Resilient Modulus Of Subgrade Soils Research Articles

English

Resilient modulus of subgrade soils is an important input in mechanistic pavement design. The primary objective of this work is to investigate the resilient modulus of four typical Victorian fine-grained subgrade soils under traffic-like repeated loading and to suggest empirical predictive models incorporating physical properties and/or strength of the soils along with the stress state. A repeated load triaxial testing procedure was developed, which is capable of collecting resilient and permanent deformation data from the same specimen.  Stress levels for testing were defined as percentages of the confined and/or unconfined soil static strengths. Stress dependency of resilient modulus was studied through the models (such as bilinear model, power model, deviatoris stress model and octahedral stress model) found in the literature and other possible combinations of deviator, confining and octahedral stresses. A semi-logarithmic model was proposed for the prediction of resilient modulus of the fine-grained subgrade soils. Calibration of model constants by soil properties was investigated.  An altervative prediction model was also developed based on unconfined compressive strength and deviator stress. Resilient modulus values were back calculated using both the semi-logarithmic model and the model based on unconfined compressive strength and deviator stress. Predicted values were compared with the measured values. Predictive capability of the proposed models were proven for use in flexible pavement design.   Key words: Resilient modulus, fine-grained subgrade soils, repeated loading, flexible pavement design.

Developing Correlation Relationship between Modulus of Subgrade Reaction and Resilient Modulus for Florida Subgrade Soils

The resilient modulus of pavement subgrade is an essential parameter for mechanistically based pavement design procedures; the modulus of subgrade reaction (k) is used as a primary parameter for rigid pavement design. The AASHTO pavement design guide (1986, 1993) suggested a theoretical relationship between modulus of subgrade reaction and resilient modulus of subgrade based on the assumption that the subgrade material is linear elastic. This paper presents an experimental study to evaluate the load–deformation and resilient modulus characteristics of the granular subgrade soils with the use of field and laboratory tests. An extensive field static plate bearing load testing program was conducted to evaluate the in situ bearing characteristics of typical Florida pavement subgrade soils. A full-scale laboratory evaluation of the subgrade performance was conducted in a test-pit facility that simulated the actual field conditions. A laboratory triaxial test was performed to evaluate the resilient modulus characteristics of the subgrade materials. Comparative studies were conducted to evaluate the resilient modulus from laboratory triaxial tests and field experimental studies. Correlation relationships were developed between the measured resilient modulus in the laboratory and the resilient modulus measured with the use of a test-pit facility and between the subgrade soil resilient modulus and the modulus of subgrade reaction. The calibrated relationship based on the experimental studies was close to the AASHTO theoretical relationship. The calibrated relationship could be used in the Florida pavement design guide to obtain realistic subgrade resilient modulus values from laboratory resilient modulus measurements.

Prediction of Resilient Modulus of Granular Subgrade Soils and Subbase Materials using Artificial Neural Network

ABSTRACT Experimental estimation of the modulus is excessively costly and complex owing to the large number of factors that affects the modulus. Thus, it is necessary to determine the resilient modulus of granular pavement materials such as subgrade soils and subbase materials from an empirical model for the design of a lower level pavement system. In this study, ANN (Artificial Neural Network) models were used to develop an empirical model for the resilient modulus of subgrade soils and subbase materials from basic material properties and in-situ conditions related to stresses in reference to data obtained from tests conducted in this study. It can be concluded that the ANN models serve as a reliable and simple predictive tool for the resilient modulus of granular subgrade soils and subbase materials.

