So far as regards tunnels actually constructed. We now come to speak of tunnels suggested. These may he divided into two classes—tunnels under rivers and tunnels under the ocean. Of the former, the first to he mentioned is that proposed to he constructed under the Mersey, to connect its Cheshire and Lancashire sides together. The scheme is propounded by Mr. John Hawkshaw, the eminent engineer, in a letter which he addressed to the Mersey Dock and Harbour Board, on the 31st of August last. Mr. Hawkshaw having stated that it is evident a bridge or viaduct over the river would interfere with the navigation, whilst the sand-stone rock which underlies its bed affords facilities for the construction of a tunnel, proceeds to show that the river should be crossed between New Brighton and Bootle, that being the best point for connecting together the dock lines of railway on each side of the river Mersey.
Total length of lines 9¼ miles; length of tunneling, 4,800 lineal yards: estimated cost £785,000.
The lowness of the estimate is owing to it not being necessary to pass through valuable property, or important commercial buildings of any kind. Nevertheless Mr. Hawkshaw feels that the usual allowance for contingencies should be increased from 10 to 20 per cent. on the outlay. Still it brings the total amount considerably under a million. Mr. Hawkshaw does not ask the Mersey Docks and Harbour Board to be at the total cost of these works, however important it is that the two several portions of the board’s establishment should be closely united in the manner which this tunnel accomplishes, but that it should only contribute a portion of the outlay—an outlay which he considers will not be more than a third of what will be required for accomplishing any other of the schemes that have been proposed for carrying a tunnel under the Mersey.
In 1864 the Dublin Trunk Connecting Railway obtained an Act for the construction of a railway in the immediate vicinity of Dublin. Part of the plan sanctioned by Parliament is a tunnel under the Liffey, less than half a mile from its mouth. The depth of the tunnel-top under the bed of the river will be 20 feet. The stratum of limestone rock is curiously placed where the tunnel is to be pierced. The bottom of it will rest upon the rock, but the tunnel itself will be constructed through the superjacent clay. It will be lined with brick in cement. Its length under the river is to be 324 yards, and the approaches to it, which are to be constructed in the manner known as “cut and cover,” are to be 430 yards each. The gradients on both sides will be 1 in 70, or 75½ feet in the mile. The cost is estimated by Mr. John Burke, the engineer for its construction, at £200 per yard forward.