New Single-Line Tunnel at Colwall
(The following account was given in the Great Western Railway Magazine 1926).
In an interesting book 'The Story of the Malverns' by GW Hastings wherein is described the geology of the Malverns mention is made of the great struggles that went on during the building of the Great Western Railway tunnel between Colwall and Malvern Wells (opened) in 1861, which took some seven years to complete. The author goes on to say:-
"Rather more than half a century ago it was decided to construct a railway from Worcester to Hereford. The line as laid down by the engineers wisely or unwisely was to pass through the hills a little to the south of Great Malvern and it thus became necessary to bore a long tunnel Perhaps the opinion of no geologist had been taken as to the quality of the plutonic rock; perhaps the engineers employed had experience of only tunnelling through sedimentary strata; but however that may be, it is certain that the expectations were falsified.
Two contractors in succession threw up the work in despair. The (hard) adamantine nature of the rock fired to a crystalline degree by volcanic heat, having defied their efforts. The third contractor was more fortunate. Not long after he had commenced operations he found that his workmen had passed into a soft layer, in fact a sedimentary deposit based deep in the Syenitic substance of the hills. The spot was examined by geologists both local and from the British Museum and its history became clear. In that period of remotest antiquity, which we have described, there had been a fissure in the submarine reef and into that fissure had drifted loose deposit with shells and other organisms of Silurian age which had been preserved as they had been laid down. At some time a future burst of subterranean energy had caused the mouth of the fissure to be sealed up and its contents were for countless ages embedded in huge rock.
The lucky contractor who thus played the part of 'Tertius Gaudens' finished the tunnel with ease, his fortune made by the accident of some million of years by-gone".
This account probably contains some picturesque exaggeration but it goes on to prove the speculative nature of the task that confronts an engineer when a tunnel has to be constructed.
For some years past the train service and other developments had rendered inadequate the accommodation provided in the old tunnel owing to the small size both in width and height coupled with the steep gradient of 1 in 80 considerable inconvenience was caused to enginemen and others by the atmospheric conditions in the tunnel. This was accentuated by the fact of the structure being used for both up and down trains.
Exhaustive tests were carried out on various tunnels on the Company's systems with the idea of ascertaining the best means of combating the atmospheric nuisance. As a result so far as the Colwall tunnel was concerned temporary relief was afforded to the enginemen by running banking engines against the gradient with the cabin first. The unsatisfactory ventilation was only one and not the chief consideration that gave rise to the question of the reconstruction of the tunnel or alternatively the building of a new one. There was the equally important fact that expenditure on repairs owing to the original indifferent construction would have to be incurred sooner or later. The construction of the old tunnel did not from an engineering point of view lead itself to enlargement and by the construction of a new tunnel interference with traffic working would be avoided and the work carried out more expeditiously and at reduced cost.
It was consequently decided to build a new single line tunnel 1589 yards in length running parallel to the old structure and situated some 44 feet away. Preparations were made to drive the headings of the new tunnel from the Malvern Wells side from one permanent shaft in either direction and from a temporary shaft some 50 yards from the Colwall face of the structure. Where the hardest rock was met with, compressed air drills and extensive blasting operations involving the use of a large quantity of gelignite were employed. In July last, the gangs of men working on the headings from their respective ends of the tunnel met and it is of interest to know that working at opposite ends for the whole distance of over three quarters of a mile the alignment was in error only a quarter of an inch and in level half an inch.
There must always be an element of romance in such an undertaking boring as it were into the "unknown" which culminates in satisfaction on the part of all concerned at the successful conclusion. During the boring operations no great trouble was experienced with water, but 200 gallons per hour had to be pumped up the permanent shaft until the meeting of the headings, when the pumping work was suspended as the whole of the water then gravitated towards the Malvern end of the tunnel.
The following sections of strata were met with. Commencing from the Malvern side, there were 170 yards of Keuper marl; here a fault was met at the junction of the Archean rock and marl. The Archean rock may be termed as gneiss a species of stratified rock composed of quartz feldspar and mica. This length extended for 541 yards when another fault was found - the junction between the igneous and sedimentary rocks. For 179 yards the strata is in this order; Woolhope shales and Woolhope limestone, when it joins the Wenlock shales - an ancient marine bed which extends for 699 yards to the Colwall face of the tunnel.. These Wenlock shales extend to another 76 yards in the Colwall cutting where they join on to the old red marl that in turn continues to near Colwall station.
The lining of the tunnel varies from three to six rings of brickwork, including the "facing up" with blue bricks. Excavations in the tunnel amounted to some 64,140 cubic yards. The building in of the brick lining accounted for 12,185 cubic yards and lastly 1,966 cubic yards of concrete were built into the tunnel and the two faces.
The new tunnel is built to a slightly larger section than that adopted by the railways in this country. The gradient in the new tunnel is an improvement on the old, being 1 in 90 as against 1 in 80. This will result in less tractive effort having to be exerted by locomotives passing through the tunnel and with the increased internal area of the new tunnel will help considerably to obviate the necessity for special ventilation
The depth from rail to top of the hill through which the tunnel passes measures some 599 feet. At the Malvern end, rail level is the same at both tunnels, but the new tunnel at the Colwall end due to the flatter gradient is some 7 ft lower. In the future it will be possible if thought advisable from a working point of view to reconstruct the old tunnel at much less expense due to non interference with the operations by passing trains, the new tunnel serving for both up and down traffic while the work is in hand. In the event of this being carried out later separate up and down lines will be made, the old tunnel in that case being used for traffic passing in the up direction and the new structure used for the down lane.
It is satisfactory to be able to say that during the constructional work that was of considerable magnitude no serious accidents took place. A considerable number of ponies were used by the contractors in the underground transport work, and a resident in Colwall looked thoroughly well after the animals. Employment was found at times for nearly 500 men, but never fewer than 200 at one time. The deposit of ballast and laying of the rails through the tunnel now proceeding and the tunnel will be opened for traffic very shortly.
Altogether the construction of the new tunnel has taken about 30 months to complete a great difference when comparing the time occupied in connecting to the old structure but one realises that since the later structure was bored the possibility of drilling by compressed air and drilling apparatus generally have been developed. In the case of the old tunnel in the days before the fuse, drilling was done by hand, bar and hammer, blasting powder being used. A skewer was inserted filled with powder and ignited by touch paper. Mechanical drills were tried but the stream and smoke were too much for the men.
Great trouble was experienced with water during the boring operations that in the case of the present structure was not of a particularly serious nature. It has been necessary to remodel Colwall station and goods yard in view of the altered working consequent upon the opening of the new tunnel, and an extra refuge siding between the station and the tunnel has been laid-in.
Messrs Wilson, Lovatt Limited of Wolverhampton were the contractors and the work on the ground was supervised on behalf of the Great Western Railway Company's Chief Engineer by the Resident Engineer Mr H Bromley.