The invention is a watertight joint seal for precast concrete box modules used to form underground enclosures to control the discharge of water runoff from roads and parking areas. The modules become sections of a tunnel structure, and the joint seal of the invention is constructed by forming the end of the walls of each module with mating offset inside and outside edge surfaces that are joined by an intermediate surface, applying sealing strips to the inside and outside edge surfaces of one end wall, compressing the modules against each other, and injecting an expansive grout into the cavity formed by the sealing strips and the modules' end surfaces. After the grout cures under the pressure resulting from its own expansion, the joint is watertight and does not suffer from leakage through cracks caused by the creep and relaxation characteristics which generally occur in joints formed with conventional grout or only rubber gaskets.