Dear sir...I have done a similar test on the rock joint samples. Moreover, on of my colleagues has carried the test for soil sample. The outcome is immensely influenced by the normal loading applied. Under the lower normal loading the effect of shearing speed has no significant effect on the shear strength.
Thanks for the research papers. One of your papers - Shariatmadari et al (2010) highlighted that increasing shearing rate from 0.8mm/min to 19mm/min at direct shear test shows 35% increase in shear strength. Interesting.
The effect of the shearing rate on the residual shear strength of soil can be very significant. It depends a lot on the soil mineralogy and normal stress.
Article Fast shearing of pre-existing shear zones in soil
Article Shear Strength of Clay at Different Shear Rates
Article Influence of Displacement Rate on Residual Shear Strength of Clays