7. Division of Work.—Engineering work on sewerage can be divided into four parts, namely: preliminary, design, construction and maintenance. An engineer may be engaged during any one or all of these periods on the same sewerage system, and should therefore be acquainted with his duties during each period.

8. Preliminary.—The demand for sewerage normally follows the installation or extension of the public water supply. It may be caused by: a lack of drainage on some otherwise desirable tract of real estate; from a public realization of unpleasant or unhealthful conditions in a built-up district; or through the realization by the municipal administration of the necessity for caring for the future. In whatever way the demand may be created the engineer should take an active part in the promotion of the work.

The engineer’s duties during the preliminary period are: to make a study of the possible methods by which the demand for sewerage can be satisfied; to present the results of this study in the form of a report to the committee or organization responsible for the promotion of the work; and so to familiarize himself with the conditions affecting the installation of the proposed plans as to be able to answer all inquiries concerning them. This work will require the general qualities of character, judgment, efficiency and the understanding of men in addressing interested persons individually and collectively on the features of the proposed plans, and the exercise of engineering technique in the survey and the drawing of the plans. The engineer should assure himself that all legal requirements in the drawing of petitions, advertising, permits, etc., have been complied with. This requires some knowledge of national, state, and local laws. Although none the less essential their description is not within the scope of this book.

The engineer’s preliminary report should contain a section devoted to the feasibility of one or more plans which may be explained in more or less detail with a statement of the cost and advantages of each. A conclusion should be reached as to the most desirable plan and a recommendation made that this plan be installed. Other sections of the report may be devoted to a history of the growing demand, a description of the conditions necessitating sewerage, possible methods of financing, and such other subjects as may be pertinent. The making of the preliminary plan and the design of sewerage works are described in subsequent chapters.

9. Estimate of Cost.—In making an estimate of cost the information should be presented in a readable and easily comprehended manner. It is necessary that the items be clearly defined and that all items be included. The method of determining the costs of doubtful items such as depreciation, interest charges, labor, etc., and the probability of the fluctuation of the costs of certain items should be explained.

The engineer’s estimate may be divided somewhat as follows:

Labor.

Material.

Overhead. This may include construction plant, office expense, supervision, bond, interest on borrowed capital, insurance, transportation, etc. The amount of the item is seldom less than 15 per cent and is usually over 20 per cent of the contract price.

Contingencies. This allowance is usually 10 to 15 per cent of the contract price.

Profit. This should be from 5 to 10 per cent of the sum of the four preceding items.

The contract price is the sum of these items. To this may be added:

Engineering. 2 to 5 per cent of the contract price.

Extra Work. Zero to 15 per cent of the contract price; dependent on the character of the work, the completeness of the preliminary information, the completeness of the plans, etc.

Legal expense.

Purchase of land, rights of way, etc., etc.

The cost of the sewer may be stated as so much per linear foot for different sizes of pipe, including all appurtenances such as manholes, catch-basins, etc., or the items may be separated in great detail somewhat as follows:

These methods represent the two extremes of presenting cost estimates. Each method, or modification thereof, may have its use, dependent on circumstances.

Reliable cost data are difficult to obtain. Lists of prices of materials and labor are published in certain engineering and trade periodicals. The Handbook of Cost Data by H. P. Gillette contains lists of the amount of material and labor used on certain specific jobs and types of construction. The price of labor and materials on the local market can be obtained from the local Chamber of Commerce, contractors and other employers of labor, and dealers in the desired commodities. Contract prices for sewerage work published in the construction news sections of engineering periodicals may be a guide to the judgment of the probable cost of proposed work, but are generally dangerous to rely upon as full details are lacking in the description of the work. A wide experience in the collection and use of cost data is the desirable qualification for making estimates of cost. It is possessed by few and is not an infallible aid to the judgment.

