Two spalling mechanisms are generally quoted in the literature. The first one is due to pore pressure buildup. The second one, which is analyzed in this paper, is generated by restrained thermal dilation. We seek to model the channel tunnel fire by a thermochemoplastic constitutive model. A simplified analytical approach allows us to express mechanical variables as stresses and strains near the heated surface (the concrete wall). This study leads to introducing a "plastification" temperature and to deduce a plastification depth that can be useful for determining the spalling localization. A comparison between different cases (with and without chemical softening or decohesion) shows that thermal spalling is due to chemical decohesion (strength degradation) and not to chemical softening (rigidity reduction).