Soil Moduli

-When are we to use Secant modulus and Tangent Modulus?
-Please where can I get any help texts or papers on secant and tangent modulus?
Geology Geophysics Mining Oil and Gas Rock Mechanics Soil Mechanics / Geotechnical Analysis Computational Geomechanics Engineering Geology Environmental Geotechnics Geotechnical Earthquake Engineering & Soil Dynamics Landslides - Slope Stability Other Shallow Foundations Structural Geology / Geomorphology Tunneling & Underground Construction