) 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.

AUTHORS' PRESENTATIONMr Kell remarked, in introducing the Paper, that the Blackwall duplication was the third large sub-aqueous tunnel to be completed in Britain in recent years, and like the others was a bored tunnel constructed largely under compressed air.80. The main interest in the tunnelling work at Blackwall probably lay in the extent to which it exemplified the trend towards the use of ground treatment processes in order to relieve part of the hazards attendant upon compressed air work.81. A measure of the success of the grouting programme in reducing the permeability of the ground could be found by comparing the compressed air consumption in the pilot tunnels and in the main tunnel. The average rates were about 5000 cu. ft/ min in both, although the face of the main tunnel was about 13 times the area of the pilot tunnel face. The average working pressure in the main tunnel through the buried channel section was about 27 Ib/sq. in., the same as had been required for the upper pilot tunnel. That was not quite as low as had been hoped for but nevertheless a significant gain in the circumstances of the work.82. It was not at present practicable to do without compressed air entirely in driving a bored tunnel through ground of the kind encountered at Blackwall. However, at a time when attention was focused upon the hitherto little known risk of bone necrosis, it was more than ever desirable to seek methods that would either reduce or eliminate dependence upon compressed air. The development of a tunnelling machine capable of working through non-cohesive water-bearing ground would be a great step forward in that direction.83. With changing techniques the Blackwall duplication might well be among the last major works to require a large force of compressed air workers over a construction period of several years., .84. The figures relating to recompressions in 0 42 were incomplete; the amended totals were 80 781 compressions and 858 recompressions, giving a bends rate of 1.06%.85. The health and welfare of the compressed air workers had been well cared for. Dr Philip Griffiths, who deserved the gratitude of everyone concerned with compressed air work for his humane and dedicated service in that field, was Resident Medical Officer on the site throughout the work.Mr Ridley explained that the traffic figure in 8 5 of the Paper was the assessment given in the London Traffic Survey Volume 2 of the two-way 24 hour average weekday flow for 1981. The capacity of the new tunnel operating in one direction only should be about 30 000 vehicles per day. Taking this figure in conjunction with the capacity of the existing tunnel operating one-way in the othel direction, even after improvement, there was a wide gap between demand and the present provision being made.87. Since the Paper was written, proposals for traffic control ($5 71-74)