An as-built survey for underground utilities documents the actual installed location (horizontal and vertical) of every pipe, structure, and fitting. Required measurements include: invert elevation and horizontal location at each manhole/structure, pipe depth and horizontal offset at all service connections and cross-street crossings, and pipe cover depth at minimum-cover areas. The survey must be completed before final backfill or paving if horizontal location is required — after backfill, measurement requires potholing or GPR.
Why As-Built Surveys Matter for Underground Utilities
Underground utilities that are not documented after installation become a long-term problem. When the next contractor digs in the area, they have no reliable information on where the pipe is. The results: struck utilities, gas releases, water main breaks, and service outages. Most municipalities now require GPS-located as-built surveys for new utility installations before the trench is backfilled or before the permit is closed. Some require full CAD deliverables (DXF or Shapefile) that are uploaded to the utility database.
What to Measure
At a minimum, an underground utility as-built survey requires:
- Every manhole and structure: GPS location (or survey coordinates), rim elevation, invert elevation at every pipe connection, pipe size and material at each connection, and direction to the next structure.
- Pipe alignment: Horizontal location at 50–100 ft intervals, or at every change of direction. Most utilities require GPS shots from the surface at each structure; some require probing or potholing to confirm horizontal location in between.
- Service connections: GPS location and cover depth at each service tee or wye, marked on the surface with a stake or APWA color-coded marker.
- Crossings: At every road crossing or crossing of another utility, record the pipe elevation at the crossing and the identity of the utility crossed.
- Cover depth at special locations: Any location where the design cover was marginal — utility crossings, near structures, at low-cover road crossings.
When to Conduct the Survey
For sewer and water mains: conduct the as-built survey before backfill is complete for any locations where horizontal location needs to be verified from the open trench. Invert elevations can be measured from inside manholes or at exposed pipe ends after backfill; horizontal location is much easier before backfill.
For storm drain: at minimum, survey all inlet structures before paving. Pipe alignment can be documented from the inlet locations if straight runs between inlets can be assumed.
Equipment for Underground Utility As-Built Surveys
- Total station + data collector: Most accurate for invert elevations and horizontal positions at structures. Operator accesses the manhole or works from grade at the structure rim.
- GPS rover: Used for surface-level structure locations (rim elevation and horizontal position) on projects where RTK accuracy (±0.05 ft) is acceptable. Cannot access underground inverts directly — pair with a level and rod for invert shots.
- Automatic level + rod: For differential leveling of invert elevations from a benchmark.
Deliverable Formats
Required deliverable format varies by municipality. Common formats:
- As-built table (PDF or spreadsheet): Structure ID, location, rim elevation, invert elevations for each pipe connection, pipe size and material. Most common for simple projects.
- Redline drawings: Markups on the design plan sheets showing actual field locations vs design.
- CAD file (DXF or DWG): Required by many utilities for GIS integration. Must be in the municipality’s specified coordinate system.
- GIS Shapefile: Required by some utilities for direct upload to their utility database.
Log all as-built shots in Gradelog and export in the required format. See also: Creating an As-Built Document: What Inspectors Actually Need.