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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:

Earth excavation, per cu. yd.

Rock excavation, per cu. yd.

Backfill, per cu. yd.

Brick manholes, 3 feet by 4 feet, per foot of depth.

Vitrified sewer pipe with cement joints, in place,

... inches in diameter, 0 to 6 feet deep

6 to 8 feet deep

8 to 10 feet deep

Repaving, macadam per sq. yd.

asphalt per sq. yd.

Flush-tanks,... gal. capacity, per tank.

Service pipes to flush-tanks, per linear foot., etc., etc.

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.

Fig. 2.—Diagram for Estimating the Cost of Sewers.

Eng. News, Vol. 76, p. 781.

Solution

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.

TABLE 2
Factors for Costs of Sewers to be Used with Figure 2
Character of Material	Factor
Clay, gravel and boulders, Medford	22–26
Mostly sand, deep trenches sheeted. Wages medium. Richland Center.	21–22
Sandy clay. Wages medium. Labor conditions good at Kiel.	15–20
Sand. Sandy clay, some water. Labor conditions good. Pipe prices medium at Manston.	14–20
Gravelly clay, ⅒th laid in concrete at Burlington.	13–22
Sandy clay, some water, sheeting at La Farge.	17–23
Sand with water.	20
Gravel and boulders. High wages.	26
Clay soil. Good digging.	17
Sandy clay. Some water.	23
Clay 2 miles inland. Laborers boarded at sanitarium, Wales	35
Clay, gravel and boulders at Plymouth.	20–27
Sand, clay and good digging at Lake Mills.	16–19
Red clay. Machine work at North Milwaukee.	20–24
Good digging. Wages medium at West Salem.	17–19
Sandy soil, bracing only required. No water. Wages and pipe medium.	14
Red sticky clay.	24
Good digging in any soil. Work scarce.	15
Red clay. No bracing.	20
Work inland from railroad. Boarding laborers and other expenses.	35

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.

TABLE 3
Cost of Sewer Construction at Atlantic, Iowa
(From Gillette’s Handbook of Cost Data)
Material: Clay, not difficult to spade and requiring little or no bracing and practically no pumping. All hand work except backfill which was done by team and scraper. Depth of trench averaged 8.2 feet; width 30 inches. Diameter of pipe 10 inches.
Item	Wage, Cents per Hour	Cost, Cents per Foot.
Pipe.		0.20
Hauling team and driver.	30	.003
Hauling. Man helping.	17	.001
Cement and sand.		.006
Pipe layers.	22	.014
Pipe layer’s helper.	17	.014
Trenching. Top men.	17	.027
Trenching. Bottom men.	17	.130
Trenching. Scaffold men.	17	.002
Trenching. Bracing men.	17	.002
Backfilling. Shovel.	17	.010
Backfilling. Team and scraper.	30	.008
Backfilling. Man and scraper.	17	.005
Water boy.	10	.006
Foreman.	30	.022
Total.		.450