The Blisworth Cutting is one of the longest and deepest grooves cut in the solid earth. It is 1½ mile long, in some places 65 feet deep, passing through earth, stiff clay, and hard rock. Not less than a million cubic yards of these materials were dug, quarried, and blasted out of it. One-third of the cutting was stone, and beneath the stone lay a
thick bed of clay, under which were found beds of loose shale so full of water that almost constant pumping was necessary at many points to enable the works to proceed. For a year and a half the contractor went on fruitlessly contending with these difficulties, and at length he was compelled to abandon the adventure. The engineer then took the works in hand for the Company, and they were vigorously proceeded with. Steam-engines were set to work to pump out the water; two locomotives were put on, one at each end of the cutting, to drag away the excavated rock and clay; and 800 men and boys were employed along the work, in digging, wheeling, and blasting, besides a large number of horses. Some idea of the extent of the blasting operations may be formed from the fact that 25 barrels of gunpowder were used weekly; the total quantity exploded in forming this one cutting being about 3,000 barrels. Considerable difficulty was experienced in supporting the bed of rock cut through, which overlaid the clay and shale along each side of the cutting. It was found necessary to hold it up by strong retaining walls, to prevent the clay bed from bulging out, and these walls were further supported by a strong invert,—that is, an arch placed in an inverted position under the road,—thus binding together the walls on both sides. Behind the retaining walls, a drift or horizontal drain was provided to enable the water to run off, and occasional openings were left in the walls themselves for the same purpose. The work was at length brought to a successful completion, but the extraordinary difficulties encountered in forming the cutting had the effect of greatly increasing the cost of this portion of the railway.
The Tunnels on the line are eight in number, their total length being 7336 yards. The first high ground encountered was Primrose Hill, where the stiff London clay was passed through for a distance of about 1164 yards. The clay was close, compact, and dry, more difficult to work than stone itself. It was entirely free from water; but the absorbing properties of the clay were such that when
exposed to the air it swelled out rapidly. Hence an unusual thickness of brick lining was found necessary; and the engineer afterwards informed the author that for some time he entertained an apprehension lest the pressure should force in the brickwork altogether. It was so great that it made the face of the bricks to fly off in minute chips which covered his clothes whilst he was inspecting the work. The materials used in the building were, however, of excellent quality; and the tunnel was happily brought to a completion without any accident.
At Watford the chalk ridge was penetrated by a tunnel about 1800 yards long; and at Northchurch, Lindslade, and Stowe Hill, there were other tunnels of minor extent. But the chief difficulty of the undertaking was the execution of that under the Kilsby ridge. Though not the largest, this is in many respects one of the most interesting works of the kind in England. It is about 2400 yards long, and runs at an average depth of about 160 feet below the surface. The ridge under which it extends is of considerable extent, the famous battle of Naseby having been fought upon one of the spurs of the same high ground about seven miles to the eastward.
Previous to the letting of the contract, the character of the underground soil was examined by trial-shafts. The tests indicated that it consisted of shale of the lower oolite, and the works were let accordingly. But they had scarcely been commenced when it was discovered that, at an interval between the two trial-shafts which had been sunk, about 200 yards from the south end of the tunnel, there existed an extensive quicksand under a bed of clay 40 feet thick, which the borings had escaped in the most singular manner. At the bottom of one of these shafts the excavation and building of the tunnel were proceeding, when the roof at one part suddenly gave way, a deluge of water burst in, and the party of workmen with the utmost difficulty escaped with their lives. They were only saved by means of a raft, on which they were towed by one of the engineers
swimming with the rope in his mouth to the lower end of the shaft, out of which they were safely lifted to the daylight. The works were of course at that point immediately stopped.
The contractor, who had undertaken the construction of the tunnel, was so overwhelmed by the calamity, that, though he was relieved by the Company from his engagement, he took to his bed and shortly after died. Pumping-engines were then erected for the purpose of draining off the water, but for a long time it prevailed, and sometimes even rose in the shaft. The question then presented itself, whether in the face of so formidable a difficulty, the works should be proceeded with or abandoned. Robert Stephenson sent over to Alton Grange for his father, and the two took serious counsel together. George was in favour of