Sealing gaskets made of thermally expanded graphite are widely used, as they provide high tightness and reliability of detachable joints of various equipment in a wide range of temperatures and pressures, and are resistant to aggressive environments. However, thermally expanded graphite gaskets can collapse or squeeze out during operation due to their relatively low mechanical strength, especially when sealing large-diameter joints. This disadvantage can be eliminated by using combined (semi-metallic) gaskets based on thermally expanded graphite. In combined gaskets, thermally expanded graphite is in a complex stress-strain state, which does not allow the use of existing methods for calculating detachable joints based on the results of uniaxial compression with unloading tests. This work is devoted to the development of a modified Cam Clay model for thermally expanded graphite, which can be used to model detachable flange joints with combined gaskets based on thermally expanded graphite. As a result of numerical simulation using the finite element method, the parameters of the modified Cam Clay model for thermally expanded graphite were determined, ensuring a good correspondence of the calculation results with experimental data. In the future, the results obtained will be used for numerical modeling of combined seals based on thermally expanded graphite.