STUDY ON DRAINGE CONDTION ON SHEAR STENGTH PARAMETERS OF EXPANSIVE SOILS IN WOLAITA SODO

Abstract

Expansive soils, commonly known as black cotton soils, are high volume change soils in which montimorillonite is the principal clay mineral. These soils cause tremendous damages to structures and, in this country; they are undoubtedly the most problematic in civil engineering constructions.In the present research work, an attempt has been made, through careful laboratory experimentation, to establish the shear strength characteristics on this expansive soil with different drainage conditions. The triaxial apparatus has been used extensively to come up with some results. In addition to the above results obtained, direct shear test results have also been used to make correlation between the index properties and shear strength. The laboratory studies have shown that most expansive soils are highly expansive, possesses lower shear strength at peak values and most of all their consolidation behavior is hard to predict with the common triaxial apparatus. It has been established that these characteristics are mainly influenced by the volume change during saturation stage (type and amount of clay minerals present) and physical state of the soil. The plasticity index and the dry density have been found to be useful indices for evaluation expansive soils. To this effect normalized shear strength was used to make correlation with plasticity index resulting in unexpected and unrelated outcome. To carry out this present project works three places were chosen, known for their Expansive soil behavior. These are Gilgel Gibe Hydro power sub station Project found 180 km south of the Arba Minch; Project found 20 km west of W/Sodo town. According to the outcome of the ptoject, the shear strength of expansive soil ranges from 22 - 83 KPa in cohesion and 12 – 39 degree in friction angle in the CU tests. While in UU tests for cohesion it ranges from 87 – 275 KPa and internal friction of 5 – 24 degrees in the unsaturated sample. For the saturated sample (with some silt and sand) in the UU test the cohesion ranged from 19- 155 KPa and 10 – 23 degrees for the internal friction. There is a dramatic decrease in strength in the saturated samples, which shows that the suction pressure can have major influence in expansive soils. For the CD test there is no cohesion but rather internal friction up to 38 degrees was obtained.