Storm Pipe Installation Starting from a Manhole Structure

[MikePhua]

Establishing Alignment and Grade from the Structure
Laying the first section of storm pipe from a newly set manhole structure is a critical step in underground utility installation. Precision at this stage determines the success of the entire run. The process begins by identifying the center of the manhole structure—typically marked on the top slab—and pulling a string line to the next grade stake. This line serves as the visual reference for alignment.
A pipe laser is then placed inside the structure, aligned with the string line using a plumb bob. The laser beam must be centered precisely to match the invert elevation of the pipe outlet. Bricks or shims are used to adjust the laser’s height to match the design invert. Once the beam is set, the laser target is positioned at the bell end of the first pipe, and the pipe is adjusted until the beam hits center.
Terminology annotation:
- Invert elevation: The lowest interior point of a pipe, critical for gravity flow systems.
- Pipe laser: A precision instrument that projects a beam along the intended pipe path to guide installation.
- Haunching: The process of compacting soil around the lower half of the pipe to provide support and prevent movement.
- Bell end: The flared end of a pipe designed to receive the spigot of the next pipe.
Setting the First Pipe and Maintaining Line
Once the first pipe is aligned and haunched, the trench is graded for the next section. Each subsequent pipe is set, homed (fully inserted into the bell), and adjusted to match the laser target. The installer checks alignment using a transit or rotating laser, such as a David White level, to confirm grade and direction.
For the first few pipes, lateral adjustments to the laser target may be necessary to ensure the run stays true to the string line. This is especially important when transitioning from the structure to open trench, where visual cues can be misleading.
Operators often use a structure clamp to mount the level instrument directly to the manhole, improving stability and accuracy. In cases where offset stakes are available, dropping a plumb bob from these points can help verify alignment.
Avoiding Common Pitfalls and Field Corrections
One of the most frequent errors in storm pipe installation is misjudging the slope. The lay of the land can deceive even experienced crews, especially when the next structure is several hundred feet away. A rotating laser must be oriented correctly to avoid laying pipe downhill when the design calls for uphill flow.
Recommendations:

• Double-check laser orientation before setting the first pipe
  
• Use a bubble level to verify slope when laser readings seem off
  
• Confirm invert elevations with benchmark shots
  
• Avoid relying solely on visual alignment over long distances
  

Digging up and relaying pipe due to grade errors is costly and time-consuming. A single misaligned section can compromise flow and require rework.
Crew Size and Workflow Efficiency
Ideally, a crew of five provides optimal efficiency: one operating the excavator, one handling pipe, one managing grade and laser, one compacting haunch material, and one supervising alignment. However, many jobs operate with two or three workers, requiring multitasking and careful coordination.
In smaller crews, using tools like pipe clamps, laser targets with built-in grade rods, and quick-set bedding material can speed up installation without sacrificing accuracy.
Equipment and Tool Selection
Essential tools for storm pipe installation include:

• Pipe laser with adjustable tripod
  
• Transit or rotating laser level
  
• Plumb bob and string line
  
• Grade stakes and paint
  
• Shovels and compactors for haunching
  
• Structure clamp for mounting instruments
  
• Laser target with fine adjustment capability
  

Some veteran installers still use manual grade rods with tape marks for quick reference, especially on small jobs. While modern lasers offer precision, traditional methods remain valuable for cross-checking.
Conclusion
Installing the first pipe from a manhole structure is a foundational task in stormwater management systems. Success depends on accurate alignment, consistent grade, and careful haunching. By combining modern tools with time-tested techniques, crews can ensure reliable flow, structural integrity, and long-term performance. Whether working on a municipal storm drain or a private development, the first pipe sets the tone—and the grade—for everything that follows.