Behavior Of Unbound Granular Materials Research Articles

Investigating the Mechanical Behaviour of Unbound Granular Material (UGM) for Road Pavement Construction Applications: A Western Victoria Case Study

The behaviour of unbound granular materials (UGMs) used in road construction is crucial in determining the longevity and performance of road pavement. Geotechnical analysis can assist engineers in selecting suitable materials and designing road pavements that meet industry standards. This paper presents the results of laboratory geotechnical tests conducted on unbound granular materials (UGMs) collected from three sites (Roses Gap, Rules East, and Polkemmet Road) in Horsham, Victoria, Australia. UGMs were investigated for their mechanical behaviour and suitability as subgrade materials for road pavements. The study utilised laboratory geotechnical tests, including particle size distribution (PSD), Atterberg limits, compaction (Proctor) test, unconfined compressive strength (UCS), California Bearing Ratio (CBR), and repeated load triaxial (RLT) tests, to evaluate the physical and mechanical properties of the UGM samples. The study indicates that UGM samples collected from different locations displayed variations in their geotechnical properties, such as particle size distribution, water absorption, and CBR strength. Roses Gap samples showed weak cohesion properties, and significant vertical displacements after repeated triaxial tests. However, among the samples in this site, samples with higher clay content (RG21) demonstrated the most promise in triaxial tests. Similarly, the Rules East samples were found to be suitable for low-traffic subgrades due to their satisfactory CBR and RLT testing results, albeit with little cohesion from clay content. Out of three locations, Polkemmet samples were identified as potential subgrade applications, with PR12 being the top recommendation overall. It satisfied PSD, CBR, and RLT test conditions due to acceptable particle size in the largest range, highest CBR strength value, and lowest permanent displacement. The study's findings provide useful information for the design of road pavements using these materials and the characterisation of rural materials around the Horsham region for future use in various other contexts.

Energy-based insight into characterization of shakedown behavior of fully weathered red mudstone

Weathered Red mudstone is widely distributed in Sichuan basin. The compacted weathered red mudstone has been used as subgrade fill materials of high-speed railway in southwestern of China. Dynamic responses of such materials under cyclic loading are critical to long-term stability of subgrade. Shakedown concept is widely employed in characterizing the permanent deformation behavior of soils. According to the evolution of axial strain (Werkmeister’s theory) or unit dissipated energy (Tao’s theory) with loading cycles, the behavior of unbound granular materials can be classified into three categories: plastic shakedown, plastic creep and incremental collapse. However, both theories are more suitable for the unbound granular materials with some limitations when used to separate the plastic creep and incremental collapse behavior. To overcome the limitations of the current theories, 26 cyclic triaxial tests were conducted on a saturated fully weathered red mudstone (SFWRM) to study the evolution of axial strain and unit dissipated energy during cyclic loading. A clear dependency of axial strain, axial strain rate on the unit dissipated energy level under various cyclic stress states were observed. A new criterion which is based on the responses of unit dissipated energy with cyclic stress ratio, was proposed to determine the limit between plastic creep and incremental collapse. Comparing with Werkmeister’s criterion and Tao’s criterion, the proposed criterion showed a better performance in identifying the incremental collapse behavior of the SFWRM.

Monte Carlo simulations of deformation behaviour of unbound granular materials based on a real aggregate library

ABSTRACT The Monte Carlo simulation (MCS) has been developed to estimate the strength distributions of heterogeneous materials influenced by different factors. The method presents a statistical correlation between the macroscopic properties and the non-uniform characteristics at meso scale. This study proposes an extensive MCS to investigate the influence of the aggregate morphology on deformation behaviour of unbound granular materials (UGMs) based on a real aggregate library and the discrete element method. Two indexes, elongation index (EI) and flatness index (FI), were used to evaluate the morphological characteristics of 3D aggregates rebuilt by the X-ray CT technique. Furthermore, repeated load triaxial tests of UGMs were simulated by 50 MCS samples. The effects of shapes and sizes of aggregates on deformation behaviours were statistically analysed. The results show that the deformation behaviour is dependent on the size and the EI distribution, but insensitive to the loading frequencies and FI distribution.

