Compacted Subgrade Soils Research Articles

Novel base predictive model of resilient modulus of compacted subgrade soils by using interpretable approaches with graphical user interface

The flexible pavement system consists of layers that are made up of a blend of aggregates and bitumen. To design flexible pavement systems that are both safe and environmentally sustainable, it is essential to have an accurate understanding of the resilient modulus (Mr) of the compacted subgrade soil. Mr refers to the ability of the soil to resist deformation under repeated loads. Thus, a critical parameter affects the performance and longevity of pavement systems. This study employs machine learning (ML) algorithms such as individual and ensemble learners using an extensive database of 2813 data points. These include dry unit weight, weighted plasticity index, confining stress, deviator stress, moisture content, and the number of freeze-thaw cycles (FT). The individual or weak learners were incorporated to create strong and robust ensemble learners by employing techniques such as bagging, adaptive boosting, and random forest (RF). Ensemble learning methods were used to improve the performance of individual learners, such as support vector machine (SVM) and decision tree (DT), by combining their predictions. To achieve the highest R2 value, a total of twenty bagging and boosting sub-models were trained and optimized. The validation of the test data was carried out through K-Fold cross-validation, utilizing metrics such as R2, MAE, and RMSE. The developed models were rigorously tested using statistical indices (MAE, MSE, RMSE, and RMLSE) to verify their predictive accuracy, reliability, and trustworthiness. The findings indicate that the integration of bagging and boosting techniques improves the efficiency of individual machine learning (ML) models. The combination of RF and DT utilizing bagging resulted in the most reliable performance, achieving an R2 value of 0.9 and demonstrating minimum errors. In general, the implementation of the ensemble algorithm in ML improved the overall prediction accuracy of the model. Sensitivity analysis reveals that the prediction of the resilient modulus (Mr) of the subgrade is primarily influenced by dry density, confining stress, and deviator stress. Moreover, a graphical user interface (GUI) is developed for practical implantation.

Indirect estimation of resilient modulus (Mr) of subgrade soil: Gene expression programming vs multi expression programming

Accurate prediction of resilient modulus (MR) in compacted subgrade soil is crucial for planning secure and environmentally friendly flexible pavement systems. This research assembled a dataset of 2813 data points from twelve compacted soils. The dry density, confining stress, deviator stress, number of freeze-thaw cycles, and moisture content were among the important variables considered for determining the MR. Subsequently, this study employs ensemble machine learning methodologies, specifically gene expression programming (GEP) and multi-expression programming (MEP), to investigate the subject further. The precision and anticipatory proficiency of both the GEP and MEP models are assessed through statistical evaluations, encompassing crucial metrics (R, RMSE, MAE, RSE, RRMSE, and ρ). The GEP and MEP models align well with validation criteria, underscoring their robustness in predicting novel data and showcasing their broad applicability. The GEP model consistently outperformed the MEP model, with higher coefficient of regression (R2) values in both training (0.992 vs. 0.983) and testing (0.981 vs. 0.972) phases, demonstrating its superior predictive accuracy and robustness. In summary, the GEP model consistently outperforms the MEP model in accuracy and prediction, making it the preferred choice for subgrade soil MR prediction. Sensitivity analysis was done, which ranked the parameters by their influence: dry density (26.6 %), confining stress (22.7 %), weighted plasticity index (15.3 %), moisture content (13.5 %), deviator stress (12.5 %), and freeze and thaw cycles (9.4 %). This research aims to enhance the utilization of GEP and MEP in civil engineering for more accurate and efficient MR prediction, ultimately reducing time and costs.

Measurement of subgrade soil permanent deformations under repeated loadings during simple in-situ test

is well known that permanent deformation has a significant influence on the performance of pavements because it leads an increase in maintenance operations and costs and reduces ride quality. Therefore, it is important to predict the permanent deformations during the design stage. Due to the fact that up to date there is no simple test equipment and procedure that enables a direct in-situ measurement permanent deformations of subgrade soil under repeated loading, the current research has been undertaken to evaluating new testing approach to fulfil this requirement. The current study deals with developing a new multifunctional light weight deflectometer (LWD) that can be used to determine the in-situ permanent deformations of subgrade soil due to repeated loading by simple, cheap and time saving testing approach. It was concluded that the suggested repeated light weight deflectometer test could give a powerful material assessment and pavement design tool for the analysis of compacted subgrade soil used in this study. The developed LWD that can measure the permanent deformations directly could be utilized to establish the risk level of permanent deformations in subgrade soil in pavement constructions during the design and construction stages.

