by Extremeophile » Jun 6, 2011 9:55 am
There are clearly a large number of people with a bias towards using nothing but 11 mm (e.g. PMI Pit Rope), but I suspect this is largely due to familiarity and inertia (i.e. if it's a system that works then why change). If I were caving only in TAG and driving up to entrance pits on the weekends then I would probably exclusively use 11 mm PMI or the like. It's reasonably priced, has a healthy safety margin. works well with most descenders, will last a long time, and won't be questioned by other cavers using it.
For push trips deep underground, where bulk and weight are critical, I believe rope as thin as 8 mm can be a safe alternative. It's true that when you take into account rope age, knots, carabiners, etc, the original rated strength is significantly reduced. Perhaps an 8 mm rope rated at 3,900 lbs is reduced to 2,000 lbs. A 200 lb caver is unlikely to generate forces exceeding 250 lbs during descent or ascent. This is still an 8x safety factor. The arguments for care on rub points should apply to any rope, regardless of diameter. I've read many stories about ropes being nearly cut through, but I've never read about a rope breaking from exceeding its tensile strength. I'm not advocating 8 mm rope for permanent rigging, rescue use, or for inexperienced vertical cavers, but I strongly believe it can be safe option for experienced cavers doing push climbs in remote areas. As with any new piece of equipment, practice under controlled conditions is wise.
Maybe the key point is that 8 mm can be very safe in the right hands and 11 mm can be extremely unsafe in the wrong hands. The overwhelming majority of vertical caving accidents are due to judgement error, not equipment failure. I'm not suggesting everyone switch to thin rope, but I don't think people should simply dismiss the use of light weight equipment as "unsafe". Being on a 20 hr trip with someone carrying 200' of 11 mm rope through crawlways and narrow canyons has its own safety risks before any rigging occurs.
Derek