There is sometimes confusion (or unwarranted conservatism) over factor B in the penetration resistance calculations required by AS 2885 and described in Appendix M. I look at it this way:
- B = 0.75 provides the borderline between penetration / no penetration if the excavator is being operated in a normal responsible manner, and is fine for most situations.
- B = 1.3 is the limiting condition for the worst possible case; penetration should be impossible regardless of how hard the excavator operator tries.
Table M5 in AS 2885 uses different words but says much the same thing.
The paragraphs following Table M5 explain where the B values came from. I was involved in the second of the field trials mentioned, when a 36 t excavator assaulted a DN 500 x 12.7 mm WT pipe – quite entertaining. Watching it gave me confidence that a 36 t excavator cannot penetrate 12.7 mm pipe, because I saw the vigour of the attack and the numerous attempts from different angles. That gave me an appreciation that doesn’t come from reading the dry descriptions in Table M5. Unfortunately only a few of us shared that experience, but I hope that by reiterating it here other pipeline engineers will overcome the fear that any B value less than 1.3 will result in punctured pipes all over the place. Even B = 0.75 provides a great deal of resistance to penetration, it just isn’t quite absolute.
Here are a few pictures from those trials (12.7 mm WT X42, 36 t excavator):
It’s not easy to put a hole in thick pipe.