Modifying Soil Shear Strength Parameters Using Additives in Laboratory Condition

Abstract

Mechanical and chemical processes and/or reinforcing materials are used in order to increase soil shear strength. Necessity for reinforcing and strengthening of soil in geotechnical and civil engineering projects requires use of new materials and reinforces. In recent years, although researchers have used new chemical compounds, however, nano-particles have not found their suitable situation. In this study, we have tried to increase soil shear strength parameters using different additives. Clay minerals are considered as problematic soils due to their engineering features. So, it is essential to reclaim them. In this research, clay with low plasticity property has been studied. Soil shear strength is an important factor for any analysis associated with stability including slope stability analysis. Slope stability analysis is used in earth dams and trenches. In this study, we have tried to increase soil shear strength parameters, i.e. cohesion coefficient (C) and internal friction angle (φ) using different additives. Direct shear test has been used for obtaining shear strength parameters as well as Mohr-Coulomb theory has been utilized for calculating of them. Although direct shear machine has its defects and its accuracy is low in comparison with tri-axial machine, however in this study it has been selected due to its simplicity and cheapness. Since this investigation aims to compare various additives and all tests have been done at same condition by direct shear machine, its deficiencies have been neglected. Additives used in present research include: Nano-silica in various percentage, Micro-silica, cement, lime (Cao) and these materials’ combination with together. The reason to choose Nano-silica is that it is a very active super-pozzolanic additive. This additive increases strength of sample significantly through chemical actions. Using silica in soil stabilization depends on type and size of silica particles so that the more finely the more continuous gradation, so property of being finer leads to decrease pores among particles and results to increase strength while light gradation has been achieved. Silica is one of the most popular materials which play a significant role in cohesion and filling. Results of experiments have shown significant effect of these additives in increase of soil shear strength parameters. The lime leads to modify behavioral features of fine-grained soils containing clay (properties such as swelling, shear strength, water absorption ability and plasticity properties) but it should not be in vicinity of sulphate ions.Since, in this condition, presence of lime not only doesn’t play an effective role but also it results to decrease in strength as well as increase in swelling. With regard to this reason, Calcium sulfate (gypsum) was added to soils containing lime in order to study swelling of soils stabilized with lime and nano-silica. Nano-silica increases Soil shear strength parameters while it is efficient in increasing of soil swelling, too.