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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.

Fig. 3.—Record Map of Underground Structures, Washington, D. C.

Eng. Record, Vol. 74, p. 263.

The various subsurface lines are differentiated by colors as follows: A—Sewers, vermilion. B—Water mains, blue. C—Potomac Electric Power Co., carmine. D—Washington Railway and Electric Co., carmine. E—Capital Traction Co., violet. F—Chesapeake and Potomac Telephone Co., green. G—Washington Gas Light Co., green. H—Western Union Telegraph Co., orange. I—Postal Telegraph Co., orange. K—Private vaults, black. L—City Electric Co., yellow.

Fig. 4.

Punch Drill.

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 ⅞ 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.

Fig. 5.—Lever for Pulling Punch Drill.

Fig. 6.—Earth Augers.

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.

Fig. 7.—Jetting Outfit.

U. S. Geological Survey, Water Supply Paper, No. 257

1. Simple Jetting Outfit. 2. Jetting Process. 3. Common Jetting Drill. 4a and 4b. Expansion Bit or Paddy. 5. Drive Shoe.

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.