Sewer Grade Check at Station 1+20
An 8-inch PVC sanitary sewer is being installed at a designed slope of 0.60%. Manhole A at station 0+00 has a pipe invert of 98.50 ft. The laser is set up over the invert at Manhole A, and the target is mounted on a rod at the pipe invert. At station 1+20, the crew reads the pipe invert 0.15 ft higher than the laser beam reference after accounting for the target height. The next manhole is at station 2+40. The pipe is supplied in 20-ft lengths, and the crew is using a transit to check line every third joint. Bedding is 6 inches of compacted gravel. The foreman wants to know whether the first 120 ft is acceptable and what invert elevation to expect at station 2+40.
1. Calculate the design invert at station 2+40 and the expected drop over 240 ft. Is the reading at station 1+20 high or low, and by how much?
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The drop over 240 ft is 0.006 × 240 = 1.44 ft. The design invert at station 2+40 is 98.50 − 1.44 = 97.06 ft. At station 1+20, the design drop is 0.006 × 120 = 0.72 ft, so the design invert is 98.50 − 0.72 = 97.78 ft. The actual invert is 0.15 ft higher than the laser reference, so it is at 97.93 ft. That is high by 0.15 ft. A 0.15-ft error over 120 ft is outside normal sewer grade tolerance and should be corrected before more pipe is laid.
2. What correction is needed before laying more pipe, and how should the laser and target be checked?
Read the worked answer
First verify the laser has not been bumped and is still set to the 0.60% design slope. Check the target height and rod, and confirm the reference elevation at Manhole A. If the laser is correct, lower the bedding at station 1+20 by 0.15 ft and adjust or re-lay the joint so the invert matches 97.78 ft. Do not bend the pipe to force grade. Recheck the next joint against the design invert before proceeding, and record the corrected reading.
3. What installation checks should keep line and grade within tolerance for the rest of the run?
Read the worked answer
The crew should check line and grade at each joint or at the specified interval, not only every third joint, using the laser or transit. Bedding must be compacted and the pipe must be uniformly supported; high spots in the bedding can hold the pipe above grade. Joint deflection and pipe alignment should be checked after assembly and again after backfill. As-built inverts should be recorded at manholes. If a joint is high or low, correct the bedding and pipe support rather than loading or bending the pipe.