Grain and cement deformation in weakly cemented sands

The role of cementation type and degree on the generation and evolution of deformation bands in consolidated soils and rocks has not been yet fully understood. This work aims at a better understanding of the interaction between cementation and strain localisation. To do so, we focus on the micromechanics of cemented sand samples coming from natural outcrops and artificially made samples.
Our field case study is a Cretaceous deposit of weakly cemented sands, located in the southern France (Wibberley et al., 2007). The sand outcrop offers a great exposure of deformation bands (Fig.1). The fieldwork is combined with laboratory production of artificially cemented sand samples that are used in comparison with the naturally cemented material collected from the field out of the deformation bands. In our artificial samples, we have chosen to test two different cement types: clay and calcite (Ismail et al., 2002). To better understand the role of cement in the deformation, we plan to perform a number of triaxial compression experiments on the artificially and naturally cemented samples.
In our work we are performing quali-quantitative measurements throughout Image Analysis, whose images are produced by two different techniques that work complementarily in the presented research: Scanning Electron Microscopy (SEM) and X-Ray Computed Tomography (XRCT). The first one provides 2D images at a resolution up to nm-scale enriched with information about the chemical composition of materials; the XRCT enables a 4D non-destructive inspection of materials at high resolution (μm) in condition of pre-, syn-, and post- in situ deformation.
First analyses of a sample coming from a deformation band in the outcrop and of material coming far from the band have shown local differences in the cement types within the weakly cemented outcrop: while a mixture of clays connects the grains with menisci bonds in the naturally cemented samples far from the deformation bands (Fig. 2), a singular 50 cm thick deformation band crossing partially the deposit reveals the presence of quartz overgrowths (Fig.3) together with the clay mixture. The quartz layers result in some parts “disturbed” by the presence of clay and of other inclusions. Moreover, the rounded to well-rounded grains outside the deformation bands (Fig. 4) are replaced by broken angular grains and tiny quartz fragments inside the band (Fig. 5), which can be considered to be regarded as traces of deformation. Trying to explain the origin of the quartz overgrowths cementation, which is weakly supported by the geothermic condition of the field area during deposition, we are now wondering whether the thinner deformation bands, broadly distributed in the deposit, share similar characteristics with the thick one analysed.