Influence of Degree of Saturation (DOS) on Dynamic Behavior of Unbound Granular Materials

The extensive application of natural unbound granular materials (UGMs) motivates studies into the mechanical properties of alternatives such as processed crushed rocks employed commonly as base or subbase layers. The rutting and settlement generated in base and subbase layers is widely restricted in many specifications and standards. In this research, the dynamic behavior including the resilient modulus (Mr) and the plastic strain (εεp) of the crushed rocks collected from Queensland in Australia will be tested by a series of repeated load triaxial test (RLT) tests to investigate the behavior of UGMs under the fluctuation of the degree of saturation (DOS) (59%–100%). In particular, the RLT specimens were prepared in the laboratory through proper gradation under optimum moisture content (OMC) and 100% standard proctor maximum dry unit weight. Results from the RLT tests showed that UGM specimens soaked at higher DOS generated lower resilient modulus and weaker resistance to heavy traffic volumes with significant accumulation of plastic strain. The Mr and εεp of the tested aggregates under different cyclic deviator stresses of 425 kPa and 625 kPa approximately linearly decreased and approximately linearly increased as the DOS increased with a certain number of cycles up to 50,000, respectively.

Experimental characterisation and numerical modelling of the resilient behaviour of unbound granular materials for roads

This research paper deals with experimental characterisation and numerical modelling of the resilient behaviour of Unbound Granular Materials (UGMs) usually used in road construction. The first part of this paper describes the main results of an experimental program that was carried out to assess the mechanical properties of two local Unbound Granular Materials (UGMs) for construction purposes in road pavement. The second part of this paper is devoted to the numerical modelling of the resilient behaviour of UGMs used in flexible pavements. For this purpose, several nonlinear unbound aggregates constitutive models are implemented within an axi-symmetric finite element code developed to simulate the nonlinear behaviour of pavement structures. In addition, deflection data collected by Falling Weight Deflectometer (FWD) are incorporated into the analysis in order to assess the sensitivity of critical pavement design criteria and pavement design life to the constitutive models. Finally, conclusions of engineering significance are formulated.

Resilient and permanent deformation of unsaturated unbound granular materials under cyclic loading by the large-scale triaxial tests

The road base and subbase are normally situated above the underground water table and thus in the unsaturated state. To study the cyclic behavior of unbound granular materials (UGMs) in the road base and subbase courses under controlled matric suction conditions, a series of cyclic tests on the unsaturated UGMs were conducted by the large-scale triaxial apparatus. The large-scale triaxial apparatus was upgraded using an unsaturated module to control the matric suction by the axis-translation technique. Emphases were placed on the effects of matric suction and cyclic stress amplitude on the accumulated axial strain and resilient modulus of UGMs. Test results showed that the increase in matric suction reduced the development of accumulated axial strain exponentially under different cyclic stress amplitudes. Based on the shakedown theory, both the plastic shakedown and plastic creep limits were determined, which tended to grow linearly with the increase in matric suction. A new empirical model was proposed to predict the accumulated axial strain of unsaturated UGMs. Moreover, the increase in both matric suction and cyclic stress amplitude led to the increase in resilient modulus, and linear relationships existed between the resilient modulus and matric suction.

The influence of moisture content and compaction level on LWD modulus of unbound granular base layers

Abstract Resilient behavior of Unbound Granular Material (UGM), as defined with stiffness modulus, is considered to be one of the major parameters in pavement analysis. This itself is affected by several other factors. In this research, the effects of compaction level, moisture content and stress state were investigated on UGM resilient behavior using a Light Weight Deflectometer (LWD). For each of these parameters, three levels were selected. Response Surface Method (RSM) was applied and based on the results 17 test designs were determined. The results showed good correlation between stiffness modulus and compaction level, moisture content, compaction-moisture interaction and stress state (drop weight of LWD in this research) at 90% significant level with R2 of 0.99. This model was more accurate than the reported models in the literature. The interaction between the effects of compaction and moisture on LWD modulus could be seen in model terms (i.e. compaction × moisture parameter). With regard to the effect of stress state it was resulted that the increased LWD weight (i.e. drop weight + instrument weight) resulted in increased LWD modulus values.