Having completed the design and summary of the bills of material and labor necessary for each structure or portion of the sewerage system, the product of the unit cost and the amount of each item plus an allowance for overhead will equal the cost of the item. The total cost will be the sum of the costs of each item. The items should be so grouped that the cost of the different portions of the system are separated in order that the effect on the total cost resulting from different combinations of items or the omission of any one item may be readily computed.

A method for estimating the approximate cost of sewers, devised by W. G. Kirchoffer^[13] depends upon the use of the diagram shown in Fig. 2. The factors for local conditions are shown in Table 2. For example, let it be required to find the cost of a 15–inch vitrified pipe sewer at a depth of 9 feet, if the unit costs of labor and material and the conditions are the same as shown in Table 3.

First: To find the factor depending on local conditions, enter the diagram at the 10–inch diameter and continue down until the intersection with the depth of trench at 8.2 feet is found. Now go diagonally parallel to lines running from left to right upwards to the intersection with the vertical line through a cost of 45 cents per foot. The diagonal line running from left to right downwards through this intersection corresponds to a factor of about 11.

Second: To find the cost of 15–inch pipe at a depth of 9.0 feet, enter the diagram at a diameter of 15 inches and continue down until the intersection with a depth of trench at 9 feet is found. Now go diagonally parallel to lines running from left to right upwards to the intersection with the diagonal line running from left to right downwards corresponding to the factor of 11 found above. The vertical line passing through this point shows the cost to be 67 cents per foot.

Methods of Financing

The construction of sewerage works may be paid for by the issue of municipal bonds, by special assessment, by funds available from the general taxes, or by private enterprise.

10. Bond Issues.—A municipal bond is a promise by the municipality to pay the face value of the bond to the holder at a certain specified time, with interest at a stipulated rate during the interim. The security on the bond is the taxable property in the municipality. The legal restrictions thrown around municipal bond issues, the value of the taxable property in the municipality, all of which may be used as security for municipal bonds, and the fact that a municipality can be sued in case of default, make municipal bonds desirable and provide a good market for their sale. The funds available from a municipal bond issue are limited by the amount that the legal limit is in excess of the outstanding issues. The legal limit varies in different states from about 5 to 15 per cent of the assessed value of the property in the municipality. In some cases the amount available from municipal bonds has been increased by forming a municipality within a municipality such as a sanitary district, a park district, a drainage district, etc., which comprises a large portion or all of an existing municipal corporation. This case is well illustrated in some parts of the City of Chicago where the municipal taxing powers are shared by the City government, the Sanitary District, and Park Commissioners. The right to create a new municipal corporation must be granted by the state legislature. Knowledge of fixed bonds, serial bonds, life of bonds, sinking funds, etc. is an important part of an engineer’s education.^[14]

Bond issues must usually be presented to the voters for approval at an election. If approved, and other legal procedure has been followed, the bonds may be bought by some of the many bonding houses, or by private individuals, and the money is immediately available for construction. The bonds are redeemed by general taxation spread over the period of the issue.

11. Special Assessment.—A special assessment is levied against property benefited directly by the structure being paid for. Special assessments are used for the payment for the construction of lateral sewers which are a direct benefit to separate districts but are without general benefit to the city. In case the construction of an outfall sewer or the erection of a treatment plant, which may be of some general benefit, is necessary to care for a separate district, a part of the expense may be borne by funds available from general taxation. The legal procedure for the raising of funds by special assessment and the purpose to which the funds so raised may be applied are stipulated in great detail in different states and their directions must be followed implicitly. Illinois procedure, which is similar to that in some other states, is as follows: a meeting of the interested property owners is called by a committee or board of the municipal government, as the result of a petition by interested persons or through the independent action of the Board. At this preliminary meeting or public hearing arguments for and against the proposed improvement are heard. The engineer is present at this meeting to answer questions and to advise concerning the engineering features of the plan. If approval is given by the Board the plan and specifications are prepared complete in every detail and incorporated in an ordinance which is presented to the legislative branch of the city government for passage. If the project is adopted it is taken to the county court. An assessment roll is prepared by a commissioner appointed by the court. This roll shows the amount to be assessed against each piece of property benefited. A hearing is then held in the county court at which the owner of any assessed property may voice objections to the continuation of the project. The project may be thrown out of court for many different reasons, such as the misspelling of a street name, an error in an elevation, an error in the description of a pavement, but most important of all is definite proof that the benefit is not equal to the assessment. The many minor irregularities which may nullify the procedure in a special assessment differ in different states and in different courts in the same state, but in general no court can approve an assessment greater than the benefits given. After the project has passed through the county court and the assessment roll has been approved, bonds may be issued for the payment of the contractor. Special assessment bonds are liens against the property assessed and have not the same security as a general municipal bond. For this reason a city which has reached its legal limit of municipal bond issues can still pay for work by special assessment.

