Excavation
126. Specifications.—The following abstracts have been taken from the specifications on Excavation by the Baltimore Sewerage Commission as illustrative of good practice. In conducting the work the contractor shall:
... remove all paving, or grub and clear the surface over the trench, whenever it may be necessary and shall remove all surface materials of whatever nature or kind. He shall properly classify the materials removed, separating them as required by the Engineer; and shall properly store, guard, and preserve such as may be required for future use in backfilling, surfacing, repaving or otherwise. All macadam material removed shall be separated and graded into such sizes as the Engineer may direct and materials of different sizes shall be kept separate from each other and from any and all other materials.
All the curb, gutter, and flag-stones and all paving material which may be removed, together with all rock, earth and sand taken from the trenches shall be stored in such parts of the carriageway or such other suitable place, and in such manner as the Engineer may approve. The Contractor shall be responsible for the loss of or damage to curb, gutter and flag-stones and to paving material because of careless removal or wasteful storage, disposal, or use of the same.
... When so directed by the Engineer the bottom of the trench shall be excavated to the exact form of the lower half of the sewer or of the foundation under the sewer.
The bottom width of the trench for a brick or concrete sewer shall be ... not less in any case than the overall width of the sewer, as shown on the plans. In case the trench is sheeted this minimum width will be measured between the interior faces of the sheeting as driven, but in no case shall bracing, stringers, or waling strips be left within any portion of the masonry of the sewer except by permission of the Engineer; and such braces, stringers and waling strips shall not, in any case, be allowed to remain within the neat lines of the masonry as shown on the plans. In case that the distance between faces of the sheeting is less than that called for by the width of the sewer to be laid in the trench, the Engineer may direct the sheeting to be drawn and redriven, or otherwise changed and altered; or he may direct that the sewer be reinforced in such manner and to such an extent as he may deem necessary without compensation to the Contractor, even though such narrower trench was not caused by negligence or other fault on the part of the Contractor.
Trenches for vitrified pipe shall be at all points at least six inches wider in the clear on each side than the greatest external width of the sewer, measured over the hubs of the pipe.... Bell holes shall be excavated in the bottoms of trenches for vitrified pipe sewers wherever necessary.
Not more than three hundred feet of trench shall be opened at any one time or place in advance of the completed building of the sewer, unless by written permission of the Engineer and for a distance therein specified....
The excavation of the trench shall be fully completed at least twenty feet in advance of the construction of the invert, unless otherwise ordered.
During the progress of construction the Contractor will be required to preserve from obstruction all fire hydrants and the carriageway on each side of the line of the work.
The streets, cross-walks, and sidewalks shall be kept clean, clear, and free for the passage of carts, wagons, carriages and street or steam railway cars, or pedestrians, unless otherwise authorized by special permission in writing from the Engineer. In all cases a straight and continuous passageway on the sidewalks and over the cross walks of not less than three feet in width shall be preserved free from all obstruction.
Where any cross walk is cut by the trench it shall be temporarily replaced by a timber bridge at least three feet wide, with side railings, at the Contractor’s expense. The placing of planks across the trench without proper means of connection or fastenings, or pipe or other material, or the using of any other makeshift in place of properly constructed bridges, will not be permitted.
This is equally applicable to certain wagon bridges to be fixed upon by the Engineer, on the basis of traffic requirements.
In streets that are important thoroughfares or in narrow streets the material excavated from the first one hundred feet of any opening or from such additional length as may be required, shall upon the order of the Engineer, be removed by the Contractor, as soon as excavated. The material subsequently excavated shall be used to refill the trench where the sewer has been built.
The preceding specifications are applicable to open-trench excavation. Rigid restrictions are placed about tunneling because of the greater difficulty of doing good work, the greater danger to life and property and the possibility of later surface subsidence if the backfilling is done improperly. A common clause in specifications is:
All excavations for sewers and their appurtenances shall be made in open trenches unless written permission to excavate in tunnel shall be given by the Engineer.
127. Hand Excavation.—Earth excavation by pick and shovel is the simplest and most primitive mode of excavation. Only small jobs are handled in this manner in order to save the investment necessary in machines or the expense of hiring and moving one to the work. The tools used in the hand excavation of trenches are: picks, pickaxes, long-handled and short-handled pointed shovels, square-edged long- and short-handled shovels, scoop shovels, axes, crowbars, rock drills, mauls, sledges, etc. The excavating gangs are divided up into units of 20 to 50 men under one foreman or straw boss, and among the men may be a few higher priced laborers who set the pace for the others. Each laborer on excavation should be provided with a shovel, the style being dependent on the character of the material being excavated and the depth of the trench. In stiff material and deep trenches requiring the lifting of the material in the shovel, long-handled pointed shovels should be used. In loose sandy material loaded directly into buckets short-handled, square pointed shovels are satisfactory. Picks are used in cemented gravels or where hard obstructions prevent cutting down with the edge of the shovel. Very stiff but not hard material can be cut out in chunks with a pickaxe and thrown from the trench or into a bucket with a scoop shovel. Scoop shovels are also useful in wet running quicksand. The number of picks, axes, crowbars, and other tools must be proportioned according to the material being excavated. Under the worst conditions of excavation in a hard cemented gravel it may be necessary to provide each man with a pick as well as a shovel, whereas in sand only a shovel is necessary. Two or three crowbars, axes, a length of chain, two or three screw jacks, etc., are provided per gang in case of an unexpected encounter with an obstruction in the trench, such as a boulder, a tree stump, a length of pipe, etc.
