Photo of the Week
March 25, 2012
In the next few weeks, you are going to be privileged to view some rare and rarely seen photos taken well over 120 years ago during the French effort of digging the Panama Canal. I started it out last week with a great engraving and documented research of the Slaven Dredges. The Slaven Dredges basically replaced the dredges that you are witness today, the High Tower Dredges.
Above is a photo showing High Tower Dredges tied up along the bank of somewhere along the old French Canal diggings between Colon and Mindi.
Photo courtesy of Ron Armstrong, CZ Images contributor. Thanks so much Ron.
The following documented research comes from a book by Charles Prelini titled, "Dredges and Dredging", 1911.
High tower Dredges. When the dredged materials are used for forming ditches or filling lowlands along the shore, the ladder dredges are built with high towers. The materials are then conveyed to the dumping place by means of a long tube and they are discharged by gravity. The height of the tower depends upon two factors, viz., the distance of the dump and the elevation of the shores from deck. They are built of different heights, but always between 50 and 80 ft. The ladder is always located at the middle of the boat in an open pit open pit.
When the buckets have reached the top of the ladder, while they revolve around the upper tumbler the materials fall into a bell leading to a closed chute, which is extended into a long tube. With dredges built to work in narrow rivers or canals, when both shores can be easily reached by the conveying apparatus, they are usually provided with two chutes and two tubes. In this manner were constructed the high tower ladder dredges used on the Panama Ganal under the French and also on the Nicaragua Canal. The conveying tubes are made of sheet iron 2 or 3 ft. in diameter. In dredges provided with only one long discharging tube this is located on one side of the boat at a right angle to the ladder. The tube is supported from the right angle supported tower by a series of gang wire ropes and also from a trussed A frame located on one side of the boat and strongly fixed to the other side by means of backstays. The conveying tube is made of varying lengths, reaching sometimes 150 and even 180 ft. To reach points at great distances from the dredge the conveying tube is extended to land, but in such a case the land section of the tube is supported by specially constructed trusses To facilitate the flow of the materials through the tube this is placed with an inclination varying from 2 to 10 per cent depending upon the quality of the dredged materials. The descent of the materials through the tube is facilitated also by means of water jets forced by a centrifugal pump placed on deck, which continuously flush the tube.
The high-tower ladder dredge presents several disadvantages; 1st, the necessity of elevating the materials to a greater height than absolutely necessary, and consequently the motive power is not economically utilized; 2d, the necessity of acquiring powerful and expensive machines, which involve high running expenses; 3d, the stability of the dredge is greatly hampered on account of having very high towers on boats usually built of small dimensions; 4th, the material being deposited in a fluid state and near the edge of water has a tendency to run back again to the point of excavation.
Many engineers object to this type of dredge, while there are others who absolutely condemn it as obsolete. But when it is considered that in the ladder dredges, the coal consumption represents only 10 or 15 per cent of the running expenses, it is easily realized that the cost per unit of volume of the dredged materials will not be greatly affected by even doubling the cost of fuel. On the other hand the great advantage of these machines consists in the fact that they convey the dredged materials to the dumping place without additional cost of transportation. Consequently the small increase in the cost of dredging is more than compensated by the free conveyance of the debris to the dumping places. Another advantage of the high tower ladder dredge consists in the fact that working day and night it will perform the work of two machines. thus greatly reducing the cost of the plant and the general expenses. Even today these high- tower ladder dredges in some classes of work will give the same satisfactory results as they did along the Suez Canal in the early days of their existence. Over one-third of the Suez Canal was excavated by means of these high-tower ladder dredges and they were found very efficient in dredging the lower sections of the Panama Canal during the French Administration.
The following description of the dredge "City of Paris" taken from the Scientific American, serves to illustrate a high-tower ladder dredge built in the United States, and used by the American Contracting & Dredging Co. of New York for the Panama and Nicaragua Canals.
The typical American dredge represented by the "City of Paris", is provided with composite hull 115 ft. long and 56 ft. wide In the forward end of the dredge is a slot, through which the lower section of the ladder 36 ft. long and 7 ft. wide, descends. An endless chain of buckets travels up and down the ladder, cutting away the bottom wherever directed, and delivering the material to the discharger on top of the tower. The upper section of the ladder for this purpose is carried up to the top on an incline. A joint is provided between the sections, so that the lower portion can be raised and lowered. The upper section is 73 ft. 10 in. long. The buckets are of 5/8-in. steel, and have a capacity of 1 cu. m. each. The links of the chain are 1 5/8 in. x 1 in. steel and are 3 ft. long. The shaft at the top of the tower around which the chain and buckets travel is 14 in. in diameter.
The chain is driven by double cylinder engines, 16x24 in., with 10 ft driving pulley with 38 in face. The dredge is anchored by wooden spuds or heavy vertical beams, 25 in. diameter, with 1800 lb iron shoes upon their lower ends. These ends are lowered to the bottom, and sinking through the earth anchor the machine securely. Besides the main engine, there are several auxiliary engines for working the spuds, raising the ladder, etc.
The material as dredged and raised to the top of the tower is emptied into the bell of one of a pair of iron chutes These are pipes 3 ft in diameter and 185 ft long which run far out on both sides of the tower Water is pumped into them along with the solid material Great banks of sand are built up by its operations.
When Mr W. P. Williams, Sr. examined this dredge in the Panama Canal in 1889 under the French Administration, it was running nineteen to twenty-one buckets per minute, three- fourths full, three buckets to the cu. m. The expansion of material in buckets was 30 per cent, occupying more volume in the buckets than in bank. He estimated in twenty four hours work it would accomplish 4800 cu. m.
He gives the average efficiency of this machine as follows:
Soft sticky clay buckets not fully
emptying at upper tumbler - 3000 to 4000 cu.yds. per day Hard
cu.yds. per day
The machine is under the complete control of the captain, who is stationed on the bow of the machine. A system of wheels at his hand connects with the different engines, namely raising and lowering the lever, controlling the main engine and velocity of revolution of buckets, the gypsy engine working the side guys, the spuds also being raised and lowered by tackles on hoisting drums. The digger may at a glance, take in the situation, and use his governing wheels accordingly.
The cost of the work can be deduced from the two most important items, which are the consumption of coal and the wages of the men handling the machine. Mr Williams gives the consumption of coal in the "City of Paris" at 10 tons per day, while regarding the men he says that at the Panama Canal, there were from 45 to 55 men on these machines distributed, as follows:
1 captain $300 per month; 3 assistant
engineers $150; 3 firemen $70; 3 oilers $50; 3 diggers $65; 3 gypsy men
$65; 1 steward and three cooks $75; 6 waiter boys $30; 22-32 seamen
The American Contracting & Dredging Co. of New York was paid 35 cents per cu. m. and notwithstanding they paid such high salaries they realized over 50 per cent profit.
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