Resumen de Hydro-mechanical behaviour of compacted binary mixtures. An experimental study
This research focusses on an experimental investigation on the hydro-mechanical behaviour of binary ‘all-one’ mixtures. These mixtures –originated from the excavation of selected materials during earthwork constructions– are basically constituted by large rock fragments immersed in a matrix with dominant finer soil fraction. These materials, which bridge the gap between coarser rockfill materials and soils, are widely accepted in different national standards for earthworks, such as dams, levees, railway and road embankments. Despite their practical interest, these mixtures have not been widely studied. The emphasis of the current study has been mainly placed on the influence of the properties of the fragments (unconfined shear strength) and the void ratio of the finer matrix to explain the hydro-mechanical behaviour of these mixtures along different stress paths, as well as to better assess the role played by fragment breakage aspects. Different mass ratios of fragments have been initially studied, which allowed selecting a matrix proportion able to fill nearly all inter-fragments voids (interactive-underfilled packing), as well as still ensuring good compaction properties (close to minimum dry density for specified water content). A constant fragment mass ratio of 40% with particles between 19 mm and 38.5 mm has been finally selected to perform a systematic experimental campaign with fragments of different uniaxial compressive strengths, ranging from natural carbonated and low-strength aggregates to high-strength slate fragments. This fixed fragment ratio has ensured reaching a mixture, in which both the density of the fine matrix and the strength of large fragments are controlling the coupled hydro-mechanical response. A procedure for fabricating artificial and cement-based fragments has been also proposed to allow obtaining an intermediate uniaxial compressive strength and thus to better fill the gap between these extreme low-strength and high-strength fragments. A real ‘all-one’ mixture retrieved under as-compacted conditions during the construction of Albagés dam (Lleida, Spain) has been also considered to compare main hydro-mechanical features with the mixtures prepared under laboratory conditions. The experimental campaign using a large-oedometer cell has included loading / unloading cycles under saturated and unsaturated conditions, as well as soaking tests at constant vertical stress. The study with different fragment strengths has been mainly focused on analysing the compressibility of the mixture on loading at different hydraulic states, the collapsible response on soaking at constant vertical stress, as well as the water retention and saturated permeability properties. These macroscopic phenomenological results have been complemented by a microstructural description that included aspects of fragment breakage, fractal dimension changes of the particle size distribution, as well as energy transfer aspects (work input per unit volume used to densify the matrix and to break large fragments). The results have shown the important role played by the strength of the fragments to achieve better compaction properties when using high-strength fragments, which allowed reducing the compressibility on loading and the collapsibility on soaking, as well as ensuring a more efficient transfer of the work input to densify the matrix (instead of breaking particles).
