EFFECTS OF SALTS ON GEOTECHNICAL BEHAVIOUR OF SOIL

Abstract

Alkaline and salts affected soils occupy 7.7% of the earth surface (Massoud 1990). These salts are primarily chlorides and sulfates of calcium, magnesium, sodium and potassium. The soils exhibit considerable changes in properties when they are exposed to salts solutions depending upon their mineralogy. High amounts of sodium relative to amounts of calcium and magnesium can cause deterioration in the physical properties of the soils. Water infiltration rates and hydraulic conductivities are also influenced in sodium affected soils due to dispersion and swelling of clays. Success or failure of engineering projects often hinges on the physical properties of soil. This thesis is aimed at to explore the effects of salts on geotechnical behavior of soils. To achieve the aim of this research, clay and sands were selected after the preliminary investigations. Two types of salts, i.e., sodium chloride, potassium chloride and urea were selected. These salts were mixed into the soils at different proportions and tested for geotechnical parameters. The results of the investigations indicated that salts affected the selected soils significantly. The Atterberg limits clay increased (5-33%) with the addition of salts till 0.2M-0.5M salts concentration. The Atterberg limits of the clay reduced with the increase of salt upto 3M and 4 M salts concentration. The reduction in plasticity index was noticed as 60% for NaCl, 71% for KCl and 42.53% in case of urea. The salt has also affected the maximum dry density and optimum moisture content (OMC). The OMC increased by about 19% for NaCl, 14.1% for KCl and 9% for urea at low concentrations; however at higher salt concentrations; the OMC decreased upto 7%.The maximum dry density of soil was also changed with the addition of salt as it has influenced the OMC. The cohesion of the clay increased with the increases in the salt contents. The increase in cohesion of KCl specimens was more as compared to the increase in cohesion value of NaCl. For 4M salt concentration, the cohesion was 39% for NaCl and 51% for KCl more than the cohesion value of the dairy farm clay (selected clay). With the increase in salt contents, the angle of internal friction in the clay reduced. The maximum reduction in the angle of internal friction was 7.4% in NaCl specimen for 4M concentration. There was a significant reduction in the friction angle of KCl mixed specimens, i.e., 33.3% at 3M and 4M KCl concentrations. The urea mixed clay samples displayed reduction in the unconfined compressive strength with the increase in urea contents. The compressive strength of urea mixed specimens was reduced upto 50% with 4M urea concentration. The analysis of e-log p curve indicated that specimen with 2M NaCl and 2M KCl concentrations exhibited less void ratio, hence were less compressible. At high salt concentration, i.e., 4M NaCl and 4M KCl, the void ratio increased and the specimens were more compressible. The increase in void ratio at 4M NaCl concentration varied between 5-8% and for KCl this variation was between 5-15%. For urea this increase in void ratio varied between 8-20%. The permeability of the clay increased for both salts and urea with 1M salt concentration. Upto 2M salt concentration of NaCl and KCl, the permeability decreased. In case of urea, the permeability of soil increased with all concentration as the urea addition exhibited reduction in strength parameters. The dry unit weights of sands have been affected with the addition of salts contents. The reduction in these densities was 6% for fine and 12% for coarse sand using NaCl and KCl. For coarse sand, the angle of internal friction increased with the increase in salt contents. For moist sand at 4M NaCl salt concentrations, the angle of internal friction increased 15% for NaCl, whereas, for the dry sand it increased by 10%. For fine sand, the angle of internal friction reduced for all salt contents for both dry and moist sand; however, the decrease in dry sand was about 20% and for moist sands it was 14-21%.