50 HAY AND FITZMAURIUE ON BLACKWALL TUNNEL. [Minutes of 13 April, 1897. WILLIAM HENRY PREECE, C.B., F.R.S., Vice-President, in the Chair. IT was announced that HENRYRICHAIW CLARKE PAULING, :Associate Member, had been transferred to the class of Members. (Paper No. 3021.) “ The Blackwall Tunnel.” By DAVIDHAY, M. Inst. C.E., and NAURICE FITZYAURICE,B.E., M. Inst. C.E. THE necessity of communicationbetween thenorth and south banks of the Thames below London Bridge other than by ferry was pressing even in the beginning of the present century, and it has become accentuatedas the manufacturing districts have extended eastward along the river. A comparison of the crossings above and below London Bridge with the respective populations east and west of the bridge, will show that the east of London is still badly provided for in thisrespect. HISTORICAL. A tunnel to connect Tilbury with Gravesend was commenced in 1798, but the work did not proceed beyond sinking a shaft at one side of the river, as the difficulty experienced with it exhausted the capital of the undertaking. In 1805 a tunnel betweenLimehouse andRotherhithe was commenced by Vazie; a shaft was sunk and a headingdriven under the river for a dista,nce of more than 1,000 feet, but the river then broke in and thework was abandoned. In 1825 Brunel commenced the tunnel between Wapping and Rotherhithe, which, as is well known, is built of brickwork, and consists of two arched passages. The outside width of the brick- work is 37 feet 6 inches, andthe height is 22 feet 3 inches. Theshield was made in twelveseparate sections, each capabJe of being driven forward separately, by screws abutting against the completed brickwork;and the ground between theshield and brickwork, as the former advanced, was supported by plates fixed tothe shield and sliding on the back of the completed Downloaded by [ University of Sussex] on [16/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.1 HAY AND FITZMAURICE ON BLACKWALL TUNNEL. 51 work.' The river broke into the tunnel several times during con- struction, and, after a serious irruption on the 28th August, 1828, the work was stopped until January, 1835. The first shield was then replaced by anew and stronger one, thoughstill on the same principle, and the work was finally completed in 1842. The length of the tunnel was about 1,200 feet, and work was in active progressfor about ten years, so that the averageprogress was only about 120 feet per annum. The cost, including shafts, was about 51,300 perlineal yard. This tunnel, originally intended for road traffic, is now used by the East London Railway. It is interesting to note that here, as at Blackwall, much of the excava- tion was through clay which had been tipped from barges on to theriver bed, andwhich came down asthe sand and ballast beneath ran into theshield. Theonly other tunnel constructed under the Thamesbelow London Bridge was the Tower Subway, built in 1869 by Mr. Peter Barlow, F.R.S., Past-President, and the late Mr. J. H. Greathead, M. Inst. C.E. This tunnel was kept at alow level so as tobe in London clay throughout, and consequently no trouble was ex- perienced from water. A shielddriven forward by screwswas used, and a cast-iron lining was adopted for the first time. In 1876 Mr. Greathead started another subway between North and South Woolwich, but the workwas never completed. Schemesfor tunnels at differentpoints below LondonBridge have been brought forward from time to time, the most important of which were thoseproposed close to the siteof the presentTower Bridge, at Nightingale Lane, and at Shadwell; and subways for foot-passengers at Limehouse and Greenwich. On account of the docks on both sidesof the river, there are fewplaces east of London Bridge where a tunnelfor vehicular trafficcould be made to tap the main lines of traffic, andat the same time allow good approaches. About the year 1875 it became generallyrecognized that the construction of more river crossings below London Bridge could notbe further delayed, andthe matter was taken up bythe Metropolitan Boardof Works, who brought forward severalschemes both for bridges and tunnels ; but it was not until 1887 that the Blackwall TunnelAct wasobtained. The design, as proposed bySir JosephBazalgette, consisted of threetunnels, twofor vehicular traffic, and one for foot-passengers ; and it was deter- mined to construct the latter in the first instance. The internal By the courtesy of Messrs. Maudslay, Sons and Field B model of this shield wm exhibited. E2 Downloaded by [ University of Sussex] on [16/09/16]. Copyright © ICE Publishing, all rights reserved. 