Following sinkings and working of several large collieries along the upper reaches of the Rhymney Valley which proved to be highly successful, most of these large undertakings being owned and managed by the early Powell Duffryn Company; the Company then decided to sink another large mine of similar layouts and surface construction in the Bedwas area. Many surveys and trial pits were made before it was finally decided to establish the site between Bedwas Station and the Tynywern Farm at what is now Trethomas.
The colliery owners were registered as the Bedwas Colliery Company who leased some 1,475 acres to begin preparatory surface works and sinkings in 1909. The acreage was allotted for colliery surface workings, sidings, offices, spoil-heaps, two explosive magazines and two quarries - one high above the site and another larger one some 250 yards to the east (which later became a reservoir for the by-product Plant in 1929 for coke quenching).
Simultaneously, housing construction for the required influx of workers had to be carried out. Prominent amongst those charged with developing the necessary housing was Sir William James Thomas from the Rhondda. He became responsible for creating and building a completely new village between Bedwas and Machen, which became known as Thomastown and later as Trethomas. The neat, uniform streets of stone terraces are still visible today. The streets of Trethomas were named after Sir William's family - Mary, Thomas, James and so on. The street leading to the colliery was named Navigation Street.
Back to the colliery itself, great difficulties were experienced due mainly to the decision to site the shafts on the steeply-inclined lower section of Mynydd y Grug, slightly to the north of the Brecon and Merthyr Railway. The steep slopes were to be overcome by the the construction of the operational sections on three different man-made descending flat areas with either rail or road-track access to each. The operational sections were as follows:
1. To the south of the main railway, extensive sidings were laid with provision for empty wagons to be shunted in from the Bedwas Station, and loaded trains taken out at the new Trethomas Station at the eastern end.
2. A large screening plant with several picking tables, feed conveyors for coal loading and for dealing with the disposal of rubbish on the available tipping area to the south of the plant, which lasted into the early 1930's were constructed.
3. A fairly large saw-mill to meet underground and other requirements was built together with suitable timber stockyards; a creeper gantry gave access to and from the yard to the higher access bridge to be built across the railway tracks to the intermediate surface level.
4.An extensive wagon weighbridge was constructed over five tracks on the eastern end of the plant.
To accommodate all of the required services considerable earthworks and levelling had to be carried out. The designed decking level of both shafts varied between 60-65 feet above the adjacent railway level below, this ground being supported for its entire length adjacent to the railway by a strong 60 feet high stone wall.
To facilitate the sinking of both shafts a winding engine was installed longitudinally between them so that it could operate in both north and south directions as required. This winding engine was retained for the life of the colliery as (a) an emergency winder, with bucket or "bowk" attachment and (b) for the changing of the rope cage-guides in the North shaft (the south shaft being installed with rail-guides)
The shafts’ headframes in each instance were similarly constructed of heavy steel sections to accommodate the large diameter winding wheels, or sheaves. The North Pit shaft being designed as an air up-cast shaft was completely covered in with riveted steel plates, with a curved concrete construction entry and exit for trams to and from the shaft decking area; both these sections were airlocks, remotely controlled from inside. The top section of the headgear in each shaft was constructed in the form of a "gallows" with small emergency pulleys and chain-blocks; these were required for changing the large main sheaves.
For coal-raising purposes the North Pit exit airlock directed the loaded trams through and along about 60 yards of track and sidings to the weighbridge at the north end of the gantry and on to the tipplers at the coal pre-Plant, the return being via twin creepers to both shafts as empties.The minerals raised in each shaft on the upper-cage deck were designed to load and unload each deck at the same time, via lifts on each side of the shaft. Double-deck cages were installed in each shaft, with a capacity for two trams on each when winding minerals, or 25 workmen on each at man-winding times.
Handling arrangements at the shaft bottom of both the North and South pits were constructed in similar fashion to the surface arrangements, with a simultaneous double-decking loading system. These systems operated in this manner for a number of years in both pits (though in the 1920s due to heavy ground movement along the "fulls" side, which caused considerable bucking of the original composite steel girder constructions with the result that "shunt" decking arrangements had to be resorted to for a number of years with a resultant reduced shaft winding capacity).
Both shafts were sunk simultaneously using the normal sinking devices and systems of the time; that is with shaft staging set up to build the various sections with brickwork of adequate thickness to form the walls and to attach fitments for pipework and cables, water garlands at specified intervals to trap any amount of water and pipe it away – large quantities of water were encountered in some sections. In the South Pit only, girders had to be built in the side walls of the shaft at predetermined intervals to which were attached later, the rail-guides for the lift.
The actual sinking works were carried out below the staging by boring holes to set patterns, dependant on the type of ground being passed through, these being charged with explosives and fired to break up the solid ground for loading into the "bowk" or bucket by hand. The materials so dislodged were then hauled to the surface for disposal. The sinkers themselves were also raised and lowered in this manner throughout the sinking of each shaft. The shafts were constructed in this manner, brick by brick to a depth of over half a mile - 854 yards.
