Abstract: A portable cofferdam and method for use in stabilizing the subsurface of box culvert structures. The portable cofferdam is inserted inside the tubular section of large box culverts, which are often used as road crossings over streams, creeks and small rivers. The portable cofferdam is designed as a chamber having an open top end and open bottom end with a floor seal to engage against the box culvert floor. Once inside the culvert, the chamber floor seal is compressed against the culvert floor by an expansion device which extends against the culvert ceiling. Once a proper seal is created against the culvert floor, the chamber is dewatered. With the culvert floor exposed, a hole can be drilled through the culvert floor bottom and grout or other hard curing material is pumped into the subsurface void for stabilization of the box culvert structure.