The building drain leaves the foundation with its invert at elevation 100.60 feet. The invert of the public sewer at the point of connection is at elevation 96.85 feet, and the building sewer will run 340 feet of developed length between those two points. Take the minimum slope for the 6-inch pipe being used as 1/8 inch per foot, given. What grade is available, and does it work?
- AAbout 0.011 inch per foot, far below the minimum, so the sewer has to be pumped
- BAbout 3.75 inches per foot, well above the minimum, so gravity drainage works easily
- CAbout 0.13 inch per foot, which falls short of the minimum, so the sewer has to be pumped
- About 0.13 inch per foot, which meets the minimum, so gravity drainage works
Where to look it up
Explicit exercise assumptions; not a universal code requirement.
Why D is correct
The elevation difference is 100.60 - 96.85 = 3.75 feet, or 45 inches. Available grade is 45 / 340 = approximately 0.13235 inch per foot, exceeding the stated 0.125-inch minimum. Required fall is 340 × 0.125 = 42.5 inches, leaving a 2.5-inch margin. Thus the supplied elevations meet this slope check; other design requirements still apply. With the upstream invert fixed, a lower downstream connection increases available fall, while a higher connection reduces it.