Horizontal directional drilling (HDD) grade control requires designing the bore path with adequate depth at all crossings (minimum 3–5 ft below crossing utilities), tracking the drill head in real time, and verifying the as-installed depth by shooting invert elevations at entry and exit pits. The drill head locator (Subsite, DigiTrak, Vermeer Navigator) provides continuous depth and steering data; log each reading at the specified tracking interval (typically every 10 ft).
Bore Path Design Elements
A bore path design specifies:
- Entry angle: The angle of the drill string entering the ground (typically 8–15° for most soil conditions)
- Depth at each tracking point: Minimum depth below all crossings, plus grade requirements for gravity drainage applications
- Radius of curvature: The minimum bend radius the pipe product can tolerate (HDPE has a much tighter minimum bend radius than steel)
- Exit angle and location: Must match the product pipe alignment and grade at the receiving pit
For gravity drainage applications (sewer crossings), the bore path must maintain the design grade continuously from the entry invert to the exit invert. For pressure applications (water main, gas), the path only needs to maintain minimum depth — grade variations are acceptable.
In-Hole Tracking
The drill head contains a sondes (transmitter) that the surface locator detects to determine depth, horizontal position, and tool face angle. The tracking operator walks above the bore path on the surface, locating the sonde signal at each drill rod interval (typically 10 ft). At each tracking point, record:
- Station (distance from entry)
- Horizontal offset from design centerline
- Depth (from surface to top of drill head)
- Clock position (tool face angle) for steering decisions
As-Built Documentation
After pullback, the final as-installed bore path is documented from the tracking log. Calculate the top-of-product elevation at each station from the surface elevation and the tracked depth. For crossings with other utilities, verify the calculated clearance meets the design minimum. Submit the as-built bore path to the owner with: GPS coordinates of entry and exit pits, top-of-product elevations at each tracking point, and a plan view and profile view of the installed bore.
Log all bore tracking data in Gradelog’s utility documentation module and generate the as-built PDF for permit close-out and the utility owner’s as-built record.