Sewerage and Sewage Treatment

118. Elements.—The principal elements in construction are: labor, materials, tools, and transportation. The lack of or inadequateness of any one of these detracts from the effectiveness of the others. The engineer should assure himself of the completeness of his plans or those of the contractor on each of these points. The disposition of labor and the handling of materials to obtain the largest amount of good with the least expenditure of money and effort are problems which must be solved by the engineer or the contractor during construction.

Work of the Engineer

119. Duties.—The duties of the engineer during construction consist in giving lines and grades; inspecting materials; interpreting the contract, specifications and drawings; making decisions when unexpected conditions are encountered; making estimates of work done; collecting cost data; making progress reports; keeping records; and in guarding the interests of the City.

120. Inspection.—In the inspection of workmanship and materials, the engineer is assisted by a corps of inspectors and assistants who act under his direction. The duties of the inspector are to be present at all times that work is in progress and to act for the engineer in enforcing the terms of the contract, the details of the drawings, and the tests applicable to the workmanship and materials that he is delegated to inspect. He should have a copy of the contract, or that portion of it which pertains to his work, available at all times. He should examine all materials as they are delivered on the job and see that rejected materials are removed at once. An ordinary recourse of some foremen will be to place rejected material to one side until a brief absence of the inspector will present the opportunity for the use of the rejected material. The methods to be followed in the inspection of materials and workmanship should be such as to discover discrepancies between the specifications and the materials delivered or the work done. Other duties of the inspector are: to record the location of house connections or to drive a stake over them for subsequent location by the engineer; to see that plugs are put in the branches left for future house connections; to inspect the workmanship in the making of joints in pipe sewers; to protect the line and grade stakes from displacement; to check the size, depth, and grade of sewers and elevations of special structures, etc.

Dishonest and unscrupulous workmen have many tricks to get by the inspector. These tricks are best learned by experience as no academic list can impress them properly on the memory. The position of the inspector is not always enviable. He must hold the respect of the workmen, of the contractor, and of the engineer. To do this he must not be unreasonable or arbitrary in his decisions, but when a decision is once made he must be firm in following up its enforcement. He must be careful not to give directions whose fulfillment he cannot enforce, nor for which he cannot give adequate reason to his superiors. His integrity must never be questioned. He must not allow himself to become under obligations to the contractor by the acceptance of favors he cannot return except at the expense of his employer, yet at the same time he must not appear priggish by the refusal of all favors or social invitations. In brief he must be friendly without being intimate, independent without being aloof, and firm without being arbitrary.

The engineer must support his inspectors in their decisions or discharge them if he cannot.

121. Interpretation of Contract.—In interpreting the contract, specifications and drawings, the engineer is supposedly an impartial arbiter between the interests of the city and the contractor. His decisions, as to the meaning of the contract, must be founded on his engineering judgment, and should aim to produce the best results without demanding more from the contractor than, in his honest opinion, it is the intention of the contract to demand. However conscientiously he may attempt to remain impartial, and in spite of the honesty of the contractor, his position, as an employee of the city will almost invariably cause him to favor the city in his decisions on close points. The experienced contractor knows this and fixes his bid accordingly, the personality of the engineer sometimes acting as an important factor in the amount of the bid. The situation arises through the character of the contract, and not through a lack of moral integrity on the part of anyone concerned.

122. Unexpected Situations.—When unexpected or uncertain conditions are encountered in construction the engineer should visit the spot at once and should advise or direct, according to the terms of the contract, the procedure to be followed. Such conditions may be the encountering of other pipes, quicksand, rock, etc. Each case is a problem in itself. Water, gas, telephone and electric wire conduits can be moved above or below the sewer being constructed with comparative ease. Other sewers, if smaller, may be permitted to flow temporarily across the line of the sewer under construction and finally discharge into the completed sewer, or one sewer must be made to pass under the other, either as an inverted siphon or by changing the grade of one of the sewers. Rock, or other material for which a special rate of payment is allowed, must be measured as soon as uncovered in order to avoid delaying the work or losing the record of the amount removed. When quicksand is met special precautions must be taken to safeguard the sewer foundation and to insure that the sewer will remain in place until after the backfilling is completed. These precautions are described in Art. 135.