In laying out the work the foreman marks the outlines of the trench on the ground by means of a scratch made with a pick, chalk marks, tape, or other devices. These marks are measured from offset or center stakes set by the engineer. Center stakes are less conducive to error but are more likely to be disturbed before use than are offset stakes, but careless foremen make more errors with offset than with center stakes. The inspector should assist or be present at the laying out of the trench. After the trench has been laid out each laborer should be given a certain specific portion of it to dig and this portion is marked out on the ground. In this way a check can be kept upon the performance of each laborer and the knowledge of this fact tends to a uniformly better performance. The amount of work that can be performed by one man with a pick and shovel is as shown in Table 49. Some men may exceed these rates, many will not attain them. The allotted task must be gaged on the character of the ground in order that the tasks may be equal and a spirit of competition fostered. The hard worker will set the pace for the lazy man. Some contractors have adopted the expedient of dismissing laborers for the day as soon as the allotted task is done.
| TABLE 49 | |
|---|---|
| Amount of Material Moved by One Man with a Pick and Shovel | |
| (From H. P. Gillette) | |
| Material | Cubic Yard per hour |
| Hardpan | 0.33 |
| Common earth | 0.8 to 1.2 |
| Stiff clay | 0.85 |
| Clay | 1.00 |
| Sand | 1.25 |
| Sandy soil | 0.8 to 1.2 |
| Clayey earth | 1.3 |
| Sandy soil (frozen) | 0.75 |
The opening of the trench may be facilitated by breaking ground with a plow. In hard ground or on paved roads it may be necessary to cut through the surface crust with a hammer and drill, although in some cases a plow can be used successfully. Frozen ground can be thawed by building fires along the line of the trench, or greater economy may be achieved by placing steam pipes along the surface with perforations about every 18 inches and either boxing them on the top and sides or burying them in the frozen earth with a covering of sand. Another arrangement is to blow steam into a line of bottomless boxes in which each box is about 8 feet long. Holes are left in the top of the boxes into which the pipe is shoved, and after its withdrawal the holes are covered. Blasting of frozen earth is sometimes successful but cannot be resorted to in built up districts where it is unsafe unless properly controlled. Once the frost crust is broken through it can be attacked from below and frequently broken down by undermining.
A laborer cannot dig and raise the earth much more than to the height of his head, and preferably not quite so high, without tiring quickly. After the trench has passed a depth of 4 feet he cannot throw the earth clear of the trench. An additional laborer is needed then at the surface to throw the earth back. He should shovel the earth from a board platform placed at the edge of the trench as a protection to the bank. When the trench passes the 6–foot depth a staging is put in about 4 feet from the top on which the lowest laborer piles his materials. It is then passed up to the surface by a second laborer on the staging, and a third laborer on the surface throws the material back clear of the trench. Stagings are put in about every 5 or 6 feet for the full depth of the trench.
When the trench has come within half the diameter of the pipe of the final grade, if the material is sufficiently firm, the remainder of the trench should be cut to conform to the shape of the lower half of the outside of the pipe, with proper enlargements for each bell.
128. Machine Excavation.—On work of moderately large magnitude excavation by machine is cheaper than by pick and shovel alone. In comparing the cost of excavation by the two methods all items such as sheeting, pipe laying, backfilling, etc., should be included, since these items will be affected by the method of excavation. The cost of setting up and reshipping the machine must be included as this is frequently the item on which the use of the machine depends. Because of the cost of setting up and shipping, which must be distributed over the total number of yards excavated, the cost per cubic yard of excavating by machine varies with the number of cubic yards excavated. The point of economy in the use of a machine is reached when the cost by hand and by machine are equal. For all work of greater magnitude, excavation by machine will prove cheaper.[[81]] Items favoring the use of machinery which may cause its adoption for small jobs are: its greater speed, reliability, ease in handling, economy in sheeting, economy in labor, and small amount of space needed making it useful in crowded streets. Continuous bucket machines, drag lines, and occasionally steam shovels are not adapted to conditions where rocks, pipes and other underground obstacles are frequently met.
The following problem is an example of the work necessary in making a comparison of the relative economy of machine and hand excavation:
It is assumed that a man can excavate 15 feet of trench 30 inches wide and 8 feet deep in 10 hours. He receives 55 cents per hour for his work. A machine costing $10,000 has a life of 6 years. It can be kept busy 150 days in the year. When operating it costs $1.25 per hour for the operator, fuel and repairs. It will excavate 800 linear feet of 30 inch trench to a depth of 8 feet in 10 hours. It is assumed that capital is worth 10 per cent on such a venture and that the sinking fund will draw 10 per cent. If the cost of moving and setting up the machine is $1,800, how many cubic yards of excavation must there be to make excavation by machine economical? Costs of sheeting, pumping, etc., are assumed to be the same for machine or hand work.
Solution.—For hand work the man excavated 1.11 cubic yard per hour at 55 cents. The relative cost of hand excavation is then 50 cents per cubic yard.
The cost of machine work will be divided into: interest on first cost; operation and repairs; and sinking fund for renewal. The interest on the first cost of $10,000 at 10 per cent is $1,000 per year. The machine works 1,500 hours in the year. Therefore the cost per hour is $0.67.
The sinking fund payment, as found from sinking fund tables or the accumulation of $10,000 in. 6 years, is $1,300 per year or per hour for 1,500 hours is $0.87.
The cost of operation per hour is given as $1.25.
The total cost per hour is therefore $2.79.
The machine excavated 59.3 cubic yards per hour which makes the cost, exclusive of moving, equal to $0.47 per cubic yard. In order to equalize the cost of machine and hand excavation the cost of moving the machine must be divided among a sufficient number of cubic yards so that the cost per cubic yard shall be 3 cents. The cost of moving is given as $1,800. This amount divided among 60,000 cubic yards equals 3 cents per cubic yard. Therefore the job must provide at least 60,000 cubic yards of excavation in order that the use of the machine shall be justifiable from the viewpoint of economy alone.
129. Types of Machines.—Machines particularly adapted to the excavation of sewer and water pipe trenches are of four types: (1) continuous bucket excavators; (2) overhead cableway or track excavators; (3) steam shovels; and (4) boom and bucket excavators. Other types of excavating machinery can be used for sewer trenches under special conditions. Machines are ordinarily limited to a minimum width of trench of 22 inches. Between widths of 22 inches and 36 inches the limit of depth for the first class of machines is about 25 feet. For other types of machines there is no definite limit, though the economical depth for open cut work seldom exceeds 40 feet.
