Spanish code NBE AE-88 recommends angle of internal friction be 30 deg (sexagesimal) and friction to concrete wall 2/3 (maximum) of this.
I think to remember AASHTO (or was other code?) limited the internal friction angle to 34 deg.
The friction factors that AASHTO table 5.5.5B gives for friction soil-concrete for sound rock, gravel etc range from about 30 to 35 deg, so it would imply somewhat higher angles of inner friction (except that friction at the interface is assumed go as high as the internal friction angle), but maybe not as high as at the 3/2 factor higher derivable from the NBE AE-88 limit for friction at the interface.
Real angles of internal friction should be in the 45 to 51 deg range for rough and dry broken sharp stones, I think to remember.
