The decrease in shear strength to soil is the cause of many geotechnical problems, especially in clay soils, which may occur due to changes in some soil properties, such as moisture content. Changing the values of moisture content and dry density are important factors that have a clear impact on the shear strength of the soil due to the sensitivity of the soil to such changes, especially expansive clay soils, which requires the study of these factors and their impact on shear strength. The present research aims to study the effect of changes in compaction properties (moisture content and dry density) on the values of shear strength parameters (cohesion and internal friction angle) using direct shear test of expansive soil. Clay soils were selected from the city of Mosul with high expansive properties, and within two sets of tests with a total of thirty compact models. Five percentages of moisture content range between(%32–% 17) With five percentages of dry density, which ranged between(13.7- 17.7 ).
The results showed that the shear strength values decrease with increasing moisture content and increasing with increasing dry density at each vertical load . From the results obtained, the cohesion values decreased by increasing the moisture content when dry density was constant and at its lowest value (12.75 ) at moisture content 32%. As for the values of the internal friction angle, they decrease by increasing the moisture content when the dry density is constant and by the lowest value for the angle of internal friction (23.30) at the moisture content 32% and the dry density 13.7 . It is noted that the values of both cohesion and the angle of internal friction increase with increasing dry density and with the highest value of cohesion ( 117.01 ) and the internal friction angle (450), respectively, at a dry density of 17.7 and a moisture content of 17%.