130. Continuous Bucket Excavators.—Continuous bucket excavators are of the types shown in Figs. 89 and 90. The buckets which do the digging and raising of the earth may be supported on a wheel as in Fig. 89 or on an endless chain as in Fig. 90. The support of the wheel or endless chain can be raised or lowered at the will of the operator so as to keep the trench as close to grade as can be done by hand work. In some machines the shape of the buckets can be made such as to cut the bottom of the trench, in suitable material, to the shape of the sewer invert. In operation, the buckets are at the rear of the machine and revolve so that at the lowest point in their path they are traveling forward. The excavated material is dropped on to a continuous belt which throws it on the ground clear of the trench, into dump wagons, or on to another continuous belt running parallel with the trench to the backfiller, by means of which the excavated material is thrown directly into the backfill without rehandling. The body of the machine supporting the engine travels on wheels ahead of the excavation and is kept in line by means of the pivoted front axle. When obstacles are encountered the excavating wheel or chain is raised to pass over the obstacle, and allowed to dig itself in on the other side.
Fig. 89.—Buckeye Wheel Excavator.
Courtesy, Buckeye Traction Ditcher Co.
Fig. 90.—Buckeye Endless-chain Excavator.
Courtesy, Buckeye Traction Ditcher Co.
Fig. 91.—Movable Sheeting Fastened to Traction Ditcher.
From Eng. News-Record, Vol. 82, 1919, p. 740.
Wheel excavators are not adapted to the excavation of sewer trenches over 3 to 4 feet in width and 6 to 8 feet in depth. The endless-chain excavators are suitable for depths of 25 feet with widths from 22 to 72 inches, and due to the arrangement permitting buckets to be moved sideways they will cut trenches of different widths with the same size buckets. This is an advantage where there are to be irregularities in the width of the trench such as for manholes or changes in size of pipe. With excavating machines pipe can be laid within 3 feet of the moving buckets and the trench backfilled immediately, thus making an appreciable saving in the amount of sheeting. In the construction of trenches for drain tile at Garden Prairie, Illinois, the sheeting was built in the form of a box or shield fastened to the rear of the machine and pulled along after it as is shown in Fig. 91.
The performance of this type of excavating machine under suitable conditions is large. A remarkable record was made by Ryan and Co. in Chicago,[[82]] with an excavating machine. 1338 feet of 32–inch trench were excavated to an average depth of 8½ feet in 7 hours, or an average of 160 cubic yards per hour. More could have been accomplished if it had not been for delays in supplies. Another crew at Greeley, Colorado,[[83]] with a Buckeye endless-chain ditcher weighing 17 tons and costing $5200, averaged 232 cubic yards per day for 300 days, and the cost was 10.7 cents per cubic yard. A 15–ton Austin excavator can be expected to remove 300 to 500 cubic yards per day.
The cost of operation of the machines is made up of items listed in Table 50. The figures given are merely suggestive.
| TABLE 50 | |||
|---|---|---|---|
| Cost of Operating Ditching Machine | |||
| Per Day | Total | ||
| Labor: | |||
| 1 Operator at $150 per month | $6.00 | ||
| 1 Assistant Operator at $120 per month | 4.00 | ||
| 4 laborers at 4.00 per day | 16.00 | ||
| $26.00 | |||
| Fuel: | |||
| 20 Gallons of gasoline at 28 cents | 5.60 | 5.60 | |
| Miscellaneous: | |||
| Oil, waste, etc. | 1.20 | ||
| Repairs and maintenance | 10.00 | ||
| Interest, 6 per cent on $10,000 for 150 days | 4.00 | ||
| Depreciation, 200 working days per year and an 8 year life | 11.11 | 26.31 | |
| Total cost per day | $57.91 | ||
| TABLE 51 | |||
|---|---|---|---|
| Comparison of Cost of Hand Excavation and Machine Excavation with Continuous-bucket Excavator | |||
| Hand Work | Per Day, Dollars | Machine Work | Per Day, Dollars |
| Foreman | 4.00 | Engineer | 4.00 |
| Timberman | 3.00 | Fireman | 2.50 |
| Helper | 2.50 | Coal | 5.00 |
| 4 Laborers at $2.00 | 80.00 | Team | 4.00 |
| Foreman | 4.00 | ||
| Pipe layer | 3.00 | ||
| Helper | 2.50 | ||
| 2 Teams backfilling | 8.00 | ||
| 2 Helpers | 4.00 | ||
| Interest, depreciation and repairs | 10.00 | ||
| Total | 95.00 | Total | 54.50 |
In making a comparison of the cost of hand and machine excavation the figures given in Table 51 are from “Excavating Machinery” by McDaniel, who quotes the cost of machine excavation from the manufacturers of the Parsons machine issued as the result of several years’ experience with their excavator. In the comparison the hand crew is assumed to dig 315 linear feet of trench 28 inches wide by 12 feet deep in a day of 10 hours. This assumes that each man will excavate 7 cubic yards per day. The machine is assumed to excavate 250 feet of the same trench. The comparison indicates that an excavator will work at about 50 per cent of the cost of hand excavation, if the cost of moving the machine is not included.
Fig. 92.—Carson Excavating Machine on Trench Excavation in South Milwaukee.
Courtesy, Mr. C. F. Henning.
131. Cableway and Trestle Excavators.—Cableway and trestle excavators are most suitable for deep trenches and crowded conditions. They should not be used for trenches much less than 8 feet in depth. They differ from the continuous bucket excavators in that the actual dislodgment of the material is done by pick and shovel, the excavated material being thrown by hand into the buckets of the machine. A machine of the Carson type is shown in Fig. 92. The machine consists of a series of demountable frames held together by cross braces and struts to form a semirigid structure. An I beam or channel extending the length of the machine is hung closely below the top of the struts. The lower flange of this beam serves as a track for the carriages which carry the buckets. All the carriages are attached to each other and to an endless cable leading to a drum on the engine. This cable serves to move the buckets along the trench. The buckets are attached to another cable which is wound around another drum on the engine and serves to lower or raise all the buckets at the same time. In operation there are always at least two buckets for each carriage, one in the trench being filled and the other on the machine being dumped. There should be a surplus of buckets to replace those needing repairs.
The machines may be from 200 to 350 feet in length, and the number of buckets which can be lifted at one time varies from one to a dozen or more. On trenches over 5 to 6 feet in width a double line of buckets is sometimes used. The entire machine rests on rollers and straddles the trench. It is moved along the trench by its own power, either by gearing or chains attached to the wheels, or by a cable attached to a dead-man ahead.
