Cyclic Behaviors of Saturated Sand-Gravel Mixtures under Undrained Cyclic Triaxial Loading

Abstract

ABSTRACT A few well-documented liquefaction case studies of gravelly soils during major earthquakes have increased concerns regarding the liquefaction susceptibility of gravelly sand soils. Understanding the failure mechanism of sand-gravel mixtures under undrained cyclic loading is important for better assessing their liquefaction susceptibility. The liquefaction mechanism and susceptibility of isotropically and anisotropically consolidated sand-gravel mixtures are investigated under undrained cyclic loading in a solid cylinder apparatus; and, the influences of the varying material properties, compactness states, and particle gradations are systematically studied. The authors introduce an index of the average flow coefficient (k a) that describes the fluidity of saturated sand-gravel mixtures, and then propose the elaboration of a new liquefaction mechanism of saturated sand-gravel mixtures. The specimens would experience cyclic liquefaction when the excess pore pressure ratio reaches 100% under undrained isotropic consolidation, whereas the cyclic mobility would be induced when the cumulative axial strain reaches 5% under undrained anisotropic consolidation. Particles with both sizes smaller than 0.25 mm and mass contents less than 30% in sand-gravel mixtures primarily play a role of the fillers of intergranular voids (fines in coarse). The skeleton void ratio (e sgk) is a unique physical state index to characterize the liquefaction susceptibility of saturated sand-gravel mixtures with the filler content less than 30%. The negative correlation between the cyclic resistance ratio (CRR) and e sgk can be expressed as the power function. Abbreviations: A cl: Closed-loop area of each loading cycle in Fig. 5; ACU: Anisotropically consolidated undrained; CRR: Cyclic resistance ratio; CRR 15: CRR in 15 cycle; CRR 25: CRR in 25 cycle; CSR: Cyclic stress ratio, Applied cyclic stress ratio in CTX test in the form of Equation (7); CTX: test Cyclic triaxial test; C u, C c: Coeffıcients of uniformity and curvature for the particle-size distribution curve; D, d: Size of coarse and small spherical particles in an idealized binary packing model; D 50: Median diameter of host coarse soils in binary mixtures; d 50: Median diameter; median diameter of host fine soils in binary mixtures; DA: Double-amplitude; D r: Relative density; initial relative density; D r0: Post-consolidation relative density; e: Global void ratio; initial void ratio; e 0: Post-consolidation void ratio; e min, e max: Minimum and maximum void ratios; e sgk: Skeleton void ratio; FC: Fines content for particles with the sizes less than 0.075 mm; FC th: Fines content threshold value; f f: Filler content by weight for particles with sizes less than 0.25 mm in the coarse-dominated grain contact sand-gravel mixtures; G c: Gravel content; G s: Specific gravityHCA Hollow cylinder apparatus; ICU: Isotropically consolidated undrained; k a: Index of the average flow coefficient; K c: Consolidation stress ratio, defined as the ratio of to m, n Fitting coefficients in Equations 11–13; N l: Number of cycles required to cause an r u = 100% for ICU CTX test or ε p = 5% for ACU CTX test; q: Deviatoric stress; q cyc: Peak cyclic deviatoric stress; q s: Initial static deviatoric stress; R: Roundness proposed by Power (1953); R d: Particle size disparity ratio in binary packing model; r u: Excess pore pressure ratio, defined as the ratio of Δ u to; Δu: Excess pore pressure change induced by the action of cyclic shear stressSA Single-amplitude; α d: Cyclic shear stress ratio of any plane in the specimen; β: Inclination angle to the horizontal axis in Mohr’s stress circle serve; : Shear strain rate; ,: Maximum and minimum shear strain rate of each loading cycle in Fig. 5; ε: Axial strain; ε p: Accumulated permanent axial strain; η: Apparent viscosity in fluid mechanics; ρ d: Dry densities; ,: Initial effective vertical and lateral consolidation stress; σ d: Applied sinusoidal cyclic axial stress amplitude; : Initial effective normal stress of any plane in the specimen; τ: Shear stress; τs : Initial shear stress of any plane in the specimen; τ d: Cyclic shear stress amplitudes of any cyclic shear effect plane; τ max, τ min: Maximum and minimum shear stress of each loading cycle in Fig. 5