Abstract: Apparatus and method for constructing an impervious underground wall in which an initial excavation, filled with an excavating slurry, is made having a length L.sub.1. A milling excavator is lowered to the bottom of the initial excavation in a collapsed condition and then expanded outwardly by a linkage mechanism in the direction of the trench portion, transmitting a reaction or bearing force to a wall of the initial excavation to excavate from the bottom upwardly. The cuttings fall to the bottom of the trench and are removed by suction or an air lift. The initial excavation is thus expanded laterally from the bottom up with the excavation forces from the excavation being transmitted to the opposing wall surface. Precast concrete panels having convave sides, one concave and one convex end, are inserted into the excavation and have formations on the end to interlock with one another and serve as an excavating guide for the excavating tool.