The Potter trench machine differs from the Carson in that only 2 buckets are used at a time and these are carried on a car which travels on a track on top of the trestle. The movement of the buckets and the car are controlled by 2 dump men who ride on the car and who can raise or lower the buckets independently.
The organization needed to operate these machines is: a lockman who locks and unlocks the buckets on the cable, a dumper, as many shovelers as there are buckets on the machine, and an engineman who is usually his own fireman. From 50 to 400 cubic yards of material can be excavated in a day with one of these machines, dependent on the character of the material and the depth of the trench. H. P. Gillette in his Handbook of Cost Data reports that about 190 cubic yards were excavated per day with a Potter machine. The machine was 370 feet long. Six ¾-yard buckets were used, 4 in the trench and 2 on the carrier. The trench was 10½ feet wide and 18 feet deep in wet sand and soft blue clay. The organization consisted of an engineman, a fireman, 2 dumpmen on the carrier, and from 17 to 21 excavating laborers depending on the kind and the amount of the excavation. In general the capacity of such machines is limited by the amount of material which can be shoveled into them by hand.
132. Tower Cableways.—These are essentially of the same class as the trestle cableway machines. They differ in that the carriage supporting the buckets travels on a cable suspended between 2 towers instead of on a track supported on a trestle. As a rule only one bucket is handled in the machine at a time. They are used in sewer work only in exceptional cases as the towers must be taken down and re-erected each time that there is an advance in the trench greater than the distance between the towers.
133. Steam Shovels.—The use of steam shovels for the excavation of sewer trenches is becoming more prevalent because of their growing dependability and durability as compared with other machines, their adaptability for small trenches, and the relatively large number of widely different uses to which they can be put. In excavating a trench the shovel straddles the trench and runs on tractors, wheels, or rollers on either side of it. The shovel cuts the trench ahead of it. As a result it is difficult to set sheeting and bracing close to the end of the trench while the shovel is operating. Steam shovels are therefore not suitable for excavation in unstable material, unless the sheeting is driven ahead of the excavation. It is only in the softest ground that ordinary wood sheeting can be driven ahead of the excavation. Steel sheet piling is more suitable for such use. Fig. 93[[84]] shows a shovel at work on a trench in Evanston, Illinois.
Shovels are equipped with extra long dipper handles to adapt them to trench excavation. The dipper handle in the picture is longer than the standard for this type of machine. The method of supporting the shovel can be seen in the picture under the machine and the method of bracing and of finishing the trench by hand work are also shown. The excavated material is taken out in the shovel and dropped on the bank or into wagons.
The limiting depth to which trenches can be excavated by steam shovels is about 20 to 25 feet, where the trench is too narrow for the shovel to enter. Wider trenches are cut in steps of about 15 feet, the shovel working in the trench for additional depths. Shovels are now made to cut trenches as narrow as a man can enter to lay pipe. The greatest width that can be cut from one position of the shovel is from 15 to 40 feet, dependent on the size of the shovel. Occasionally a combination of a drag line and a steam shovel can be used, as on the construction of the Calumet sewer in Chicago. On this work the first step was cut by a steam shovel. It was followed by a drag line resting on the step thus prepared, and excavating the remaining distance to grade. The depth of the trench in this work averaged about 25 to 30 feet.
Fig. 93.—Steam Shovel at Work on Sewer Trench for North Shore Intercepting Sewer, Evanston, Illinois.
Steam shovels are rated according to their tonnage and the capacity of the dipper in cubic yards. Both are necessary as the size of the dipper is varied for the same weight of machine, dependent on the character of the material being excavated. For rock the dipper is made smaller than for sand. Gillette in his Hand Book of Cost Data gives the coal and water consumption of steam shovels as shown in Table 52. The performance of steam shovels is recorded in Table 53. The conditions of the work have a marked effect on the output of the shovel. A shovel in a thorough cut, i.e., in a trench just wide enough for the shovel to turn 180 degrees but too narrow to run cars or wagons along side of it, will perform less than one-half of the work that it can perform in a side cut, i.e., where the cars can be run along side the shovel which turns less than 90 degrees.
| TABLE 52 | ||||||
|---|---|---|---|---|---|---|
| Coal and Water Consumption by Steam Shovels | ||||||
| (From Handbook of Cost Data, by H. P. Gillette) | ||||||
| Weight in tons | 35 | 45 | 55 | 65 | 75 | 90 |
| Dipper, cubic yards | 1¼ | 1½ | 1¾ | 2 | 2½ | 3 |
| Coal, tons per 10 hour day | ¾ | 1 | 1¼ | 1½ | 2 | 2¼ |
| Water, gallons per 10 hour day | 1500 | 2000 | 2500 | 3000 | 4000 | 4500 |
| TABLE 53 | |||||||
|---|---|---|---|---|---|---|---|
| Performance by Steam Shovels | |||||||
| Weight in Tons | Dipper Cubic Yards | Depth of Cut, Feet | Width of Cut | 10–Hour Performance | Cost in Cents, per Cubic Yard | Authority | Remarks |
| 25 | 1 | 9 | 36 in. | 85 | 22.6 | R. T. Dana Eng. Rec., 69:581 | 1 |
| 25 | 1 | 8 | 35 in. | 96 | 23.5 | do. | 2 |
| 70 | 2 | 26 | 16 ft. | 569 | 6.7 | do. | 3 |
| 30 | 1 | 15–18 | 60 in. | 300 | A. B. McDaniel Excavating Machinery | 4 | |
| 15 | ⅝ | 14 | 134 ft. | 400 | Eng. Cont’r, 8–25–09 | 5 | |
| 8 | 36 | Very wide | 16 yd. cars | Marion Steam Shovel Co. | 6 | ||
| 55 | 296 | H. P. Gillette’s Cost Data | 7 | ||||
| 65 | 2¼ | 280 | do. | ||||
| Greater than 78 in. | 700 | 30.6 | G. C. D. Lenth, Eng. News-Record, 85:22 | 8 | |||
Remarks: 1. One runner at $5.00, one fireman at $2.31, two laborers at $1.70 each, supplies at $4.50, and interest and depreciation on 200 days per year, $4.00. Total per day, $19.21. Material, clay and gravel. 2. Average of 11 jobs with the same shovel. 3. Cost per day, one runner at $5.00, one crane-man at $3.60, one fireman at $2.00, 7 roller men at $1.50 each, supplies $9.00 and interest and depreciation on $9000 at 200 days per year $8.00. Total, $38.10. 4. Hard clay. 5. Stiff clay for the basement of a building in Chicago. 6. Stripping ore. This is a maximum record. The average was about three hundred and twenty 16 cubic yard cars per day. 7. Blasted mica-schist. 8. General average.
