Jon said:
I just want some history \ context about bolt failures.
With the first few uses of SRT it quickly became apparent that one needed to obtain better hangs than could be obtained using natural belay points due to rope rub. So anchors were placed. A popular version was the spit which required you to take your own hanger and bolt to screw into each placed spit. (Plus in the pre electric drill days, spits were simple to place with just a hammer.) As I recall, a book was written on a set of popular routes using red headed spits. The problem then arose with the popularity of these routes, when cavers using dirty threads on their bolts, the thread in the spit became unusable. That lead to other spits being placed near by hence giving rise to a rash of spits.
Les Sykes, Glenn Jones and others sought a way around this problem and with the help of a Hilti rep found in the early 90s the DMM Eco anchor together with a resin would work in cave. The technique was 'exported' to other regions and adopted by the then National Caving Association (NCA). The technique was handed onto BCA.
In the mid 2000's DMM gave up making Eco anchors. After a fair amount of effort by Les and others, the Bolt Product (BP) resin anchor was chosen as a replacement. However, the twisted legs of a BP anchor fill the hole in the rock. When there was a question over an installed Eco anchor, one could just drill down each side of the straight legs mostly within the resin and get sufficient reduction in strength that one could then extract the Eco anchor albeit with some effort. There is still an unproven concern that a BP anchor will require a larger hole which is worrying on the basis of conservation of belay points. So Simon Wilson came up with the IC anchor. There has also been a suggestion to use titanium anchors but the experience has been mixed and that is disregarding their price.
There has been changes in the resin used over the years and yes there has been a number of concerns raised over certain anchor being loose. In a fair number of cases the concern was due to the person failing to take a true measure of movement, instead sensing from the movement of their fingers that the bolt was loose, rather than that their fingers were flexing. A few were replaced.
If you think an anchor is loose, then please report it ASAP to your regional caving council.
It is worth noting that the bonding of a resin anchor is not just based on the chemical bond of surface of resin to rock and resin to anchor but also on the mechanical interaction of the resin with the features of the anchor and also with the irregularity of the hole in the rock. (You do not use diamond drills to place anchors!)
Eco, BP and IC anchors are made from 316 Stainless Steel (SS). That was chosen to avoid the potential for Chloride Stress Corrosion Cracking (CSCC - not the region) in UK caves. However there have been a few cases in high temperature countries where SS bolts on sea cliff climbs have cracked. There has also been a case or two where poorly welded SS anchors have cracked under what is thought to be CSCC. There was one case in the UK where the location was found to be flow stone over mud over rock - so the surface flow stone cracked when it was tested.
There is a large amount of information at
https://cncc.org.uk/fixed-aids/ ,
http://british-caving.org.uk/wiki3/doku.php?id=equipment_techniques:anchor_scheme and
https://www.thebmc.co.uk/articles/tag/bolt%20technical . Simon also reported his work at
http://www.resinanchor.co.uk/ .