Belgian method (or also called Classical Madrid method or technically key gallery method[1]​) is a method for the construction of tunnels. It is characterized by the progressive excavation of the elements that make up the tunnel, in such a way that the most stable elements of the tunnel are removed, avoiding collapse or lack of stability of the front.[2]​ The method is called classical Madrid method because it is the most used method in the construction of Madrid metro tunnels. It was used in the first stage of construction of Line H "Line H (Buenos Aires Subway)") and in the last extensions of Line A "Line A (Buenos Aires Subway)") of the Buenos Aires subway.[3]​.

The first excavated element is the tunnel vault (it is usually called vault advance or shell). The vault is supported on the ground by a progressive wooden framework. The vault is secured with formwork and when it is secured, the lower part is excavated as progress is secured. In this way, the gallery is built as progress is made without putting workers at risk due to the collapse of the tunnel. This method has been used in the construction of different subway tunnels in the world, such as Madrid and Chicago.