52 HAY AND FITZMAURICE ON BLACKWALL TUNNEL. minutes of diameter was to be 15 feet, and the tunnel was to be made of cast iron, andto be linedwith brickwork. A cross-section of the tunnel, and of the shield which Sir Joseph Bazalgette proposed to use in itsconstruction, are shown in Fig. 9, Plate 2, and Fig. 12, Plate 3. Tenders for this work were invited, and it was decided to let it t,oMessrs. S. Pearson and Son ; but owing to the Dletropolitan Board of Works being replaced by the London County Council, and the latter body considering that a larger tunnel ought to be made at once, the contract was never carried out. It was then three years before the final decision was made, and at the end of 1891 thecontract for thepresent tunnel designed by Mr. Alex. R. Binnie, M. Inst. C.E., Chief Engineer to the Council, was obtained byMessrs. S. Pearson and Son, the amount of their tender being 5871,000. The total length of the tunnel is 6,200 feet, 1,220 feet of which are below the river. The lengths adopted for each type of construction are shown in Fig. 1, Plate 2, that of the cast- iron lined portion being 3,112 feet. The gradients on each side rising to existing roads involved interference toa large extent with the sewers, which had to be diverted and reconstructed. A large amount of this work was carried out, but, with the exception of the reconstruction of the Isle of Dogs low-level sewer, an egg- shaped sewer 10 feet by 6 feet 8 inches, where difficulty was experienced with water, and where the flow of sewage could not be interfered with whileconnecting up theold and the newwork, it was of ordinary character. This sewer was constructed partly in tunnel and partly indeep open-cut, for a long length of which a 6-foot barrelwas carried on thearch of thelarger sewer. Severalsmaller brick sewerswere constructed, aswell as a number of pipe-sewers of diameters of 2 feet and less. The total cost of the work of this kind was about 540,000. OPEN APPROACHES. The open approaches on each side of the river extend from the ends of the cut-and-cover work to the points where the roadway attains the existingroad-levels. The construction adopted consists of concrete and brick retainingwalls ,with a concrete invert between them. On the Middlesex side the excavation was almost entirely in clay or made ground, and there was little difficulty with water. The excavation was taken out to thefull width before any concrete or brickwork was inserted, and, except at the deep end, very little timbering was necessary. On the Eent side the work at thedeep end was almost whollyin water-laden ballast. Downloaded by [ University of Sussex] on [16/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.] HAY AND FITZNAURICE ON BLACKWALL TUNKEL. 53 Theside walls were first builtin timbered trenches, andthe dumpling was afterwards excavated and the invert inserted. By this mode of construction there is no danger of a longitudinal fracture between the walls and the invert, due to unequal settle- ment, and lighter timbering is required for the narrow trenches than for a trench embracing the whole of the structure. Skew- backs were formed at the bottom of the walls toreceive the invert, and as this temporarilyreduced the base, timber strutswere placed between the two walls at about theroad-level when the dumpling was excavatedto that point, untilthe invert was constructed. A cross-section at the deep end of the open approach is shown in Fig. 2, Plate 2, the section being the same on both sides of the river. The brickwork has a white glazed face and is bonded into the concrete, being alternately 9 inches and 18 inches thick, in equal lengths of 3 feet. As water-tightness was absolutely neces- sary, the walls were coated with 14 inch of asphalt, which was carried down the back to within 3 feet of the bottom and thence across throughthe invert. This gave a continuous water-tight layer completely surrounding the work, which has proved satis- factory. At one or two places, owing probably to expansion and contraction due to changes of temperature, cracks have appeared, but the amount of leakage is almost inappreciable and is carried away in smallpipes connected to the road gully-pipes. CUT-AND-COVERPORTION. The original intention was to adopt this mode of construction for lengths of 436 feet and 335 feet on the Middlesex and Kent sides respectirely, in accordance with the cross-sections shown in Fig. 3, Plate 2. In carrying out the work, however, on the Eent side the ground was foundto be better, and less difficulty was encountered from water than was anticipated ; so it was decided, after inquiry as to the relative cost, etc., to extend the trench- work by 582 feet. This brought it to No. 4 shaft, the point from which it was arranged to drive the shield northwards.