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.
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.
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 ────────────────────────────────────────────────────────────────┴──────