Hi,

When modeling air packets, you should think about what will happen with these air packets.

1. Are these compressible, but the air cannot flow out? This means with compressing the packets, the stiffness should increase. If the air can flow out (this cannot be simulated in PLAXIS yet), it means that the air packet should have a very small stiffness.
2. Do these air packets add to the shear stiffness? Or are these air packets only loaded under compression? If they are loaded only under compression and the air can flow out, a linear elastic material would suffice. But since bulk modulus and shear modulus are coupled via Hooke's Law in the linear elastic material, this might not give the correct behavior. 

For more details, our experts are ready to help you here:

https://apps.bentley.com/srmanager/ProductSupport 

Kind regards,

Micha van der Sloot
PLAXIS | Bentley Systems, Geotechnical