Determining the Bimodal Soil–Water Characteristic Curve of Fine-Grained Subgrade Soil Derived from the Compaction Condition by Incorporating Pore Size Distribution

The soil–water characteristic curve (SWCC) is a key constitutive relationship for unsaturated soil which can be unimodal or bimodal. For the fine-grained compacted subgrade soil with a bimodal pattern, the determination of SWCC is complicated and needs a wide-range suction measurement. In this paper, the bimodal SWCC of a subgrade soil derived from the compaction condition was measured and determined by incorporating pore size distribution. For this purpose, a series of laboratory tests were conducted, including the pressure plate method, filter paper method, and vapor equilibrium method, which were used to measure SWCC at the low, medium, and high suction range, respectively. The pore size distribution (PSD) data were obtained by mercury intrusion porosimetry (MIP) tests and used to predict SWCC. Based on the analysis of hydraulic paths and SWCC-PSD correlations, the SWCC of the subgrade soil should be determined to follow the actual hydraulic path. SWCC within a low suction range can be filled by PSD-based data to improve the fitting accuracy. Then, a graphical method is applied to predict the bimodal SWCC by combining the filter paper method, vapor equilibrium method, and PSD-based data. The prediction curves fit well with the test data for all selected compaction conditions. Furthermore, the prediction method can still provide good prediction performance in the absence of high suction section data, which is beneficial for the application of bimodal SWCC.

Prediction of the resilient modulus of compacted subgrade soils using ensemble machine learning methods

The accurate estimation of resilient modulus (MR) of compacted subgrade soil is imperative for the safe and sustainable design of flexible pavement systems. The aim of this study is to explore the potential of ensemble machine learning techniques for predicting the MR of pavement subgrade soil. For this, 2813 data points from twelve compacted subgrade soils were collected which consists of the following inputs parameters: dry unit weight, weighted plasticity index, deviator stress, confining stress, number of freeze–thaw cycles, and moisture content. Four commonly used machine learning (ML) methods, namely, gradient boosting regression (GBR), decision tree regression (DTR), K nearest neighbour regression (KNR), and random forest regression (RFR) were developed and implemented for forecasting the MR value. Thereafter, several ensemble ML techniques including voting ensemble (VO-ENSM), voting ensemble with RF as a meta-model (VO-ENSM (RF)), stacking ensemble (ST-ENSM) and bagging ensemble (BG-ENSM) were utilised to amalgamate the outputs from the developed standalone ML models. Additionally, a multiple linear regression model was also developed as a baseline. The predictive veracity, reliability and trustworthiness of the developed ensemble models were corroborated using rigorous statistical testing, ranking technique, and uncertainty analysis. The results as obtained have shown that the BG-ENSM outperformed its counterparts in predicting the MR of subgrade soil. Hence, it can be a part of portfolio of predicting tools utilised by the practitioners in evaluating the strength of the pavement subgrade soil. Finally, the sensitivity analysis was performed to assess the strength of input variables on the MR.

Prediction of resilient modulus for subgrade soils based on ANN approach

The resilient modulus (MR) of subgrade soils is usually used to characterize the stiffness of subgrade and is a crucial parameter in pavement design. In order to determine the resilient modulus of compacted subgrade soils quickly and accurately, an optimized artificial neural network (ANN) approach based on the multi-population genetic algorithm (MPGA) was proposed in this study. The MPGA overcomes the problems of the traditional ANN such as low efficiency, local optimum and over-fitting. The developed optimized ANN method consists of ten input variables, twenty-one hidden neurons, and one output variable. The physical properties (liquid limit, plastic limit, plasticity index, 0.075 mm passing percentage, maximum dry density, optimum moisture content), state variables (degree of compaction, moisture content) and stress variables (confining pressure, deviatoric stress) of subgrade soils were selected as input variables. The MR was directly used as the output variable. Then, adopting a large amount of experimental data from existing literature, the developed optimized ANN method was compared with the existing representative estimation methods. The results show that the developed optimized ANN method has the advantages of fast speed, strong generalization ability and good accuracy in MR estimation.

