EXCAVATION. The excavation of material is divided into two categories. Wet excavation is subaqueous excavation and the work is performed as described under DREDGERS AND DREDGING. Dry excavation is performed by machines which do not float on water.
This is by far the most important machine used in dry excavation. It is essentially a fixed frame, mounted most often on crawling treads, and supporting a revolving frame upon which is the power unit and gearing. A boom projects upward at an angle of about 45° from the forward end of the revolving frame and revolves as the frame revolves. A split dipper handle either passes through the mid-point of the boom or the two sticks pass on either side of the boom. The handle may be forced back and forth. A dipper is rigidly fastened to the forward end of the handle and is supported from the boom upper end by a wire cable or two wire cables extending from the dipper or dipper bail over a sheave at the point of the boom and down to a grooved drum on the revolving frame. The dipper has a door in the bottom which can be opened by the shovel operator by pulling a rope. It latches itself shut when the motion of the dipper causes the door to swing inward. The motions controlled by the operator are : to move the shovel forward and back and to turn, to revolve the up per frame, to thrust out and pull back the dipper handle, to hoist and lower the dipper handle and dipper, and to open the dipper door. The vertical angle of the boom is fixed while working. The power unit is usually an internal combustion engine or electric motors. One man operates the machine and an assistant oils the machinery and aids in moving the machine. There are three classifications as to size. The smallest ones, with dippers with a capacity from to 21- cu.yd., are used for loading trucks with earth and clay. The intermediate sizes, with 3 to 5 cu.yd. dippers, are used for quarry work, loading blasted rock into trucks or cars, and for stripping and mining copper and hematite iron ore. The largest ones, with 3o cu. yd. dippers, are used mostly for stripping coal. The largest operate as easily as the smallest and have the same size crew. The smallest will make a complete cycle in 18 seconds, the largest in 48 seconds. The theoretical capacity ranges from 64 cu.yd. per hour to 2,025 cubic yards. The output used in estimating steady performance is three-fourths the theo retical. The largest electric shovels have 2,300 h.p. in motors.
The dragline machine has a walk ing device for moving ahead or is identical with the revolving shovel as to base and revolving frame, but it has a long light boom at a
to 3o° angle above the ground with no dipper stick and, instead of the dipper hoisted by a rope, the dragline has a bucket hoisted by one rope over a sheave at the upper point of boom and thence to a drum on the revolving frame. Another rope, called the dragline, pulls the bucket toward the machine, all the strain of digging being on this rope and none on the boom. The two ropes can be pulled or slackened so as to cause the bucket to slide over the ground, to dig in, to tilt up for carrying the load in the air and to dump the load where desired. The crew is the same as for the shovel. The cycle is slower because of the inertia of the long boom, the sizes range from a s cu.yd. bucket and 28ft. boom with an output of 3o cu.yd. per hour to a 20 cu.yd. bucket and 2ooft. boom with an output of i,000 cu.yd. per hour. These ma chines are very useful for canal building and the very large ones are used exclusively for stripping phosphate in Florida. They are not so good as shovels for loading trucks or cars because they have a slower cycle and less perfect control over the bucket. The longest boom in use is 25oft. with a i 2cu.yd. bucket.
This is used for excavating and transporting earthy material a distance of over 75o feet. It is es pecially useful in building up old levees where the material for the levee has to come from beyond old borrow pits. It consists of a head tower on self propelling crawler tread mounting a power unit, gearing, and drums. From the top of this tower a steel track cable extends to the tail tower which is self-propelling on crawler treads. The drag bucket is suspended at its rear to a trolley which runs upon the track cable. A drag rope pulls the bucket toward the head tower for digging and pulls the loaded bucket along the ground to the dumping point where the track cable is tightened, thus dumping the bucket. The empty bucket slides back down the track cable assisted by a back-haul line from bucket to tail tower to head tower. The loaded bucket is never off the ground. The output with a io cu.yd. bucket is 300 to 48o cu.yd. per hour, depending upon the length of span. The crew consists of operator and helper on the head tower and opera tor on the tail tower. The dumping takes place just in front of the head tower.
This machine is a simple modi fication of the tower excavator in that there is no tail tower and no back-haul line. Instead the end of the track cable is anchored in the ground and the anchor shifted as required. The loaded bucket is suspended from the track cable instead of being pulled along on the ground.
These take the place of the horse-drawn scrapers, wheeled scrapers, Fresnos, etc., which dug and transported earth. The modern scrapers are two or four-wheeled and are pulled by a tractor. They consist of a large scoop which can be lowered by the tractor operator for digging and can be raised for carrying, or tilted up for dumping. The two-wheeled scrapers have a ca pacity from 5 to 8 cu.yd. and require a tractor of 4o to 7o horse power. The four-wheeled scrapers are of 5 to io cu.yd. capacity and are hauled by a 4o to 75 h.p. tractor. The tractor runs i oof t. at 2.4mi. per hour to load and thereafter runs at 4mi. per hour for loaded and empty hauls. The one operator drives the tractor and controls the functions of the scraper.
The elevating grader is especially useful in the case of a long shallow cut where trucks are to be loaded with the material excavated. It is pulled by a tractor. As the spreader is pulled forward, a disc plow digs a furrow and forces the material upon a moving belt. When plenty of trucks are available, the output averages 25o to 30o cu.yd. per hour, and when casting upon a pile, 50o cubic yards. The tractor should have 95–I oo h.p. and the digging speed is 2.4mi. per hour.
