• Journal of the Korean GEO-environmental Society
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• v.13 no.9
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• pp.61-68
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• 2012

The main objective of this study is to develop an analytical model for the resilient modulus of EPS geofoam when it is applied for flexible pavement as a subgrade material. This analytical model has been developed based on the results from triaxial compression tests. And this model can be used to analyze the flexible pavement structure using the finite element method by developing a program or modifying an existing program for any desired purposes. The results of this study show that the EPS geofoam as a replacement material for subgrade in flexible pavement is a feasible alternative to natural weak soils.

• International Journal of Highway Engineering
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• v.9 no.4
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• pp.55-63
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• 2007

Deformational characteristics of subgrade soils are important properties in the mechanistic analysis and design of pavement system. In this study, to evaluate the effect of specimen construction methods on deformational characteristics of subgrade soils in Korea, resonant column tests were performed for specimens constructed by various methods. Specimen construction method affected to the modulus value but the variation in the normalized modulus reduction curve was almost identical. The effects of specimen construction method on modulus are decreased with increasing confining pressure. The average maximum variation in the modulus value with different specimen construction methods was estimated as 16.8%. The differences in the modulus value of the specimens with same water content and dry density conditions that made by gyratory compaction and impact compaction were very small within 5.2%. The impact compaction method was proposed as a specimen construction method for determining the design input parameter testing considering that impact compaction method is much simpler and require less expensive specimen construction equipment and setup than gyratory compaction method.

• International Journal of Railway
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• v.6 no.2
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• pp.53-58
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• 2013

In this study, cyclic triaxial tests were carried out with the coarse granular materials used in Korean railway infrastructure (reinforced trackbed, gravel of transition zone, upper subgrade of railway) and Young's modulus for the target materials in small strain level were suggested. And the result of elastic modulus suggested in this study is expected to be effectively applied to dynamic analysis of the railway embankment structure using similar material, since the grain size distributions and unit weight of the material tested in this study are specified in Korean Railway Design Criteria.

• Journal of the Korean Geotechnical Society
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• v.20 no.2
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• pp.5-13
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• 2004

Conventional railroad roadbeds constructed with soils can easily deteriorate with time due to the increase of repeated traffic loading, increase of train speed, built-up of ground water on the roadbed and decrease of permeability in the roadbed layer, etc. In this study, performance of reinforced railroad roadbeds with the crushed stones was investigated through the real scale roadbed tests and numerical analysis. It was found that the reinforced roadbed with crushed stone had less elastic and plastic vertical displacement(settlement) than general soil roadbed regardless of the number of loading cycles. It was also found through the actual testing that for the roadbed with the same thickness, the displacement of reinforced roadbed decreases with the increase of subgrade reaction modulus. The settlement of reinforced roadbed with the same subgrade reaction modulus also decreases with the increase of thickness of the reinforced roadbed. However, the subgrade reaction modulus is a more important factor to the total plastic displacement of the track than the thickness of the crushed stone roadbed.

• Geomechanics and Engineering
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• v.28 no.4
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• pp.385-396
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• 2022

The accurate determination of the subgrade reaction modulus (K_s) of soil is an important factor for geotechnical engineers. This study estimated the K_s of soft soil improved with floating deep cement mixing (DCM) columns. A novel prediction model was developed that emphasizes the accuracy of identifying the most significant parameters of K_s. Several multi-layer perceptron (MLP) models that were trained using the Levenberg Marquardt (LM) backpropagation method were developed to estimate K_s. The models were trained using a reliable database containing the results of 36 physical modelling tests. The input parameters were the undrained shear strength of the DCM columns, undrained shear strength of soft soil, area improvement ratio and length-to-diameter ratio of the DCM columns. Grey wolf optimization (GWO) was coupled with the MLPs to improve the performance indices of the MLPs. Sensitivity tests were carried out to determine the importance of the input parameters for prediction of K_s. The results showed that both the MLP-LM and MLP-GWO methods showed high ability to predict K_s. However, it was shown that MLP-GWO (R = 0.9917, MSE = 0.28 (MN/m²/m)) performed better than MLP-LM (R =0.9126, MSE =6.1916 (MN/m²/m)). This proves the greater reliability of the proposed hybrid model of MLP-GWO in approximating the subgrade reaction modulus of soft soil improved with floating DCM columns. The results revealed that the undrained shear strength of the soil was the most effective factor for estimation of K_s.

