) said that the Authors and their colleagues were to be congratulated on the successful completion of this notable work in the face of very difficult conditions. 155. The Authors of the first Paper said in 5 17 that because of hazards to, and from shipping it was not possible to sink boreholes within the width of the river and it was realized that much uncertainty must exist as to the nature of the ground there until the first pilot tunnel had been driven. As it happened there was a good cover of boulder clay in that part of the tunnel, but conditions were not always as favourable as this and the lack of boring in the river bed could sometimes give rise to tunnelling hazards that were considerably greater in terms of the risk of life than those which would have been accepted in getting borings in the first place.156. He thought that port and river authorities were becoming more understanding in this respect as time went on, and rightly so, because although geophysical sounding methods could be extremely useful, actual borings were very often indispensable.157. The internal diameter of the cast-iron lining was given as 29 ft 6 in. It would be interesting to know what considerations determined the choice of this dimension. It was normally possible with a diameter of this order to accommodate a 22-ft carriageway with a full standard clearance of 16 ft 6 in. at the kerb. What had evidently prevented this from being the case was the need for sufficient head room for cyclists and pedestrians under the road deck. He could not help wondering why in this case the diameter of the tunnel was not increased in order to give the full clearance over the full width of the carriageway. Whether the standard 16 ft 6 in. was excessive might be a matter of opinion. Other countries used a lower value, but it was still the standard in Great Britain, and with the increasing tendency of road users to take full advantage of it in the shape of tall vehicles and high loads, he thought it was advisable that new road tunnels should be free of the kind of restrictions which the Authors had described in 5 72.

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