123. Cost Data and Estimates.—Cost account keeping and the making of monthly or other estimates are closely connected. Cost accounts are of value in estimating the amount of work done to date, and in making preliminary estimates of the cost of similar work. Although the engineer is not always required to keep such accounts, they are usually of sufficient value to pay for the labor of keeping them. Under some contracts the contractor’s accounts are open to examination by the engineer. Usually, however, he must depend on reports from the inspectors for information concerning the man-hours required on different pieces of work, and on his own measurements of materials used and his knowledge of their unit costs, in order to make up an estimate of total cost.

The measurement of a completed structure and a summary of the materials used in its construction may act as a check on the use of proper materials as called for in the contract. For example, if it is known that 2,000 bricks are required for the construction of a manhole and if only 15,000 have been used in the construction of ten manholes, it is probable that some or all of the manholes have been skimped. Similar conditions may show in the proportions of concrete, backfilling in tunnels, sheeting to be left in place, etc.

The statement of a few principles of cost accounting, and the illustration of a few blanks in use should be sufficiently suggestive to lead a resourceful engineer in the right direction.^[79] Costs should be divided into four general classifications: labor, materials, equipment, and overhead. Labor should be subdivided under its several different classifications arranged in accordance with rates of pay. The number of laborers under each classification and the amount of work done per day should be recorded. Fig. 86 is an example of a form which may be used for such a purpose.

Materials may be recorded as they are delivered on the job, as they are used, or in both cases. Measurements are usually easier to make at the time of delivery, but records made at the time materials are used are more serviceable. For example, 100 barrels of cement may be delivered on a job in November, 50 of them are used before the job freezes up and the other 50 are held over until spring. It would be misleading to charge 100 barrels used in November. Fig. 87 is a form in use for an inspector’s report on materials. The total cost must be made up in the office from these records and a knowledge of unit costs.

Equipment consists of tools, animals, machinery, and apparatus used in construction. Only equipment that is actually used should be charged to the job and a credit should be made at the completion of the job for the fair value of the equipment remaining after the completion of the work.

Overhead charges include the expense of the office force, superintendence, and miscellaneous items such as insurance, rent, transportation, etc., which cannot be charged to any particular portion of the work but are equally applicable to all portions. It happens frequently that many jobs are handled in the same main office. The division of overhead becomes more difficult and is frequently arranged on an arbitrary basis, e.g., each job may be charged the proportion of overhead that its contract price bears to the total contract prices being performed under that office. This rule may be modified when it becomes evident that some job is taking distinctly more than its share of the overhead.

Estimates of work done in any period can be made with the above data in hand by subtracting the total costs of the work up to the beginning of the period from the total costs up to the end of the period. Fig. 88 shows a sample blank from the final estimate sheets used at Scarsdale, N. Y.

124. Progress Reports.^[80]—These are kept by the engineer in order that he may see that the work is progressing as called for in the contract, and any portion which is lagging behind without reason may be pushed. Such reports are most useful when the information is expressed graphically, as the eye quickly catches points where the work is falling behind schedule.

125. Records.—The contract drawings are supposed to show exactly where and how construction is to be done. Due to unexpected contingencies changes occur, of which a record should be made and preserved. These records may be kept in a form similar to the contract drawings, or if the changes are not extensive, they can be recorded on the original contract drawings. The location of house and other connections should be recorded in a separate note book available for immediate consultation. The engineer should keep a diary of the work in which are recorded events of ordinary routine as well as those of special interest and importance. This diary should be illustrated by photographs showing the condition of the streets before and after construction, methods of construction, accidents, etc. Such accounts are of great value in defending subsequent litigation and their existence sometimes prevents litigation. A contractor may wait a year or so after the completion of a piece of work until the engineer and other city officials have broken their connection with the city. Suit is then brought against the city and unless good records are available the administration may be forced to buy the claimant off or may elect to enter court, only to be beaten.

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.

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.

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.

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.

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.

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.

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.