Thus, then, the engineer had, to commence with, a strong and reliable brickwork shaft, 3 feet thick, by which men and materials could ascend and descend in safety. A smaller shaft was also sunk deeper for drainage.
And now the actual boring of the tunnel commenced. It was to be 38 feet wide and 22½ feet in height. On New Year’s Day, 1826, the boring-shield was placed below in the shaft. The shield was composed of 36 cells, 3 cells in height and 12 in breadth, with a workman to each.
The huge “shield” was placed before the earth to be excavated, and a front board being removed, the soil behind it was dug out to a specified extent, and the board was propped against the fresh surface thus made. When the boards had all been placed thus, the cells were pushed forward into the hollow then made. This was accomplished by means of screws at the top and bottom of the shield, and which were set against the completed brickwork behind.
For, while the labourers were working in front, the bricklayers behind built up the sides and roof, and formed the floor of the tunnel, the soil at the roof being supported by the shield until the masons had completed their task.
For nine feet, the tunnel proceeded through clay, but then came an unwelcome change. Wet, loose sand prevailed, and the work progressed with peril for thirty-two days, when firmer ground was reached. Six months passed and substantial headway was made, the tunnel being completed to the extent of 260 feet.
Then, on the 14th of September, the startling intelligence came that the engineer feared the river would burst in at the next tide. He had found a cavity over the shield. Sure enough, at high tide, when the river was brimming full, the workmen heard the ominous rattle of earth falling on their shield, while gushes of water followed.
So excellent were the precautions, however, that no disastrous effects followed, and Father Thames himself rolled earth or clay into the hole and stopped it up. It was a warning, and emphasised the fear that haunted the men’s minds all through the hazardous undertaking—the fear that the river would break through and drown the tunnel.
In October, another small irruption took place, and was successfully combated. Then, in the following January (1827), some clay fell, but still no overwhelming catastrophe occurred. The ground grew so moist, however, that it was examined on the other side. That is, the river bed was inspected by the agency of a diving-bell, and some ominous depressions were found. These were promptly filled by bags of clay.
It may be asked, Why had Brunel not gone deeper? Why had he not placed a greater thickness of earth or clay between his work and the waters of the Thames?