Compacted soils can change volume when wet or loaded, causing issues in engineering projects. Existing theories for unsaturated soils are incomplete, assuming soils are elastic when they are actually elastic-plastic. This study introduces a new method to calculate excess pore water pressure in unsaturated soils under undrained conditions, treating compacted soils as elastic-plastic materials. This method allows for the calculation of one-dimensional consolidation in collapsible soils, providing solutions for excess pore water pressure, settlements, and consolidation process. The theory can be applied to unsaturated collapsible soils under isotropic loading, with a real collapsible soil used to demonstrate the approach.