134. Drag Line and Bucket Excavators.—A drag line excavator is shown in Fig. 94. The back of the bucket is attached to a drum on the engine by means of a cable passing over the wheel in the end of the long boom. The front of the bucket is attached by another cable directly to another drum on the engine. In operation the bucket is raised by its rear end and dropped out to the extremity of the boom. It is then dragged over the ground towards the machine, digging itself in at the same time. When filled the bucket is raised by tightening up on the two cables, swung to one side by means of the movable boom, and dumped.
Fig. 94.—Drag Line at Work on Trench for Drain Tile.
Drag line excavators will perform as much work as steam shovels under favorable conditions. They are less expensive in first cost and operation, and are equally reliable but they are not adapted to the more difficult situations where steam shovels can be used to advantage. Drag lines are suitable only for relatively wide trenches in material requiring no bracing, and in a locality where relatively long stretches of trench can be opened at one time.
The bucket excavator differs from the drag line in that the bucket can be lifted vertically only and the types of buckets used in the two types of machine are different. The bucket may be self filling of the orange-peel or clam-shell type, or a cylindrical container which must be filled by hand. A drag line can be easily converted into a boom and bucket excavator. Boom and bucket excavators are well adapted to use in deep, closely braced trenches and shafts.
135. Excavation in Quicksand.[[85]]—A sand or other granular material in which there is sufficient upward flow of ground water to lift it, is known as quicksand. Its most important property, from the viewpoint of sewer construction, is its inability to support any weight unless the sand is so confined as to prevent flowing of the sand, or unless the water is removed from the sand.
Excavation in quicksand is troublesome and expensive and is frequently dangerous. The material will flow sluggishly as a liquid, it cannot be pumped easily, and its excavation causes the sides of the trench to fall in or the bottom to rise. The foundations of nearby structures may be undermined, causing collapse and serious damage. These conditions may arise even after the backfilling has been placed unless proper care has been taken. The greatest safeguard against such dangers is not only to exercise care in the backfilling to see that it is compactly tamped and placed, but to leave all sheeting in position after the completion of the work.
The ordinary method of combating quicksand and in conducting work in wet trenches is to drive water-tight sheeting 2 or 3 feet below the bottom of the trench, and to dewater the sand by pumping. When dry it can be excavated relatively easily. A more primitive but equally successful method is to throw straw, brickbats, ashes, or other filling material into the trench in order to hold the excavation once made, or this may supplement the attempts at pumping, or the wet sand may be bailed out in buckets. Successful excavation in quicksand requires experience, resourcefulness. and a careful watch for unexpected developments. The well points described in Art. 142 are used for dewatering quicksand.
136. Pumping and Drainage.—Ground water is to be expected in nearly all sewer construction and provision should be made for its care. Where geological conditions are well known or where previous excavations have been made and it is known that no ground water exists it may be safe to make no provision for encountering ground water. Where ground water is to be expected the amount must remain uncertain within certain rather wide limits until actually encountered.
In order to avoid the necessity for pumping, or working in wet trenches it is sometimes possible to build the sewer from the low end upwards and to drain the trench into the new sewer. The wettest trenches are the most difficult to drain in this manner as the material is usually soft and the water so laden with sediment as to threaten the clogging of the sewer. It is undesirable to run water through the pipes until the cement in the joints has set. This necessitates damming up the trench for a period which may be so long as to flood the trench or delay the progress of the work. If it is not possible to drain the trench through the sewer already constructed the amount of water to be pumped can be reduced by the use of tight sheeting.
Fig. 95. Improvised Trench Pump.
Pumps for dewatering trenches must be proof against injury by sand, mud, and other solids in the water. For this purpose pumps with wide passages and without valves or packed joints are desirable. The types of pumps used are: simple flap valve pumps improvised on the job, diaphragm pumps, jet pumps, steam vacuum pumps, centrifugal pumps, and reciprocating pumps. All are of the simplest of their type and little attention is paid to the economy of operation because of the temporary nature of their service.
137. Trench Pump.—A simple pump which can be improvised on the job is shown in section in Fig. 95. Its capacity is about 20 gallons per minute but its operation is backaching work. It is inexpensive, quickly put together and may be a help in an emergency. It is to be noted that the passages are large and straight, that there are no packed joints, and that the velocity of flow is so small that it is not liable to clogging by picking up small objects.
Fig. 96.—Diaphragm Pump
Courtesy, Edson Manufacturing Co.
138. Diaphragm Pump.—The type of pump shown in Fig. 96 is the most common in use for draining small quantities of water from excavations. It is known as the diaphragm pump from the large rubber diaphragm on which the operation depends. The pump is made of a short cast-iron cylinder, divided by the rubber diaphragm or disk to the center of which the handle is connected. The valve is shown at the center of the disk. As the diaphragm is lifted the valve remains closed, creating a partial vacuum in the suction pipe and at the same time discharging the water which passed through the valve on the previous down stroke. When the valve is lowered the foot valve on the suction pipe closes, holding the water in place, and the valve in the pump opens allowing the water to flow out on top of the disk to be discharged on the next up stroke. Table 54 shows the capacities of some diaphragm pumps as rated by the manufacturers. The smaller sizes are the more frequently used and are equipped with a 3–inch suction hose with strainer and foot valve. They are not adapted to suction lifts over 10 to 12 feet. Where greater lifts are necessary one pump may discharge into a tub in which the foot valve of a higher pump is submerged.
| TABLE 54 | |||
|---|---|---|---|
| Capacities of Diaphragm Pumps | |||
| Diameter of Cylinder, Inches | Diameter of Suction, Inches | Length of Stroke in Inches | Capacity per Stroke, Gallons |
| 6 | 3 | 4 | 0.49 |
| 8½ | 4 | 6 | 1.47 |
| 9[[86]] | 2½ | 0.75 | |
| 12½[[86]] | 3 | 1.25 | |
| 12½[[86]] | Power driven by 1 horse-power engine | 0.58[[87]] | |
Fig. 97.—McGowan Steam Jet Pump.
