DREDGING. 341 



Variation of Quantities. The quantities, as taken from 

 the cross-sections, will of course show the amount of work to 

 be done in the solid ; but, owing to the increase in bulk in 

 breaking up, the silting that inevitably goes on during the 

 operation of dredging, slips on the slopes, and the quantity 

 of water mixed with and lifted with the dredged mud, silt, or 

 clay, the quantity estimated from the cross-sections in the 

 solid will be increased from 25 to 50 per cent., and in some 

 cases to double the quantity as dredged material. 



At Dunkirk, observations made with reference to the 

 bucket-dredgers working in silty material with occasional 

 boulders, showed that the difference between the quantities, 

 as measured from the cross-sections, and in the hopper-barges, 

 after the excess water had flowed off, varied from 25 to 45 

 per cent., according to the age of the deposit and the propor- 

 tion of sand contained in the silt. 1 



As regards the actual quantity of material lifted by the 

 suction-pump dredgers, tests made from time to time in a 

 graduated measure showed that the mixture brought up by 

 the pumps contained from 4 to 10 per cent, of sand, which 

 settled in the hopper, the average being 6 per cent., sometimes 

 the proportions extended to 15 or 20 per cent. ; this, however, 

 indicated that the slopes were slipping, and that a block of the 

 suction-pipe was imminent. 2 



In dredging in sand at the mouths of the ports of Amster- 

 dam and Rotterdam, a proportion of 1 part sand to 7 parts 

 water has been recorded equal to about 14'28 per cent, of the 

 total quantity lifted; of this about 6 per cent, remained in 

 suspension, and was carried away with the surplus water. 3 



At the Oaklands Harbour Works in California, where the 

 material was reduced to the consistency of mud by mechanical 

 means before being dealt with by the pumps, as much as 40 per 

 cent, of material was carried in suspension. Experience, however, 

 showed that about 15 per cent, was the amount that could be 

 most conveniently dealt with by the pumps. 4 



The following are the results of observations on the efficiency 

 of a sand-pump dredger, fitted with underwater suction, made 

 whilst working on the Liverpool Bar. 5 



1 M.P.I.C.E., vol. Ixxxix. pp. 74, 75. * Ibid., p. 83. * Ibid., p. 70. 



4 Engineering, vol. xxxviii. p. 30. 



1 Communicated by Messrs. Lobnitz, of Renfrew, Scotland. 



