Sewerage and Sewage Treatment

by Harold E. Babbitt

Line and Grade

159. Locating the Trench.—In order to locate a trench a line of stakes should be driven at about 50–foot intervals along the center line of the proposed sewer before excavation is commenced. Reference stakes or reference points to this line are located at some fixed offset or easily described point, or the stakes marking the center line of the trench may be driven at some constant offset distance one side of the trench, in order to avoid danger of loss or disturbance of the stakes. Grade or cut is seldom marked on the line of preliminary stakes, although the approximate cut may be indicated.

For hand excavation the foreman lays out the trench from these stakes. In machine work the operator guides the machine so as to follow the line of the stakes.

160. Final Line and Grade.—After the excavation of the trench has proceeded to within a foot or two of the final depth, the grade and line are transferred to markers supported over the center of the trench. The markers are horizontal boards spanning the trench and held in position either by nails driven into stakes at the side of the trench, by nails driven into the sheeting, or by weights holding the boards on the ground. Two stakes driven in the ground at the side of the trench as shown in Fig. 113 are the common method of support. If the banks are too weak to stand under the jarring of the driving of the stakes, or pavement or other causes prevent their use the horizontal cross piece may be weighted down by bricks or a bank of earth. The cross pieces are located about every 25 feet along the trench and at any convenient distance above the surface of the ground. The nearer the ground the stronger the support but the greater the interference with work in the trench. The center line of the sewer is marked on the cross pieces after they are set, and vertical struts are nailed on them with one edge of the strut straight, vertical, and on the center line as shown in Fig. 1. The corresponding edge should be used on all struts in order to avoid confusion. The edge is placed in a vertical position by means of a plumb bob or carpenter’s level.

Fig. 113.—Methods for the Support of the Grade Line.

The cut to the invert of the sewer is recorded to an even number of feet where practicable by driving a nail in the upright strut so that the top edge of the nail is at the desired elevation above the sewer, or the upright is nailed with its top at the proper number of feet above the sewer invert. The cut is marked on the upright in feet, tenths, and hundredths from the recorded point to the elevation of the invert.

The inspector should watch these grade markers with care by sighting back along them to see that they are in line and have not moved. In quicksand or caving material the marks may move during the setting of the pipes and the levelman should be on the job constantly.

When excavation is being done by machine the depth of the excavation is controlled by the operator who maintains a sighting rod on the machine in line with the grade marks on the uprights.

Fig. 114.—Diagram Showing the Use of the Grade Rod for Fixing the Elevation of a Sewer.

161. Transferring Grade and Line to the Pipe.—In transferring grade and line to the sewer a light strong string is stretched tightly from nail to nail on the uprights marking the line and grade. A rod with a right angle projection at the lower end, as shown in Fig. 114, is marked with chalk or a notch at such a distance from the end that when the mark is held on the grade cord the lower portion of the rod which projects into the pipe will rest on the invert. The pipe is placed in line by hanging a plumb bob so that the plumb bob string touches the grade and center line cord. These marks are taken only as frequently as may be necessary to keep the sewer in line. An experienced workman can maintain the line by eye for considerable distances. Measurements should never be taken to the top of the pipe in order to determine position and grade as the variations in the diameter of the pipe may cause appreciable errors.

The position and elevation of the forms for brick, concrete, and unit block sewers are located by reference to the grade line, or they may be placed under the immediate direction of the survey party, or by specially located stakes. For large sewers requiring deep and wide excavation the grade and line stakes are driven in the bottom of the trench about a foot above the finished grade. This requires the constant presence of an engineer who is usually available on work of such magnitude.

162. Line and Grade in Tunnel.—In tunnels, line and grade are given by nails driven in the roof, the progress of excavation or the shield being followed by eye and the forms set by direct measurement to the nails.