An experimental investigation of the mechanical behavior and microstructural features of a volcanic soil (Isparta, Turkey) and stability of cut slopes in this soil

Abstract

Volcanic soils are widely distributed group of soils, which cover significant parts of the world's surface and include areas occupied by urban settlements, structures and infrastructures, and may create geo-engineering problems. These soils exhibit a distinctive behaviour that is a consequence of their formation history, mineralogy and structure. Some part of the subsoil of a large area surrounding the city of Isparta (western Turkey) mainly consists of volcaniclastic deposits. This paper presents the results of the part of a research programme aiming at geotechnically characterizing the uppermost layer of this volcaniclastic sequence, particularly focusing on its collapse potential, shear behaviour and the influence of microstructure on these behaviours. In addition, its main futures were compared with those of some similar volcanic soils at different parts of the world, and the stability of sub-vertical cuts in this soil was also assessed. The experimental investigation mainly consisted of index and classification tests, double oedometer and direct shear box tests to assess the collapse potential and shear behaviour of the soil, respectively. Matric suction of the samples was also determined for preliminary evaluation of the influence of suction on collapse and shear behaviours of the soil. In addition, thin-section studies, and X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were also conducted to determine mineralogical and microstructural features and to evaluate their influences on the mechanical behaviour of the volcanic soil. The deposit is composed of weakly bonded firm silty sand with an open structure, low unit weight and high porosity, generally unsaturated in-situ. The results of the double oedometer tests suggest that the volcanic soil generally falls in the category of slight collapse potential at normal stresses ranging between 5 and 100kPa, while at normal stresses greater than 100kPa its collapse potential increases and ranges between moderate and moderately severe collapse potential classes. At normal stresses lower than 100kPa, matric suction may have an effect on collapse behaviour. The direct shear box test results indicated that the volcanic soil has two different types of shear strength behaviour represented by bi-linear failure envelopes. In other words, the shear strength parameters of the volcanic soil within a low normal stress range (generally between about 6 and 200kPa) exceed those of determined at higher normal stresses. Since no matric suction was measured during the direct shear box tests, it could not be possible to evaluate the effect of suction on the bi-linear failure envelopes. However, some differences between the suction and degree of bonding of the samples taken from different locations might probably have been contributed to the separation of the bi-linear behaviour at different normal stress levels. The destruction of the bonding (cementation) between the particles is probably more responsible from the bi-linear form of the failure envelope. Comparisons between the SEM micrographs taken from natural and tested specimens revealed that the mechanical behaviour of the volcanic soil is strongly influenced by the bonding material. The results of the stability analyses showed a good agreement with the current situation of the cut slopes observed in the study site and suggested that vertical cut slopes with a height of 14–15m will maintain their stability based on a reasonable factor of safety such as 1.5.