Predicting resilient modulus of compacted subgrade soils under influences of freeze–thaw cycles and moisture using gene expression programming and artificial neural network approaches

This study aims at developing a gene expression programming (GEP) model and an artificial neural network (ANN) model to predict the resilient modulus (MR) of compacted pavement subgrade soils based on their physical properties, external stress states, and environmental factors. A database of 2813 MR measurements derived from 12 subgrade soils with different moisture-temperature histories was established for model development and validation. Influencing factors considered in this database include the weighted plasticity index (wPI), dry unit weight (γd), confining stress (σc), deviator stress (σd), moisture content (w), and the number of freeze–thaw cycles (NFT). Sensitivity analysis was conducted to evaluate the importance of each factor. The order of influencing factors by decreasing importance was found to be wPI, γd, w, NFT, σd, σc and the importance of the σc and σd is remarkably lower than other factors. This indicates that the MR of compacted subgrade soils is strongly dependent on the soil type (wPI, γd) and is more sensitive to environmental factors (NFT, w) than external stress states (σc, σd). The developed GEP and ANN models reasonably predicted the MR in the database and achieved better performance compared to several existing empirical models.

Performance comparison of different electrode materials for electro-osmosis treatment on subgrade soil

During the service life of subgrade, the moisture content increase on compacted subgrade soil can lead to bearing capacity serious degradation of the soil and exaggerated deformation. Electro-osmosis drainage is an effective method in reducing the moisture content of soil, which also serves to improve the subgrade. However, in practice, the low corrosion resistance and poor contact condition of conventional electrode materials make their energy inefficient. And the insufficient understanding of unsaturated soil electro-osmosis also limits the further application of this method. In this paper, a new type of electro kinetic geosynthetics (EKG) electrode was designed and used in electro-osmosis tests on unsaturated soil which was sampled from a reconstruction field in comparison with graphite and iron electrodes. The energy consumption, effective voltage variations, current variations and change in moisture contents under different voltage gradients were analyzed. A one-month field mesoscale test was conducted to verify the feasibility and efficiency of the new EKG electrode. Test results indicated that electro-osmosis drainage performed well with three kinds of electrode materials and the moisture reduction had reached 8.5–15.4%. The contact resistance of iron electrode would reach 1.8–4.1 times that of the EKG and graphite electrodes due to material corrosion. The newly-designed EKG electrode could reduce contact resistance by 19.7–40.8% for the layered fiber structure could enlarge the contact area between soil and electrodes.

Characterizing hydro-mechanical behaviours of compacted subgrade soils considering effects of freeze-thaw cycles

Abstract This paper presents a series of experimental studies for evaluating the effects of closed-system freeze–thaw (FT) cycles on the hydro-mechanical behaviours of two subgrade soils (a low plastic lean clay, SS and a lean clay with higher plasticity, HC). Investigated hydro-mechanical behaviours include the soil–water characteristic curve (SWCC) obtained from the filter paper method, resilient modulus (MR) determined from cyclic triaxial tests, unconfined compression strength (qu) and reloading tangent modulus (E1%) and stress (Su1%) at 1% strain measured from unconfined compression tests, with emphasis on the SWCC and MR. Specimens compacted at the maximum dry density (ρdmax) and optimum moisture content (wopt) were firstly subjected to multiple FT cycles (number of FT cycles NFT = 0, 1, 3, 6 and 10) and then dried or wetted to different moisture contents before determining hydro-mechanical behaviours. Experimental results revealed that (i) FT cycles reduce the magnitude of volumetric strain upon moisture variation for the HC but have little impact on the SS; (ii) FT cycles reduce the water retention capacity of both soils. For each soil, the void ratio (e)-moisture content (w)-suction (s) relationships after different FT cycles are possibly distributed on a unique surface; (iii) Reductions in the mechanical properties (i.e. MR, qu, E1% and Su1%) are more significant at NFT = 1 and vary with the post-FT cycle moisture content. Reductions in the MR are most serious at a threshold w level on the wet side of wopt; (iv) FT cycles reduce the sensitivity of the mechanical properties to moisture content for the HC but exert minor influence on that of the SS; (v) Relationships of the MR to the qu, E1% and Su1% are not influenced by the NFT and moisture content for both soils. They are non-linear and can be well described by quadratic polynomials. Soils with higher plasticity such as the HC is, in general, more vulnerable to effects of closed-system FT cycles at wopt than low plastic soils such as the SS.