Variations of Resilient Modulus of Subgrade Soils with Postcompaction Moisture Contents

Environmental conditions such as temperature and moisture can significantly influence the performance of a pavement structure. These conditions vary within the pavement layers, degrading or enhancing the properties of pavement materials. The AASHTO 1993 design guide accounts for this influence by incorporating a single resilient modulus ( MR) value that represents the effective roadbed resilient modulus; the revised AASHTO Mechanistic–Empirical Pavement Design Guide (MEPDG) uses models that predict changes in modulus due to moisture, temperature, or both. The effect of changes in postcompaction moisture content (i.e., wetting or drying) on the resilient moduli of subgrade soils is evaluated. The effects of wetting and drying were examined by conducting resilient modulus tests on specimens compacted at various initial moisture contents: optimum moisture content (OMC), 4% drier than OMC (OMC – 4%), and 4% wetter than OMC (OMC + 4%). Results showed that MR–moisture content ( MRMC) relationships caused by drying exhibited higher values than the corresponding MRMC curves for specimens subject to wetting. The most significant finding was that changes in MR values depended on the initial compaction moisture contents; the MR values compacted at OMC exhibited a different drying and wetting trend than the ones compacted at OMC ± 4%. Finally, the MEPDG MR–moisture model was modified in this study to better predict the variations of resilient modulus with moisture changes.

Performance evaluation of silty sand subgrade reinforced with fly ash and fibre

In this paper, the effectiveness of fibre reinforcement (coir fibre and synthetic fibre) in subgrade soil has been studied from the point of view of strength. The permanent strain, resilient strain behaviour and resilient modulus of subgrade soil have been determined in the laboratory. A value of 10% (20 mm) strain is taken as the failure criterion for the subgrade for pavement in rural area. A subgrade soil of silty sand mixed with optimum content of fly ash and two different types of fibres varying in their tensile strength and coefficient of frictions were used. Repeated triaxial tests on samples, unreinforced and reinforced at the optimum content of fibre, were carried out at a confining pressure of 25, 50 and 75 kN/m 2 and the stress levels of 153 and 204 kN/m 2, producing six different deviator stresses. It is concluded from this study that both the permanent and resilient strains in all materials decrease with confining pressure but increase with the number of load cycles and deviator stress in reinforced and unreinforced conditions. Further, the resilient modulus decreases with the number of load cycles and deviator stress and increases with the confining pressure. Coir fibre shows better resilient response against synthetic fibre by higher coefficient of friction. Fly ash is also used in this study and for maximum dry density, the 30% fly ash and 70% sand mix is tested for various parameters.

Rapid Alternative for Laboratory Determination of Resilient Modulus Input Values on Stabilized Materials for AASHTO Mechanistic-Empirical Design Guide

NCHRP is in the final stages of developing and implementing a new mechanistic-empirical pavement design guide (MEPDG) to be used by all state highway agencies for layer thickness design. Once adopted, it will become the national procedure required on all federally aided projects. A key material property to be used in the design will be the resilient modulus value, which either can be obtained from laboratory testing or can be backcalculated from deflection data collected with falling weight deflectometers. It is important that realistic design moduli values be used. The standard test method for resilient modulus of subgrade soils and untreated base and subbase materials is AASHTO T307-99. However, the setup for this procedure is both difficult and time-consuming, and the equipment required to run this test is expensive. With the low strain levels used, it is likely that accurate instrumentation will be problematic for most state departments of transportation. The free-free resonant column test is a simple, nondestructive laboratory test method for determining the seismic modulus of pavement materials; it takes approximately 5 min to setup and complete. The equipment costs less than $4,000. For the tests conducted, it appears that the seismic modulus device is a better and more repeatable test for estimating the resilient modulus of stabilized materials, and it shows great potential for acquiring reasonable resilient modulus input values for the new AASHTO MEPDG.

Variation of Resilient Modulus with Soil Suction for Compacted Subgrade Soils

The variations of resilient modulus with the postconstruction moisture content and soil suction for cohesive subgrade soils were evaluated. In particular, the effects of relative compaction of the subgrade on the suction and resilient modulus were investigated. To simulate subgrade soils at in-service conditions, soil specimens were compacted at various relative compactions and optimum moisture content and then saturated to equilibrium moisture content to test for resilient modulus and soil suction. The filter paper method was used to measure the total and matric suctions of two cohesive soils. Test findings demonstrated that resilient modulus correlated better with the matric suction than with total suction. Matric suction was found to be a key parameter for predicting the resilient modulus of cohesive subgrade soils. A prediction model incorporating deviator stress and matric suction for subgrade soil resilient modulus was established.

