Answers to the most common questions from field contractors.
City engineers typically require: manhole rim and invert elevations (invert in and out for each pipe at every manhole), pipe size and material, calculated slope between manholes, depth of cover at key stations (especially road crossings), GPS coordinates for each manhole (increasingly required), and photographs of pipe before backfill. The as-built must be submitted by the engineer of record or a licensed surveyor in most jurisdictions. The format is usually a standard city drawing sheet with a manhole schedule table.
Per IPC Section 704.1, the minimum slope for a 6-inch gravity sewer is 1/16 inch per foot, which is approximately 0.52% (or about 5 feet of fall per 1,000 feet of pipe). Some jurisdictions specify 0.6% or 1.0% minimum. The minimum slope is set to achieve self-cleaning velocity of approximately 2 feet per second at half-full flow conditions. A 4-inch pipe requires a minimum of 1/8 inch per foot (approximately 1.04%). Always verify with the applicable local code and project specifications - many municipalities exceed the IPC minimum.
Set up a level or total station on a known benchmark referenced to the project control. Shoot the invert elevation at each manhole by measuring to the inside bottom of the pipe at the manhole wall. Record the elevation for each pipe entering and leaving the manhole. Calculate slope between manholes: slope = (upstream invert minus downstream invert) divided by distance. Compare to design invert. Flag any deviation greater than the project tolerance (typically plus or minus 0.10 ft for new installation). Document while the pipe is visible before backfill.
A complete manhole-to-manhole as-built includes: manhole IDs (upstream and downstream), rim elevation (surveyed), invert elevation in and out at each manhole (all pipes), pipe size (diameter in inches), pipe material (VCP, PVC, RCP, HDPE, etc.), slope between manholes (calculated), depth of cover at each manhole and at road crossings, GPS coordinates, and flow direction arrow. Many cities also require CCTV inspection log reference numbers and photos of connections.
Review time varies widely by jurisdiction: 2-5 business days for smaller cities with efficient review processes, 2-4 weeks for larger cities with backlogs, and up to 6-8 weeks in some California jurisdictions. As-builts with missing data or format errors are typically returned without review, restarting the clock. Submitting complete, organized as-builts in the city-required format significantly reduces review time.
In order of frequency: (1) Missing GPS coordinates - many cities now require northing and easting coordinates and return submittals that omit them. (2) Incomplete invert data - missing inverts for one or more pipes at a manhole. (3) No depth of cover data at road crossings. (4) Incorrect elevation datum - project using a local datum that does not match the city datum reference. (5) Illegible records or poor scan quality. (6) Wrong plan version referenced. Addressing these systematically before the first submittal avoids most rejections.
Increasingly yes. Most cities now require GPS coordinates (northing and easting) for each manhole location as part of the as-built, because the data is imported into the city GIS system. Without GPS coordinates, the as-built may not be processable by the city GIS team even if the engineer of record accepts it. Accuracy requirements vary: most cities accept GPS coordinates to 0.1-foot accuracy. RTK GPS provides this easily; handheld GPS units (typically plus or minus 10-15 feet) are not acceptable.
Document the offset in the as-built: note the distance and direction from the design centerline (example: 0.8 ft east of design). Some engineers include a dimensioned sketch showing the deviation. Notify the engineer of record immediately when an off-line condition is discovered - depending on the magnitude, it may require correction before proceeding. Minor offsets within the city tolerance (typically plus or minus 1.0 ft for manhole location) may be acceptable with documentation. Major offsets may require relocation.
The standard minimum depth of cover for gravity sewer is 36 inches from the top of pipe to finished grade in most US jurisdictions. Some cities require 48 inches. In northern states, frost depth overrides this minimum: Minnesota requires 7 feet, Michigan 5 feet. Pipes under roadways with less than 24 inches of cover typically require concrete encasement per AASHTO H-20 loading requirements. Always check the project specifications and local standards - the 36-inch figure is a common baseline but not universal.
Document any grade deviation at the station where it occurs: record the station, the design invert at that station, the as-built invert, and the deviation (as-built minus design). Note whether the deviation is within or outside the project tolerance (typically plus or minus 0.10 ft). If outside tolerance, document the corrective action required and taken. For deviations that cannot be corrected (for example, a manhole set to design elevation with an existing utility obstruction that required the pipe to deviate), document the engineering direction you received to accept the deviation.
Sitemark logs MH-to-MH inverts, calculates slopes, verifies depth of cover, and generates as-built packages in city-accepted format.
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