Courtesy, The John H. McGowan Co.
139. Jet Pump.—The simplicity of the parts of the jet pump is shown in Fig. 97. It has a distinct advantage over pumps containing valves and moving parts in that there are no obstructions offered to the passage of solids as well as liquids through the pump. It is not economical in the use of steam, however. It operates by means of a steam jet entering a pipe at high velocity through a nozzle. This action causes a vacuum which will lift water from 6 to 10 feet. The lower the suction lift, however, the greater the efficiency of the work. The sizes and capacities of jet pumps as manufactured by the J. H. McGowan Co. are shown in Table 55.
| TABLE 55 | ||||
|---|---|---|---|---|
| Capacities of Jet Pumps | ||||
| (J. H. McGowan Co.) | ||||
| Size of Pump and Suction Pipe, Inches | Discharge Pipe, Inches | Steam Pipe, Inches | Capacity, Gallons per Minute | Approximate Horse-power Required |
| ¾ | ½ | ⅜ | 8 | 2 |
| 1 | ¾ | ½ | 15 | 3 |
| 1¼ | 1 | ½ | 20 | 4 |
| 1½ | 1¼ | ¾ | 30 | 6 |
| 2 | 1½ | ¾ | 40 | 8 |
| 2½ | 2 | 1 | 50 | 10 |
| 3 | 2½ | 1 | 60 | 15 |
| 4 | 3½ | 1¼ | 85 | 25 |
140. Steam Vacuum Pumps.—This type of pump depends on the condensation of steam in a closed chamber to create a vacuum which lifts water into the chamber previously occupied by the steam and from which the water is ejected by the admission of more steam. The best known pumps of this type are the Pulsometer, manufactured by the Pulsometer Steam Pump Co., the Emerson, manufactured by the Emerson Pump and Valve Co., and the Nye Pump, manufactured by the Nye Steam Pump and Machinery Co.
Fig. 98.—Pulsometer Steam Vacuum Pump.
A section of a Pulsometer is shown in Fig. 98. It consists of two bottle-shaped chambers A and B with their necks communicating at the top and each opening into the outlet chamber O through a check valve. Steam is admitted at the top and enters chamber A or B according to the position of the steam valve C as shown. This steam valve is a ball which is free to roll either to the right or left and forms a steam-tight joint with whichever seat it rests upon. In normal operation chamber A would be filled with water as the steam enters the cylinder. At the same time a check valve at the top opens to admit a small quantity of air which forms a cushion insulating the steam from the water, reduces the condensation of the steam, and serves as a cushion for the incoming water on the opposite stroke. The pressure of the steam depresses the surface of the water without agitation and forces the water through the check valve F into the discharge chamber O. When the water falls to the level of the discharge chamber the even surface is broken up and the intimate contact of the steam and water condenses the former instantaneously. This forms a vacuum in chamber A which, assisted by a slight upward pressure in chamber B caused by the incoming water, immediately pulls the ball C over to the other seat and directs the steam into chamber B. The vacuum in chamber A now draws up a new charge of water through the suction pipe into the chamber.
Fig. 99.—Emerson Steam Vacuum Pump.
A section of the Emerson pump is shown in Fig. 99. The pump consists of two vertical cylinders B and C. Each chamber has a suction valve L at the bottom, opening upward from a common chamber from which the discharge pipe U extends. On the top of each chamber is a baffle plate G which operates to distribute the steam evenly to the two chambers and to prevent it from agitating the surface of the water in the chambers. A condenser nozzle F is connected with the bottom of the opposite chamber by a pipe into which a check valve opens upward. As the pressure in the chamber alternates water will be injected through F into the opposite chamber and condense the steam therein, promptly forming a vacuum. An air valve P admits a small quantity of air while the chamber is filling with water, the air acting as an insulating cushion as in the Pulsometer. Valve O, just above the top connection S is used to regulate the amount of steam that enters the pump. The top connection S has two ports, one leading to each chamber. An oscillating valve enclosed in it admits the steam through these ports to the two chambers alternately. This valve is driven by a small three-cylinder engine, the crank shaft of which extends into the top connection in the center of the bearing on which the valve oscillates. A positive geared connection is made between the valve and the engine and so arranged that the engine will run faster than the valve.
The action of these pumps consists of alternately filling and emptying the two chambers. They will continue operation without attention or lubrication so long as the steam is turned on. In view of the simplicity of their operation and make-up, their ability to handle liquids heavily charged with solids, and their reasonable steam consumption these pumps are widely used for pumping water in construction work. They have an added advantage that no foundation or setting is required for them as they can be hung by a chain from any available support.
These pumps are manufactured in sizes varying from 25 to 2500 gallons per minute at a 25–foot head, and with a steam consumption of about 150 pounds per horse-power hour. They reduce about 4 per cent in capacity for each 10 feet of additional lift. They will operate satisfactorily between heads of 5 to 150 feet, with a suction lift not to exceed 15 feet. Lower suction lifts are desirable and the best operation is obtained when the pump is partly submerged. The steam pressure should be balanced against the total head. It varies from 50 to 75 pounds for lifts up to 50 feet, and increases proportionally for higher lifts. The dryer the steam the lower the necessary boiler pressure.
141. Centrifugal and Reciprocating Pumps.—The details of these pumps, their adaptability to various conditions, and their capacities are given in Chapter VII. The centrifugal is better adapted to trench pumping as it is not so affected by water containing sand and grit, but for clear water, high suction lifts and fairly permanent installations, reciprocating pumps can be used with satisfaction.
142. Well Points.—In dewatering quicksand a method frequently attended with success is to drive a number of well points into the sand and connect them all to a single pump. Figure 100 shows a well point system used on sewer work in Indiana. The well points are 3 feet apart and are connected to a 2½-inch header which in turn is connected to six Nye pumps, each with a capacity of 200 gallons per minute for a lift of 50 feet. The number and size of well points and pumps to use will depend on conditions as met on the job. On a piece of work in Atlantic City[[88]] the equipment consisted of two complete outfits each comprising one hundred 1½ inch by 36–inch No. 60 well points, one hundred 6–foot lengths of rubber hose, about 600 feet of suction main, one hundred valved T connections, and a 7 × 8–inch Gould Triplex Pump with a capacity of 200 gallons per minute, belted to a 7½ horse-power motor.
