Liquefaction of Nearly Saturated Sand under Undrained Isotropic Cyclic Loading

Abstract This study investigates liquefaction in nearly saturated sand subjected to undrained isotropic cyclic loading using a conventional triaxial apparatus. Loose and nearly saturated silica sand specimens were prepared, minimizing suction effects. Under sinusoidal isotropic cyclic loading with an amplitude of 100 kPa, the mean effective stress <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="p prime" display="inline" overflow="scroll"> <mml:msup> <mml:mi>p</mml:mi> <mml:mo>′</mml:mo> </mml:msup> </mml:math> of all specimens residually reduced from an initial value of 100 kPa. Liquefaction was identified as the loss of initial stiffness during subsequent undrained shear cycles. The relationships among initial Skempton’s coefficient <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="upper B" display="inline" overflow="scroll"> <mml:mi>B</mml:mi> </mml:math> , initial degree of saturation <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="upper S Subscript r" display="inline" overflow="scroll"> <mml:msub> <mml:mi>S</mml:mi> <mml:mi>r</mml:mi> </mml:msub> </mml:math> , and ratios of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="p prime" display="inline" overflow="scroll"> <mml:msup> <mml:mi>p</mml:mi> <mml:mo>′</mml:mo> </mml:msup> </mml:math> to its initial value after 1, 20, 50, and 100 cycles were examined. The amount of residual decrease in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="p prime" display="inline" overflow="scroll"> <mml:msup> <mml:mi>p</mml:mi> <mml:mo>′</mml:mo> </mml:msup> </mml:math> after 100 cycles maximized when <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="upper B" display="inline" overflow="scroll"> <mml:mi>B</mml:mi> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="upper S Subscript r" display="inline" overflow="scroll"> <mml:msub> <mml:mi>S</mml:mi> <mml:mi>r</mml:mi> </mml:msub> </mml:math> were approximately 0.193 and 0.972, respectively, and when deviating from these values, it diminished. Additional undrained cyclic shear tests were conducted on sand specimens with varying degrees of saturation. The results indicated that the low shear stress generated during isotropic loading was insufficient to induce liquefaction in nearly saturated sand. Finally, to interpret the mechanism responsible for residual reduction in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="p prime" display="inline" overflow="scroll"> <mml:msup> <mml:mi>p</mml:mi> <mml:mo>′</mml:mo> </mml:msup> </mml:math> under undrained isotropic cyclic loading, a conceptual model consisting of an elastoplastic soil spring and an elastic pore-fluid spring was proposed.