1896.] on the Tunnel under the Thames at Blackwall. 83 



the north side of the Thames. The section of the tunnel shows 

 that at this point the river is 1200 feet in width and 46 feet in 

 depth at high water ; and borings revealed the fact that although 

 the London clay was present on both banks of the river, yet 

 that the tunnel must pass below it into the sands and clays of the 

 Woolwich series, and for a considerable distance through a bed of 

 gravel which apparently filled an older and deeper river bed. Not 

 only had the river to be passed under, but it will be noticed that it is 

 embanked, and that what were the old marshes on each side are below 

 the level of high water in the river. Further, it will be observed from 

 the section that the soil of these marshes, to a depth considerably 

 below that of low water in the river, consists of vegetable soil, peat, 

 sand and gravel, all of them highly charged with water. Under this 

 is the London clay with its base beds of impure limestone full of 

 fossils, and then the sands, clays, &c., of the Woolwich beds ; all these 

 beds were under the full pressure of water due to the varying tidal 

 level, and in their natural state so saturated were some of the beds of 

 sand as to convert them into quicksands. 



But undoubtedly the most serious obstacle was the large deep bed 

 of coarse gravel with but little sand. This gravel was open and fully 

 saturated with the river water, and as the bottom of the tunnel was 

 to be 80 feet below high water it was certain that a pressure of about 

 35 lbs. on the square inch would have to be encountered. This, 

 however, was not the only difficulty, for it was clear that if the water 

 could find an easy entrance to, and flow amoug the gravel, air would 

 also as easily escape from it. It was obvious, therefore, from the 

 outset that the tunnel would have to be constructed under difficulties 

 never before contended with, either in the construction of Brunei's 

 tunnel at Kotherhithe or elsewhere. Moreover, it was evident that 

 no ordinary mode of tunnelling could be adopted, and that some 

 description of shield would be required. Also, from the difficulties 

 met with by Brunei at the much easier site at Rotherhithe, that some 

 more than ordinary measures would have to be resorted to to keep 

 out the inflow of water in passing under the river and through the 

 gravel bed above referred to. It was consequently determined to use 

 compressed air, as had been first suggested, in his patent of 1830, by 

 Admiral Lord Cochrane (Earl Dundonald), and which had been 

 successfully used under Lake Michigan and the Hudson River at 

 New York, as well as at the tunnel under the Saint Clair Eiver, and 

 on a portion of the City and South London Railway at Stockwell. 

 After consultation with Sir Benjamin Baker and Mr. Greathead, the 

 final design was determined upon, and the contract was let by the 

 London County Council to Messrs. S. Pearson and Son for 871,000Z. 

 early in 1891. 



The whole work is 6200 feet in length ; the incline on the south 

 or Kent side of the river is on a gradient of 1 in 36 and has a run of 

 2408-6 feet ; the portion under the river for a distance of 1212 feet is 

 level, and the north or Middlesex incline has a gradient of 1 in 31 for 



G 2 