Cumulative strain characteristics of compacted soil under effect of freeze-thaw cycles with water supply

Abstract The cumulative strain characteristics of compacted subgrade soil were investigated by considering the effects of both freeze–thaw (FT) cycles and repeating loads. The soil samples were obtained from the seasonally frozen area in Northeast China. First, FT cycle experiments were carried out in an open system, following which dynamic triaxial tests were conducted on the same specimens. The influences of the number of FT cycles, dynamic stress, confining pressure, loading frequency, initial moisture content and compaction degree on the cumulative strain of the compacted soil were investigated systematically. The experimental results demonstrate that the cumulative plastic strain is positively correlated with the number of FT cycles, dynamic stress and initial moisture content, while the correlations between the cumulative plastic strain and the factors of confining pressure, loading frequency, and compaction degree are negative. Furthermore, the contribution rate of every factor on the cumulative plastic strain indicates that the dynamic stress and number of FT cycles have the significant effect on the cumulative plastic strain, while the initial moisture content effect is the weakest. Finally, a multifactor prediction model was established to evaluate the cumulative plastic strain of soil subjected to FT cycles with water supply, and the calculated results agree well with the experimental data, implying that the model proposed in this study is sufficiently reliable.

Effect of Wetting‐Drying Cycles on Mechanical Behaviour and Electrical Resistivity of Unsaturated Subgrade Soil

Compacted soil is widely used in road and railway subgrade, while alternation of seasons can cause fluctuations in moisture content of soil (i.e., wetting‐drying cycles) and influence the performance of soil. In order to research the effect of wetting‐drying cycles on mechanical behaviour and electrical resistivity of compacted unsaturated subgrade soil, wetting‐drying tests considering different number and cyclic amplitude were conducted on compacted unsaturated clay specimens, and the electrical resistivity and unconfined compressive strength of soil were measured in this study. The AC (alternative current) two‐electrode method was applied in the resistivity measurement. The experimental results show that increasing number and cyclic amplitude of wetting‐drying cycles can both reduce the strength and electrical resistivity of the compacted unsaturated specimens. After 3‐4 wetting‐drying cycles, the strength and electrical resistivity tend to be constant value. The change of pore structure can be the key factor leading to the reduction of electrical resistivity of soil subjected to wetting‐drying cycles and consequently causing the decrease of soil strength in the present study. Thus, the electrical resistivity can be adopted to indirectly assess the mechanical behaviour of unsaturated compacted soil after wetting‐drying cycles.

Experimental Simulation and Quantification of Migration of Subgrade Soil into Subbase under Rigid Pavement Using Model Mobile Load Simulator

AbstractRigid pavement structure typically consists of a surface layer (concrete), underlying granular layers (subbase and/or base), and a compacted subgrade soil layer. Because the subgrade is sat...

Characterizing cyclic and static moduli and strength of compacted pavement subgrade soils considering moisture variation

Compacted soils are widely used as the subgrade layer for pavements. Knowledge of the mechanical properties of subgrade soils under cyclic and static loading conditions and their variation under the influence of environmental factors is required for the rational design of pavements based on mechanistic methods. This paper presents an experimental investigation of the cyclic and static moduli and the strength properties of seven different compacted Canadian subgrade soils considering the variation in the post-compaction moisture content. Cyclic triaxial tests were performed to reliably determine the resilient modulus (MR). Unconfined compression tests, which allow an unloading-reloading loop at 1% strain, were performed to determine the deviator stress (Su1%) at 1% strain, the reloading elastic modulus (E1%) at 1% strain and the unconfined compressive strength (qu) at failure. The physical properties, the chemical and mineralogical compositions, and the soil-water characteristics of these soils were also determined. Relationships were developed to predict the MR from the Su1%, E1%, qu and soil physical properties for the investigated subgrade soils because the experimental determination of MR is both expensive and time-consuming. The studies presented in this paper provide useful information and approaches that can be used to promote the implementation of mechanistic pavement design methods using simple techniques.