Fig. 100.—Well Points Pumped by Nye Steam Vacuum Pump.
143. Rock Excavation.—A common definition of rock used in specifications is: whenever the word Rock is used as the name of an excavated material it shall mean the ledge material removed or to be removed properly by channeling, wedging, barring, or blasting; boulders having a volume of 9 (this volume may be varied) cubic feet or more, and any excavated masonry. No soft disintegrated rock which can be removed with a pick, nor loose shale, nor previously blasted material, nor material which may have fallen into the trench will be measured or allowed as rock.
Channeling consists in cutting long narrow channels in the rock to free the sides of large blocks of stone. The block is then loosened by driving in wedges or it is pried loose with bars. It is a method used more frequently in quarrying than in trench excavation where it is not necessary to preserve the stone intact. In blasting, a hole is drilled in the rock, and is loaded with an explosive which when fired shatters the rock and loosens it from its position.
Fig. 101.—Plug and Feathers for Splitting Rock.
In drilling rock by hand the drill is manipulated by one man who holds it and turns it in the hole with one hand while striking it with a hammer weighing about 4 pounds held in the other hand, or one man may hold and turn the drill while one or two others strike it with heavier hammers. In churn drilling a heavy drill is raised and dropped in the hole, the force of the blow developing from the weight of the falling drill. Hand drills are steel bars of a length suitable for the depth of the hole, with the cutting edge widened and sharpened to an angle as sharp as can be used without breaking. The drill bar is usually about ⅛th of an inch smaller than the diameter of the face of the drill.
Wedges used are called plugs and feathers. They are shown in Fig. 101 which shows also the method of their use. The feathers are wedges with one round and one flat face on which the flat faces of the plug slide.
144. Power Drilling.—In power drilling the drill is driven by a reciprocating machine which either strikes and turns the drill in the hole, or lifts and turns it as in churn drilling, or the drill may be driven by a rotary machine which is revolved by compressed air, steam, or electricity. There are many different types of machines suitable for drilling in the different classes of material encountered and for utilizing the various forms of power available.
A jack hammer drill is shown in Fig. 102. In its lightest form the drill weighs about 20 pounds and is capable of drilling ⅞-inch holes to a depth of 4 feet. Heavier machines are available for drilling larger and deeper holes. The same machine can be adapted to the use of steam or compressed air. When in use the point of the drill is placed against the rock and a pressure on the handle opens a valve admitting air or steam. The piston is caused to reciprocate in the cylinder, striking the head of the drill at each stroke. The drill is revolved in the hole by hand or by a mechanism in the machine. A hollow drill can be used by means of which the operator admits air or steam to the hole, thus blowing it out and keeping it clean. These machines have the advantage of small size, portability and simplicity. They can be easily and quickly set up and the drills can be changed rapidly. Their undesirable features are the vibration transmitted to the operator and the dust raised in the trench.
Fig. 102.—Jack Hammer Rock Drill.
Fig. 103.—Tripod Drill.
A type of drill heavier and larger than the jack hammer drill is shown in Fig. 103. It requires some form of support such as a tripod, or in tunnel work it can be braced against the roof or sides. Some data on steam and air drills are given in Table 56. The effect of the length of the transmission pipe, temperature of the outside air, pressure at the boiler or compressor, etc., will have a marked effect on the amount of steam or air to be delivered to the drill. Compressed air is affected more than steam by these outside factors, but it has an advantage in that as it loses in pressure it increases in volume so that the loss of power is not so marked. Gillette states:
We may assume that a cubic foot of steam will do practically the same work in a drill as a cubic foot of compressed air at the same pressure, because neither the steam nor the air acts expansively to any great extent in a drill cylinder, due to the late cut-off. This being so ... one pound of steam is equivalent to nearly 30 cubic feet of free air ... all at the same pressure of 75 pounds per square inch. If a drill consumes at the rate of 100 cubic feet of free air per minute ... it would therefore consume 240 pounds of steam (at 75 pounds pressure) per hour.... Where not more than three or four drills are to be operated, probably no power can equal compressed air generated by gasoline. It will require 12 horse-power to compress air for each drill, hence 1½ gallons of gasoline will be required per hour per drill while actually drilling.
| TABLE 56 | ||||||
|---|---|---|---|---|---|---|
| Data on Rock Drills | ||||||
| (From H. P. Gillette) | ||||||
| Diameter of cylinder in inches | 2¼ | 2½ | 2¾ | 3⅛ | 3¼ | 3⅜ |
| Length of stroke in inches | 5 | 6 | 6½ | 6⅝ | 6⅝ | 7¼ |
| Length of drill from end of crank to end of piston | 36 | 43 | 50 | 50 | 50 | 52 |
| Depth of hole drilled without change of bit, inches | 15 | 20 | 24 | 24 | 24 | 24 |
| Diameter of supply inlet. Standard pipe, inches | ¾ | ¾ | ¾ | 1 | 1 | 1¼ |
| Approximate strokes per minute with 60 pound pressure at the drill | 500 | 450 | 375 | 350 | 325 | 300 |
| Depth of vertical hole each machine will drill easily, feet | 6 | 8 | 10 | 14 | 16 | 20 |
| Diameter of holes drilled, inches | ¾ to 1½ as desired | |||||
| Diameter of octagon steel, inches | ¾ to ⅞ | ⅞ to 1 | 1 to 1⅛ | 1⅛ to 1¼ | 1⅛ to 1¼ | 1¼ to 1⅜ |
| Best size of boiler to give plenty of steam at high pressure, horse-power | 6 | 8 | 8 | 9 | 10 | 12 |
| Best size of supply pipe to carry steam 100 to 200 feet, inches | ¾ | ¾ | ¾ | 1 | 1 | 1¼ |
| Weight of drill unmounted, with wrenches and fittings, hot boxed, pounds | 128 | 190 | 265 | 315 | 385 | 390 |
| Weight of tripod, without weights, not boxed, pounds | 80 | 160 | 160 | 160 | 210 | 275 |
| Weight of holding down weights, not boxed, pounds | 120 | 270 | 270 | 285 | 330 | 375 |
| Cubic feet of free air per minute required to run one drill at 100 pounds | 92 | 104 | 126 | 146 | 154 | 160 |
| For more than one drill, multiply the value in the above line by the following factors: For 2 drills, 1.8; 5 by 4.1; 10 by 7.1; 15 by 9.5; 20 by 11.7; 30 by 15.8; 40 by 21.4; 70 by 33.2. | ||||||
Since gasoline air compressors are self regulating, when the drill is not using air very little gasoline is burned by the gasoline engine driving the compressor. A gasoline compressor possesses other very important economic advantages over a small steam-driven plant. First, there is the saving in wages of firemen and second, there is the saving in hauling and pumping of water and the hauling of fuel. The cost of gasoline is often less than the cost of coal for operating a small plant.
