Unravelling the role of natural imperfections on the mechanical behaviour of cemented granular systems: insights from naturally weakly cemented sands

Weakly cemented sands can be classified as soft rocks; their difference from hard rocks (well-cemented sandstone equivalents) is their nature to disintegrate within a short time (from days to several years) when being exposed to water and climatic changes. This loss of strength is irreversible under normal conditions [Nickmann et al., 2006]. Understanding the mechanical behaviour of weakly cemented sands is crucial for several geotechnical engineering applications (e.g., shallow foundations, offshore construction, slope stability) because of the potential hazards posed by these materials. Previous work mainly focused on capturing how different cement types (i.e., concrete cement [Li et al., 2015], bio cement [Terzis & Laloui, 2019]) enhance the mechanical behaviour and durability of artificially cemented materials. This work has aimed to study the micromechanics of natural soft rocks exposed in a French outcrop, where numerous deformation bands have been observed locally.
Soriano’s PhD thesis was carried out for this purpose focusing on a) the textural characterisation of the outcrop material (matrix and deformation bands); b) the mechanical behaviour of the natural weakly cemented material; and c) the manufacturing of an artificial material that mimics the mechanical behaviour of the natural one. The outcrop material has ~300 μm grain size and contains both quartz and clay cement. Several imperfections related to previous diagenetic and/or deformation processes captured in the tested material. In this presentation, we focus only on results from triaxial compression experiments conducted on natural weakly cemented sand samples containing a) regions of enhanced porosity; b) an elongated (greater than the sample’s radius) pore, and c) a pre-existing dilation band [Figure 1]. Several non-destructive (x-ray CT, Digital Volume Correlation) and destructive (ESEM) methods were used to capture textural changes and strain fields due to lab-induced deformation. Our experimental results [Soriano, 2019] demonstrate that these imperfections locally trigger or halt laboratory-induced strain localisation. Samples with the elongated pore or with the deformation band are stiffer than those containing zones of enhanced porosity. The presence of local imperfections affects, thus, the system behaviour. However, their local position within the studied samples seems to leave unaffected the inclination of the (new) lab-induced deformation bands. Moreover, the pre-existing dilation band within one of the samples temporarily reactivates but then deactivates as new preferred orientation deformation bands develop.
To better understand fluid migration and flow patterns within weakly cemented sand samples containing dilation bands [Figure 1], we ran flow experiments coupled with High-Speed Neutron Tomography [Charalampidou et al., 2019]. To avoid total disintegration of the samples, we performed initially an oil injection (decane) followed by water (heavy water) injection. Time maps visualised complex flow patterns. Clay particles, aligned along the rims of a pre-existing dilation band, appear to retard oil saturation within the dilation band, while they promote matrix oil migration.
Our results show how complex the mechanical behaviour and the fluid-soft rock interaction can be in the presence of natural imperfections within the tested samples. An in-depth understanding of the local deformation processes and fluid migration pathways can facilitate the constitutive modelling of those materials.