KEEPING THE CHANNELS CLEAR.

Pukekohe & Waiuku Times, Volume 4, Issue 22, 19 March 1915, Page 3 (Supplement)

KEEPING THE CHANNELS CLEAR.

THE WONDERFUL WORK OF THE DREDGERS. The general public are certainly not aware of the important part that wonderful contrivance, known as a dredger, which scoops up mud and sand from the river's bottom, plays in our commercial life, says an English paper. Indeed, were it not for these machines which delight to grovel in the dirt, our principal ports, such as London, Liverpool. "Glasgow, Cardiff, etc., would be absolutely inaccessible to modern ships. Lying a considerable distance from the "open sea, vessels can only reach them "through channels in which sand, silt and mud are being constantly deposited bv the action of the river an 1 tides. If"this material were not removed the rivers would soon be blocked to everything except craft of light draft. Furthermore, ships are rapidly increasing in draft, and also in tonnage, with the result that estuaries deep enough for vessels of a decade ago would be use'ess to the merchantmen of todav. Thus not only has it become necessary to remove the continual formin-' deposits of sand and mud from the rivers' bottoms, but to deepen them by scooping out their beds so that the great ships may navigate them without fear of stranding. THE MERSEY BAR. What dredging has done for our leadin" ports is "obvious when it is stated that the estuary of the Mersey, on which Liverpool stands, was, until about thirty-five years ago, almost blocked at low tide by a great sandbar, over which there was only about eight feet of water. To make Liverpool a first-class port, it was necessary to cut a huge trench through this sandbar, and a fleet of dredgers was set to do the work. Bv 1907 the total quantity of sand excavated, carried away, and dumped where it would not impede navigation had risen to nearly 110,000,000 tons; and the largest ships could reach the port even at the lowest spring People who are acouainted with the great ship-building trade of the River Clvde and the sea-borne commerce ol Glasgow, may be surprised to learn th:u in the middle of the eighteenth century the river was so constricted by sandbanks that even at high tides boats drawing more than three feet of water could not reach the Glasgow quays, lne opening up of the river is due almost entirelv to" dredgers. A decade ago vessels drawing more than seventeen or eighteen feet of water could not reach many of the docks on the Thames at low tide. . The Port of London Authority spends about £30,000 a vear on dredging, while Liverpool's annual bill for similar work, bv which the Mersey is kept open, amounts to £12,500. In a like manner it costs Bristol £10,500 a year, on an average to keep the Avon deep enough so that boats may reach the Bristol docks at anv state of the tide. On both the Tyne and the Clvde are situated the great' ship-building yards, and the increase in the size of warships, liners and cargo steamers generally has meant that both these rivers have had to be considerably deepened, with the result that -r-50,000 is spent in a single twelvemonth on the former and £-10,000 on the latter river in dredging. LONDON'S WATERWAY.

