Abstract: Under certain assumptions, a rigidity formula for a combined support composed of a segment and pea gravel is derived based on elastic strain theory for a thick-wall cylinder, proving a theoretical foundation for the application of the convergence-confinement method in the design of a shield type TBM segment structure. In order to further study the mechanical action of pea gravel backfilling layer in this combined support, three indexes (the stress sharing ratio Up, the deformation ratio of the pea gravel backfilling layer Uu, and the reduction coefficient of the combined support rigidity K) are defined. The case study and parameter analysis suggest that: 1) the grouted pea gravel backfilling layer could neither effectively bear the pressure nor absorb the deformation of the surrounding rock, and its main function is to transfer the pressure and deformation between the surrounding rock and segment lining; and2) there is a critical elastic modulus for the backfilling layer, which leads to abrupt changes to the combined support rigidity, while an increase of the backfilling layer′s thickness increases rigidity of the combined support.