Soil Compaction Methods Development

A process of soil compaction methods development including new authors’ methodology is described. The importance of soil compaction for engineering purposes is substantiated. Preconditions for Proctor compaction test appearance are highlighted. Proctor’s approach and suggestions for the degree of soil compaction assessing are analyzed. Soviet version of Proctor’s equipment and Modified Proctor compaction test are given. Principal differences between Proctor test, Standard compaction test and Modified Proctor test are presented. The problems and disadvantages of existent soil compaction tests are revealed. New authors’ physical experiment methodology for patterns establishment of water migration in subgrade embankment depth, in the capacity factors of what it is accepted: clay soil type (its number plasticity); moisture, at what the soil was compacted; soil skeleton density; embankment height; «rest» time after subgrade erection and before it’s operation is developed and realized. By laboratory and field tests water migration patterns in compacted subgrade soils depth are established. As a result of statistical processing of research results, the empirical dependence of compacted clay soil stabilized moisture is obtained. Empirical dependence parameter corresponds to maximum molecular moisture capacity at what it is advisable to do the subgrade clay soils multilayer consolidation for their long-term strength ensuring.

MOISTURE CONDITIONS PATTERNS IN ROAD EMBANKMENT CLAY SOILS DEPTH

The article deals with optimal compaction criteria of road embankment soils improving, which provide their long-term strength. Physical experiment methodology for patterns establishment of water migration in subgrade embankment depth, in the capacity factors of what it is accepted: clay soil type (its number plasticity); moisture, at what the soil was compacted; soil skeleton density; embankment height; «rest» time after subgrade erection and before it’s operation are developed and realized. By laboratory and field tests water migration patterns in compacted subgrade soils depth are established. As a result of statistical processing of laboratory and field research results, the empirical dependence of compacted clay soil stabilized moisture for their multilayer consolidation in relation to soil skeleton density and plasticity number values is obtained.

Geogrid for unsealed forest roads: installation considerations and bearing capacity testing in New Zealand

ABSTRACTThis study established 10 field trials on corporate forest roads in New Zealand to demonstrate geogrid installation procedures and test for differences in bearing capacity, hereafter referred to as road strength, for road segments with and without geogrid reinforcement. The primary objective of this research was to determine if thinner aggregate surface layers could be used in conjunction with geogrid reinforcement without significant reductions in road strength. Each trial consisted of three 25-m long road segments randomly configured with the following pavement designs: (1) Control section that consisted of a single aggregate layer overlying a compacted subgrade soil; (2) Geogrid reinforcement that used the same aggregate thickness as the Control; and (3) Geogrid + Reduced Aggregate treatment that used a thinner aggregate layer. Road strength was measured with a Clegg Hammer for the prepared subgrade, finished road surface (i.e. before traffic), and after one winter of log truck traffic. Overall, there were no clear differences in road strength among treatments before or after trafficking. Several factors related to studying operational forest roads are thought to have contributed to this finding, including relatively low traffic volumes (285–1220 loaded trucks) and variability in aggregate thickness within and among sites. Geogrid-reinforced roads may perform better over time and with more traffic. In terms of cost, this finding supports the common practice of simply using thicker aggregate layers to achieve a desired strength as long as aggregate is cheap, local, and readily available.

Integrated approaches for predicting soil-water characteristic curve and resilient modulus of compacted fine-grained subgrade soils

This paper combines a series of approaches for predicting the soil-water characteristic curve (SWCC) and the variation of the resilient modulus (MR) of compacted fine-grained subgrade soils with moisture content, which is the key information required in mechanistic pavement design methods. The presented approaches for the SWCC and MRare integrated, as (i) they are developed following the same philosophy, (ii) they require only the measurements of the suction and moisture content or MRat saturated and optimum moisture content conditions for prediction, and (iii) the predicted SWCC is used for predicting the MR– moisture content relationship. Experimental studies have been performed on five fine-grained subgrade soils that were collected from different regions in Ontario, Canada, to determine their MRat various external stress levels and post-compaction moisture contents, as well as their SWCCs after the MRtests. Experimental measurements are predicted using the integrated approaches and the empirical approaches currently used in the mechanistic–empirical pavement design guide (MEPDG). It is demonstrated that the integrated approaches are easy to use and show improved reliability in predicting both the SWCC and MRfor the investigated subgrade soils in spite of using limited experimental data.

