Experimental investigation and analysis of monotonic and cyclic behaviour of a natural sand in intact and reconstituted states

Abstract

Sandy soil structure formed during geological deposition critically controls its mechanical properties, directly impacting engineering design. However, challenges in obtaining undisturbed specimens hinder comprehensive understanding of in-situ sand behaviour. In this study, high-quality intact specimens were obtained from a deep caisson, frozen on-site, and precisely trimmed using specifically designed apparatuses. For comparison, reconstituted specimens with identical gradation and density were prepared via air pluviation. Systematic element tests were conducted using a hollow cylinder torsional shear apparatus to compare the monotonic and cyclic behaviour of both specimen types. Furthermore, anisotropic monotonic tests, unconfined drying tests, and X-ray Computed Tomography (XRCT) analyses were performed to identify the origins of the intact sand's distinctive mechanical properties. The results indicate that under monotonic loading, intact specimens exhibit a shear modulus more than twice that of the reconstituted specimens, accompanied by significantly greater initial dilatancy, while attaining comparable peak strength. Under undrained cyclic loading, intact specimens demonstrate more than twice the liquefaction resistance of their reconstituted counterparts. Notably, the normalized accumulation of excess pore pressure and its relationship with shear strain during undrained cyclic loading follow nearly identical trends for intact and reconstituted specimens. Under drained cyclic loading, intact specimens exhibit weaker contraction and greater modulus. The observed similar anisotropic mechanical patterns between intact and reconstituted soils contribute only marginally to their mechanical disparities. Rather, particle interlocking emerges as the dominant factor governing the behaviour of this sand. This study offers insights into the mechanical behaviour of sand at engineering sites with analogous geological origins.