The expansion of shocked interstellar medium around an OB association is an example of the astrophysical blastwave (Ostriker and McKee, 1988). The thin, cold, shell-like structure expanding supersonically accumulates the ambient medium. We assume that the shell has the zero thickness at radius of the shockwave rs and that it expands at velocity vs. The shell is divided into elements described with equations of motion: d/dt(Mv s ) = dS[(P–Po ) + novo (v s –vo )]–Mg(R,z), where M and dS are the mass and the surface of it, P, Po are the inside and outside pressures, no,vo are density and velocity of the ambient medium and g(R,z) is the gravitational acceleration in the Galaxy. R, z are the galactocentric cylindrical coordinates. The mass of an element increases as long as the expansion velocity component normal to the shell, v⊥ exceeds the velocity of sound in the ambient medium M = v ⊥ nodS. The 3D model using the above infinitesimally thin shell approximation was developed by Palouš (1990, 1992) and Silich (1992).