The failure was undoubtedly NOT in the bolt, rather in the epoxy attachment to the concrete. The 1.5X rule is valid for a threaded fastener into similar materials (steel into steel), but in this application, it does not apply.
I would be interested to know if the whole epoxy plug pulled out of if the threads in the plastic pulled out of the hole. The issue here is not the length of the bolt but the strength of the plastic threads and the strength of the glue to the concrete and dirt. Hind sight being 20/20 I think Studs should of been placed in the concrete.
JB Weld is great stuff,but I do not think it wise in this situation.It may very well have been a stronger epoxy,I was being sarcastic.
Standard Heavy Hex Nuts are one bolt diameter thick. Bolts used on many high pressure connections are considerably higher strength than the nuts. Standard tests require the bolt to fail in tension before the threads between bolt and nut strip. In applications I am familiar with the stud bolts were threaded into the body of a part one diameter deep. The body material strength was somewhere between that of the bolt and the nut used on the other end of the bolt.
The above has nothing to do with the big gouge problem. Just pointing out that there are many "rules."