The study aims to investigate the strength characteristics, as represented by shear parameters, of various soil types that pose numerous engineering problems in and around Mosul, Iraq. Five soils, including low-plasticity clay, high-plasticity clay, silty organic soil, fill organic materials (10.2%), and gypseous soil (63.4%), were selected for the study, having serious geotechnical problems like swelling, collapse, high organic content, and high gypsum content. The testing program includes direct shear, residual shear, unconfined compression, oedometer, chemical tests, and the index properties of the study soils. The results generally showed that direct shear tests yield strength parameters that lead to overvaluation of bearing capacity. In contrast, unconfined compression tests yield more conservative results, and residual shear tests yield the smallest values due to the test conditions. The results indicated that shear properties can vary significantly across test methods, highlighting the need for careful interpretation of bearing capacity values. Taking safety considerations into account, the bearing capacity may range from 50 to 150 kN/m2. Also, for the design of shallow foundations on Mosul soils, any available field or laboratory strength test for soil should be used in combination with tests that identify geotechnical issues in the soil, relying primarily on the test that yields the lowest value—or at least a reasonable value with engineering judgment—for the strength parameters.
