Unbound Granular Materials Research Articles

Simulation of Permanent Deformation of Unbound Granular Materials Under Repeated Loading by DEM

This paper presents a numerical investigation of permanent deformation of unbound granular materials under repeated loading using the distinct element method (DEM). Rigid walls are used as the boundaries in the simulation. Stress-controlled loading scheme is designed to perform biaxial compression testing using a commercial DEM code, Particle Flow Code (PFC2D). Three runs with different confining stress levels are conducted for simulation of the permanent deformation of randomly generated unbound granular materials. Simulation results show that both confining and cyclic axial stresses can be effectively applied to the testing sample. In addition, preliminary results of mechanical response imply that simulated permanent deformation trend of unbound granular materials is consistent with the classic results in real laboratories.

Model for prediction of resilient modulus incorporating matric suction for recycled unbound granular materials

This paper presents experimental results on the effect of matric suction on the resilient modulus of four recycled unbound granular materials. The recycled materials were prepared at moisture contents ranging between 70% and 90% of optimum moisture content (OMC) and tested in a repeated load triaxial test (RLTT) apparatus under various stress regimes. Soil-water characteristic curves (SWCC) were established for each material by preparing samples at various moisture contents and measuring matric suction with filter papers. To obtain the wet end of the SWCC, further samples were conditioned on a tension plate at suctions controlled by the hanging water column method. Some published models for prediction of resilient modulus were applied to the experimental data, but the correlations were unsatisfactory generally, and so an improved model was sought. Subsequently, a model with four terms and six constants was developed, which followed the general power law. A single set of material constants was found for all recycled materials to provide satisfactory predictions of resilient modulus (R2 = 0.88), over a wide range of stresses and moisture states.

Accurate Determination of Equivalent Modulus of Nonlinear Anisotropic Granular Base Layer

A variety of mechanistic models have been proposed to characterize the resilient behavior of unbound granular materials as a function of applied stress states including confinement and shear effects. However, significant computation effort is needed to implement the nonlinear stress-dependent modulus in the pavement structure response analysis. In this study, an equivalent modulus of granular base layer was developed to represent the nonlinear anisotropic behavior of aggregate base layer using three-dimensional finite element modeling analysis. The analysis results indicate that the equivalent modulus is not a constant value but changes with temperature, vehicle speed, and load. The equivalent modulus increases as the temperature increases but decreases as the vehicle speed increases due to the viscoelastic nature of asphalt surface layer. The equivalent modulus results in comparable results in the prediction of critical pavement responses, compared to the responses predicted using the real nonlinear anisotropic model for the granular base. The developed equivalent modulus can be used as an alternative approach when the sophisticated model is not available or the analysis needs to be conducted in a quick manner.

Mechanistic modelling of tests of unbound granular materials

Various tests are used to characterise the strength and resilience of granular materials used in the subbase of a pavement system, but there is a limited understanding of how particle properties relate to the bulk material response under various test conditions. Here, we use discrete element method (DEM) simulations with a mechanistically based contact model to explore influences of the material properties of the particle on the results of two such tests: the dynamic cone penetrometer (DCP) and the resilient modulus tests. We find that the measured resilient modulus increases linearly with the particle elastic modulus, whereas the DCP test results are relatively insensitive to particle elastic modulus. The DCP test results are also relatively insensitive to inter-particle friction coefficient but strongly dependent on the particle shape. We discuss strengths and weaknesses of our modelling approach and include suggestions for future improvements.

Comparing Results between the Repeated Load Triaxial Test and Accelerated Pavement Test on Unbound Aggregate

