fix check/timestep/gran command

hi all,

dt_h = 2.87*(m_eff^2/(r_eff*Y_eff^2*v_max))^0.2

I understand the concept/calculation of effective mass,radius and young modulus for particle-particle.
Eg: 1/m_eff = 1/m1+1/m2; (m1, m2 are masses of two particles).

How is liggghts code calculating for particle-wall? What can the mass of wall be? Is it going to be infinity?

Please help.
Thank you.