point potentiometer. However, thermal conductivity sensors and filter paper test data showed significant sensitivity to the quality of contact with the surrounding soil and measurement time and procedure, respectively, resulting in orders of magnitude deviations from the otherwise consistent in-situ psychrometer and dew-point potentiometer readings for the lean clay examined in this study. Findings of this study highlight the challenges involved, and level of care and due attention required in sensor installation and the testing procedure in order to obtain satisfactory suction data in unsaturated soils.

A large-scale test of reinforced soil railway embankment with soilbag facing under dynamic loading

Geosynthetic reinforced soil retaining walls can be employed as railway embankments to carry large static and dynamic train loads, but very few studies can be found in the literature that investigate their dynamic behavior under simulated wheel loading. A large-scale dynamic test on a reinforced soil railway embankment was therefore carried out. The model embankment was 1.65 meter high and designed to have a soilbag facing. It was reinforced with HDPE geogrid layers at a vertical spacing of 0.3 m and a length of 2 m. The dynamic test consisted of 1.2 million cycles of harmonic dynamic loading with three different load levels and four different exciting frequencies. Before the dynamic loading test, a static test was also carried out to understand the general behavior of the embankment behavior. The study indicated the importance of loading frequency on the dynamic response of reinforced soil railway embankment. It also showed that toe resistance played a significant role in the dynamic behavior of the embankment. Some limitations of the test were also discussed.

Seismic behavior of model embankment affected by seepage water and frozen surface

Seismic behavior of model embankment affected by seepage water and frozen surface

Suffusion-induced change in spatial distribution of fine fractions in embankment subjected to seepage flow

Suffusion describes the phenomenon whereby finer particles are eroded through the voids of coarse particles by the seepage flow. This may cause the deterioration of the hydraulic structure and, in the worst case, result in the failure of the hydraulic structure. The suffusion process is presented in this paper in an embankment under transient and steady seepage conditions. A series of physical model tests on seepage-induced suffusion on a small-scale model embankment is performed under constant boundary head conditions. A binary mixture, consisting of two Silica sands (Silica sands Nos. 3 and 8), which is categorized as “internally unstable material” by several previous criteria for the seepage-induced internal stability, is used for the model embankment. The cumulative eroded soil mass and the discharged rate of water are recorded during the seepage tests. The spatial extent of the variation in erosion-induced fines contents is discussed through sieve analyzes on subdivided areas of the model embankment after seepage testing. The test results reveal that a decrease in fines propagates along the phreatic surface from downstream in the embankment. Below the phreatic surface, the eroded fines not only move laterally by the seepage flow, but also vertically due to the gravitational force, and are deposited in the foundation. This deposition of the fines results in the expansion of the fine-rich region in the foundation and causes a decrease in the permeability of the whole embankment.

Study of a small scale tyre-reinforced embankment

Tyre-reinforced soil, used to improve slope stability, retaining walls, etc., has an excellent mechanical performance, and has the capability of a wider application and of reducing waste disposal costs. This article studies the stress and deformation characteristics, as well as the influencing factors related to the reinforcing arrangement, through small scale model embankment tests. It is shown that tyre reinforcement highly improved the strength of the model embankments; much higher stresses were mobilised inside the soil mass (around 2 times higher in comparison with the unreinforced embankment). There is an obvious plastic flow in the unreinforced embankment, while the plastic zone, on the reinforced embankments, was difficult to determine. Comparisons between the vertical settlement of the embankments show that the settlement of the reinforced embankment is roughly half of the settlement of the unreinforced embankment, for the same vertical load applied. These results also show that the tests with the top layer of reinforcement nearer the load application area and a smaller distance between the intermediate layers have a better performance, particularly in dense fabrics. The location of the top reinforcement layer seems to dominate the failure modes of the reinforced and unreinforced embankments, the horizontal deformations and the location of the shear bands in the embankment.