An electric drill[[89]] operated on the principle of the solenoid does away with motor, valves, pipes, vapor, freezing, and other difficulties attendant on the use of steam or air.
The rates of drilling in different classes of rock are shown in Table 57. Frequent changes of drills and relocation of tripods will materially reduce the performance of a drill, for as much as 45 minutes may be lost in making a new set up. In this the jack hammer drills show their advantage as no time is lost in a set up.
| TABLE 57 | ||
|---|---|---|
| Rates of Rock Drilling | ||
| Rates in Feet per Ten-hour Shift. Vertical Holes 10–20 Feet Deep. | ||
| (From Gillette) | ||
| Hard Adirondack granite | 48 | |
| Maine and Massachusetts granite | 45–50 | |
| Mica-schist of New York City. Possible | 60–70 | |
| Mica-schist of New York City. Average | 40–50 | |
| Hard, Hudson River trap rock 40 | ||
| Soft red sand stone of Northern New Jersey | 90 | |
| Hard limestone near Rochester, N. Y | 70 | |
| Limestone of Chicago Drainage Canal | 70–80 | |
| Douglass, Indiana, syenite. Difficult set ups | 36 | |
| Canadian granite on Grand Trunk R. R | 30 | |
| Windmill point, Ontario limestone: | ||
| 3⅝-inch drills | 75 | |
| 2¾-inch drills | 60 | |
| 2¼-inch drills | 37 | |
145. Steam or Air for Power.—The choice between steam or air is dependent on the conditions of the work. Steam is undesirable in tunnels on account of the heat produced. In open cut work it is at a disadvantage because of the loss of power due to radiation from the hose or pipe. The life of the hose is not so long as when air is used, escaping steam causes clouds of vapor which obscure the work, and serious burns may occur due to hot water thrown from the exhaust. It is advantageous since leaks may be easily discovered and remedied, it requires less machinery than air, and it is sometimes less expensive. With compressed air, gasoline or electric motors can be used for operating the compressors.
| TABLE 58 | ||||
|---|---|---|---|---|
| Rock Blasting | ||||
| (From Gillette) | ||||
| Character of Material | Powder Used per Hole | Depth of Hole, Feet | Distance Back of Face, feet | Distance Hole to Hole, feet |
| Limestone of Chicago Drainage Canal | 40 per cent dynamite | 12 | 8 | 8 |
| Sandstone | 200 pounds black powder | 20 | 18 | 14 |
| Granite | 2 pounds 60 per cent dynamite | 12 | 1½ | 4½ to 5 |
| Pit mining, Treadwell, Mine, Alaska | 12 | 2½ | 6 | |
146. Depth of Drill Hole.—The depth of the hole is dependent on the character of the work. The deepest holes can be used in open cut work where the shattered rock is to be removed by steam shovel. The face can be made 10 to 15 feet high. The depth of the hole in center cut tunnel facings are from 6 to 10 or even 12 feet. In the bench the depth is equal to the height of the bench. In narrow trenches where the rock is to be removed by derrick or thrown into a bucket by hand, the hole should be sufficiently deep to shatter the rock to a depth of at least 6 inches below the finished sewer. Frequently shooting to this depth at one shot cannot be done due to the built up condition of the neighborhood or other local factors. The depth of the hole in trench work should not much exceed the distance between holes. Deep holes are usually desirable as a matter of economy in saving frequent set ups, but the holes cannot be made much over 20 feet in depth without increasing the friction on the drill to a prohibitive amount.
147. Diameter of Drill Hole.—The diameter of the hole should be such as to take the desired size of explosive cartridge. The common sizes of dynamite cartridges are from ⅞ inch to 2 inches in diameter. In drilling, the diameter of the hole is reduced about one-eighth of an inch at a time as the drill begins to stick. This reduction should be allowed for, and experience is the best guide for the size of the hole at the start. In general the softer or more faulty or seamy the rock, the more frequent the necessary reductions in size of bit.[[90]] For hard homogeneous rock the holes can be drilled 10 feet or more without changing the size of the drill bit.
148. Spacing of Drill Holes.—The spacing of holes in open cut excavation is commonly equal to the depth of the hole. The character of the material being excavated has much to do with the spacing of the holes. The spacing, diameter and depth of holes used on some jobs is shown in Table 58. Gillette states:
It is obviously impossible to lay down any hard and fast rule for drill holes. In stratified rock that is friable, and in traps that are full of natural joints and seams, it is often possible to space the holes a distance apart somewhat greater than their depth, and still break the rock to comparatively small sizes upon blasting. In tough granite, gneiss, syenite, and in trap where joints are few and far between, the holes may have to be spaced 3 to 8 feet apart regardless of their depth for with wider spacing the blocks thrown down will be too large to handle with ordinary appliances. Since in shallow excavations the holes can seldom be much further apart than one to one and one-half times their depth we see that the cost of drilling per cubic yard increases very rapidly the shallower the excavation. Furthermore the cost of drilling a foot of hole is much increased where frequent shifting of the drill tripod is necessary.
The common practice in placing drill holes is to put down holes in pairs, one hole on each side of the proposed trench; and if the trench is wide one or more holes are drilled between these two side holes[[91]] but in narrow trench work, such as for a 12–inch pipe, one hole in the middle of the trench will usually prove sufficient.
The holes are spaced about 3 feet apart longitudinally. After the holes have been completed they should be plugged to keep out dirt and water.