In 1886 the most populous part of Stockholm was situated on the north shore of Lake Mälaren and divided into two districts (Norrmalm and Östermalm) by an elevated ridge of stones and gravel.
Unsurprisingly, the ridge was an impediment to traffic and in 1886 The Engineer published a report on work to resolve the issue.
According to our coverage, an officer in the Swedish Royal Engineers, a certain Captain Knut Lindmark, had applied to the municipality of Stockholm to construct a tunnel so that pedestrians could pass through the hill for a fee of two öre.
As part of the deal, Captain Knut would pocket the takings for 50 years, after which the Brunkebergs tunnel would be returned to the city.
“This application was strongly opposed, not only by the owners of adjacent houses, but by engineers who stated that driving a tunnel through loose stones and gravel…would necessarily cause great subsidence…and, consequently, damage to the buildings above,” said The Engineer. “The municipality, however, considering the great benefit that would result from the realisation of the project, granted the concession and in the summer of 1884 the works commenced.”
The tunnel is 758 feet long, 12 feet eight inches high, and has a width of 13 feet two inches. The works were commenced from the eastern end by driving a heading at the bottom level, which was straightforward as the heading path was entirely granite and blasted by dynamite.
“The enlargement of the heading, however, caused considerable trouble because the crown of the tunnel in several instances passed into fine sand lying close to the rock,” said The Engineer. “At such places explosives could not be used.”
The driving of the tunnel from the west end introduced difficulties of a more serious nature, because the ground to be pierced consisted entirely of coarse gravel, intermixed with large stones and a small quantity of wet clay.
Fifty feet from the mouth, the tunnel passed between two five storey houses built and the distance between them was so small that the sidewalls of the tunnel had to be constructed right under their foundations, which extended down to within 10 feet of the top of the arch.
“The foundations of those houses could not in this case have been brought down to the bottom of the tunnel by underpinning - partly on account of the great depth, but chiefly from the loose nature of the ground,” said The Engineer.
An iron wall of plates, 12 inches square, was made to place against the face of the tunnel as the excavation advanced.
“Notwithstanding these precautions, the results were not satisfactory,” continued our reporter. “It was found that the gravel, on account of the water and clay it contained, had no cohesion whatsoever, and would pass freely through even a very small opening. The consequence was that a subsidence took place in the ground above; and the excavations had not advanced more than 40 feet when the works had to be stopped.”
To overcome this, Captain Lindmark decided to freeze the earth before making the excavation by means of cold air, and for this he obtained a ‘Lightfoot’ dry air refrigerator capable of delivering about 25,000 cubic feet of cold air per hour.
The inner part of the tunnel was formed into a freezing chamber by means of a partition wall made of double planking filled in with charcoal.
By running the refrigerator continuously for 60 hours the gravel inside the freezing chamber was frozen into a solid mass to a depth varying from five feet near the bottom of the tunnel to one foot near the top.
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“The work was now proceeded with as before, in sections of five foot, the excavation commencing at the top, and the iron wall being built up from above downwards as fast as possible,” said The Engineer. “The great difference was that now the whole mass of gravel and stones was solid; indeed, for some eight feet or nine feet from the bottom the iron wall was dispensed with, the gravel forming such a hard and compact mass that it had to be cut away with special tools.”
The machine – run on average for 10 to 12 hours - delivered the air at a temperature of 67 degrees below zero Fahrenheit, ‘and worked admirably all the time without a single hitch or stoppage of any description.’
The temperature in the freezing chamber was generally from six degrees to 15 degrees below zero Fahrenheit after 10 or 12 hours working, but it soon rose to freezing point when the workmen commenced their operations inside.
After two sections had been excavated, the partition wall was removed forward, and the contents of the freezing chamber varied from 3000 to 6000 cubic feet.
About 80 feet of the tunnel was driven by the aid of the Lightfoot refrigerator ‘with perfect success’.
“The total cost of the undertaking including all expenses, amounts to about £14,000 pounds, and to pay 10 per cent on this sum, after clearing the working expenses, will require that 4400 persons should pass through it every day,” concluded The Engineer.
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