To stake out a sewer, you obtain data from a plan and profile that shows (1) the horizontal location of each line in the system, (2) the horizontal location and char-acter of each manhole, (3) the invert elevations at each manhole, and (4) the gradient of each line. You will also have detail drawings of each type of appurtenance. If manholes in the same category are of different types, you may identify them by letter symbol, as CI "A," and so on. In addition, identification of a particular appurtenance may be by consecutive number, as CI "A" #3.

The stakeout consists of setting hubs and stakes to mark the alignment and indicate the depth of the sewer. The alignment may be marked by a row of offset hubs and stakes or by both offset hubs and a row of center-line stakes. Cuts may be shown on cut sheets (also called grade sheets or construction sheets) or may be marked on the stakes, or both. The cuts shown on the center-line stakes guide the backhoe operator or ditcher operator; they are usually shown to tenths; they generally repre-sent the cut from the surface of the existing ground to the bottom of the trench, taking into account the depth to the invert, the barrel thickness, and the depth of any sand or gravel bed. The cuts marked on the stakes next to the hubs are generally shown to hundredths and usually represent the distance from the top of the hub to the invert; these cuts guide the pipe crew. The use of these cuts in transferring the information to batter boards or various types of offset string lines was de-scribed in chapter 14 of the EA3 TRAMAN.

If the survey party stakes only the offset hubs, then the construction crew usually sets center-line stakes for line only and uses the hubs as a guide for the depth of excavation. The extent of the stakeout and computa-tions performed by the survey party and the correspond-ing extent of such work done by the construction crew depend on the capabilities and the availability of per-sonnel and the work load. In any case, hubs and/or stakes are generally set at 25-foot intervals, though 50-foot and even 100-foot intervals have been known to suffice.

Sewer hubs are usually offset from 5 to 8 feet from the center line. Before you enter the field, you compute from the profile the invert elevation at every station where you will set a hub. Consider fig-ure 10-19, for example. This is a plan showing a line running from a curb inlet through two manholes to an outfall. The dotted lines are offsets (greatly exaggerated for clearness) to points where you will set the hubs. Note that at stations 5 + 75 and 1 + 70.21, you set two hubs, one for the invert in and the other for the invert out.

Suppose now that you are starting the stakeout at CI "A" #2. The final-location party left a center-line stake at this station. You occupy this point, turn 90 degrees left from the line to MH "A" #1, and measure off the offset; for example, 8 feet. This is presuming that, if the ground slopes across the line, the high side is the side on which the hubs are placed in figure 10-19. Hubs are always placed on the high side to prevent them from being covered by earth dozed off to form a bench for the trench-digging rig. You drive a hub 8 feet offset from station 9 + 00 and determine the elevation of the top of the hub. The vertical distance from the top of the hub to the invert at station 9 + 00 is the difference between the invert elevation and the elevation of the top of the hub. The invert elevation at station 9 + 00 is 122.87 feet. Suppose the elevation of the top of the hub is 126.94 feet. Then you would mark the guard stake for this hub, CI "A" #2 inv. C 4.07. Suppose the elevation of the top of the hub driven at station 8 + 50 is 127.33. The invert elevation at this station is 121.78; therefore, you would mark the guard stake for this station, 8 + 50, C 5.55.

The manner in which the construction crew will use these hubs to dig the trench to grade will vary according to the preference of the supervisor for one of several methods. One method involves the erection of a batter board across the trench at each hub. The top of each board is placed on the posts at a set distance above invert elevation; for example, 10 feet. Fig-ure 10-20 illustrates this method.

Take station 9 + 00 in figure 10-19, for example. The elevation of the top of the hub is 126.94 feet and the invert elevation is 122.87 feet. To be 10 feet above invert elevation, the top of the batter board must be placed on the post 5.93 feet above the top of the hub. To get this distance, the field constructor would simply

subtract the specified cut from 10 feet. At station 8 + 50, for example, the height of the top of the batter board above the top of the hub would be 10 5.55, or 4.45 feet.