Checking Grade with a Level Instrument

Figure 15-49.Backsight reading to determine instrument height.

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

Equipment Operator Basic - Beginning construction equipment operators manual
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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 eleva- tion 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 in- strument height of 376.57 feet. Then you take a fore- sight 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 re- quired 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. 15-30

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