Variation of Resilient Modulus with Soil Suction for Compacted Subgrade Soils

The variations of resilient modulus with the postconstruction moisture content and soil suction for cohesive subgrade soils were evaluated. In particular, the effects of relative compaction of the subgrade on the suction and resilient modulus were investigated. To simulate subgrade soils at in-service conditions, soil specimens were compacted at various relative compactions and optimum moisture content and then saturated to equilibrium moisture content to test for resilient modulus and soil suction. The filter paper method was used to measure the total and matric suctions of two cohesive soils. Test findings demonstrated that resilient modulus correlated better with the matric suction than with total suction. Matric suction was found to be a key parameter for predicting the resilient modulus of cohesive subgrade soils. A prediction model incorporating deviator stress and matric suction for subgrade soil resilient modulus was established.

Seasonal Variations of a Subgrade Soil Resilient Modulus in Southern Brazil

The influence of suction on the resilient modulus of a shale residual lateritic soil was analyzed. Laboratory tests were carried out to obtain the moisture content-suction relationship and determine the resilient moduli of specimens submitted to drying, wetting, or wetting-after-drying paths. A supplementary study on the effects of compaction method on soil resilient modulus was carried out. Soil suction was measured in situ, with jet-fill tensiometers installed along test sections built in a pavement testing facility. Test sections were loaded by a traffic simulator, and periodically deflections were measured for modulus backcalculation. Laboratory and in situ results confirmed the consensus that suction remarkably affects soil elastic deformability. Wetting-after-drying paths proved to be an extremely severe condition that may lower resilient modulus up to four times. Static compaction led to resilient moduli higher than those of specimens compacted dynamically or by kneading. In situ results matched reasonably well with laboratory moduli of specimens compacted by kneading and tested at optimum moisture content. In general, the importance of drainage design and maintenance was clearly confirmed. In well-drained pavements, subgrade soils will not be saturated for long periods. Suction will control stress state and soil deformability and guarantee that the pavement will carry the designed traffic before failure.

Alternative method of determining resilient modulus of subgrade soils using a static triaxial test

The resilient moduli (MR) of subgrade and subbase soils are very important properties in the analysis and design of a flexible pavement system. However, difficulties and complexities in performing cyclic MRtesting and the high cost of the testing system have prevented the cyclic MRtest from becoming a routine test. Therefore, the development of an alternative simple and reliable MRtesting technique is essential to the application of the mechanistic design of a flexible pavement system. In this study, an alternative MRtesting technique for subgrade soils was developed using a static triaxial compression (TX) test. For the development of the alternative testing method, the effects of strain amplitude, loading frequency, mean effective stress, and number of loading cycles on the resilient modulus of subgrade soils were fully investigated. Cyclic MR, static TX, and resonant column – torsional shear tests were performed to evaluate the deformational characteristics. Synthetic specimens of known stiffnesses ranging from those of soft subgrade soils to those of subbase materials were developed, and all of the testing systems used in this study were calibrated. The alternative MRtesting procedures were proposed considering deformational characteristics of subgrade soils. The reliability of the proposed test method was verified by comparing the moduli determined by the proposed alternative MRtesting method with those determined by the standard MRtests.Key words: resilient modulus (MR), alternative MRtest, subgrade soils, static triaxial test, deformational characteristics.

