Integral Equation based MoM for Planar Circuits

I have a Question on simulating a grounded substrate via using MPIE and solving with MoM. How is the z-directed basis function actually "grounded"? Is it just taken care of with the planar satisfied medium Green's function? I have a simple code and when I just add a z-directed basis function that touches where the ground is specified, my results indicate that it's actually still an open and not grounded. Any insight is appreciated. Thanks.