Effect of vertical stress rest period on deformation behaviour of unbound granular materials: Experimental and numerical investigations

Repeated load triaxial test is used to assess the deformation behaviour of unbound granular materials (UGMs) in flexible road pavements. Repeated load pulse characteristics (i.e. shape, loading period and rest period) are the stress configurations used in the experimental set-up to simulate the passing axle loads. Some researchers and standard testing protocols suggest a rest period of varying durations after a loading phase. A thorough review of existing literature and practises has revealed that there is no agreement about the effect of the rest period of vertical stress pulse on the deformation behaviour of the UGMs. Therefore, the main objective of this study is to investigate the effect of repeated stress rest period on the deformation behaviour of UGMs experimentally. Experiments are conducted, both with and without rest period, using basalt and granite crushed rocks from Victoria, Australia. Furthermore, in order to gain insight into the effect of the rest period, finite element modelling is also developed. Both the experimental and modelling results show that the rest period has a noticeable effect on both resilient and permanent deformation behaviours of UGMs. It is, therefore, recommended to take extra precautions while adopting a particular standard testing protocol and to supplement the results by additional tests with different loading configurations.

Investigation of unbound granular material behavior using precision unbound material analyzer and repeated load triaxial test

Abstract This study aims to investigate the mechanical behavior of unbound granular materials (UGM) the using precision unbound material analyzer (PUMA) and repeated load triaxial test (RLTT). Both single-size and mixed-size aggregate and two graded materials were used for multi-stage loading tests in this study. The confining stress, resilient modulus, and permanent strains of six granular materials were analyzed considering the effect of particle size on the performance of UGM. The results of PUMA tests showed that the confining stress became smaller with more large-size particles and denser structure. Resilient modulus was found to be sensitive to particle size when the axial load stress was greater than 40 kPa. The resilient modulus of high-density materials calculated from the variable confining pressure (VCP) model was greater than that from the constant confining pressure (CCP) model. The results suggested that axial load, material type, and the largest particle size needed to be considered for analysis of confining stress and resilient modulus. Without considering confining stress, resilient modulus might be underestimated for high-density materials but overestimated for low-density materials using the CCP model. The resilient modulus and permanent strains measured using the PUMA were similar to those measured using the RLTT in the range of 10 kPa to 20 kPa confining stress. The predicted permanent strains from the CCP model and RLTT had greater error than the predictions from the VCP model and PUMA tests compared to experimental measurements. The dynamic confining stress in PUMA tests was found effectively to simulate the actual loading condition of UGM in a pavement. Although RLTT and PUMA tests present consistent results for characterizing the behavior of UGM, the PUMA can provide more reasonable results for simulating confining stress and measuring resilient modulus.

Predicting the behavior of unbound granular materials under repeated loads based on the compact shakedown state

Abstract The behavior of unbound granular material under repeated loads primarily depends on the nature and arrangement of constituent particles, particle size and shape. However, very few analytical models are available for predicting the behavior of unbound granular material under repeated loads. This paper presents a generalized failure criterion with a non-associated flow rule which uses from the concept of compact shakedown state for predicting the stress–strain behavior of unbound granular material. At the compact shakedown state, the response of the unbound granular sample to a stress change is reversible and behaves as an elastic material. In an extension to original CANAsand constitutive model, two concepts namely critical state and compact shakedown state play paramount roles as all of the moduli and coefficients are related to these states. In contrast to the critical state, the unbound granular material samples do not experience any plastic deformation at the compact shakedown state and behaves as a purely elastic material and the response to a stress change is reversible. For stress reversal two additional concepts as reflected plastic potential and bounding surface are employed which distinguishes between virgin loading and secondary loading. The extended model is examined by simulating different typical drained tests in conventional tri-axial tests. The applicability of the present model is evaluated through comparisons between the predicted and the measured results.

Shakedown analysis of cyclic plastic deformation characteristics of unbound granular materials under moving wheel loads

Cyclic plastic strain behavior of unbound granular materials (UGMs) exhibits significant stress path dependency. Using a customized triaxial apparatus capable of applying stress path loading, a series of laboratory repeated load triaxial (RLT) tests were conducted on two typical UGMs by varying simultaneously the axial stress and the radial stress. Effects of realistic in-situ stress paths due to a passing wheel on cyclic plastic strain behavior of unbound granular base and subbase materials were investigated and quantified. The analyses of experimental results revealed that the accumulated plastic strain responses of both UGMs subjected to different stress path loadings can be described by the shakedown approach. The amount of accumulated permanent strain, permanent strain rate, and resilient strain were used to classify the plastic deformation accumulation patterns of tested materials under moving wheel loads. Finally, the significance of the research findings on pavement analysis and design practices was highlighted to evaluate permanent deformation resistance of UGMs and their suitability for use in rut resistant and stable pavement foundation layers.

