SCOURING AND SLUICING. 289 



injury to the gates. The trials were resumed and continued 

 at suitable intervals, when the gates of the south sluicing- 

 chamber were carried away in a similar manner to the former. 



"From sections taken across the basin after every set of 

 trials, it was soon discovered that a portion of the bottom, 

 immediately beyond the stone apron, was being gradually 

 washed out, and that a hole 6 feet deep had thus been formed. 

 All attempts to fill up and reduce this hole by discharging large 

 blocks of rubble stone into it, were ineffectual. It was found 

 that the sheet-piling in front of the apron on the margin of the 

 hole was bare on the face to a depth of 4 feet, sloping down 

 suddenly to a depth of 9 feet below the finished bottom of the 

 basin. The piling had also parted from the masonry of the 

 apron, leaving a space between the stonework and the heads of 

 the piles of from 3 to 9 inches. 



" The water was pumped out of the north chamber, and an 

 examination showed that the masonry therein had sustained 

 considerable damage. The side walls and upper portions of the 

 interior were uninjured, but a large quantity of the floor 

 immediately at the back of the sluices had been torn up, and 

 the concrete and the piles laid bare in several places. No 

 remains of the broken-up masonry were found in the chamber, 

 all had been carried out into the basin, except one sandstone- 

 ashlar block, which, being too large for the opening, had become 

 jammed in one of the central sluices. A subsequent examina- 

 tion of the channel leading to the south sluices showed that 

 similar damage, though not to the same extent, had taken 

 place in the floor of that chamber. 



" Prior to the commencement of the sluicing operations, a 

 deposit had accumulated over the bottom of the basin to an 

 average thickness of 32 inches. 1 The result of the operations, 

 which extended over a period of ten months, was that the 

 deposit was reduced from 32 inches to 14 inches ; equal to an 

 average of 28 inches over the whole area of the basin, viz. 

 14 acres. The quantity of material removed was therefore 

 51,800 cubic yards, in addition to the quantity carried into the 

 basin daily by the recurring tides, which was estimated to 

 amount to 1264 cubic yards 2 per week ; equal to 54,352 cubic 

 yards deposited within the period of 43 weeks over which 

 tha sluicing operations extended, and making a total of 106,100 



1 M.PJ.C.E, vol. xxviii. p. 528. * Hid., p. 529. 



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