Flat cross-section tunnels became popular in the mountain road tunnels and their cross-section area also became larger. Flat cross-section tunnels with large cross-section area are necessary to consider about the heaving. The heaving sometimes occurs in the expansive natural ground and causes severe issues in the tunnel constructions. Although the invert and the ground improvements are usually installed against the heaving, the effective countermeasures have not been established. In this study, the shapes of tunnel linings with invert were verified by conventional compression tests, and the importance of invert and the effectiveness of ground improvement methods were discussed through the model tunnel tests simulating the heaving behavior. Consequently, the ground improvement under the invert was effective against the heaving.