Unbound granular materials (UGM) are extensively used as basecourse materials around the world as they are capable of bearing relatively high traffic loads and are an economical option in comparison to bound materials. The unbound materials performance as basecourse determines the life-cycle costs of a pavement. The extent to which the repeated load triaxial test can predict the performance of unbound granular materials in the laboratory is an important parameter for road designers. Moreover, the performance of the unbound basecourse materials depends upon the moisture conditions when they are being loaded, gradation curve of the material, in situ density, permeability, and the nature of the aggregate fines (clays). There is a need to find the factors that cause the variation in the performance of the materials both in the laboratory and in-field pavement conditions to enable appropriate selection and use. This research utilizes accelerated pavement tests (APT) alongside repeated load triaxial (RLT) tests to test differently graded unbound granular materials at higher moisture contents. The objective of the research is to find the similarities and contrasts in basecourse material in both tests and to find the root causes of variation in the aggregate performance. If results of the RLT test can truly represent the results from full-scale APTs for basecourse materials, then the pavement materials industry can use the much lower costing RLT tests with confidence knowing it is representative of performance tests from APTs. The accelerated tests are performed on test pavements at the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) with the introduction of surface water, which replicates typical rainfall events. An increase in moisture in the basecourse layers is observed during the APTs due to entry of surface water through the surface seal, which allowed the performance of the basecourse material at higher moisture contents to be assessed. The RLT tests were conducted in parallel with the same basecourse materials at higher moisture conditions. The relative performance ranking of APTs and RLT tests is found to be the same for some materials; however, it differs in some cases. The similarities and differences in the rankings from both procedures are highlighted, and the causes of these similarities and differences are inferred and discussed.

Statistical Modelling of the Resilient Behaviour of Unbound Granular Material

Abstract The resilient behaviour of an unsaturated, unbound granular material is a primary input used in the mechanistic analysis of pavements incorporating such layers. Various models exist for the determination of the resilient behaviour, mainly based on the output of tri-axial laboratory testing. This paper describes an investigation where basic engineering properties such as grading, laboratory compaction characteristics and optimum moisture content are incorporated into the resilient behaviour model to quantify the effect of basic material properties on the resilient response of unsaturated, unbound granular materials. Such a resilient behaviour model will enable practitioners to estimate the behaviour of specific material, which might enable the use of available quality material that was discarded in the past. Data from tri-axial laboratory tests on materials originating from the Long Term Pavement Performance test sections are combined with basic engineering parameters of typical unbound granular material through a statistical modelling process to develop a model for predicting resilient behaviour, which can be used as a practical predictor of the expected behaviour during a Level 2 and/or Level 3 Mechanistic Empirical Pavement Design analysis. The work illustrates the process and the potential to develop a general resilient behaviour model for unbound granular materials incorporating saturation effects.

Fast layered elastic response program for the analysis of flexible pavement structures

One of the key components in analysing pavement structural behaviour is the response model which is used to estimate the stresses, strains and displacements of the pavement structure subjected to the existing traffic, taking into account the material properties and prevailing environmental conditions. Multilayer elastic theory (MLET) is often preferred over other methods such as the finite element method, due to its computational performance for repeated applications. A new elastic response analysis program has been developed based on the Burmister MLET theory to calculate the response of flexible pavement structures. In the development of the program, the time-consuming part of MLET processes was optimised. To improve the convergence and accuracy of responses in the vicinity of the surface of the top layer, an approach based on Richardson's extrapolation was employed. Moreover, an iterative approach to model stress dependency of unbound granular materials was incorporated. A comprehensive comparison of the program with two frequently used programs demonstrated an excellent agreement and improved performance.

Evaluation of permanent deformation models for unbound granular materials using accelerated pavement tests

Mechanistic-empirical (M-E) pavement design methods have become the focus of modern pavement design procedure. One of the main distresses that M-E design methods attempt to control is permanent deformation (rutting). The objective of this paper is to evaluate three M-E permanent deformation models for unbound granular materials, one from the US M-E pavement design guide and two other relatively new models. Two series of heavy vehicle simulator (HVS) tests with three different types of base material were used for this purpose. The permanent deformation, wheel loading, pavement temperature, and other material properties were continuously controlled during the HVS tests. Asphalt concrete layers were considered as linear elastic where stress-dependent behaviour of unbound materials was considered when computing responses for the M-E permanent deformation models with a nonlinear elastic response model. Traffic wandering was also accounted for in modelling the traffic by assuming it was normally distributed and a time-hardening approach was applied to add together the permanent deformation contributions from different stress levels. The measured and predicted permanent deformations are in general in good agreement with only small discrepancies between the models. Model parameters were also estimated for three different types of material.

