Dear shruthip,
1. please see ATENA Troubleshooting, 2.2.8 How can I include the influence of shrinkage? for basic information about representing shrinkage in ATENA. The document is included in ATENA installation and also available for separate download from our web (Products - ATENA - Documentation).
I suspect if the spatial differences in shrinkage (gradient) are important, you probably need to use ATENA Science to run your analysis using the Creep(+Shrinkage) module?
2. For basic information about Springs and Interfaces, I suggest to see the 2D User's Manual, 4.4.3.1 Spring Material and 4.4.3.2 Interface Material + Troubleshooting, 2.2.14 I have problems when using the Interface (GAP) material on contacts.
3. The simplest way to represent the soil under a slab is a surface spring with low tensile striffness - see Troubleshooting, 2.2.16 How can I model a compression-only support with nonlinear springs?
4. A more detailed (and complicated and labour-intensive) solution is to model some close part of the soil with volume elements (elastic or Drucker-Prager or possibly another material model), the "far" with springs (or just a normal support), and interface elements between the slab and the soil volume.
Regards.