what is shear strength of a soil?

Answer 1

There are tables in reference books that give specific values. As the previous answerer said composition is key. If there is a lot of organic matter connecting the soil particles (like in clay) then the soil is very resistant to shear. Sand on the other hand has little connectivity and can easily slide apart. The worst is quicksand where sand particles are lubricated by water between them and slide apart very easily.

Answer 2

I don't really know, but I would imagine that it would range significantly with the composition of soil. Think about it... sandy soil verses a soil with a high clay content.