![]() Ground freezing is considered to be the most suitable technique to obtain undisturbed samples of non-frost-susceptible sands, although the costs are restrictively high. Nonetheless, any substance introduced into the soil voids must also be somehow removed prior to testing, which requires special procedures and care. Frost, 1989 Schneider et al., 1989 Sutterer et al., 1996 Yang, 2002 Evans, 2005), but in situ impregnation still has not found its way into practice. Soil impregnation using agar, agarose and Elmer's glue has been used in research studies (e.g. To avoid disturbances, stabilisation methods such as soil impregnation or freezing have been proposed. ![]() Moreover, disturbances during sample transportation, handling, extrusion and specimen preparation may change the initial sand fabric before a specimen is tested in the laboratory. However, these sampling methods may cause either densification or loosening of sand samples, depending on the sand's initial fabric. Allegedly improved sampling techniques include block sampling (on cemented locked sands with fines), the Bishop sampler ( Bishop, 1948) and the Gel-Push sampler ( Mori & Sakai, 2016). ![]() It has been claimed that high-quality undisturbed samples of sands can be obtained either by improving sampling techniques or by stabilising a portion of the soil in situ. However, as has been known for about 50 years, every reconstitution technique imposes a different initial fabric and, consequently, a different stress–strain behaviour on reconstituted sands ( Oda, 1972 Ladd, 1974 Arthur & Phillips, 1975 Ladd et al., 1977 Mulilis et al., 1977 Silver et al., 1980 Miura & Toki, 1982 Kuo & Frost, 1996 Vaid & Sivathayalan, 2000 Yamamuro & Wood, 2004 Madhusudhan & Baudet, 2014). As a result, sands are commonly reconstituted in the laboratory to target void ratios empirically derived from in situ tests, such as the cone penetration test (CPT). State-of-the-practice sand sampling techniques induce sample disturbance. Sands are very difficult to sample in an undisturbed state so that their initial fabric remains intact. Initial fabric controls key features of a soil's mechanical response such as its small-strain stiffness, undrained instability, phase transformation and peak strength, as well as the soil's undrained cyclic strength ( Wanatowski & Chu, 2008 Shi et al., 2020). Fabric can be defined as the particle sizes, shapes and their distribution, along with the arrangement of grains and their contacts in a given soil ( Mitchell & Soga, 2005). Granular soils are complex assemblies of discrete particles with a certain fabric. This has major implications for geotechnical testing and analyses of liquefaction of sands deposited under water such as fluvial, offshore and tailings sands. This observation explains why previous rigorous studies on the macro-mechanical behaviour of sands deposited under water have systematically demonstrated that slurry deposition is the most suitable method to reconstitute in the laboratory natural sands deposited under water. However, the slurry deposition method managed to reproduce the inherent particle orientation, anisotropy and the variations of void ratio and particle sizes of the undisturbed fluvial sand. Fabric analysis results suggest that none of the laboratory reconstitution techniques used captures the true three-dimensional initial fabric of undisturbed fluvial sand. Initial fabric features were assessed in terms of the particle orientation, anisotropy, void ratio distribution and particle sizes within the specimens. The initial fabric of the specimens was evaluated using X-ray micro-computed tomography and advanced image analysis. In the laboratory, the in situ void ratio of these high-quality undisturbed frozen samples was replicated using four different reconstitution methods: dry deposition, moist tamping, water sedimentation (by spooning) and slurry deposition. High-quality undisturbed samples of fluvial sand were obtained from the field using the ground freezing technique.
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