Effect of illite on the mechanical properties of subgrade soil under varying surcharge loads

Due to its non-expanding properties, presence of Illite mineral in subgrade soil is investigated particularly on California bearing ratio (CBR), resilient modulus (M_R) and swell potential. Multiple samples of stiff and weak subgrade soils with varying illite percentages were tested under six different surcharge loads ranging from 2.27 to 13.8 kg. Mineralogical analysis is performed using X-ray diffractometer and M_R of soil is assessed using Ultrasonic pulse velocity (UPV) technique. Results showed a positive correlation between Illite percentage and both CBR and M_R value. The soil with higher Illite content tends to exhibit higher CBR and M_R values while those with higher montmorillonite content show lower values even with more Illite content. The CBR and M_R values increases from 8.4% to 19 % and 139 MPa-315 MPa for stiff soil and 3.8%-11.7 % and 23 MPa-83 MPa for weak soil, respectively when the surcharge load was increase from to 2.27-13.8 kg. Additionally, a decrease in swell potential was observed from 1.64% to 1.09 % for stiff soil and 1.39%-0.84 % for weak soil with an increase in Illite percentage. The study also developed an improved relationship for predicting resilient modulus based on CBR value, showing a strong correlation with equations developed by many researchers in the past.