The bulldozer is a heavy steel blade fas tened to the front of a tractor and capable of being raised and lowered and in some cases swung into a beveling position. With this blade, the tractor can excavate earthy material, push it along and over the edge of a spoil bank or into a pile. For excavating, moving a short distance and depositing fairly soft material, this equipment is most effective.
If a deep narrow trench of some considerable length is to be excavated, a bucket or ladder type of sewer-excavator is employed. This machine propels itself and occupies a position just ahead of the trench. A ladder supports an endless chain of buckets mounted on steel chains. The material falls upon a belt conveyor which stacks it on one side of the trench. This machine is special for this class of work and is not adaptable to other kinds of work and therefore more and more of the trench digging is done by the back hoe equipment on a standard shovel.
Except where the excavated material may be dis posed of directly by the excavating machine, it is necessary to transport it to a distance. This was formerly done by narrow gauge cars running on tracks and hauled by steam locomotives. At present this work is done by belt conveyor, gasoline or Diesel truck, or by tractors and big dump wagons. With all but the belt conveyor, there is complete adaptation to the changing conditions at the loading and dumping locations. The belt conveyor is the cheapest in operating cost, but has not the adaptability of the others, as the loading end and the dumping end have to be shifted and lengthened as work proceeds.
Although the modern quarry shovel is very rugged, being built to withstand the great strains of loading rock, this rock must first be drilled and blasted so that the shovel can loosen up the piles of blasted rock of all sizes and load it into cars. So drills are employed, usually electric portable well drills using a heavy drill bit and drill stem on a drill rope. The holes drilled are of various sizes, but usually between five and six inches in diameter. The crew consists of the driller and helper. In medium hard limestone, the speed will average 8ft. per hour. Where a seam of soft stone is located at the elevation of the bot toms of the vertical holes, horizontal holes are sometimes drilled instead. This reduces the cost of drilling and greatly increases the quantity of powder which can be used to best advantage. These horizontal holes are drilled by an electric machine with a screw bit. Powder, dynamite or liquid oxygen are used as the explosive. The crew of the drill consists of an operator and a helper.
The hydraulic method is sometimes used for excavating dry earth and depositing it at some distance. A monitor or nozzle is fitted to the end of a pipe through which water flows under pressure. As the stream or jet issues from the nozzle, it is directed against the material to be moved. The material displaced by the jet flows with the water to the desired location by gravity, or to a pump which forces it to its destina tion. The monitor jet can be moved so as to impinge upon dif ferent sections of the bank being excavated. This method is used almost exclusively in mining pebble phosphate.
Low grade mineral deposits may sometimes be uncovered (or stripped) by simply overcasting the overburden with a large power shovel or dragline machine, mining out the uncovered strip of deposit and using that area for depositing the overburden from the next cut of the machine. To use this method, the thickness of the overburden must not exceed the limitations of the stripping machine. The thickness of the deposit usually varies from Oft. to i 5ft. with a maximum probably of 3o feet.
Pebble phosphate, bituminous coal, ironstone, etc., are mined in this manner. The maximum depth which can be stripped by a large shovel alone is 58ft., uncovering a strip 3oft. wide as in fig. 5. A width of coal of 97ft. can be exposed if desired, as in fig. 6, but the width of cut is the same-3o feet. By using a shovel and dragline with the dragline located on the bench, a depth of 93ft. can be stripped as in fig. 7. If the dragline is equipped with a clamshell bucket instead of a dragline bucket, and the dragline is located on the top of the spoil pile made by the shovel two cuts before, i oof t. depth can be stripped. With the largest dragline alone, a cut 3oft. wide and 8I ft. deep may be taken. To reach the maximum depths indicated, the original ground must be firm to stand up steeper than the angle of re pose. The angle of repose of the spoil pile must be one vertical to I+ horizontal or about 38°. The first cut or pilot cut must be made, no matter how troublesome and expensive it may be, so that all subsequent cuts can be ideal. If at all possible, the shovel should be used alone in hard digging and the dragline alone in easy digging.
Resume.—For overcasting large areas of overburden to re cover mineral deposits below, the large stripping shovel is most efficient up to its piling limit. For greater thicknesses of over burden, the shovel must be helped by a dragline. The dragline is the favourite in soft digging, as it digs and transports a dis tance equal to twice the length of its boom. The tower excava tor is good for collecting material over a great width of ground and depositing it in a ridge or levee. The slackline cableway does much the same on a smaller scale. Scrapers are efficient in easy digging where the haul is not too great. The scrapers dig and transport. Where the haul is longer and the digging very light, an elevating grader is used for loading trucks. If the haul is long and the digging heavy, the machine loading the trucks would be a shovel. Bulldozers excavate and transport shorter distances than scrapers. For trenches, a back-hoe or a ladder type trench machine is used. Transportation of the excavated material is by large trucks and tractor-trailer equipment, but also by electri cally propelled unit cars on tracks and by belt conveyor. The hydraulic method is used where other means of transportation would be difficult, such as crossing a main line railway, or where segregation of rock from earth is to be made. (G. B. M.)