Neural network based model for estimation of the level of anisotropy of unbound aggregate systems

Abstract Directional dependency of the materials properties in granular soils is an integral component of the analysis and design of pavement foundations. Laboratory determination of such anisotropic properties is often overlooked by design engineers due to the lack of available protocols, equipment, time constraints, and budgetary issues. This research strives to bridge this gap by investigating an alternative approach to provide a practical and reliable framework to estimate the level of anisotropy of unbound granular materials. To achieve this objective, an experiment design was developed to establish a comprehensive aggregate feature database. Nonlinear and cross anisotropic material properties were determined using Variable Dynamic Confining Pressure (VDCP) stress path tests in the laboratory. Particle geometry was characterized using the Aggregate Imaging System (AIMS). Scale parameters and shape parameters of the form, angularity and textural properties of the particles were incorporated in the aggregate database to account for the shape-induced anisotropy of particulate soils. Moisture state and density parameters were also incorporated in the database for further post processing. Several neural network models with different architectures were developed, and the performances of the models were assessed based on an unseen set of data. The impact of the neural network topologies on the performance of the models were also investigated in this research effort. The results indicate that increasing the number of hidden layers has negligible impact on the performance of the models. Additionally, a series of sensitivity analyses were performed to investigate the level of contribution of aggregate features on the anisotropic behavior of unbound granular materials. The results of the sensitivity analysis underscores the influence of aggregate angularity, fines content, surface macro-texture, as well as the parameters of the stress path tests, on the orthogonal load distribution capacity of aggregate layers.

Resilient Behavior of Unbound Granular Materials Subjected to a Closed-System Freeze-Thaw Cycle

AbstractThe resilient modulus of a base course granular material is an important input parameter for pavement design and analysis. In recent decades, numerous studies have been performed to charact...

Development of a New Mechanistic Empirical Rutting Model for Unbound Granular Material

AbstractThis paper proposes a new mechanistic-empirical rutting (MER) model to evaluate the permanent deformation (PD) behavior of unbound granular material (UGM). To characterize the stress dependence of rutting behavior in UGM, the MER model incorporated a softening stress term and a hardening stress term into the Tseng-Lytton model, which is based on the Drucker-Prager plastic yield criterion. Repeated load triaxial tests were performed on two types of UGMs in this study, employing seven stress states to calibrate the model coefficients, and two stress states to validate the accuracy of the model predictions. The correlations of the two incorporated stress terms with the accumulated permanent strains were established based on the triaxial test results. It was found that the correlations are fitted by power functions with 0.97–0.99 R2 values. The proposed MER model was compared with the existing UGM rutting models, including the MEPDG model, Korkiala-Tanttu model, and UIUC model in terms of differences ...

Feasibility of Using 4th Power Law in Design of Plastic Deformation Resistant Low Volume Roads

A low volume road (LVR) structural design has two phases: first one is selection of appropriate construction materials and second is the determination of layer thicknesses under the certain traffic and environmental conditions with considering the subgrade bearing capacity. Pavements are prompted to serve the traffic without reaching the terminal serviceability index over its design life. Rut accumulation (plastic deformation) is the most common pavement deterioration type of flexible pavements. Therefore the main goal of the design is prevent rutting. Many low volume road design manual assume that plastic deformation occurs only in subgrade. Construction of overlying layers by selective high performance materials according to the related material and construction specifications is the reason of this assumption. In fact, the assumption is not much reasonable especially for with no, or thinly overlaid low volume road pavements, where the major structural strength is comprised of unbound granular pavement materials and where the principal distress mechanism is rutting in the aggregate layers.Subgrade bearing capacity and the traffic are the main input parameters in the design stage of low volume roads. Subgrade bearing capacity is expressed with California Bearing Ratio (CBR) or Resilient Modulus (Mr). The “traffic” term is determined by Equivalent Standard Axle Load (ESAL) repetitions which is often admitted as 80kN single axle load. Although it is not too difficult to determine an axle load for an individual vehicle, it becomes quite complicated to determine the number and types of axle loads that a particular pavement will be subjected over its design life. For calculation of Load Damage Factors of different vehicle types, which have various axle load and configurations, a generalized fourth-power law has been used for more than a half century. The objective of this study is to indicate the limitations and difficulties faced on reliably applying a power law relationship in design of LVRs with no, or only thin seals. If a power law relationship to be used due to its simplicity, several parameters must be considered in selection of the power value. Such as stress dependent behavior of unbound granular materials and the selected distress type.