Evaluation of Permanent Deformation Characteristics of Unbound Granular Materials by Means of Multistage Repeated-Load Triaxial Tests

Rutting in flexible pavements is often associated with permanent deformation of the unbound granular layer. The current permanent deformation models are applicable only to a single stress path in repeated-load tri-axial (RLT) tests, in which the load pulses are of constant amplitude. In this paper, a general approach using the time-hardening concept was introduced to model the permanent deformation of unbound granular materials (UGMs) continuously in multistage (MS) RLT tests, in which load pulses of a range of different amplitudes are applied, to represent field conditions realistically. With this formulation, three existing permanent deformation models were reconstructed, and one of the models was slightly modified, to suit MS loading conditions better. The material parameters of these models were then optimized for three UGMs used in pavement construction with data from MS RLT tests and application of a least squares curve-fitting method to the test data. The goodness-of-fit statistics were computed to evaluate and compare the quality of fit achieved with these models. The shakedown ranges were also calculated for each stress path of the MS RLT tests to compare these models in simulating these ranges. Generally, the time-hardening approach, for which the quality of fit is dependent on the chosen model, was found to work successfully. In this study, the selected models performed quite well, with the modified model showing the closest agreement to the test data. Thus, this approach has the potential to be applied for better modeling and prediction of pavement performance.

Strengthening Of Subgrade by Using RBI Grade-81 A Case Study.

Road infrastructure in India is developing at a very fast pace. A good pavement is needed for the safe, comfortable and economical movement of traffic. If the in situ soil has no adequate strength, either soil from other sites are to be used or the available soil has to be stabilized so that it attains sufficient strength to carry the traffic load. Soil can be stabilized with RBI grade 81 and then can be used in Sub grade and also as Sub base and base Layers. The whole Pavement can be constructed by using RBI grade 81 thus reducing energy consumed and placing of unbound granular material (WBM/WMM) without compromising on Strength and durability. Various samples have been made by taking soil with 0% RBI Grade-81 content; 2% RBI Grade-81 content; 4% RBI Grade-81 content; 6% RBI Grade-81 content; 8% RBI Grade-81. The CBR sample is prepared by 7 days curing and 4 days soaked period. The comparison of the strength results with or without RBI Grade 81 has been done. It has been found that RBI Grade 81 is a unique and innovative material which results in saving the extra cost of the pavement.

Estimation of Resilient Modulus of Unbound Granular Materials from Senegal (West Africa)

The resilient modulus (Mr) is an important parameter which describes the mechanical behavior of unbound granular materials. However, this parameter can be determined from physical properties. This paper presents the relationship between resilient modulus and physical properties of Quartzite from Bakel (GB), Basalt from Diack and Bargny and Bandia limestones. Simple and multiple regression method by stepwise are used to establish linear and nonlinear relations to predict the resilient modulus. The results showed no significant correlation for Basalt, a weak estimation of the modulus for GB and good prediction of resilient modulus for limestone. These results also showed that the model of Uzan is more suitable to predict the resilient modulus than NCHRP model and the resilient modulus is better predicted in nonlinear relationship.