Soft soil improved by stone columns and/or ballast layer

Iraq has the oldest railway network in the region. Currently the Iraqi railway company is rehabilitating the existing network in different regions and planning to expand it for the future. About 30–40% of the total length of the network is located in the southern part of Iraq, passing over soft, saturated, sedimentary deposits and in many areas close to the marshland. Stability and settlement are the major challenges to the safety and serviceability of rail tracks in these areas, thus ground improvement is essential to achieve the required level of performance. The paper presents results of tests of three treatment patterns. The first investigates the presence of a ballast embankment overlying a bed of soft, saturated clay. The second pattern focuses on the improvements achieved in load-carrying capacity and settlement as a result of eight stone columns added at an area replacement ratio of 0·196. The third pattern investigates the improvements achieved when patterns one and two are combined. In all tests, the ballast model embankment is loaded gradually by stress increments up to failure and stress deformation measurements are recorded and analysed in terms of bearing improvement ratio and settlement reduction ratio. Optimum outcomes are deducted from the third pattern, revealing bearing improvement ratio of 2·3 and settlement reduction ratio of 0·17.

Measured Performance and Stability Analysis of Large-Scale Reinforced Model Embankments at Different Moisture Contents

This paper describes construction and instrumentation of two 1.65-m high reinforced embankment models that were tested at two different gravitational water contents (GWC) on the dry and wet sides of the optimum moisture content (OMC); i.e. OMC−2 % and OMC+2 %. The embankment models were constructed using a clayey sand that was reinforced with four layers of instrumented geotextile with a uniform vertical spacing of 300 mm. Each model embankment was subjected to a line surcharge load (strip footing) near its crest until failure. The magnitudes of GWC, earth pressure, geotextile strains, footing settlement and embankment deformations were measured using EC-5 sensors, earth pressure cells, wire potentiometers, linear variable deferential transformers and reference plates, respectively during the construction and loading phases of each test. The GWC values in each model embankment were measured using EC-5 sensors and the oven drying method to ensure that they were close to the target values. Slope stability analysis was carried out to study the equilibrium of embankment models and to validate the predicted bearing capacity and factor of safety results against the experimental data. Results of this study show that the embankment model constructed on the dry side of OMC (i.e. OMC−2 %) had a 10 % greater serviceability failure load than the model built at OMC+2 %. Meanwhile, stability analysis results indicate that the bearing capacity of an unreinforced model embankment at OMC−2 % would be 72 % lower than that of the corresponding reinforced model. It is also found that compaction-induced energy could increase the earth pressure within the embankment by up to 70 % of the vertical stress in the soil mass due to gravity.

Experimental Study on the Piping Erosion Process in Earthen Embankments

Experimental results on the piping erosion process in an earthen embankment emplaced in a laboratory flume are reported in this paper along with the details of the experimental set-up and procedures. A mixture of sand, silt, and clay with different compaction rates is used for constructing the embankment. An image processing technique is successfully applied to track the erosion process from both side-looking and bottom-up views. The paper discusses changes in the depth, area, and volume of erosion during the piping phenomenon in a model embankment using a visual technique. The results show that increasing compaction of the construction layers significantly increases the time required for erosion but has little effect on the final-average depth of erosion. The ratio of the average depth of erosion to the average bottom width of piping is close to one at different time intervals. Exponential equations to estimate the depth of erosion, side area of the piping zone, and volume of eroded material are presented. Two approaches are investigated to estimate the volume: the approach based on the processing of images from both the side–view and bottom–view gives more accurate results than that based on processing the side–view.

Study of a railway embankment reinforced with jute tassels

This paper presents the results of laboratory model tests and corresponding numerical analyses carried out on a model slope representing an actual railway embankment which failed on several occasions after overnight heavy rainfall. In the absence of any field data, the displacement behaviour and failure pattern of the model embankment slope are observed in the laboratory under fully saturated condition and under a static load at the crest simulating the actual rail loads. A study is also performed by reinforcing the slope with thin jute tassels of 1 mm diameter. The numerical simulation of the model tests is performed by a commercial program called FLAC. The responses of the model slope with and without jute tassels are observed in fully saturated condition. Significant improvement in deformation of the slope is observed both in the case of numerical analyses and laboratory experiments when jute tassels are utilized as a slope protection measure. The study indicates that the stability of the railway embankment can be improved significantly at low cost by reinforcing it with jute tassels.