The funds available from special assessments are limited only by the benefit to the property assessed. The amount of the benefit is difficult to fix and may lead to much controversy. It should not exceed the amount demanded for similar work in other localities, unless unusual and well-understood reasons can be given.

12. General Taxation.—In paying for public improvements by general taxation the money is taken from the general municipal funds which have been apportioned for that purpose by the legislative department of the municipal government. This method of raising funds for sewerage construction is seldom used unless the political situation is unfavorable to the success of a bond issue or special assessment and the need for the improvement is great. It is usually difficult to appropriate sufficient funds for new construction as the general tax is apportioned to support only the operating expenses of the city, and statutory provisions limit the amount of tax which can be levied.

13. Private Capital.—Private capital has been used for financing sewerage works in some cases because of the aversion of the public in some cities to the payment of a tax for the negative service performed by a sewer. Sewers are buried, unseen, and frequently forgotten, but knowledge of their necessity has spread and the number of privately owned sewerage works is diminishing because of the better service which can be provided by the municipality.

Franchises are granted to private companies for the construction of sewers only after the city has exhausted other methods for the raising of capital. The return on the private capital invested is received from a rental paid by the city, or paid directly by the users of the system, an initial payment usually being demanded for connection to the system. To be successful the enterprise must be popular and must fill a great need. This method of financing sewerage works is seldom employed as favorable conditions are not common.

Preliminary Work

14. Preparing for Design.—Methods for the design of sewerage systems are given in Chapter V. Before the design is made certain information is essential. A survey must be made from which the preliminary map can be prepared as described in Art. 42. Other necessary information which is the basis of subsequent estimates of the quantity of sewage to be cared for must be obtained by a study of rates of water consumption and the density and growth of population, the measurement of the discharge from existing sewers, and the compilation of rainfall and run-off data. If no rainfall data are available estimates must be made from the nearest available data. Observations of rainfall or run-off for periods of less than 10 to 20 years are likely to be misleading. Methods for gathering and using this information are explained in subsequent chapters.

Underground surveys are desirable along the lines of the proposed sewers to learn of obstructions, difficult excavation and other conditions which may be met. All such data are seldom gathered except for sewerage systems involving the expenditure of a large amount of money. For construction in small towns or small extensions to an existing system the funds are usually insufficient for extensive preliminary investigation. The saving in this respect is paid unknowingly to the contractor as compensation for the risk in bidding without complete information.

15. Underground Surveys.—These may be more or less extensive dependent on the character of the district in which construction is to take place. In built-up districts the survey should be more thorough than in sparsely settled districts where only the character of the excavated material is of interest and no obstructions are to be met.

