subaqueous tunneling, surmised in later years thai 

 Brunei had chosen a rectangular configuration for 



actual use, as one better adapted to the sectional 

 type of shield. The English civil engineer, Peter W. 

 Barlow, in 1864 and 1868 patented .1 circular shield, 

 of one piece, which was the basis ol one used b) him in 

 constructing a small subwa) ol 1 >50 feel beneath the 

 Thames in 1869, the firs! work to follow the li 

 Brunei, (irc.11hc.nl. acting as Barlow's contractor, 

 was the designer ol the shield actuall) used in the 

 work, but it was obvioush inspired In Harlow's 

 patents. 



The reduction ol the multiplicity of parts in the 

 Brunei shield to a single rigid unit was of immense 

 advantage and an advance perhaps equal to the 

 shield concept of tunneling itself. The Barlow- 

 1 Iri tthead shield was like the cap ol .1 telescope with 

 a sharpened circular ring mi the front to assist in 

 penetrating the ground. The diaphragm functioned, 

 .is did Brunei's breasting boards, to resist the longi- 

 tudinal earth pressure ol the face, and the cylindrical 

 portion behind the diaphragm bore the radial pressui e 

 ul roof and walls. I lere also for the lirst time, a 



permanent lining formed of east-iron segments was 



used, a second major advancement in soft-ground 

 tunneling practice. Not only could the segments be 

 placed and bolted together far nunc rapidly than 

 masonry lining could be laid up, but unlike the 

 green masonry, they could immediately bear the full 

 tunc ul the shield-propelling screws. 



Barlow, capitalizing on Brunei's error in burrowing 

 so close to the riverbed, maintained an average cover 

 of 30 feet over the tunnel, driving through a solid 

 stratum of linn London clay which was virtually 

 impervious to water. As the result <ii tins, combined 

 with the advantages of the solid shield and the 

 rapidly placed iron lining, the work moved forward 

 at a pace and with a facility in startling contrast to 

 that of the Thames Tunnel, although in fairness it 

 must be recalled that the face area was far less. 



The cla\ was found sufficiently sound that it could 

 be readily excavated without the support of the dia- 

 phragm, and normally three miners worked in hunt 

 of the shield. 1 1 i •_; u i n U out the clay and passing it 

 back through a doorway in the plate. This could be 

 closed in case of a sudden settlement or break in. 

 Following excavation, the shield was advanced is 



inches into the excavated area by means of (> screws, 

 and a ring of lining segments IS inches in length 

 bolted to the previous ring under cover of the over- 



Figure 21, Eni vrc \n ol Brunei's 



tunneling shield, vciiiial sniion. I lie 

 lnsi two and pan ol the third of the 

 twelve frames are shown, to the left is 

 the tunnel's completed brick lining and to 

 the right, the indiv idual breastii 

 and -i rews for suppoi ting the fa< e l"h< 

 propelling si i ews ai e seen al top and 

 bearing against the lining. 

 Three miners worked in each frame, one 

 .ili ive the other. Mill model 

 (Smithsonian photi 



lapping rear skirt of the shield. The small annular 

 space left between the outside of the lining and the 

 clay by the thickness and clearance of the skirt about 

 an inch was Idled with thin cement grout. The 

 tunnel was advanced 18 inches during each 8-hour 

 shift. The work continued around the clock, and the 



I'AI'I K 11: M WEI. ENGINE! RIM, A MUSEUM TKEAIMIM 



221 



