An interesting possibility for a larger Tube is if Brunel's original tunneling shield was more like Gratehead's later shield was.
Innovation and the Rise of the Tunnelling Industry – Graham West
The first of Brunel’s shield patents (1818) described a cylindrical shield about 14ft diameter, divided across the horizontal diameter by a working platform into an upper and a lower half. Each half was further divided into five vertical cells each of which accommodated a miner and each of which had its own set of poling boards to support the face. Each cell could move independently, pushed forward by a hydraulic ram which reacted against a frame erected further back in the completed section of the tunnel. The lining, which was to be of cast iron segments, was erected under protection of a tailskin at the back of the shield. The hydraulic rams were specified to be double acting so that they could also pull the reaction frame forward from time to time as the tunnel progressed. The whole arrangement has a thoroughly workmanlike appearance and looks much more practicable than the cumbersome rectangular shield. Brunel eventually used when he came to drive the Tames Tunnel. Maybe the cylindrical shield with independently moving cells, each a different size and shape was too difficult to manufacture whereas a rectangular shield in which all cells were identical was easier to make. Whatever the explanation, the first shield in Brunel’s patents of 1818 is much more like the tunnelling shields as they eventually developed than is his Thames Tunnel shield. Also the cast iron segmental lining is clearly shown and its safe erection described whereas the Thames Tunnel was constructed with the less safely built and more time consuming brick lining.
Then there is the possibility of immersed tubes (from the same book)
Immersed tube tunnels
It will be recalled that when the Thames drift way was flooded in February 1808, Trevithick proposed to recover the works by sinking a caisson in the river, but the Thames Archway Co. did not accept this plan. In July 1808 Trevithick put forward an alternative scheme for constructing a tunnel under the Thames. A watertight wooden caisson, 50ft long by 30ft wide and reaching above high water level, was to be placed on the river bed. Wooden piles were then to be driven into the river bed just inside the caisson and to a depth just below the level of the bottom of the proposed tunnel, to form a close-boarded watertight cofferdam inside the caisson and projecting beneath it. The water in the caisson was to be pumped out and the earth inside the cofferdam was then to be excavated down to the depth of the bottom of the proposed tunnel. Any surplus water which had leaked into the excavation was to be drained away through a pipe into the drift way underneath, which was to be used as a drainage gallery for the new works. In the excavation it was then intended to construct a 50ft length of twin-tube brick tunnel, which when complete was to be covered with earth back up to river bed level. The piles forming the cofferdam were then to be withdrawn and the caisson moved 50ft further on across the river. The whole proves was to be repeated as many times as was necessary to cross the river, 50ft of tunnel being added at each stage. The caisson was to be fitted with a platform at the top carrying a steam crane which was to be used for drawing the piles and for hoisting the spoil and lowering material for constructing the tunnel. Each tube was to be 12ft in diameter and was to accommodate an 8ft wide waggon road and a 4ft wide footpath. Only 50ft of water would be occupied at any one time during construction, which Trevithick said was less than a 400 ton ship laying at anchor.
Trevithick’s plan, had it been put into practice, would have amounted to a cut and cover tunnel with the novel feature of being constructed beneath a river bed instead of on dry land. However, in September 1808 Trevithick proposed that instead of brickwork, the section of tunnel should be made of cast iron. Here, we have then an early proposal for a method of tunnel construction that approached closely the concept of an immersed tube tunnel. If it had been proposed that the cast iron cylinders could have been sunk into position in trench in the river bed rather than placed in a cofferdam excavation, then the idea would have been a true immersed tubes tunnel. As it was, this concept quickly followed, but it sprang from the mid of another engineer.
The Thames Archway Co rejected Trevithick’s plan for a cut and cover subaqueous tunnel just as it had earlier rejected his scheme for recovering the driftway. Instead in March 1809 the company advertised for plans to be submitted for completing the tunnel project: engineers were invited to suggest how the tunnel could be built. One of the plans submitted was by Charles Wyatt.
Wyatt’s plan was to excavate a trench across the river bed using steam excavators and ballast lighters and then to sink into the trench a series of cylinders, each about 50ft long, made from brickwork. The ends of the cylinders were to be provided with temporary spherical walls so that each one would be a watertight floating vessel. Each cylinder was to be provided with a cock to admit water so that it could be sunk, and a pump to empty it of water after it was in position. The trench was to be deep enough so that the cylinders could be covered with 6ft of earth without raising the level of the river bed, this thickness of cover being considered sufficient protection form ships anchors. There remained however, four vital questions upon which success or failure of the plan would depend. These were (i) Could cylinders be made from brickwork that would be strong enough to be towed into position and sunk? (ii) Could the cylinders be placed in the trench on the river bed with sufficient accuracy to be joined together to form a tunnel? (iii) When the cylinders were joined together and the temporary ends removed, would the joints be watertight? (iv) Could the whole operation be carried out in a busy waterway without being imperilled by collision form passing ships?
The company decided to begin preliminary trials to see if solutions could be found and authorised John Isaac Hawkins to begin preliminary trials.
The book goes on to say that in September 1810 tests were carried out that concluded that brickwork cylinders of sufficient strength could be made if the walls were 13 ½” thick. Tests in 1811 to deploy the tubes were successful although there were numerous incidents of boats hitting the scaffold that was doing the deploying and three quarters of the workmen’s time was spent repairing damage. There were leaks in the joins but it was considered that these could easily be sealed with puddled clay.
Hawkin’s trials had showed that questions (i) – (iii) could all be answered affirmatively and that Wyatt’s plans for an immersed tube were feasible technically. However, question (iv), the problem of posing an obstruction to navigation had proven troublesome. The Company blamed the’ accidental and unforeseen circumstances arising from the crowded state of the River’ for the fact that the cost of the trials had far exceeded the estimate.
In November 1811 the Directors of the Thames Archway Co declared that they considered it proper to suspend operations, being deterred from proceeding with the main project by the cost of the trials.
The book also points out that the use of compressed air for tunneling was suggested by Thomas Cochrane in 1830.
Earlier successful tunnels across the Thames might have resulted in a larger network.