The launch of the 5th edition of Mendip Underground caused somewhat of a surge of interest in certain caves in which it contained updated descriptions, photos and rigging diagrams, Mangle Hole near Sandford being one of them. It was the rigging topo which caught my attention as it suggests some nice free hanging pitches with multiple rebelays and a sump at the bottom, what more could you ask for !
The reality is somewhat different, its reputation does seem to precede it and it is seldom visited but I quite enjoy a trip there and have undertaken a few club trips, several digging trips, several surveying trips plus a failed dive trip.
As there was no decent published survey of the cave (and there still isn’t) I decided with my new found cave survey skills I would have a go myself. I was joined by Pete Hall and we set about surveying the cave, I don’t remember much detail of the trip itself but I can remember feeling quite frustrated when sitting down at my computer with the data afterwards knowing what shape the passages and chambers actually were and seeing the shapes that I was to draw on the survey to represent them.
The problems lies in that the cave is very steep in nature, the entrance rift is very narrow floor to ceiling but wide in the wall to wall dimension, the chambers are large and vertical in nature with lots of alcoves and side rifts, the route to Aldermaston Chamber is small, complex and muddy. When the splay shots are viewed in plan view the whole cave looks very different to the reality.
Attempt at Plan view in CAD, Splays in yellow, survey legs in red.
At the time I was trying to learn how to draft surveys in Therion but perhaps this multilevel cave was a bit to challenging for my basic skills. Instead I found a handy export function of PocketTopo where the raw data is exported as a 3d *.dxf file, this can then be loaded in CloudCompare where it approximates the walls based on splay shots and shows a 3d model which can be spun around and viewed and understood more readily then a plan and elevation view.
One of the exquisite mud formations, this one looks like a mans head
I bought an Aquazepp LT30 (short body, single speed, 12v with headlight) in the summer of 2017. Once a new battery was installed it was fully functional despite being well used and quite old. I was getting around one hours use out of the battery and measured my underwater speed at 33 m per minute, faster than swimming but not that fast for a scooter.
Standard Aquazepp LT30 attached to a back plate for transport
Though this was ‘slow’ around the same time Marcus Blatchford was scootering around Vobster Quay with a camera attached to the nose of his vehicle at similar speeds taking photos to be processed into a model (via the process of photogrammetry) so it was ideally suited to the task of accompanying him for which many a pleasant dive was had cruising through the wintry atmospheric waters of Vobster Quay.
I was already planning on upgrading my batteries to lithium based chemistry and had been searching Hobbyking for suitable packs, some 16 ah packs came up at a good price so I purchased six of theses giving me 96 ah Vs the standard 33 ah plus the benefit of a higher voltage as these were 4 s packs which give 16.8 v when full and 12 v when empty.
Lead acid Vs Lipo batteries
I was unsure if the standard 12v motor would be able to handle the extra power but installed the batteries anyway and took the scooter for a dive. A very enjoyable 90 minutes was spent whizzing back and forth to the end of Brixham Breakwater and back, and back and back…until the scooter seemed to loose power, I had assumed the packs were just flat so ended my dive very happy that my modification had improved both the speed and the duration of the scooter. Brixham breakwater is about 900 m in length and it had taken me just under 20 minutes to reach the end so a big improvement in speed over the lead acid batteries had been achieved.
The scooter was taken home and the batteries charged, and I took it for another dive at Vobster Quay expecting to cruise around faster than before, unfortunately once the scooter was in the water the propeller was making a feeble effort to turn much like it had at the end of my Brixham dive so a bit miffed I left it at the surface and went for a swim without it. Once I got home I took it apart and began to check the batteries had actually charged which they had, on opening the rear motor compartment a burnt smell was apparent and the insides were covered with a thin layer of black dust, clearly the motor had burnt out given the extra power that it was being asked to convert from electricity to rotation of the propeller.
A source of motors from the larger Aquazepp models which run on 24 v was provided by a friend and an order quickly placed. The motor was a direct replacement for the 12 v motor, it just needed the drive gear fixing to the shaft which was achieved using a small screw fixed through the motor shaft. The 24 v motor is heavier than the 12 v motor so I had to remove some of the lead ballast that was required when swapping from lead acid to lipo batteries.
Drive gear fitted to 24v motor (metal gears are why the Aquazepp has such a distinctive sound)
Confident that a motor rated to work at 24 v could handle 4s lipo packs some more test dives were performed using a power meter to assess the average current draw. I found that I was now able to travel at around 44 m per minute with the motor drawing 16 amps so I now had a maximum range of nearly 16 km (6 hrs) ! Not bad for an old Aquazepp !