Underground surveys furnish to the engineer and to prospective bidders on contract work information on which the design and estimate of cost and the contractor’s bid may be based and without which no intelligent work can be done. By removing much of the uncertainty of the conditions to be met in the construction of the sewer, the design can be made more economical and the contractor’s bid should be markedly lower, sufficiently so to repay more than the expense of the survey. The information to be obtained consists of the location of the ground-water level, and the location and sizes of water, gas, and sewer pipes, telephone and electric conduits, street-car tracks, steam pipes, and all other structures which may in any way interfere with subsurface construction. These structures should be located by reference to some permanent point on the surface. The elevation of the top of the pipes, except sewers, rather than the depth of cover should be recorded, as the depth of cover is subject to change. The elevation of sewers should be given to the invert rather than to the top of the pipe.

A portion of the map of the subsurface conditions at Washington, D. C., is shown in Fig. 3. Many of the dimensions and notations are not shown to avoid confusion on this small reproduction.^[15] Colors are generally used instead of different forms of cross hatching to show the different classes of pipe and structures. In addition to a record of the underground structures the character of the ground and the pavement should be recorded. A comprehensive underground survey is seldom available nor does time usually permit its being made preliminary to the design of a sewerage system. The character of the material through which the sewer is to pass should be determined in all cases.

Underground pipes and structures are located by excavations, which may be quite extensive in some cases. Their position is fixed by measurements referred to manholes and other underground structures which are somewhat permanent in position. A city engineer should grasp every opportunity to record underground structures when excavations are made in the streets. The character of the material through which the sewer is to pass is determined by borings.

16. Borings.—Methods used for the investigation of subsurface conditions preliminary to sewer construction are: punch drilling, boring with earth auger, jet boring, wash boring, percussion drilling, abrasive drilling, and hydraulic drilling. The last three methods named are used only for unusually deep borings or in rock.

Punch drills are of two sorts. The simplest punch drill consists of an iron rod ? of an inch to 1 inch in diameter, in sections about 4 feet long. One section is sharpened at one end and threaded at the other so that the next section can be screwed into it without increasing the diameter of the rod, as shown in Fig. 4. The drill is driven by a sledge striking upon a piece of wood held at the top of the drill to prevent injury to the threads. The drill should be turned as it is driven to prevent sticking. It is pulled out by a hook and lever as shown in Fig. 5. It is useful in soft ground for soundings up to 8 to 12 feet in depth. Another form of punch drill described by A. C. Veatch^[16] consists of a cylinder of steel or iron, one to two feet long split along one side and slightly spread. The lower portion is very slightly expanded and tempered into a cutting edge. In use it is attached to a rope or wooden poles and lifted and dropped in the hole by means of a rope given a few turns about a windlass or drum. By this process the material is forced up into the bit, slightly springs it, and so is held. When the bit is filled it is raised to the surface and emptied. Much deeper holes can be made with this than with the sharpened solid rod.

Types of earth augers about 1½-inches in diameter are shown in Fig. 6. They are screwed on to the end of a section of the pipe or rod and as the hole is deepened successive lengths of pipe or rod are added. The device is operated by two men. It is pulled by straight lifting or with the assistance of a link and lever similar to that shown in Fig. 5. The device is suitable for soft earth or sand free from stones, and can be used for holes 15 to 25 feet in depth. For deeper holes a block and tackle should be used for lifting the auger from the hole. It is not suitable for holes deeper than about 35 feet.

In the jetting method water is led into the hole through a ¾-inch or 1–inch pipe, and forced downward through the drill bit or nozzle against the bottom of the hole. The complete equipment is shown in Fig. 7.^[17] It is not always necessary to case the hole as shown in the figure as the muddy water and the vibration of the pipe puddle the sides so that they will stand alone. The jet pipe may be churned in the hole by a rope passing over a block and a revolving drum. In suitable soft materials such as clay, sand, or gravel, holes can be bored to a depth of 100 feet and samples collected of the material removed. An objection to the method is the difficulty of obtaining sufficient water.

Methods of drilling in rock up to depths of 20 feet are described in Chapter XI under Rock Drilling. For deeper holes percussion, abrasive, or hydraulic methods as used for deep well drilling must be employed.