입상재료의 지오셀 보강효과 분석을 위한 재료모형 거동연구

지오셀은 지반재료의 보강을 위한 효과적인 도구중의 하나이다. 본 연구에서는 도로포장구조의 보조기층으로 사용되는 입상재료의 지오셀 보강효과를 이론적으로 고찰하였다. 이를 위해 원통형 지오셀로 보강된 입상재료의 거동을 모사할 수 있는 수학적 모형을 정식화 하였으며, 수치적 해석을 위해 MATLAB 프로그램 코드를 개발하였다. 지오셀이 보강된 입상재료의 역학적 거동 영향인자로서 지오셀의 탄성계수(<TEX>$E_G$</TEX>=800, 1600, 2400 MPa), 지오셀의 반지름(R=50, 100, 150, 200 mm), 지오셀의 두께(t=1.2, 2.4, 3.6mm)를 설정하였다. 수치적 해석결과를 바탕으로 지오셀의 보강효과는 지오셀의 탄성계수와 두께가 증가하고, 지오셀의 반지름이 감소할수록 그 효과가 증가하며, 동시에 채움재인 입상재료의 재료적 비선형성으로 인해 보강효과의 증가가 비선형적임을 알 수 있었다. 특히, 입상 채움 재료의 응력상태가 낮은 경우가 응력 상태가 높은 경우에 비해 지오셀 보강효과가 상대적으로 더욱 뚜렷한 것을 알 수 있었다. Geocell is one of the effective tools for reinforcing geotechnical materials such soil, sand, and so on. In this study, we investigated the reinforcing effect of geocell on unbounded granular material (UGM), which is used for subbase of pavement structures. For this purpose, a mathematical model for UGM reinforced with cylindrical geocell was formulated. A MATLAB program code for the mathematical solution was developed. In oreder to investigate the reinforcing effect of geocell on the UGM, a parametric study was conduced using the MATLAB code. The parameters used for the study were Young's modulus of geocell(<TEX>$E_G$</TEX>=800, 1600, 2400 MPa), radius of geocell(R=50, 100, 150, 200 mm), and thickness of geocell(t=1.2, 2.4, 3.6 mm). The results showed that the reinforcing effect increased with the increase of Young's modulus and the thickness, and with the decrease of the radius. In addition, lower stress level showed a better reinforcing effect of geocell compared to higher stress level in terms of relative strain reduction ratio.

Analytical Model for Resilient Modulus and Permanent Deformation of Geosynthetic-Reinforced Unbound Granular Material

To consider the benefit of geosynthetic reinforcement in a mechanistic-empirical pavement design method, the resilient modulus and permanent deformation behavior of a geosynthetic-reinforced unbound granular material (UGM) must be considered. Many researchers conducted repeated-load triaxial (RLT) tests to investigate the resilient and permanent deformation behavior of the geosynthetic-reinforced UGM. However, these tests are difficult to perform, and the results are often interpreted empirically. Thus, implementation of the research results is limited. In this study, an analytical model was developed to predict the resilient modulus and permanent deformation of the geosynthetic-reinforced UGM in RLT tests. The analytical model is compatible with the resilient modulus and permanent deformation models in the current mechanistic-empirical pavement design guide. Both planar and three-dimensional geosynthetics can be analyzed using this model. RLT test results from two published studies were selected to validate the proposed analytical model. In general, the analytical results confirmed and explained the typical test observations from previous studies that geosynthetic reinforcement is more effective in reducing the permanent deformation than increasing the resilient modulus of the UGM sample. A parametric analysis was conducted to investigate the effect of input parameters (i.e., material properties, sample dimensions, and stress level) on the analytical model. The limitations, assumption, and implementation of the model are also discussed.

The effective stress concept in the cyclic mechanical behavior of a natural compacted sand

Road pavement structures are generally composed of granular materials. The influence of the unsaturated state, or suction, on the mechanical behavior of unbound granular materials in roads during a large number of cycles has not been given sufficient attention and is not generally considered in models used for these materials.This article presents an experimental study on compacted natural sand in Missillac, France that used the direct shear test and the soil water retention curve (SWRC) to measure the effective stress, and repeated load triaxial tests to determine the material's resilient behavior.The effective stress concept was used in a nonlinear elasticity model to predict the material's resilient behavior under unfavorable moisture conditions, which are one of the most common mechanisms of deterioration in low-traffic pavements.

Water sensitivity of resilient modulus of compacted unbound granular materials used as pavement base

Resilient modulus of granular materials is significantly influenced by the degree of saturation. As in northern climates pavement materials may reach saturation and as their water content may significantly vary through the yearly cycle, this water sensitivity of the unbound layers becomes a major factor to quantify. Yet, few models are available to quantify the water sensitivity of resilient modulus of typical Canadian unbound granular materials used in pavement bases. Therefore, the objective of this study is to develop, through resilient modulus testing, a generalised model quantifying this water sensitivity. Six gradation curves for three aggregate sources were used to measure the resilient modulus variations with water content. Data from previous studies have also been used to develop the model. A generalised model is proposed, which introduces a new parameter describing the water sensitivity of resilient modulus, which was found to be a function of stress and granular assemblies' volumetric and frictional properties.

