It may not help, but there has been quite a lot of work done on STABL by professors at UW Madison WI.
Also, I see that the US Dept of Transportation, Federal Highway Administration has a publication Jan 1985 FHWA -TS- 85-212 on STABL4, apparently written by Turner-Fairbank Highway Research Center, 6300 Geoegetown Pike, McLean, VA 22101. Since this was a government work job, maybe it is not restricted as to giving out information.
With regard to Purdue, my old stuff shows that a IBM/PC compatible source code for STABL13 can be had from Civil Engineering Shareware, P.O. Box 472, Lee's Summit, MO 64063 for $75.
Good luck