Cyclic Behaviors of Saturated Sand-Gravel Mixtures under Undrained Cyclic Triaxial Loading. Issue 4 (21st March 2021) - Guoxing, Chen
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 σ 3 c ′ to σ 1 c ′ 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 σ 3 c ′ ; Δ u : 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; γ ˙ : Shear strain rate; γ ˙ max, γ ˙ min : 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; σ 1 c ′, σ 3 c ′ : Initial effective vertical and lateral consolidation stress; σ d : Applied sinusoidal cyclic axial stress amplitude; σ s ′ : 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