Evaluation of Resilient Modulus of Subgrade Soil by Cone Penetration Test

Pavement design based on the resilient modulus of subgrade soil has been adopted by many transportation agencies following the recommendations of the AASHTO guide for design of pavement structures. Laboratory and field nondestructive tests are generally used to evaluate the resilient modulus of subgrade soil. These methods have shortcomings and limitations and are considered laborious, time-consuming, and expensive. The difficulties associated with the existing methods signify the need for a popular in situ technology for evaluating the resilient modulus of subgrade soil. Among other methods, the cone penetration test (CPT) is fast, simple, and economical and provides repeatable and reliable results. The results of a pilot investigation to assess the possibility of predicting the resilient modulus of subgrade soil from the CPT soundings are presented here. Field and laboratory testing programs were carried out on two types of cohesive soils at the Louisiana Transportation Research Center/Pavement Research Facility. Field tests consisted of CPT soundings using the 15-cm2 friction cone penetrometer and the 2-cm2 miniature friction cone penetrometer. Laboratory tests included the resilient modulus and physical properties of the investigated soils. The results of the miniature CPT were evaluated and compared with the soundings of the 15-cm2 cone at the same site. Both laboratory and field tests were analyzed. Based on statistical analyses, a model was proposed to estimate the resilient modulus from the CPT data and basic soil properties. Predicted values of the resilient modulus are consistent with laboratory measurements.

Regression Model for Resilient Modulus of Subgrade Soils

Subgrade soil is an important part in both flexible and rigid pavement structures. AASHTO recommends the use of bulk stress and deviatoric stress models to characterize granular and cohesive soils, respectively. However, these models oversimplify the fundamental behavior of subgrade soils. Being proposed is the use of an octahedral stress-state model to characterize the resilient modulus of different soils. Eight different soils representing major soil types in Louisiana were selected to validate the model and to calibrate the model parameters. Additional analysis was then performed to develop correlations between the model parameters and other soil properties. Three types of correlation were produced: ( a) model parameter with soil properties, ( b) model parameters with California bearing ratio, and ( c) model parameters with unconfined compressive strength. Statistical analysis indicated that the correlation between model parameter and soil properties yielded the best results.

Alternative Method of Determining Resilient Modulus of Compacted Subgrade Soils Using Free-Free Resonant Column Test

An alternative procedure of determining the resilient modulus ( MR) of compacted subgrade soils using a free-free resonant column (FF-RC) test is proposed. The FF-RC test was used to determine a small-strain Young’s modulus ( Emax) and Poisson’s ratio on the basis of the elastic wave propagation theory. Resonant column (RC) and torsional shear (TS) tests were performed to investigate the effects of loading frequency and strain amplitude on the resilient modulus of subgrade soils. By performing FF-RC, RC/TS, and MR tests on the synthetic specimens of known stiffnesses, the feasibility of using the FF-RC test to measure the MR was evaluated and the MR testing equipment was calibrated. Moduli of subgrade soils determined by FF-RC tests match well with values obtained from RC and TS tests at small strains, indicating that the FF-RC test can provide a reliable estimation of Emax, if the effects of loading frequency are considered. In the proposed method, Emax obtained from the FF-RC test can be combined with the effect of loading frequency and the modulus reduction curve determined by the data base of the RC and TS tests for the compacted subgrade soils of various plasticity indexes. Moduli obtained from the proposed method overlapped nicely with MR values determined by standard MR testing, showing the capability of the proposed method being used in determining MR values, provided that the effects of loading frequency and modulus reduction curve are considered.

Seasonal Variation of Resilient Modulus of Subgrade Soils

Results of a study to evaluate the seasonal variation of the resilient modulus of granular soils for flexible‐pavement design are presented. The field temperature and moisture content underneath the pavement were monitored at two sites using soil moisture‐temperature cells over a period of one year. A new laboratory testing procedure and system was used to test reconstituted soil specimens under monitored ranges of temperatures, moisture contents, dry densities, and stress conditions. These laboratory data allowed the development of multiple regression equations to determine resilient moduli under Rhode Island environmental conditions. In addition, a theoretical model was developed to predict resilient moduli due to the change of temperature and moisture conditions.