Validation experiments for lower-bound shakedown theory applied to layered pavement systems

Experiments involving the application of moving-wheel loads to a series of two- and three-layered pavement systems, involving various soils and unbound granular materials, were used to determine the applied contact pressure ranges within which the structures may be assumed to have developed a ‘shakedown' condition. Shakedown implies that the pavement responds to loading in a resilient manner after some limited plastic deformation during the early load applications. The data were used to assess the validity of applying three-dimensional lower-bound shakedown theory to pavements as a basis for improving pavement design computations. The research indicated that this approach has considerable promise, and that further experimental validation work is desirable.

Hollow cylinder apparatus for testing unbound granular materials of pavements

Unbound granular materials used in pavement structures are subjected to a complex stress path which includes rotational stresses. Hollow cylinder apparatuses (HCAs) are suitable laboratory devices for reproducing stress paths found in the field. However, as the size of the HCA depends on the size of the granular particles, their use for testing unbound granular materials for pavements has not been reported in the literature. This paper presents the development of a new large size hollow cylinder apparatus designed to study the response of unbound granular materials subjected to stress rotation. This large HCA has hydraulic actuators for vertical movement and torsion while shear and vertical stresses are servo-controlled to reproduce stress paths produced by a heavy vehicle moving on a pavement structure. Confining stress is applied through the use of rings with controllable stiffness. This paper highlights the role of ring stiffness on stress paths. The results show the capabilities of the large HCA for studying the behaviour of unbound granular materials used in pavement structures.

Relating resilient behaviour of compacted unbound base granular materials to matrix and interlock characteristics

Compacted unbound granular materials used in pavement bases present a complex elastoplastic behaviour under wheel loading. This behaviour was found to be influenced by numerous factors, but the effect of aggregate matrix and interlock is not yet fully understood in the available literature. A comprehensive resilient modulus laboratory study on 18 samples revealed significant variations of the effect of gradation, which are associated with the evolution in the frictional properties within the grain fractions. The results also emphasized the importance of matrix interlock and non matrix aggregate void-filling capacity.

Shakedown Analysis of Geogrid-Reinforced Granular Base Material

AbstractThis research study was performed to examine the shakedown behavior of geogrid-reinforced unbound granular materials and evaluate the factors that affect the shakedown stress limits. To achieve this objective, the study was performed through conducting repeated load triaxial (RLT) tests on both unreinforced and geogrid-reinforced unbound granular specimens. A multistage RLT test, in which only one sample is needed to determine the shakedown stress limits, was selected in preference to a single-stage RLT test, in which several tests on multiple samples are required. Five geogrids of different tensile modulus and different aperture geometries (three rectangular or biaxial and two triangular or triaxial) were used. The test results clearly demonstrated the potential benefits of placing the geogrid within the unbound granular base specimen in terms of permanent deformation reduction. The benefits of the geogrid are more prevalent at higher stress levels and for higher tensile modulus geogrids. The inc...

Portuguese unbound granular materials characterisation using cyclic triaxial tests

Crushed materials with extensive granulometry – unbound granular materials (UGM) – are often used as unbound granular layers of road pavements, namely as granular sub-base and base. In spite of several studies having already been undertaken, the behaviour of these materials on pavement layers has not been sufficiently characterised, especially due to reasons connected to the heterogeneity of the rock masses from which they come. This has special importance for Portuguese pavement technology and has led to the development of two PhD theses in different schools (University of Coimbra and Technical University of Lisbon) using different approaches but both aiming at the characterisation of the aptitude of the same types of material (crushed limestone) being used as UGM. In addition, in the same terms, one of the theses has also characterised a crushed granite material. Basically, both theses included laboratory testing using cyclic triaxial tests and in situ mechanical characterisation to establish behaviour models for the crushed materials investigated. This paper describes the main results obtained from both theses and discusses the results on an integrated basis, pointing out the main indications that can be extracted from both in terms of the use of UGM on Portuguese pavement technology.