Resilient Modulus Characterization of Compacted Cohesive Subgrade Soil

Soil investigations concerning cyclic loading focus on the evaluation, in particular, of design parameters, such as elastic modulus, Poisson’s ratio, or resilient modulus. Structures subjected to repeated loading are vulnerable to high deformations, especially when subgrade soils are composed of cohesive, fully-saturated soils. Such subgrade soils in the eastern part of Europe have a glacial genesis and are a mix of sand, silt, and clay fractions. The characteristic of, e.g., Young modulus variation and resilient modulus from repeated loading tests, is presented. Based on performed resonant column and cyclic triaxial tests, an analytical model is proposed. The model takes into consideration actual values of effective stress p′, as well as loading characteristics and the position of the effective stress path. This approach results in better characterization of pavement or industrial foundation systems based on the subgrade soil in undrained conditions. The recoverable strains characterized by the resilient modulus Mr value in the first cycle of loading was between 44 MPa and 59 MPa for confining pressure σ’3 equal to 45 kPa, and between 48 MPa and 78 MPa for σ’3 equal to 90 kPa. During cyclic loading, cohesive soil, at first, degrades. When pore pressure reaches equilibrium, the resilient modulus value starts to increase. The above-described phenomena indicate that, after the plastic deformation caused by excessive load and excess pore water pressure dissipation, the soil becomes resilient.

Application of lignin-based by-product stabilized silty soil in highway subgrade: A field investigation

Lignin is a by-product of paper and timber industry, and it has not been fully utilized in both developed and developing countries. Improper disposal or storage of lignin is not only a waste of natural resources, but also posing significant risk to public health and the environment. Sustainable reuse options for lignin in civil engineering infrastructures, such as road embankments and dam foundations, have been recently evaluated by laboratory testing. However, up to now studies on the actual filed performance of lignin in stabilizing silty soils in highway subgrade have been noticed to be quite limited. With this in view, a field trial was carried out to verify the viability of using lignin stabilized silty soil as a highway subgrade course material. Traditional soil stabilizer, quicklime, was selected as a control chemical mixture in the field trial for comparison purposes. The construction procedures of the subgrade silty soil stabilized by lignin and quicklime in field Sections were presented. A series of field tests, including California Bearing Ratio (CBR) test, resilient modulus (Ep) test, Benkelman beam deflection test, and dynamic cone penetrometer (DCP) test were carried out after the subgrade construction to investigate the effects of the curing time and additive content on the mechanical properties and bearing capacity of the stabilized silt. In addition, moisture content and compaction degree tests were conducted to evaluate the quality of the compacted subgrade soils. The test results indicate that at the bottom zone of the filled soil layers with 96% degree of compaction, the 12% lignin stabilized silt exhibits superior mechanical performances (i.e., higher value of CBR and Ep, and lower values of resilient deflection (Hr) and DCP Index) than the 8% quicklime stabilized silt after 15 days of curing. Under the same additive content (i.e., 8%), the bearing capacity of lignin stabilized silt is slightly lower relative to the quicklime stabilized one. The field trial results reveal that as a stabilizer of the subgrade soil, lignin has negligible environmental influences and induces low construction costs. The use of lignin as a stabilization chemical mixture for silty soil may be one of the viable answers to the reuse of biobased organic by-product in civil engineering. The outcome of this study is of great significance for the development of nontraditional, cost-effective, and environmentally friendly soil stabilizer in solidification/stabilization technology.

Relationship between resilient modulus and suction for compacted subgrade soils

The resilient modulus (MR) of pavement materials is a key parameter required in the mechanistic pavement design methods. Seasonal moisture regime and suction (s) fluctuations exert remarkable influence on the MR which should be measured or predicted to assist in the analysis of pavement distress and service life. This paper presents extensive experimental investigations to determine the MR-s relationship for three Canadian subgrade soils (a silty clay from Toronto, a marine clay from Ottawa and a glacial till from Indian Head) using two different experimental methods. Following the first method, glacial till specimens were consolidated to known stress and suction levels using the axis-translation technique before subjecting to cyclic loading tests to determine the MR. In the second method, equilibrium conditions were achieved with respect to different moisture contents for specimens of the silty clay and marine clay before cyclic loading tests. Suction was measured after the completion of cyclic loading tests using the filter paper method. Experimental results suggest varying degrees of non-linear relationships between the MR and the suction, gravimetric water content and external stress for different soils. A simple model, which is based on the soil-water characteristic curve and MR values at saturated and optimum moisture content conditions for prediction, is used to predict the measured MR-s relationship for the three soils. Same measurements were also predicted using two rigorous models from the literature that were proposed based on different approaches. It is shown that the simple model, compared with other two models, provides equally good predictions but requires remarkably less experimental data. The simple model therefore can be used in practice as a reliable tool for predicting the influence of suction and moisture content on the resilient modulus of subgrade soils.