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Coral sand is the main filling material for the island–reef foundation. Under tidal actions, the saturation (<i>Sr</i>) of coral sand layers varies with the specific depths in the reclaimed foundation. Studying the <i>Sr</i> effect of coral sand’s mechanical behaviors is crucial for the stability of the reclaimed foundation of island–reefs. In this study, a “quantitative injection method” was designed to prepare coral sand with saturation ranging from 90% to 100%, and unconsolidated–undrained (UU) triaxial shear tests were conducted on coral sand under different effective confining pressures (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mo>σ</mo><mn>3</mn><mo>′</mo></msubsup></mrow></semantics></math></inline-formula>). The results indicated that the stress–strain curves of coral sand under various conditions were of the strain-softening type. When <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mo>σ</mo><mn>3</mn><mo>′</mo></msubsup></mrow></semantics></math></inline-formula> = 200, 400, 600, and 800 kPa, the shear strength of coral sand decreased exponentially by 13.1, 9.1, 16.8, and 15.2%, respectively, with the increase in <i>Sr</i> from 90% to 100%. As <i>Sr</i> rose, the internal friction angle (<i>φ</i>) dropped by 3.77°. The cohesion (<i>c</i>) was not significantly affected by <i>Sr</i> compared to <i>φ</i>. In consideration of the physical susceptibility of coral sand to breakage, relative breakage ratio (<i>B_r</i>) and modified relative breakage index (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>B</mi><mi>r</mi><mo>*</mo></msubsup></mrow></semantics></math></inline-formula>) were introduced to evaluate the particle breakage behaviors of coral sand samples with different <i>Sr</i> levels in the triaxial shear process. It was found that <i>B_r</i> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>B</mi><mi>r</mi><mo>*</mo></msubsup></mrow></semantics></math></inline-formula> increase linearly with increasing <i>Sr</i>; the effect of <i>Sr</i> on the particle breakage of coral sand weakens significantly when <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mo>σ</mo><mn>3</mn><mo>′</mo></msubsup></mrow></semantics></math></inline-formula> is sufficiently large. The median particle size (<i>d</i>₅₀) of coral sand decreases with increasing <i>Sr</i>, and presents a negative linear correlation with both <i>B_r</i> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>B</mi><mi>r</mi><mo>*</mo></msubsup></mrow></semantics></math></inline-formula>. Based on comparing the strength and particle breakage characteristics of coral sand samples with varying <i>Sr</i> levels, this study suggests that 92.5% should be considered as the <i>Sr</i> value of coral sand available for testing.