Volume change response and fabric evolution of granular MX80 bentonite along different hydro-mechanical stress paths

Abstract

Despite the increasing understanding of bentonite behaviour, there is still missing evidence on how different hydro-mechanical loadings, including sequences of hydration and compression, affect the fabric and the volume change behaviour of the material. It is generally assumed that the interplay between the behaviour of clay assemblages and the overall fabric of the material is the reason of having final states that are dependent on the stress path followed. Here the results of an experimental campaign aiming to study these factors are reported and discussed. Free swelling and swelling pressure tests were performed, both followed by compression to a relatively high stress. The experimental program involved various samples that were dismantled at intermediate states in order to perform microstructural observations by means of mercury intrusion porosimetry and electronic scanning microscopy. It was observed that while the void ratio at a given stress level depends on the stress path, subsequent compression led to a unique virgin compression line. The data obtained at the microscale gave further insight for an interpretation of the volume change behaviour observed at the macroscale, showing that at high stress the material tends to recover the same fabric regardless of the path to saturation.

Microstructure and hydro-mechanical behaviour of compacted granular bentonite

“…1a). The montmorillonite content of this bentonite approximates 90%, and the total specific surface is 523 m 2 /g [5][6]. The solid density (𝜌 𝑠 ) at 20°C was determined to be 2.75 Mg/m 3 (ASTM D854-14) using a pycnometer with non-polar ethyl alcohol.…”

“…In the service period of these systems, bentonites likely encounter complicated hydromechanical HM stress paths [3]. For example, from the assemblage of buffer/backfill materials to the running of repositories, bentonites will suffer from compaction, desiccation under tunnel environments, hydration with water from near host rocks, and subsequently swelling in free/confined conditions [5][6]. Furthermore, bentonite constituents in GCLs, initially displaying high total suction, must be hydrated to achieve a certain degree of saturation before emplacing the wastes [4].…”

“…Granular bentonite GB manufactured by crushing heavily-compacted bentonite units (pellets) at low water content and dry densities close to 2.0 Mg/m 3 has been proposed to be a reference buffer/backfill material in * Corresponding author: hao.zeng@upc.edu recent years [5][6][7]. The granular bentonite includes powder and high-density granules with an extended grain size distribution (maximum grain sizes around 10 mm).…”

“…Therefore, this material has a more optimal particle size distribution than other bentonite types (e.g. powder and pellet bentonites) [5][6]. Moreover, the granular bentonite with better pourability and workability facilitates backfilling and compaction operation in horizontal drifts or geotextiles, while avoiding the creation of more dust [8].…”

Modelling the density homogenisation of a block and granular bentonite buffer upon non‐isothermal saturation

“…One of the main assumptions is that the parameters defining the saturated state behaviour of both GBM and block are coincident and that, at isothermal saturated state, the position of the normal compression line (NCL) does not depend on the initial state nor on the stress history of the material. 4,26,28 The same parameters calibrated and reported in Bosch et al 26 for the MX80 at saturated states were used in this study (Table 1). Swelling pressure tests in mixtures of pellets and powder were used to determine the loading collapse curve parameters of the GBM.…”