St Cuthbert’s Sump II - Where do we go from here?

By Stuart McManus

Summary and Short History

Ever since the discovery of St Cuthbert’s by the BEC in 1953, the major efforts to discover new passage along its streamway have always been fraught with difficulty as the mud and lead tailings that choked the sumps have prevented divers from pushing through in to the passages that lie beyond. The usual method employed on these occasions required the construction of dams and the bailing of the water back into them to drain the sumps to enable the removal of the build-up of silt and mud by diggers.

Since the breakthrough in sump I on Halloween night in 1969 by diggers during a dry spell, and the discovery of over 275 metres of streamway that is Cuthbert’s II, and which terminates in Sump II there have been numerous attempts to pass the sump both by digging and diving but to no avail.

The basic problems with Cuthbert’s II Sump are similar to Sump I, in that the sump is heavily silted with lead tailings from the washing ponds from the old mineries used by the lead miners of old and muds/silt brought in by the streams as well as the continuing erosion of silt banks within the cave. The water retained within the catchment area is far greater than can be drained by damming the Mineries pond and therefore stored on the surface.  Here is an excerpt for the last push made on Sump II from 1982 – 1985.

In 1982 the attention of Butcher (SMCC) and McManus (BEC) returned to Sump II.  Ideas were discussed and plans were afoot to repeat a similar exercise to that of 1967. Rock drills had been used several years previously, but new attempts were made in 1982 to blast a way across the top of the sump.  This very slow procedure forced the diggers to re-consider the situation.  The conclusion reached was that the most effective way to overcome the problem was to bail Sump II and remove the infilling.  The number of dams in the cave was reassessed.

The 1977 dam used for draining the sump remained at Sump II but an additional one would be required. So, in April 1982, the sand-bag dam was built doubling the storage capacity to about 4,000 gallons.  Trial digs were attempted; the Mineries dam would be inserted some three weeks before the event and the Plantation Stream diverted into the St. Cuthbert's Depression, allowing the water to flow down into Maypole Sink and overflow into the floor of the depression.  The surface dams prevented most of the water flowing into the cave entrance.  The general idea was to drain the 'spongy' ground over which the surface Plantation Stream flowed.  However, as on previous occasions, this storage medium held far too much water and, although Plantation Stream had been diverted from Plantation Swallet and was flowing into the valley, too much water from the 'sponge' was flowing into the cave at Plantation Junction long after the dams had been put in.

With the dams operating in the cave holding back the water draining into the system, the sump bailing teams were able to empty Sump II to the infill level.  When the stream is prevented from flowing into Sump II the sump partially drains itself reducing the water level by some 1.5ft indicating that this sump is a true siphon draining through gravels at the downstream end. It took a dozen cavers about eight hours to drain the sump of water. All this effort allowed an inordinately short time for digging.  It was realised that the greater the volume of infill removed, the greater would be the volume of water to be bailed on subsequent occasions.  Continued digging pushed the choke face still deeper. 

To make matters worse bad air formed because of the large numbers of cavers working in such a confined space.

The ingenuity of Mendip diggers never fails to amaze the onlooker.  To overcome the problem of bailing the sump and reduce the volume of water, hundreds of plastic bottles were obtained and placed in the sump making the place look, to quote one caver, more like a "Moroccan bazaar than a cave passage".  The use of the bottles and two 1500 gallon double diaphragm hand pumps gave some success but the actual digging time was still only an hour or so.  With only a couple of men operating the pumps the foul air problem was considerably reduced.

A 'big-push' was arranged for the summer of 1985.  This time, ideas of driving a 110 Volt  submersible pump took shape. 3,000ft of cable would be required.  With this in mind, all the dams were refurbished, both on the surface and underground and a 5ft third dam – “The Kariba” was built close to Sump II, designed to holdback a further 4,000-5,000 gallons of water!  By the autumn of 1984,the Mendip Rescue Organization was preparing for the 1985 National Cave Rescue Conference to be held on Mendip.  Suggestions were made that it might be possible to borrow sufficient fire hose from the Somerset Fire Brigade to convey the necessary air to drive a submersible centrifugal pump at Sump II.  The Chief Fire Officer, Nigel Musslewhite, agreed and the tremendous task of transporting 60 fire hoses into and eventually out of the cave was a major task in its own right.  The logistics for the event were considerable and included the setting up of kitchen facilities, laying of telephone cables and the transportation of the pump. McManus wrote:

 “With everybody keeping an eye on the weather the Mineries dam was inserted ... to reduce the water retained in the catchment area ... The operation was probably the biggest pumping operation that had been carried out by cavers at that time, a case of the BEC 'doing it to excess' again”.

Suffice to record the air-driven pump worked successfully on 18th May 1985.  The pumping capacity was extremely high (more than 16,000 gal/hr) and the sump was drained in less than thirty minutes!  Digging now commenced within an hour of the start of pumping! On the first day over eighty cavers went to the dig site in teams of six.  The initial task, once the sump had been drained was to remove the hundreds of plastic bottles that had been placed in the sump during the course of the previous year.  This took a couple of hours though it had taken a year to put them in!

Once the water had been pumped out digging commenced.  The infill being removed consisted mainly of lead tailings from the washing operations of the miners.  Water was continually draining back from the downstream end of the sump.  Consequently it was necessary to keep the pump running in order to keep the digging face reasonably free of water.

So successful was the weekend's activity that it was agreed to repeat the entire operation weekly until the sump had been passed.  However, though great strides had been made the choke was not cleared.  By the middle of July 1985 digging had to come to an end when the borrowed pumping equipment had to be returned.  However Sump II had been excavated to a depth of 25ft and 65ft in length but still with no indication of the roof rising.

A further 10,000 gallon dam “The Aswan”, was constructed during 1986 – 1988 though this has still to be used in a successful pumping operation at Sump II.

Future Operations

The pumping exercise of 1985 demonstrated that the use of compressed air provided the best means of pumping the sump, for two reasons. The use of compressed air as the motive power, rather than electrical power, enables a relatively low weight submersible pump (DIP 25 Atlas-Copco) for its pumping rate can be used when compared with the equivalent weight of an electrically driven pump. Air also allows for flushing/purging of bad air that always accumulates at the sump area due to the number of and time that cavers need to spend there during the pumping and digging operation.

What’s required For a Further Push?

Dams

Since 1985, Sump II has refilled with silt and mud and the dams will need to be refurbished, though this should not constitute a major problem since additional capacity in the Aswan dam is available. The dams at the Beehive and Gour Hall will  need refurbishing as will the other smaller dams dotted along the active streamway. The major expenditure here will be cement.

Pumps & Equipment

The ideal system would be the loan, or purchase of an Atlas Copco air driven submersible (DIP –25) pump direct from the manufacturer – sponsorship would be quite attractive. The obtaining of over 1,000 metres of standard fire-hose from Angus Fire Armour again by sponsorship would also be desirable. The major expense with the fire hose is the couplings that connect the 15 metre lengths of hoses together. The use of longer lengths could be considered, though this would need to be assessed from any offer.

The final piece of kit would be the standard 7 barg road compressor that would be needed to drive the pump from the Belfry car-park. The diesel to drive it costing some 25p per litre.

Manpower

I would consider it possible to muster sufficient cavers and volunteers to work on a six week-end period to re-establish the sump to its 1985 condition, once the dams had been refurbished and there was a definite dry spell. These dry spells are normally early spring and early autumn. The autumn is more favorable as summer flows into the cave should be somewhat lower than the early spring.

Stuart McManus

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