Abstract: The fundamental drawbacks and their disastrous consequences of the current geotechnical safety design theories for slopes are pointed out, analyzed and evaluated. The drawbacks are as follows: (1) The shear strength parameters of soils (effective cohesion and angle of internal friction) are assumed to have constant values in the design theories. However, they are changeable and can be reduced to zero due to the progressive tensile deformation and increase in void ratios and pore-water contents in the slope soils. (2) The classical theories of soils (such as the effective stress principle and soil consolidation theory) are valid only under the condition of compressive-shearing loading with reduction of soil voids and pore-water contents. They are not suitable to the condition of tensile-shearing loading with increase of soil voids and pore-water contents. This condition is actually what happened during the process of slope failure and landslide. (3) The factor of safety is a ratio of the soil shear strength to the downward sliding shear stress. As the shear strength of soils is small, its increase according to the ratio is very small, which cannot make substantial improvements to the soil quality and resistance to failure. Accordingly, failures and landslides in engineered slopes can commonly occur around the world although geotechnical engineers put tremendous efforts in preventing and controlling their occurrences. The classical soil mechanics is only applicable to the soils subjected to complete compression and shear loading condition, and is a completely compression-shear soil mechanics theory. A new soil mechanics and new geotechnical design methods applicable to soils subjected to either the compression-shear or tension-shear loading conditions shall be developed.