leveling
chapter seven
Standard Practices
No Mistakes!!
It must be recognized that setting grade must be absolutely correct. The elevation that is placed on the stakes or forms for excavating, building formwork, setting anchor bolts, etc., is the guide for where something is going to be built. There can be no mistakes in the setup, the rod readings, the notekeeping, calculating the elevations, or writing the elevations on the stakes, as these crafts are going to use what is written to build from or to build to. Any mistakes that result in a footing being a foot high, anchor bolts being at the wrong elevation, or a bridge abutment being 4 feet low, is going to cost someone a lot of money to fix. The bottom line is that when it comes to construction elevations, there is only one answer . . . elevations must be 100 percent right.
Try to Balance BS and FS
Setting grade often requires just one setup of the leveling instrument. If possible, it is crucial that the instrument be set up so that the distance from the benchmark to the instru-ment, and the instrument to the point where grade is needed, are the same. Remember, balance backsights and foresights. Unfortunately on the construction site, it is often impossible to have them balance because of obstacles that are in the way. Therefore, it is necessary to make sure the leveling instrument is in proper calibration. See Chapter 10, Equipment Calibration, for the procedure for checking leveling instruments.
Obtain Your HI From Two BM's
One very important step for ensuring that mistakes in setting grade are eliminated is to always sight onto two BM’s in order to determine the HI of the instrument. This simple procedure will provide a triple-check of the leveling process if the HI is the same when obtained from two benchmarks. The three checks are:
• Benchmark movement. If one of the benchmarks has been run over or moved in some way, the calculated HI’s will not be the same.
• Reading check. If the rod is read incorrectly on one of the benchmarks, the calculated HI’s will not be the same.
• Incorrect addition. If one of the readings is added incorrectly, the HI’s will not be the same.
Only when the benchmarks are undisturbed, the readings are correct, and the calcu-lations are correct will the HI’s be the same. When they are the same, setting grade can proceed with great confidence that the HI is correct. Now, avoid mistakes on the foresights so the grade stakes will be correct.
ch 10
Always obtain the HI from
2 BM's.