Soil at rest has a lateral factor of k(o) associated with it. How much movement (or soil strain) of a retaining wall is required to change the factor to k(a), assuming that the soil is granular. Or are there other factors to take into account?
Check out elementary soil mechanic books - they have a graph, usually, showing the deflection/height required to cause "movement" in the k values. Sorry, but my books are packed away as I'm moving.