Deformation Of Unbound Granular Materials Research Articles

Investigation on permanent deformation of unbound granular material base for permeable pavement: Laboratory and field study

• Deformation of Unbound Granular Materials was investigated in laboratory and field synchronously. • The confining pressures and Shear Stress-strength Ratios affect the deformation greatly. • The deformation prediction model was consistent with the trends of laboratory test results. • A deformation model based on laboratory triaxial load tests, dynamic triaxial repeated load tests and filed LWD tests was developed. Unbound Granular Materials (UGM) base is widely used in permeable pavement structure for infiltration, water storage, and load bearing. However, because of large void content, UGM base easily deforms under repeated loads, resulting in surface rutting and even reducing durability of permeable pavement. Thus, research on practical deformation prediction model of UGM base is necessary to improve the pavement structure design and evaluate the base layer construction quality. This study conducted laboratory and field tests to develop a deformation prediction model of UGM base under varied stress states. The deformation of UGM specimen was tested and analyzed through triaxial shear test and dynamic triaxial repeated load test, with varied confining pressures and Shear Stress-strength Ratios (SSR). A laboratory deformation model based on SSR was developed, and the linear relationship between total deformation and permanent deformation was obtained. The deformation of UGM base in situ was tested with Light Weight Deflectometer (LWD). The trends of laboratory deformation model and field test results were similar, and deformation prediction model for field was fitted. The values of parameters in permanent deformation model were recommended in varied stress ranges, based on mathematical analysis. Accordingly, the deformation of UGM base can be estimated. This study developed a model to estimate the permanent deformation of UGM based on deformation data from field and laboratory tests.

Modelling the permanent deformation of unbound granular materials in pavements

ABSTRACT An existing permanent deformation (PD) model for the unbound granular materials (UGMs) in pavements, developed based on multistage (MS) repeated load triaxial (RLT) tests, was modified to better suit to field conditions and software applications. The model was calibrated for a few UGMs with a series of moisture contents (w) using MS (to include the stress-history effect) RLT tests and was validated by predicting the PD under different sets of stress levels and w’s. Generally, it exhibited improved predictions over the Mechanistic-Empirical Pavement Design Guide (MEPDG) model. The model was further applied for field conditions and was implemented in a pavement design software entitled ERAPave PP, using data from accelerated testing of three structures employing a heavy vehicle simulator (HVS). Based on additional MS RLT tests, a guideline for the ranges of the material parameters for different types of UGMs was presented.

Neural network-based model for prediction of permanent deformation of unbound granular materials

Abstract Several available mechanistic-empirical pavement design methods fail to include predictive model for permanent deformation (PD) of unbound granular materials (UGMs), which make these methods more conservative. In addition, there are limited regression models capable of predicting the PD under multi-stress levels, and these models have regression limitations and generally fail to cover the complexity of UGM behaviour. Recent researches are focused on using new methods of computational intelligence systems to address the problems, such as artificial neural network (ANN). In this context, we aim to develop an artificial neural model to predict the PD of UGMs exposed to repeated loads. Extensive repeated load triaxial tests (RLTTs) were conducted on base and subbase materials locally available in Victoria, Australia to investigate the PD properties of the tested materials and to prepare the database of the neural networks. Specimens were prepared over different moisture contents and gradations to cover a wide testing matrix. The ANN model consists of one input layer with five neurons, one hidden layer with twelve neurons, and one output layer with one neuron. The five inputs were the number of load cycles, deviatoric stress, moisture content, coefficient of uniformity, and coefficient of curvature. The sensitivity analysis showed that the most important indicator that impacts PD is the number of load cycles with influence factor of 41%. It shows that the ANN method is rapid and efficient to predict the PD, which could be implemented in the Austroads pavement design method.

Effect of loading frequencies on permanent deformation of unbound granular materials

ABSTRACT Unbound granular materials (UGMs) show both permanent and resilient deformations under repeated traffic loads. Permanent deformation (PD) is the irreversible part of the deformation, commonly used for rutting prediction. Several factors, both stress and material related, are found to have influences on PD but some of these factors have not been thoroughly investigated. For example, there is no consensus in existing pavement design standards, guides and literature about the effect of loading frequencies and consequently, they have suggested different frequencies for testing. In order to address this disparity, this study investigates the influence of loading frequencies on PD of UGMs. Three frequencies (0.2, 0.33, and 0.5 Hz) were applied on two types of crushed rocks (basalt and granite) using the repeated load triaxial test (RLTT). The experimental results show that there is negligible effect of these loading frequencies for the tested materials under specified testing conditions. This result has an important implication in that by increasing loading frequency, RLTT testing duration could be drastically reduced. The experimental data were further modelled using regression and artificial neural network (ANN) modelling. The sensitivity analysis from the ANN modelling showed that the loading frequency has only 3% importance on the PD of the tested UGMs among the other factors.