So far as the Thames is concerned, two grab dredgers are requisitioned to keep the channel clear of mud and sand between Tedclington and London Bridge, a distance of 134 miles. Both these machines have a capacity of 20 cubic vards ner hour. As the name indicates, this particular type of dredger consists of a grab, mounted on a long arm, which is lowered into the stream. When it touches the bottom the teeth are made to close-biting great holes several feet wide and two feet more in depth into the river bed. This material is then lifted and deposited either on the dredge or in a barge or upon the river s bank. Between London Bridge and uravescnd, a distance of 26 miles, the more powerful bucket dredger is employed. There are six of these, the largest being capable of lifting 765 cubic yards of material per hour when working at a maximum depth of 55 feet. No less than eight steam hopper barges of 1,000 cubic yards capacity each wait upon this single vessel, while each of the other dredgers has several hoppers which convey the excavated material out to sea and there dump it. STRANGE FINDS. These dredgers are capable of dealing with sand and soft clay in which there may !>e boulders and other hard material. In many respects they are wonderful vessels. 'The hull, aft of midships, lias a central opening or well extending to the stern, where it is bridged over. In this well is a very strong steel girder, pivoted at the forward end on the top of a strong steel superstructure, and suspended near the other end by powerful tackle supported by a kind of tower. An endless belt of steel buckets, joined together by strong links, runs round flat-sided pulleys or tumblers at the extremities. The buckets travel down the under-side of the ladder, out into the material as they pass upwards round the bottom end, and carry their loads up the upper face of the ladder, which is provided with rows of rollers. As they pass round the tumbler at the highest point of their elevation they discharge their contents into a shoot leading into troughs that feed the hopper barges. The biggest buckets used on the Thames are capable of holding 27 cubic feet of material, which is small compared with the 54 cubic feet capacity of some of the dredgers on the Suez Canal. Yet those in the Thames ha', e brought to the surface stones weighing as much a? two tons apiece. Indeed, it is surprising what these buckets are capable of raising. Heavy anchors, long lengths of chain, and great nieces of iron, steel plates, etc.. are objects that are occasionally raised from the Thames bottom !>y the larger buckets. Strang, too, are some of the articles they bring to light. Recently one dragged up an iron bedstead and another a chest of valuable tools. Shins' compasses, field-glasses. Indies' handbags and purses have been fished up bv the buckets and returned to their owners. So far as eels and fish are concerned they are too common to cause comment. ROCK-BREAKING RAMS. Should a bucket strike a stone or piece of rock which it cannot move danger would be caused to the lip of the bucket, while excessive strain would be thrown upon the chain and drivinir gear. To prevent accidents of this description a special clutch is hrnticht into plav which releases the bucket when" the strain exceeds a certain limit. There are special dredgers, bowever, for removing rock. These are fitted with rock-breaking rams. These rams, which are in principle huge chisel-pointed hammers, weighing four tons CH'b. r.e raised bv hydraulic power nnd allowed to fall through from 10 to T> feet. There are five on each side of

thu well, and an ordinary bucket dredger working between them, raises tiie rock bv the hammers. Off Southend, operating in what is known as the Yantlet Channel, some nine miles in length, there is a suction hopper dredger with a hopper capacity of 70,000 cubic feet. This interesting type of dredger is capable of raising 3,500 tons of fine mud and sand in 50 minutes from a maximum depth of 60 feet. We get an idea of what these machines are capable of accomplishing in removing sand from the bottom of the sea b\ a description of the Leviathan, on the Mersey, the largest sandpump dredger in existence. It is W feet long and 69 feet wide, and has twelve hoppers, each able to hold nearly 1.000 tons of sand. Large cylindrical valves, operated hy hydraulic power, discharge the whole of the contents in ten minutes. The vessel has four pumps, each driven by an 800-horse-power steam engine. The suction pipes are 90 feet each in length and 42 inches in diameter. About fifty minutes' hard pumping fills up the hoppers. The Leviathan's twin screws then propel her at ten miles an Lour to the dumping ground, where the valves are opened and the contents poured out. Then she returns to the dredging ground for another load. On her trial the Leviathan dredged 20,000 tons of sand and deposited it at a point ten miles from the scene of operations in five hours' working time. It is estimated that this single • dredger could, under fair conditions, raise and dump 20,000,000 tons in the course of a year. THE CLAY-CUTTER. An interesting typo of suction dredger, used for making channels rather than for keeping them open, is the claycutter. Clay is too stiff a material to be detached* from its bed by suction alone; it must first he loosened. The nozzle end of the suction pipe is therefore furnished with a revolving cutter —a cage of curved knives —which slices off lumps of the clay and delivers them to the pipe. As clay dredging is usually done close to shore, the "spoil' 1 can be forced through floating pipes and deposited where it will be usefully employed for claiming land from the sea, so the change of the material's position brings two advantages with it. The greasy nature of clay in water makes it slide'through pipes more easily than sand. Two clay-dredgers, the "Jinga" and "Kalu," were recently sent to Bombay for the improvement of the harbour there. Shortly after her arrival the

" Jinga" cut a channel 300 feet wide, 21 feet deep, and 1,000 feet long in 85 hours, at an average of 2,700 cubic yards per hour. During these operations a quantity of quarried stone, including pieces of over 4001b. weight, were encountered, sucked up and forced bv the pump to the general dump-ing-ground. The "Jinga'' and her companion have entirely ousted the bul-lock-carts previously used in the district for reclamation work; and no wonder, as they can do as much work in a single hour as 5,000 carts could do in a whole day!