Considine's Cave Dig (North End)
Alternate name :-
M 10940 x 00277
510925 x 700307
Total Man-Hours at the site = 1061 = 151 separate visits.
Considines Cave is an elongated rift that normally takes a very minor stream, which, after rain, can become a cataract. To facilitate spoil removal from such depths a level platform of twenty square metres has been installed with associated tramway and hauling infrastructure covering, virtually, the entire extent of the entrance. This initial effort is pursuant to normal P.C.N. practice; to create the most efficient work environment in which a small team can function and achieve results. The lengthy preparation period of the site has indeed increased efficiency of spoil removal and significantly enhanced safe working practises both above and below ground.
During the initial excavation the access ladder was fixed to the South end shoring as shown in the earlier photos on this page. The ladder now divides the shaft into two areas, one for hauling and one for climbing. This provides some protection in the form of a brattice should anything be dropped down the hauling shaft; the digger always taking refuge in the climbing area during hauling. The loose fill in the South end of the rift is held back by one metre lengths of telegraph pole placed just beyond a widening of the rift. Each new piece of pole being inserted from the bottom as the dig gets deeper. The North end is much narrower so is not a problem. The above two photos are taken from the hauling side of the ladder.
The "Plank" at -14 metres. CC about to descend to the dig site.
View from the "Plank" to the dig site 9 metres below.
South rift shoring from -14 metres before the fixed ladder was extended.
Development of a shoring procedure at Considines Cave, County Clare.
As digging has progressed, vertically, in the confined rift it became necessary to use timber shoring to support the exposed face of the southern end; the fill is a clay matrix with occasional areas of clean washed boulders. Until a depth of around three metres was attained the method of shoring was to remove an area of debris from below, (one metre), and behind the vertical line of shoring, (half a metre), to then insert several sections of pole; either end resting against the convenient, vertical natural ribs of rock. At this depth safety issues became manifest as rock could suddenly tumble from both above and behind the shoring; particularly so in the areas where little or no clay matrix held the debris together. Also difficulties arose as the weight of the heavy pieces of timber shoring increased with depth. Therefore it required a simple manageable system to stabilize the ever increasing weight of shoring, and, to prevent debris tumbling from the rift. The two methods, which evolved and used successfully, without incident, are described below.
Shoring, vertical support mechanism.
At surface, on the south edge of the working platform, a small enclosure was constructed through which access to the fixed ladder is approached. This “box” effectively closes off the area of rift, not covered by the working platform, preventing possible injury to the curious, unaccompanied visitor. This “box” rests upon heavy horizontal timbers that span the rift. Across the top of this construction a galvanized scaffold bar is secured, around it a 6mm steel cable. This cable is fed vertically down, in line with the front of the timber shoring, to the lowest piece of shoring and secured to it as a “Y” hang, (“D” fig 1), assemblage of galvanized turnbuckles and heavy metal bars ultimately fixed to the lowest piece of shoring with heavy duty coach screws. Thus a continuous steel cable securely holds the shoring in place as the area directly beneath is removed prior to insertion of the next section. By this method, almost, total control is maintained; by use of the “Y” hang arrangement of turnbuckles horizontal adjustment of the new shoring may be achieved.
Rift fill support mechanism.
With the timber shoring secured and the area of debris beneath now removed the next three or four pieces of shoring may be inserted; these initially rest upon two short lengths of pole, (“A” fig 1), the final piece of shoring to be inserted is left until later. The lowest piece of shoring now has two holes drilled into it and two threaded eye bolts screwed into them, (“B” fig1). From these bolts hang two adjustable threaded eye bolts which have inserted between and through their eyes a steel bar, (“C” fig 1). Two, three or four lengths of 12mm rebar are now driven deep into the rift fill behind forming a “stable floor” upon which the rift fill may “rest” during operations. At least one lathe of timber is now placed behind the lowest shoring on top, and across, the inserted lengths of rebar. Upon this lathe a back packing of stone is placed and brought up level to the penultimate piece of shoring, The final piece of shoring is now inserted leaving a three inch gap between it and the existing lowest secured piece of shoring. Through this gap the final packing is levelled off.
Positioning and securing the shoring.
With the new shoring resting on the two short pieces of pole, (“A”), and having been finally back packed with stone this section of shoring is ready to be lifted up into position tight against the existing shoring. This is accomplished by using a two metre timber lever to “jack” up each side in turn. During each lift the gap exposed between the shoring and the short piece of pole has a piece of timber inserted, and so on, until the space between the new and existing shoring is almost closed. With the new shoring supported by the inserted timber on top of the short poles the “Y” hang assembly is dismantled and secured to the new lowest piece of shoring, before final tightening the pieces of rebar previously driven in are withdrawn, the final lifting of the entire shoring is finished and the final tensioning of the support cable completed by adjusting the turnbuckles.
The Winch, Comms & Hauling System.
The winch which was purpose built for this dig has a continuous rated, ½ Horse power clutched/braked motor with a two stage belt speed reduction. This drives a 40:1 David Brown worm gear box to give a rope speed of approximately forty five feet or 13·7 meteres per minute.
Showing the winch with its cover in place, the No-Go-Backascope and emergency stop button. The winch is engaged by applying slight foot pressure to the silver coloured lever shown at centre, bottom.
The photo shows a modified “CLOG” ascender fitted in-line on the hauling rope. This prevents the load from descending down the shaft in the event of the hauling rope becoming slack during winching.
With the bottom lever placed in the left hand position as shown, the left hand spring compliments the Clog’s own spring to keep the cam pressed against the rope. Once the load arrives at surface and the safety lid is closed on the shaft it is now necessary to lower the load to be emptied into the barrow. The lever is then moved to the right, and with tension still held on the hauling rope the winch is engaged momentarily causing the cam to open by means of the stronger right-hand spring. The load can now be lowered into a position for emptying.
The Mark 4 Kibble.
Constructed from ground-water drainage pipe it has the advantage over the Mark 3 in having the rounded top edge which prevents catching on the numerous small projections in the shaft.
Showing the comms system which consist of an amplified telephone circuit plus an independent signalling system that uses SUD. 1 ‘Stop’, 2 ‘UP’, & 3 ‘Down’.
The raised platform which is attached to the sliding shaft cover. This is at barrow height to allow almost effortless emptying of the kibbles. There is a safety fence around the shaft when the lid is open with the platform forming one side of this. Shown is the now obsolete Mk. 3 Kibble.
The excavation of the Northern half of the rift has all but concluded. Upon reaching a final depth of some 26.5 metres a small east trending down slope passage developed, here the diverted northern stream rejoined the shaft. After some four metres the passage and stream entered a parallel, immature joint, here the stream turned toward the south; hence the decision to dig out the previously shored up southern rift. The 25.5 metre direct surface pitch is believed to be the second deepest in Clare. Outstanding tasks in the northern half of the rift remain its survey, remove the infrastructure and secure the entrance on conclusion of the entire project.
To follow the progress of the "Considine's Cave South End Dig" (click Here)