incorporation of cracks in ansys

Dear all,

I am using ANSYS 12 for analysing nonlinear dynamic  behaviour of concrete gravity dam.
Due to chemical components in water, in the long run, the dam has developed strucutral cracks. So I need to incorporate these cracks (may be 5 % or 10 % of total contact surface upto a certain depth into the body and of certain length) and do the analysis and find the difference in mode shapes, stresses etc.
I am new to Ansys usage, so please help me how to accompolish my work.

Thanks,