Modelling the Moisture Dependent Permanent Deformation Behavior of Unbound Granular Materials

The impact of moisture on the permanent deformation (PD) behavior of unbound granular materials (UGMs) was investigated based on multistage (MS) repeated load triaxial (RLT) tests. Two UGMs with different particle size distributions were tested for a range of moisture contents and the accumulation of permanent strains for the different moisture contents were modelled using a simple predictive model. Moisture was found to increase the accumulation of PD in the materials. Analyses of the variation of the parameters of the model with respect to moisture showed that it is possible to capture the moisture dependent PD behavior of the materials assuming a simple linear relationship between one of the parameters of the model and the moisture content.

Improving Rutting Resistance of Flexible Pavement Using Geosynthetics

Unbound base layers deform under load and contribute to the accumulation of ruts. Therefore, this study was concerned with studying the effect of reinforcement on the behavior of unbound granular material that used in flexible pavement layers as a base c...

Use of linear viscoelastic theory to predict resilient behavior of unbound granular materials

This paper presents a methodology to estimate the stress-strain relationship of an unbound aggregate base using linear viscoelastic theory. Current Mechanistic-Empirical (ME) pavement design procedure adopts the resilient modulus concept to explain the behavior of granular materials for flexible pavement design. The resilient modulus is a stress dependent material property of granular materials that is different from strength. Although California Bearing Ratio (CBR) test results (i.e., stress and strain) can be used to estimate the strength of a granular material, it is not possible to estimate the resilient modulus directly. Therefore, it is necessary to estimate stress along with strain changes. The convolution integral enables the stress to be estimated from the given strain changes only if the relaxation modulus is measured. Aggregate specimens prepared from two different sources in Georgia were subjected to the relaxation modulus test. From the test data, the time-dependent stress due to a known strain rate was computed as a convolution integral of the strain. The computed stress-strain relationship was compared with that from the resilient modulus (MR) test. The results indicate that the stress-strain relationships from the MR test and the convolution integral are similar with nearly the same slopes when horizontal stress is assumed to be approximately 45% of vertical stress. This observation supports the use of the proposed methodology by state highway agencies to validate the MR test results for quality control and quality assurance of aggregate base material selection for pavement design and construction.

Characterisation of unbound aggregate materials considering physical and morphological properties

The objective of this paper is to evaluate the factors affecting resilient and permanent deformation behaviour of unbound granular materials, with a focus on the aggregate physical and morphological characteristics. To evaluate the behaviour of base course, repeated load triaxial testing is commonly used to establish the stress-dependent resilient modulus properties of unbound aggregate base and subbase materials. Although resilient modulus of aggregates is a critical input into mechanistic-empirical pavement design methods, the resilient modulus of unbound base material is often estimated from empirical correlations with index properties in the AASHTOWare Pavement ME design procedure for its simplicity. Since actual field stress conditions and resilient modulus stress states are generally quite different from those generated in the empirical test methods, use of an empirical correlation could lead to an unreliable prediction of resilient modulus and permanent deformation. In order to properly assess the stability of an unbound aggregate layer, it is necessary to establish a proper process to understand the factors affecting fundamental and performance-related properties of unbound granular materials. In this study, aggregate samples from four different sources were tested for resilient modulus and Poisson’s ratio measurements using the Precision Unbound Material Analyzer equipment. Morphological or shape properties of aggregate samples were also measured using an image analysis device. The results demonstrate that aggregate physical and morphological properties affect aggregate resilient and permanent deformation. Further, it is suggested that the resilient modulus of the aggregate should not be used as the sole indicator of rutting performance of aggregate base.

Stress–Strain Response of Unbound Granular Materials Under Static and Cyclic Loading

Resilient modulus is an important parameter in flexible pavement engineering design. The stress–strain behavior of unbound granular base materials under static and cyclic loading and the effects of the state of stress, moisture content and loading conditions have been analyzed in the present study. In this paper, the resilient behavior of unbound granular materials and the applicability of different constitutive models are examined in order to understand the behavior of granular material under static and cyclic loading. Since the aggregate material exhibits a complex stress–strain behavior, more than one model will be required to predict the stress–strain response of granular materials. Hence, different constitutive models have been considered in the present study which can adequately predict the stress–strain response of aggregate materials. Considering the constitutive models used in the present study, it could be said that the Lentz and Baladi model and the normalized degradation model suitably bring out a relation between static and cyclic tests. This is a useful input in pavement design and explains the variation of modulus with load repetitions.