The SOFT=2 contact uses node numbers to identify the elements that are attached to each segment to get information like the thickness and density. When solids and shells share nodes, the default behavior is that only the shell segments are found. If the shells erode, then the segments are deleted and the contact with underlying solids will not be detected. If I2D3D on the *CONTACT optional card B is set to 1, then only the solid elements will be found. Of course now the shells will be passed through and contact will occur later with the solid element unless you give the solid segments some thickness using SLDTHK which is also on optional card B. Another thing one could do when SOFT=2 is define one interface that includes the shells and a second that includes the solids. This way you the contact segments on both shells and solids would be found and contact with the solids will be detected after the shells erode.