Checking Grade with a Level Instrument

An example of checking ground spots for desired grade with a level instrument is shown in figure 15-51.

The hubs and stakes at the side of the construction represent offset grade stakes. In figure 15-51, view (A), the grade stake calls for a cut of 7.5 feet. You set up your level and take two readings: first on the hub and then on the excavation. Your first reading is 5.0 feet. Since the excavation is supposed to be 7.5 feet below the hub, your second reading should be 12.5 (5.0 plus 7.5 as shown). But the rod reads only 12.2; therefore, you must cut 0.3 feet more to get to finished subgrade.

In figure 15-51, view (B), your first reading is 12.0 feet on the hub. Since the stake calls for F 7.0, you should read 5.0 on the completed fill. But the rod reads 5.5; therefore, you must fill another 0.5 feet to finish the subgrade.

MISSING GRADE STAKE.- Another leveling procedure is to compute a cut-or-fill requirement from a missing grade stake. In figure 15-52, the finish elevation from the project drawings at point B is supposed to be 378.75. You setup your level, take a backsight shot on the bench mark at point A, and get a direct reading of 11.56 feet. The 11.56 feet backsight reading plus the bench mark elevation of 365.01 feet gives you an instrument height of 376.57 feet. Then you take a foresight shot at point B, and get a direct reading of 1.42 feet. You now subtract the foresight reading of 1.42 feet from the instrument height of 376.57, and find that the existing ground is at elevation 375.15. You now take the required finish elevation of 378.75 and subtract the existing elevation of 376.15 and get a FILL requirement of 3.6 feet at point B. If the existing elevation is greater than the required finish elevation, you would be required to cut.

Another example of a missing grade stake is shown in figure 15-53. Suppose the stake at station 4 + 50 has been knocked out, and there is no bench mark

Figure 15-51.-Checking cut and fill.

Figure 15-52.-Computing cut-or-fill requirement from a missing grade stake.

Figure 15-53.-Replacing a missing grade stake. 15-31

nearby, but you do have a nearby grade stake at station 4 + 00 and a set of the project drawings. At station 4 + 00, the project drawings call for a subgrade elevation of 240.0 feet. The stake at station 4 + 00 calls for a fill of 7.0 feet; therefore, the existing elevation at station 4 + 00 is 233.0 (7.0 feet below 240.0).

You set up your level and take a backsight direct reading on station 4 + 00 of 4.0 feet. The backsight reading of 4.0 feet plus the existing known elevation of 233.0 feet gives an instrument height of 237.0. Then you take a foresight shot on station 4 + 50 and get a direct reading of 4.5 feet. You subtract the foresight reading of 4.5 feet from the instrument height of 237.0 feet, and you get at station 4 + 50, the existing elevation of 232.5 feet.

The project drawings show that the finished subgrade elevation at station 4 + 50 is 239.0 feet. With the existing elevation at 232.5, you must FILL at station 4 + 50 a total of 6.5 feet to reach the 239.0 feet required subgrade elevation. Therefore, you should place a grade stake at station 4 + 50 that is marked F 6.5.