Cambridge Underground 1983 pp 53-55


Dave Brindle

An unfortunate consequence of being a final year engineer in Cambridge is the necessity of doing a project in the Lent Term, which counts towards the degree one is (hopefully successfully) working for. Like most people, I wasn't looking forward to doing this at all, because the set projects usually had some undesirable feature, which could often be spotted from the title, for example:-

Investigate the drag on a sphere suspended in a fluid. There's probably a formula to tell you the answer to this problem, and you can never make experiments agree with theory.
Set up a computer simulation of a chemical plant ... Computing is white man's magic to me, and involves hours of typing at VDUs, generally at half one in the morning.
Analyse the resonance of impacting gear teeth. This looks very much like hard work!

Eventually I decided on what looked like a quick and simple project, but found that others had had the same idea - it had been taken a fortnight ago. However, the supervisor pointed out that there was the possibility of doing a "roll your own" project, if I could think of one that was acceptable. So, while thinking (of course) along caving lines, the idea of testing the strength of badly placed rock anchors came. This may not have been unconnected with the state of some of the bolts I had seen, and would rather not have seen, on expedition.

At first, everything went smoothly. The basic idea got the green light from the Department, so I wrote to Troll, and they were sufficiently interested to supply a large bag of bolts to be tested. The testing rig itself was simple, a frame of giant "Meccano" with a hydraulic ram (in JCB yellow) to pull out the bolts. With a capacity of 4OOOkg, I felt there was probably power to spare. This was set up in the Structures Research Lab of the Engineering Department. Obtaining the limestone to place the bolts in proved less straightforward, Cambridge being rather far from any caving area. I was considering organising a midnight rock stealing raid on the "Examples of Karst plants and landscape" in the Botanical Gardens, when at this point I had a lucky break. During an awkward pause in a Structures supervision, when it was becoming more and more apparent that I hadn't done any work, the rather one-sided conversation was diverted onto the subject of projects. I was expanding at length on my lack of rocks, so to speak, and it turned out that the supervisor knew someone who worked for Rattee and Kett, who are a local firm doing a steady business restoring the the many and splendid buildings in Cambridge, which being quite old are always falling down. A word to the wise is sufficient, and soon I was standing in Rattee and Kett's yard (surrounded by old church gargoyles and quarried limestone blocks the size of cars!) and asking if they had any smaller stones to spare. They did indeed have some handy sized pieces, already squared off, and these were later collected in a battered Transit.

The practical work now started in earnest. The first bolt was placed in a lump of Clipsham limestone, and the hydraulic ram connected. The bolt was covered with an old coat in case it shot off at high speed across the lab when it broke. As I started to operate the hand pump it felt rather like the party game where you have to blow up a balloon until it bursts in your face, a game I have always found unpleasant. As the load reached 1000 kg a small crowd gathered in anticipation. The failure was expected to be at around 1800kg, but at 1500kg there was a dull thud and the load dropped back to about 900kg. Something had clearly broken, but unfortunately not the bolt. I took off the coat to find that the anchor was still firmly in place, but the crab connecting it to the ram had ripped its gate out! I hasten to add that it was an very rusty steel one which had been found underground. A new one was put in, and the bolt finally went.

The next few weeks passed smoothly. I had fifty anchors to test, in eight weeks, and having done ten in the first week I thought I was not going to be pushed for time. All the bolts had to be placed by hand, and although I couldn't, for some reason, interest anyone else in lending a hand to drill them, it became quite therapeutic to sit and hammer for fifteen minutes, place the bolt, pull it out, and then drill another. A positive lack of enthusiasm was displayed by the other occupants of the lab. though, as the hammering resounded in the air and made conversation difficult. Apparently it could also be heard in the adjacent lecture rooms. Actually recording the results was easy, with no awkward calculations to be done, as the write up of the project was simply to be a tabulation of the measured strengths of each bolt.

There were some minor setbacks. Several more crabs broke, before the bolt they were supposed to be testing, and as this was getting tiresome a rather large alloy one not belonging to me was substituted. On one test in a very soft piece of rock, the anchor could be pulled out by hand. Then, while testing a bolt in a rather small lump of rock, the whole rock fell in half around the bolt. About this time I thought it would be useful to try some bolts in real Yorkshire limestone, and as it happened the following weekend CUCC were up north in the Dales again. On Sunday we were nearly stranded in the Hill Inn at lunchtime because of a freak blizzard, but several of us left our pints to battle with the elements outside, and we captured some rocks which were taken back to Cambridge in an old Land Rover. A star-drill I drew from stores to produce a countersunk hole was as blunt as a baby's bottom, and needed major surgery with a grindstone to make it work. And after every project session I was covered with rock dust from head to foot, and my clothes suffered accordingly. Finally however, with about a fortnight to go before the end of term, the practical work was all but finished, and as the results looked reasonable, I thought the write-up would go easily.

Of course it didn't, in fact it was the hardest part of the whole project. I had written several rough drafts in pencil, and intended to type it onto the IBM computer in Cambridge and print it on the Diablo, partly to right justify the text (for appearance) and partly to reduce Tippex costs (a RUBOUT key being much handier). At first I used the CUCC file-space but combined with the journal it started to overload. With about a week to go before I was due to go to France, and before the deadline for "Caves and Caving", I scrounged a bit of file-space for myself and moved what I had written onto it. On Sunday the computer was off and I paced up and down my room in frustration. Monday saw quite a bit done, but trouble with the software, which centres titles and right justifies the text. It does this by using symbols such as $>, $T, and # as flags. This caused endless problems, especially tabulating results. The columns of figures just wouldn't stay in line. Everytime a new copy was run off, I noticed something else that was wrong. Tuesday evening the computer went down again, I could hardly believe it. Things were looking serious! To make up, I had to work until two in the morning, something which I had not intended! The console room became deserted as I laboured on into the night. On Wednesday we printed some photos for the report. One of the negatives got scratched. Despite all this, on Thursday morning it was ready, or at least ready to be output. The Diablo printer took only an hour, and after a slight contretemps with the Xerox machine (no double siding except by hand feeding the paper and the auto-collating device was broken) all that was left was to stick in the photos and bind up the pages, a straightforward if tedious task. On Friday morning copies were posted to Troll and "Caves and Caving", and twenty four hours later I had made my escape to Chamonix. Troll were quite pleased with it, and the actual results are in "Caves and Caving". Now, all that remains is for the Tripos examiners to be favourably impressed.


Although the strength of each and every badly placed bolt was recorded, the individual strengths are less important than the general trends observed. The most common fault seen underground is the anchor protruding from the rock, because the hole has been drilled insufficiently deep. This is the most serious mistake that can be made, and underdrilling by only 2mm reduces the strength from about 2000kg to under 1000kg. This is because when the hanger does not rest on the rock surface, bending stresses are set up in the anchor, and it is relatively weak in bending. Inserting the hanger at an angle also reduces the strength to around 1000kg, as again the hanger does not rest flat on the rock. Careless drilling, which leads to the start of the hole being funnel shaped, is less serious as long as the amount of the countersink is under a couple of millimetres, as the hanger should still rest flush. Loading outwards does not reduce the strength of a well driven anchor, but should be avoided in case the anchor works loose. As the breaking load for rope and other gear is likely to be around 1000kg as well, a slight incorrect placement may not be disastrous, but by placing with care, the safety of the bolt can be considerably increased.

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