Mar 07, 2011 I am trying to evaluate the internal angle of friction of crushed limestone as a backfill material (max. particle size of 25mm). I had 30cm x 30cm (1ft x 1ft) direct shear box tests on the material. The angle of internal friction came out pretty high (65 to 80 degrees). We are testing at very low normal stress ( 10 to 50kPa)