Contact between two surfaces in Abaqus

Hi,

I have made a 2d model between foundation and soil in Abaqus 6.10. The foundation (concrete) is 1 m in length and 0.1 m thick whereas the soil is 30 m in length and 30 m in width. The soil is considered undrained saturated clay which means poisson's ratio is equal to 0.49 (i.e 0.5). The boundary conditions are FIXED for the 3 sides of soil except the top. All sides of foundation are free and no boundary condition is applied. The boundary condn gets activated in INITIAL step. A purely vertical load is applied on the top of foundation which gets activated in 2nd step.  The problem I am facing is that of displacement increment. It says that displacement increment is too large. This implies that there is some problem between the contact. How can I troubleshoot this problem? The contact I have used is frictionless (tangential behavior) and hard default (normal behavior). I used job diagnostics also. I see that maximum displacement increment and displacement correction are at the nodes which are at extreme right or left of the contact surface between foundation and soil. Given below are the properties of soil and concrete.

Concrete

Elastic E=60 GPa v=0.15

Soil

Elastic E=60 MPa v=0.49

Mohr Coulomb Plasticity Friction Angle=Dilation Angle=0 i.e. associated flow. Cohesion=100 kPa Abs plastic strain=0

In the message file, I am also getting numerical singularity and negative eigenvalue.

Thanks.