A vast majority of the geotechnical infrastructures (retaining walls, shallow foundations, piles, etc) made of construction material are always surrounded by fine-grained soil. The behaviour of interface shearing between soil and construction material is essential for providing safe and optimal designs within the field of geotechnical engineering. Considerable experimental investigations have been conducted by researchers to enhance understanding of the interface shearing between soil and soil nailing, geosynthetic-soil interaction, and pile-soil friction through a variety of laboratory interface shear tests (Yin and Zhou, 2009; Su et al., 2010; Su et al., 2018). The shear behaviour of the interface soil-construction material has been often examined by using direct shear and simple shear tests. (Goh and Donald, 1984) have investigated the interaction between three different soils and two concrete surfaces of different texture using simple shear apparatus. The test results showed that both the concrete surface texture and the clay content played an important role in the shear resistance at the interface (Tsubakihara et al., 1993) tested cohesive soils against mild steel interfaces using a simple shear device. They observed three failure modes: shear failure within the soil, full sliding at the interface and a simultaneous failure at the interface and within the soil specimen. Moreover, (Fligha et al., 2015) conducted a direct shear tests to investigate the effect of roughness interface on the friction angle between cohesive soils and different structural materials (steel and abrasive paper). They concluded that the behaviour at the soils–solid interface varied according to surface roughness and soil type. The main disadvantage of the direct and simple shear tests is the stress concentration at the end of the interface, namely nonuniform distribution of the stress or strain along the interface (Zhang and Zhang, 2009; Haydar et al., 2005).In addition to theses devices, the torsional ring shear device has been used in a large number of tests of the interface between different building materials and fine-grained soils (Hammoud and Boumekik, 2003; Lejane and Jardine, 1992; Lemos and Vaughan, 2000; Meehan et al., 2007). This device can impose shear displacements as large as those experienced by pile shafts. (Hammoud and Boumekik, 2003) conducted tests by means of the Bromhead ring shear apparatus on two kinds of cohesive soils and steel interfaces with different roughness, sheared to large displacements. They demonstrated that the surface roughness characteristics and the particle size distribution played an important role in estimating the shear strength of granular materials. According to roughness, two failure modes exist: shear failure at the interface and within the soil. A series of Bromhead ring shear tests were performed to measure the residual strength of clay soils (Meehan et al., 2007). As a result, the top platen should be modified to minimize the effect of wall friction in the Bromhead ring shear test. The major advantage of the torsional ring shear device is that it shears the sample without changing the shearing direction and the stresses and strains within the specimen are nearly uniform (Zhang and Zhang, 2009).Accordingly, the interface shear behaviour of soil-construction materials has been extensively studied both numerically and experimentally. To date, however, there are very limited experimental investigations devoted to microstructural analyses of laboratory produced shear zones consisting of different types of soils (Lupini et al., 1981; Klukanova and Modlitba, 1990; Frost and Jang, 2000; Li et al., 2012; Chen et al,. 2014).The present work aims at providing deeper insight into the failure mechanisms at cohesive soils-steel interfaces under large shear displacement by means of the Bromhead ring shear apparatus. The focus is placed on describing and analysing shear zones features which are structure and thickness as well as texture orientation angle. The Scanning Electron Microscopy (SEM) analysis were the main source of information in assessing theses features.
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