We numerically minimized the Landau-de Gennes free energy to investigate the layer structure of the twist-grain boundary (TGB) phase of chiral liquid crystals. We compare the obtained structure with Scherk's minimial surface. For large twist angles, an unlocking of the director and the layer orientation reduces the effective layer bending rigidity, which causes a large deviation of the mean curvature from zero. At temperatures close to the TGB-cholesteric transition, we find a melting of the smectic layers in the whole grain boundary.