At the 330 yard and 680 yard levels, in each shaft, a tunnel connection was made from one to the other, each about 10 feet in diameter. In the South pit only a large bay or sump was constructed to act as a reservoir at each level. Also, a large pump house was made to accommodate two high-powered electrically driven Sulzer-turbine pumps in tandem at each level, capable of dealing with the expected high quantity of water which it was anticipated would need to be dealt with during the expansion of coaling operation in all districts. Similarly, a large bay or sump was made near to the shaft bottom, where again two further pumps of high capacity were installed. This large bay was designed to accept the water made from each working district pumped back by smaller electric and compressed air pumps. The water was thus pumped via the South shaft in three stages to first, the original round reservoir at the upper surface level for feeding the boiler and generating plants and so on. Some time later, in 1928, the large square spray-fed reservoir was built to provide water for the British Benzol Plant.
The tunnel connections between the shafts were constructed to serve as part of the mine’s ventilation system, and that at the 330 yard landing also served as an emergency or separate means of egress or ingress for the workings in each pit.
Whilst the sinking operations were being carried out together with construction of a level area from the proposed eastern wall of the large engine house to the western end entrance road to the site, a large diameter tunnel was constructed (the fan drift) from some 10 yards below the north pit decking level into a very large brick-built chimney called an Evasee, at the base of which was installed a large diameter Walking Manufacturing Co. fan. This ventilated the underground workings by drawing air through the fan drift up the North shaft, (the upcast shaft) and out through the evasee into the atmosphere.The North shaft headframe was airtight, so air was sucked into and down the South shaft, the downcast shaft amd through the underground roadways in a complex system of airtight doors. When the colliery came into production, the fan operated continuously year in, year out for many years, except for a few short periods for examination and overhaul.
The engine house when completed was, of necessity, extremely large for its day and extended longitudinally from north to south for some 70-80 yards, and east to west, 35-40 yards. The steam-driven winding engines were sited at each end of the building, in line with the headgear of each shaft, each housing a scrolled drum for winding the ropes, like a hose reel. Each drum had a large diameter in the centre, reducing in diameter to the outer edge to the drum flanges on each side. The large diameter winding ropes – 2 ½ to 3 inches –were secured from the small diameter scrolls so that one half had to be wound on first up to the large diameter at the centre of the drum, at which point it was attached to the shaft cage. The other rope was then secured to the other small scroll, taken slowly off the reel with the cage on the other side being lowered at the same time to the pit bottom. The rope was then capped and attached to the other cage left at the surface. Then, following precise measuring and other mandatory actions being carried out, the shaft-winding operations were ready to be carried out.
The original plant in the engine house consisted:
Immediately behind the eastern side of the engine house a large boiler plant was installed containing a number of Lancashire Type Boilers and a battery of Bulcock and Wilcox high pressure boilers with the attendant furnaces being belt-fed with small coal from the Screening Plant, the actual feed to the furnaces being controlled from hoppers by limited manpower.
Further east and adjacent to the Boiler Plant, three large wood constructed coating towers were erected into which the exhaust steam from the winding engines and the various other steam driven plants was conducted via a pipe system into each at the bottom. The steam would then rise and pass through an intricate system of limited crossings, which caused condensation to take place and fall like rain into the concrete catchment bay over which the towers were built. This water was re-used where possible and the surplus was piped and channelled into a stream to the Rhymney River.
To the north-west of the North pit, large machine shops were built with direct tram-way access from the main surface tracks system. The first section being the tram repair section in which several blacksmiths with their furnaces were employed. The next section housing a further blacksmith area, which dealt with tool sharpening and general metals fabrication for all purposes. This section having direct access into the larger next area of the fitting or machines shop, with large and small lathes, millers and grinders, welders etc., with a number of well experienced operatives. The end section of the building was the Electric Shop, where generators, motors and other equipment were overhauled and maintained by highly-qualified and experienced electricians.
The large building to the north of the machine shops was the Colliery Stores, which carried a large quantity of many of the pits’ requirements as well as a large oil storage area. The timber yard, which was also controlled by the Head Storekeeper, assisted by Teddy Humphreys and Reg Perry, was situated on the lowest level on the southern side of the railway tracks for some 40-50 years until it was transferred as part of the major reorganisation in the 1960s.
Immediately to the north of the north pit a 30 feet high supporting wall was constructed over a considerable length, which supported a large land-fill area which had been consolidated and upon which the main Lamp room with Ambulance facility room adjoining – a very primitive facility – and the Underground Managers’ and Officials; briefing rooms, together with the Time Keepers’ Office. All these offices etc., were housed under one roof. These offices and services were connected to the lower colliery levels by a wide concrete stairway, at the bottom of which in 1940 during World War II a large air raid shelter was constructed behind the high wall.
The rubbish or spoil from the underground workings and the Screening Plant was disposed of by large end-tipping wagons being shunted by the colliery loco, and tipped over a large area to the south of the colliery sidings, being some half-mile or so in length in the Bedwas direction and varying in width from 0 at the Bedwas end to about 250 – 300 yards to the south of the Plant. Spoil was tipped on this site from its inception in 1910 in the manner described until about 1934, when at its highest point it was about 150 – 200 feet in height, with its southern limit being a retaining wall following the course of a footpath from Trethomas to the Church at Bedwas. This tipping area was locally known as the Trethomas Tip.
With the colliery sinking complete, the first coal was raised in 1913 the year of the peak output of coal from the South Wales Coalfield. Unhappily, the colliery's story continues against the start of a decline in the South Wales coal industry which was to see its extinction over the next 75 years.