A model for predicting permanent deformation of unbound granular materials

A simple model has been proposed to characterise the accumulation of permanent deformation (PD) in unbound granular materials (UGMs) under cyclic loading of variable magnitudes. The model was developed based on multi-stage (MS) repeated-load triaxial (RLT) tests. The material parameters of this model can be evaluated using an MS RLT test. The model was validated by calibrating it for a few UGMs with a range of grain size distributions, moisture contents and degrees of compaction. The calibrated model was further validated by predicting the PD behaviour of some of these UGMs for different stress conditions. Generally, quite satisfactory predictions were obtained with this model with the advantage of reduced effort required for its calibration compared to some existing models. Additionally, the sensitivity of the parameters of this model to moisture, degree of compaction and grain size distribution was investigated with the aim of incorporating them into the model in future.

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.

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.

Packing theory-based framework to evaluate permanent deformation of unbound granular materials

Permanent deformation of unbound granular materials plays an essential role in the long-term performance of a pavement structure. Stability of unbound granular materials is defined by the particle-to-particle contact of the system, the particle size distribution and the packing arrangement. This paper presents a gradation model based on packing theory to evaluate permanent deformation of unbound granular materials. The framework was evaluated by using 10 unbound granular materials from different countries. The disruption potential, which determines the ability of secondary structure (SS) to disrupt the primary structure (PS), is introduced. This study also identified the amount of PS and SS that may eventually be used as a design parameter for permanent deformation of unbound road layers. The evaluation of the model regarding permanent deformation behaviour of granular materials is found to compare favourably with experimental results.

Modeling Permanent Deformation of Unbound Granular Materials under Repeated Loads

Finite-element analysis on a pavement structure under traffic loads has been a viable option for researchers and designers in highway pavement design and analysis. Most of the constitutive drivers used were nonlinear elastic models defined by empirical resilient modulus equations. Few isotropic/kinematic hardening elastoplastic models were used but applying thousands of repeated load cycles became computationally expensive. In this paper, a cyclic plasticity model based on fuzzy plasticity theory is presented to model the long-term behavior of unbound granular materials under repeated loads. The discussion focuses on the model parameters that control long-term behavior such as elastic shakedown. The performance of the constitutive model is presented by comparing modeled and measured permanent strain at various numbers of load cycles. Calculated resilient modulus from the complete stress-strain curve is also discussed.

Ratcheting of Granular Materials

We investigate the quasistatic mechanical response of soils under cyclic loading using a discrete model of randomly generated convex polygons. This response exhibits a sequence of regimes, each one characterized by a linear accumulation of plastic deformation with the number of cycles. At the grain level, a quasiperiodic ratchetlike behavior is observed at the contacts, which excludes the existence of an elastic regime. The study of this slow dynamics allows exploration of the role of friction in the permanent deformation of unbound granular materials subjected to cyclic loading.

Influences on Permanent Deformation Behavior of Unbound Granular Materials

The results of a research program aimed at characterizing the permanent deformation behaviors of different types of unbound aggregates under repeated loading are summarized. Tests were performed in a triaxial or hollow-cylinder apparatus, depending on the grading of the materials. The scope of the research was to assess the relationship between total permanent axial strain and both the number of load applications and the stress level. The test results were used to verify a model recently developed in France in which attempts are made to predict the long-term behaviors of granular materials. The French model is examined with particular reference to its validity on the effect of stress level on permanent deformation of unbound granular materials. The results of the study indicate that the French model is generally successful in predicting the permanent strain for a given number of load applications. For one of the materials, however, the model did not seem to fit the data well. The French model also suggests that the variation in total permanent strain with stress level is related to the static failure line and could be determined by comparing the maximum shear stress ratio with the slope of the estimated failure line. This is discussed and shown to be questionable because it results in either unreasonable failure parameters or a very low level of correlation with the observations. An attempt was made to modify the French model, but that also proved to be unsuccessful. The concept of relating the permanent deformation behavior to the static failure condition of the material is therefore questioned.

Influences on Permanent Deformation Behavior of Unbound Granular Materials

The results of a research program aimed at characterizing the permanent deformation behaviors of different types of unbound aggregates under repeated loading are summarized. Tests were performed in a triaxial or hollow-cylinder apparatus, depending on the grading of the materials. The scope of the research was to assess the relationship between total permanent axial strain and both the number of load applications and the stress level. The test results were used to verify a model recently developed in France in which attempts are made to predict the long-term behaviors of granular materials. The French model is examined with particular reference to its validity on the effect of stress level on permanent deformation of unbound granular materials. The results of the study indicate that the French model is generally successful in predicting the permanent strain for a given number of load applications. For one of the materials, however, the model did not seem to fit the data well. The French model also suggests that the variation in total permanent strain with stress level is related to the static failure line and could be determined by comparing the maximum shear stress ratio with the slope of the estimated failure line. This is discussed and shown to be questionable because it results in either unreasonable failure parameters or a very low level of correlation with the observations. An attempt was made to modify the French model, but that also proved to be unsuccessful. The concept of relating the permanent deformation behavior to the static failure condition of the material is therefore questioned.