In bedrock including expansive clay minerals, the pore fluid composition affects the mechanical behavior of the bedrock such as swelling. It is necessary to elucidate the mechanism of the behavior and evaluate the deformation caused by the swelling quantitively because the swelling behavior of expansive bedrock causes problems when tunnel excavation and earth cutting. To simulate the issues regarding swelling behavior, a constitutive model that considers the electro-chemo-mechanical phenomena on the surface of expansive clay mineral crystal, stiffness of bedrock, and the degradation of the internal structure is required. This research proposes a new model targeting expansive bedrock by means of combining an elastoplastic model considering electro-chemo-mechanical phenomena with the Cam-clay model introducing cementation and its degradation due to plastic deformation. The proposed model describes the process of degradation of cementation due to the swelling caused by the electro-chemo-mechanical interlaminar action occurring between crystal layers, which can lead to the transition of a rigid rock mass to soft soil. Tunnel excavation and swelling analyses are demonstrated by incorporating the proposed model into a finite element analysis code. The simulation results indicate that the proposed model is applicable to the geotechnical problems occurring in expansive bedrock.