Nonlinear equivalent linear design method to determine unbound aggregate base layer modulus

Mechanistic-Empirical Pavement Design Guidelines (MEPDG) recommends use of material modulus in lieu of structural number for pavement base layer thickness design. Soil modulus is nonlinear with respect to effective confinement stress, loading strain and moisture (suction); and modulus nonlinearity need to be considered for an efficient base layer design and analysis. For practical design purposes, single effective modulus value of base layer need to be known and this modulus value should be able to approximately account for nonlinearities of the whole base layer. However, MEPDG does not describe a standard procedure for determining this single modulus value. This research study focuses on laboratory characterization of base soil modulus nonlinearity, developing a nonlinear response model for nonlinear pavement analysis and developing a practical linear design methodology to determine nonlinear equivalent single effective design modulus value for whole base layer.;First, Fixed-Free and Free-Free resonant column tests are conducted on two gravelly base soils used in the State of Florida, to characterize shear modulus (G) nonlinearity in the strain range of 10-5% to 10 -1%, including small-level strains, under different loading confinements and moisture contents. Moisture suction effect on nonlinear modulus is evaluated. It is found that unsaturated gravelly soils modulus is linear at strains lower than 10-5% and nonlinear thereafter. Compared to dry soils, presence of moisture in unsaturated soils makes it more nonlinear with respect to strain. Suction effect can increase G in the strain range of 10 -5% to 10-1%, but very significantly at strain levels less than 10-3%. At any given moisture content, additional confinement due to suction does not decrease with increase in strain. Empirical equations are developed to calculate very small-strain modulus (Gmax ) of dry soils. A procedure to calculate approximate G value at given water content, confinement and strain magnitude is developed.;Second, using laboratory nonlinear modulus characterization data as material parameter inputs, a stress and strain dependent nonlinear response model for base layer analysis is developed via Plaxis-HSsmall model. Nonlinearity of the response model is verified with respect to stress and strain variation. Considering maximum surface deflection of nonlinear analysis as matching factor between nonlinear and linear design methods, a practical design methodology to determine equivalent single effective elastic modulus for base layer is proposed. Effect of moisture, subgrade modulus and layer thickness on base single effective modulus is analyzed. Stress and strain responses for both nonlinear and equivalent linear analysis are compared at critical locations. Influence of base nonlinearity on pavement performance is evaluated. It is found that use of single effective modulus in place of nonlinear modulus over estimates rutting performance, when structure thickness and base water content decreases together. Last, effect of subgrade modulus nonlinearity on pavement performance is also analyzed for limited cases. Equivalent single effective modulus for all layers is back calculated via Falling Weight Deflectometer (FWD) analysis of nonlinear analysis surface deflection. It is found that, subgrade nonlinearity may significantly influence rutting performance and elastic based rutting performance criterion overestimates rutting performance.