RESPONSE OF GEOMATERIALS OF TWO LAYERED PAVEMENT STRUCTURES UNDER STATIC AND CYCLIC LOADINGS

Abstract

Flexible pavement is idealized as a multi-layered elastic structure resting on subgrade soil, constructed to facilitate the movement of vehicles. Further, these flexible pavements are classified as High and Low volume roads based on traffic intensity and utility. High volume roads are constructed of 3 to 4 granular layers with thick bituminous surfacing (eg. national and state highway roads); whereas, low volume roads are rural or village roads, generally considered as two layered pavement systems are mainly comprised of thick granular layer with thin bituminous surface or without bituminous surfacing. The behavior of a two layered pavement structure is dependent upon the strength and stiffness of both granular base course and subgrade layer. The pavement responses i.e., deformation at the surface, deformation and stress at the interface of granular-subgrade are the governing factors for the analysis and design of a two layered flexible pavement system. A number of research have been conducted to investigate the deformation behavior of flexible pavement systems for different characteristics of granular and subgrade soil; however, most of the studies were performed on the material characterization of the pavement layers. The overall design of pavements depends on the composite behavior rather than the individual layer; hence, analysis of the entire pavement structure as a composite system is required for the design of low volume roads based on the subgrade soil conditions and availability of granular materials for base course. In this thesis, numerical analysis was performed for a two-layered flexible pavement system for different granular layer thicknesses with different material characteristics of granular layer and subgrade layer subjected to two different wheel load configurations. Based on the numerical results, a new mechanistic framework was proposed to determine the critical pavement responses i.e. the surface and interface deformations, and interface stresses of a two layered flexible pavement system. For performing the analysis, a single lane rural road in India of a two-layered system consisting of the granular layer and subgrade was taken into consideration, and a 3D finite element analysis was carried out using ABAQUS to overcome the limitations related to two-dimensional analysis, The computations were performed for five different values of pavement thickness to tire width ratio (h/b = 1, 1.5, 2, 2.5 and 3) and five modular ratios (E1/E2 = 5, 10, 15, 30, 50; where E1 = elastic modulus of granular layer; E2 = elastic modulus of subgrade layer). From the numerical solutions, design charts and prediction models have been developed to compute the deformations and interface stresses for a two layered pavement system. It was observed that pavement-subgrade interface stress is dependent on modulus ratios and thickness of pavement. With the increase in thickness of base course and modulus ratios, interface stress was observed to decrease. Similar to the interface stresses, deformations at any point within the pavement system were observed to be strongly dependent on the modulus ratios and pavement thickness. The magnitude of surface and interface deformations decreases as the pavement thickness increases; whereas, it increases for a higher modulus ratio of the same pavement thicknesses. The deformation contributed from subgrade has been found to be higher when the difference between modulus of subgrade soil and granular base course is greater. Surface and interface deformations were observed to increase sharply with the modulus ratio, which indicates that merely providing a strong and thick granular base course for a two-layered pavement structure is not sufficient. Most of the pavement design practices are based on the resilient modulus value of the soil. Various soils undergo large amount of permanent deformations even though they exhibit higher resilient modulus values and hence subgrade fails in the early stage as reported in previous studies. Therefore, both elastic and plastic strain responses are required to be studied for the complete characterization of subgrade soils. A significant amount of research has been done and a number of empirical models have been developed for predicting cumulative plastic deformation in different types of soil under cyclic loading; however, in most of the models, the accumulated plastic strain has been expressed as the function of the number of load applications and deviatoric stress. The influences of other factors such as soil type, moisture content, and level of confinement are required to be addressed as they also impose a significant influence on the deformation characteristics of subgrade soils under cyclic loading. To study the deformation behavior of subgrade soil under cyclic loading, three different types of soils i.e. low plastic clay (CL), high plastic clay (CH), and clayey sand (SC) were used. In order to investigate the influence of compaction moisture content, four different moisture content levels were selected for the study i.e., two levels on dry of side of optimum, optimum, and one level on wet side of optimum. Unconsolidated undrained static and cyclic triaxial tests were conducted on the soil specimens at the selected water content levels in order to determine the undrained shear strength and deformation characteristics of these subgrade soils at different confining pressures and deviatoric stresses. The major proportion of deformation was observed to take place in the first few numbers of cycles and then the rate of strain development decreased slowly with an increase in the number of load cycles. Corresponding to a given compaction state and confining pressure, the total and plastic strain increased with an increase in the magnitude of cyclic deviatoric stresses and number of load repetitions. For all the soils under consideration in this study, the maximum deformations were obtained on the wet side of the optimum at compaction state of lowest undrained shear strength. However, the minimum deformation was obtained for clayey sand (SC) compacted at the optimum moisture content, and for cohesive soils (low and high plastic clay) compacted on the dry side of optimum moisture content corresponding to compaction state of highest undrained shear strength. The deformation behavior of subgrade soils was observed to be dependent upon the combined influence of confining pressure, applied deviatoric stresses, number of repetitions, and compaction moisture content. Based on the variation of total and plastic strains with a number of load cycles, simple logarithmic strain models have been developed taking into the combined influence of water content, stress levels, and number of load repetitions at different compaction states from the test results of the three soils. The model developed from the present study was also verified by comparing the values of plastic strain predicted from this model with other model using the experimental data reported in the literature. The strength and compressibility characteristics of weak subgrade soils are improved by stabilizing them with different traditional admixtures; however, the use of such traditional stabilizers has been observed to impose various environmental issues and makes the soil excessively brittle which is undesirable in the case of structures subjected to cyclic loading. Thus, an alternative material that can improve the strength and durability without causing any ill effect to the soil and environment needs to be established. Few researches have recognized lignosulfonate (LS) as a promising agent for stabilizing problematic soils; however, these studies have been carried out to study the effect of lignosulfonate on mechanical performances such as unconfined compressive strength, swelling, erosion resistivity, moisture susceptibility, compaction, and compressibility properties. Very limited research studies on the behavior of lignosulfonate treated soil under cyclic loading seem to be available in the literature. An attempt has been made to study the effectiveness of lignosulfonate treated high plastic clay on its different parameters such as unconfined compressive strengths and undrained shear strength under static loading, and deformation characteristics i.e., total and plastic strain behavior, and resilient modulus under cyclic loading. In addition, the time-dependent strength development of high plastic clay at different lignosulfonate contents and the influence of LS stabilization on the rate of strain development to evaluate the longevity of LS treated soil under cyclic loading were also investigated. Soil specimens were prepared at two different compaction moisture contents and the admixture LS was added in various percentage of dry soil mass. The analysis of test results indicated that the unconfined compressive strength of soil samples increased with an increase in lignosulfonate content up to certain optimum content after which it decreased. A three-parameter compressive strength development model as a function of lignosulfonate content and time has also been developed to predict the compressive strength of lignosulfonate treated high plastic clay at any curing period for a given lignosulfonate and water content. A significant increase in the deviatoric stress at failure in the case of static triaxial tests and a significant amount of reduction in the axial and rate of axial strain development for stabilized soil samples in comparison to the untreated specimens was observed for both the compaction states. A considerable reduction in the rate of strain development for stabilized specimens indicates the slow rutting of subgrade soil which would result in increased longevity and durability of soil specimens under cyclic loadings.