• Journal of the Korean Geotechnical Society
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• v.24 no.4
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• pp.37-46
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• 2008

Application of resilient modulus, representing mechanical behavior of pavement materials, has become general concept for pavement design, analysis and maintenance after '86 AASHTO selected it as a basic input property of subgrade. It is known that resilient modulus of domestic subgrade soil is affected greatly by material factors, such as water content and dry weight unit, and stress components, such as deviatoric stress and confining stress, while effects of loading frequency and loading repeat were regarded negligible. If design based on resilient modulus is to be successfully implemented, design input variables of relevant models should be able to reflect local conditions. In this study, generalized mechanical model for subgrade is proposed. Model parameters are estimated from test results. Verification of the model was performed through finite element analysis using the proposed model, which showed good agreement with measured results of pavement deflections.

• Journal of the Korean Society for Railway
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• v.11 no.1
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• pp.54-60
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• 2008

In the trackbed design using elastic multilayer model, the stress-dependent resilient modulus $(E_R)$ is an important input parameter, that is, reflects substructure performance under repeated traffic loading. However, the evaluation method for resilient modulus using repeated loading triaxial test is not fully developed for practical purpose, because of costly equipment and the significantly fluctuated values depending on the testing equipment and laboratory personnel. The this study, the paper will present an indirect method to estimate the resilient modulus using dynamic properties. The resilient modulus of crushed stone, which is the typical material of sub-ballast, was calculated with the measured dynamic properties and the range of stress level of the sub-ballast, and approximated with the power model combined with bulk and deviatoric stresses. The resilient modulus of coarse grained material decreases with increasing deviatoric stress at a confining pressure, and increases with increasing bulk stress. Sandy soil (SM classified from Unified Soil Classification System) of subgrade was also evaluated and best fitted with the power model of deviatoric stress only.

• Journal of the Korean Society for Railway
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• v.18 no.2
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• pp.127-138
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• 2015

It is important to evaluate the stiffness characteristics of compacted subgrade soil under track that is loaded dynamically. Using a mid-size Resonant Column test apparatus, normalized shear modulus and shear modulus variation with changing of confining pressure were investigated with change of degree of saturation (DOS). From an analysis of the test results, it was verified that the maximum shear modulus decreased with increases of DOS. However, normalized shear modulus increased with increases of DOS. Using the test results, a relation of G~${\gamma}$~DOS can be constructed and characterized. In the future, by performing tests with soils used as trackbed broadly in the field, a prediction model for DOS~G~${\gamma}$ can be proposed.

• Proceedings of the KSR Conference
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• 2007.11a
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• pp.1042-1049
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• 2007

Recently, a theoretically-sound design approach, using an elastic multilayer model, is attempted in trackbed designs for the construction of high speed railways and new lines of conventional railways. In the elastic multilayer model, the stress-dependent resilient modulus($E_R$) is an important input parameter, that is, reflects substructure performance under repeated traffic loading. However, the evaluation method for resilient modulus using repeated loading triaxial test is not fully developed for practical purpose, because of costly equipment and the significantly fluctuated values depending on the testing equipment and laboratory personnel. In this study, the paper will present an indirect method to estimate the resilient modulus using dynamic properties. The resilient modulus of crushed stone, which is the typical material of sub-ballast, was calculated with the measured dynamic properties and the range of stress level of the sub-ballast, and approximated with the power model combined with bulk and deviatoric stresses. The resilient modulus of coarse grained material decreases with increasing deviatoric stress at a confining pressure, and increases with increasing bulk stress. Sandy soil(SM classified from Unified Soil Classification System) of subgrade was also evaluated and best fitted with the power model of deviatoric stress only.

• Proceedings of the Korean Geotechical Society Conference
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• 1995.03a
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• pp.175-180
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• 1995