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The Fernhill Project - Part II
By Tony Audsley

 
(Continued from BB 530).
By May, the bottom of the concrete pipe shaft had been secured and the diggers were ready to break out of the bottom of the shaft and get into the Fernhill bedding-plane.  However the way on into the top of the bedding-plane was blocked by some massive stal and this needed removing before we could get in.  The nearness of the pipes and the general fragility of the surroundings meant that extremely namby-pamby charges had to be used, but because the angled shaft behaved like a cannon and directed the sound waves at the opposite quarry face even these charges produced some very satisfying reverberations.  Gave the climbers something to think about anyway.
After a few such sessions, the bedding-plane was a comfortable digging width and a few evening's worth of hauling had generated a working space sufficient to accommodate two or three people, or more if they happened to be particularly friendly.  At this stage, the ungrouted material around the outside of the base of the shaft, i.e. above our heads, looked a bit iffy, but we thought that it would be good enough for now and we could deal with it later if necessary. 

And then, over the May bank holiday, it rained. Good honest bank holiday type rain.   The floor of the quarry was flooded and the clay and rock backfilling round the concrete pipes turned into porridge and slumped, completely blocking off the digging space below the pipes.  Attempts to dig out the slump failed, for as fast as it was removed, more material fell down from behind the pipes.
So what to do?  Well, there are probably more solutions to this problem than there are diggers, but the one used did actually work surprisingly well.  Originally, I didn't think that there was much point in outlining it in any great detail, as it seemed a one-problem technique and not one likely to be used again.  So much for thought. 

One of the nice things about digging is that you never know what the ground is going to throw at you next and recently, Home Close Hole came along saying "dig me".  So having dug it a bit and then installed a concrete pipe shaft, we now come to tragic bit ... there is nothing solid under the pipes.  The bottom ring of the Home Close Hole shaft sits on semi-liquid goo, with any solid rock being some distance away to the sides and an unknown distance away below.  In other words, the situation at the bottom of the Home Close Shaft is similar, just a little bit worse, than that at the bottom of the Fernhill shaft.   So, the technique is likely to get used again and with that as an excuse, I'll witter on about it for a bit.

The requirement at Fernhill was to push through the collapse material to reach the rock wall behind and then to drill about four inches into the solid.  The drill bit would then be extracted and replaced with a length of bar.  This done, the whole would then be moved sideways a bit and the process repeated (ad nauseam).  Not difficult in principle, but there are a couple of practical problems.  The first being to locate the end of the drill so that it doesn't skid all over the place before the hole get going and the second is to locate the hole once it has been drilled and to poke the bar into it.
The solution arrived at is the ACME 'Miracle' Drilling Jig, a wonderful bit of kit made up entirely of bits of scrap iron that were lying around the back of the Belfry.  (Tidy minded Belfryites, please take note, you never know when such stuff will come in useful).

The 'AMDJ' clamps under the bottom concrete ring and is held in position by wooden wedges and supported by a car bottle jack underneath.  The AMDJ's tube is then hammered through the fill until it reaches the rock wall.  The clamps are tightened, the drill bit is inserted and the hole drilled to the required depth.  The drill bit is then withdrawn.  At this point, the purist will insert a length of plastic tube and blow down it, thus getting a eyeful of limestone dust, but leaving a beautifully clean hole into which the bar can be poked. 

Next comes the tricky bit.  The bar is poked into the hole then held in position while the outer tube is withdrawn.  The sticky-down bits of the AMDJ are unbolted; the AMDJ is hammered sideways to the next position, assembled again and the process repeated.  By the way, if you think that this description is tedious, you should try doing it in practice.

This is all very good in theory, but what happened in the cave?

The initial work was carried out over two days, 1st and 14th July.  On the 1st, the team was Mandy Voysey, Alan Gray, Rich Witcombe Alice and myself.  We ferried what seemed like hundredweights of kit (generator, drills, steel bars, angle grinders, the AMDJ, eats, drinks, etc.) over to the shaft and then there was nothing for it but to start work.  I went down to play with my toys and the rest of the team variously tended the generator, planted trees, sunbathed and discussed the merits of Darwinism / Creationism.  (Honestly, it was good to be underground).

The second daylong session was somewhat similar, except that Clive North replaced Alan Grey and I forgot to bring the drill, which delayed the start a bit.  By the afternoon, the bars were in position and they supported the fill sufficiently to dig out enough of the collapse material to lie underneath the bars and work some steel lagging (cut up lengths of 'trident' fence posts from the Belfry digging store) on top to make a permanent support.

The evening of 16th July saw the removal of 32 loads of clag and the installation of a permanent support to hold the bars in position.  This was followed by another all day session on Monday 21st July, when we were joined by Jrat, on what I think was probably his last active digging session. 

Twenty six more loads came out, sufficient to make a Witcombe-sized working space, so he was pushed in to the hole, fed cement and stones and told to get on with wall building to meet up with the right hand edge of the steel-work.  Meanwhile, the rest of us had a pleasant day on the surface, sitting in the sun drinking Jrat's beer and chatting, while Jrat wrote up his digging log.

Right - Looking up the bedding-plane at the shoring, the bottom concrete ring and Alan Gray.

Digging could now restart in earnest in the bedding-plane, with wall building and cement shoring taking place on the left of the bedding plane and the exposure of a void on the right hand side.  The void itself is fine, but the general quality and arrangement of the roofing material leaves something to be desired.  Rich Witcombe has inserted timber, steel and cemented walling under the most offensive candidates, but a glimpse of the rest of it makes Home Close Hole seem ever so attractive, so I'm keeping out of the way for a while. 

Diggers and esteemed visitors May - October 2008

Alice Audsley, Alan Gray, Alison Moody, Clive North, Dave King, Duncan Price, Fiona Burchell, Geoff Dawson, Kate Lawrence, Mandy Voysey, Mark Lumley, Martin Grass, Matt Voysey, Paul Stillman, Pete Moody, Rich Witcombe, Rob Taviner, Steve Shipston, Tony Audsley, Tony Jarratt, Tony Littler.
26 November 2008