Power meter used to asses battery usage post dive
Next on the improvement list was a rear handle like most other scooters have, the scooter needs two hands to drive comfortably in its standard configuration. A delrin handle and bracket was made and fixed to the shroud, a small PVC trigger box was welded onto the main body which contains a magnet and spring which are actuated by a lever on the trigger. This dramatically improved the comfort when scootering, its now easy to control with one hand and only needs light steering input to instigate a turn.
The scooter is now capable of running at different speeds, a 50 amp Syren brushed DC motor controller has been installed which is controller by an Arduino UNO. The sketch loaded onto the Arduino reads the output from a hall effect sensor mounted to the inside of the hull. On the outside of the hull in the same location is a small plastic rod with magnets embedded which can slide back and forth to increase or decrease the speed. Whilst noise is always going to be loud when using an Aquazepp due to the metal gears the motor seems to run better now it is being supplied through the controller. The speed as its currently set can be run anywhere between around 65 m/ m and 40 m/ m, this can be adjusted further on the surface by editing the Arduino sketch if required. The controller outputs a PWM signal and even running at my current top speed I’m not at 100% duty cycle yet but 65 m/ m seems fast enough for now if dive kit is worn, if snorkelling it is even faster !
The downside to using a variable speed over fixed speeds is monitoring battery consumption, with fixed speeds battery usage can easily assessed post dive and then used as reference in the future, with variable speed unless it is run at fastest or slowest then its hard to gauge where you have been on the scale.
The Cheddar Caving Club was responsible for creating the link between Bath Swallet and Rod’s Pot (after work by other clubs) creating an entertaining through trip on Burrington Combe, in the Mendip Hills, Somerset. The next logical step was to attempt to link Rod’s Pot with its neighbour the opposite side; Drunkard’s Hole.
I was involved in the dig for a while in Rod’s Pot starting at the bottom of what was known as the ‘Blind Pots’. This progressed horizontally for a short while before a route was excavated vertically upwards to enter a small chamber. From here further digging occurred in the upwards direction.
Pete Hall near the entrance of Rod’s Pot
At a similar time myself and my regular caving partner began re-examining a few areas of interest in Drunkard’s Hole, a short section of passage was found after climbing up an ever tightening aven and a dig was started here feeling that we were heading right for our dig in Rod’s Pot.
Whilst we thought we were heading in the right direction it is hard to know for sure. I purchased a Disto x310 and the conversion kit which allows accurate and fast cave surveying to be performed. The goal was to establish the distance between the two digs in the cave to motivate us to further our efforts (after all we had dug an estimated 120 m of passage in Rod’s Pot and the gap between the two caves was thought to be about 80 m in a straight line).
Arriving early before each digging session I set myself the task of surveying between both entrances on the surface so that the data collected in both caves could be accurately linked. Lengths of PVC pipe stuck into the ground were used to clear the vegetation acting temporary survey stations.
The process was completed three times to check the accuracy of the work; misclosure between the traverses was within 0.2 m in the horizontal plane and within 0.3 m in the vertical plane.
Pete Hall near the start of the dig
Now the task of surveying from both cave entrances to the dig faces was all that was required, this took a few trips in both caves to achieve and was a good excuse to escape the physical work of digging, bagging, dragging and stacking of mud in its various forms.
A no frills approach to the survey was taken meaning that only centre line data was recorded to speed up the process in what are difficult size and shaped passages to accurately survey.
The results were suprising, both ends of both digs in Rod’s Pot and Drunkards Hole were still separated by over 50 m horizontally despite over 120 m of passage having been dug from solid mud fill.
Profile view of the two caves and digs
Plan view of the two caves and digs
Not long after the surveying was completed we retired to the Crown Inn at Churchill for the final time and focused efforts elsewhere thwarted by the complex geology of the Mendip Hills.
Upper Dinas Silica Mine is often used by divers interested in mine or cave diving when visiting South Wales, it offers all weather diving and the opportunity for a nice long swim, exactly how long can you swim for or more importantly how far ….?
The current owners are the Cambrian Mines Trust, they have an excellent website containing all the relevant information and more importantly for this post the abandonment plans.
For those familiar with CAD the plan can be imported, scaled to measure in metres (or feet if so desired) then passages lengths or routes of interest can be plotted and measured.
Abandonment Plan of the Upper Dinas Silica Mine
Those familiar with the site will recognise that diving takes place in the levels below ‘A’. Some common routes and distances are described below:
‘The Grand Tour’ – Starting and ending at the main dive base and swimming each level to its end before returning and changing levels is a massive 3965 m (or over 13,000 feet if training for Floridian diving).
‘The Western Tour’ – Starting and ending at the main dive base but staying to the West of the main passage is a more manageable swim of 962 m (just over 3,200 feet).
‘The Eastern Tour’ Starting and ending at the main dive base but swimming all levels to the East back and forth is 3240 m (just over 10,700 feet).
Simplified layout of underwater passages and length of tunnels
