This study presents the results of investigations and analyses for two failed slopes and one proposed embankment slope involving clay shales in Iowa. The in-situ Borehole Shear Test (BST) was the primary test method for obtaining the soil shear strength parameter values in the investigations, which were supplemented with laboratory ring shear tests for the three slopes and other laboratory tests including direct shear tests for the embankment slope. The limit equilibrium method was used for the slope stability analyses. The major findings in the study include (1) the estimate of the mobilized shear strength parameter values for the slope failures could be improved by considering the BST measurements compared with empirical method of using "good engineering judgment or experience"; (2) geotechnical information including in-situ BST measurements was effective in the characterization of the weathered shales with emphasis on the different weathering grades for slope stability analyses; (3) the relatively large amount of in-situ shear strength parameter values was particularly useful for probabilistic slope stability analysis, which provided a check and comparison to the probabilistic analyses using conventional shear strength parameter values of indirect field measurements or laboratory measurements.