How to Install Underground HDPE Pipe
HDPE pipe is made from High Density Polyethylene, and is used as an underground conduit for cable, communications, electrical, water, gas, and others. There are 2 common methods for installing HDPE pipe, trench installation and horizontal directional drilling.
Trench installation involves digging a trench in which the HDPE pipe is placed, then back-filled after installation. There are several steps to this process.
When constructing trenches, the size of the pipe, surface conditions, burial depth, surface loads, SDR of the pipe, trench length, and groundwater elevation will all need to be factored in order to meet code requirements.
The below table gives minimum trench widths, assuming stable ground conditions:
|Nominal Pipe Size (inches)||Minimum Trench Width (inches)|
|3-24||Pipe O.D. + 12|
|24-63||Pipe O.D. + 24|
Safety is of utmost importance on any job site. To prevent injuries from collapsed trenches, shoring or bracing can be done. For unsupported excavations, sloping of trench walls to a safe angle can be done. Local codes can provide information about sloping of trench walls for particular areas. Additionally, de-watering may be needed to keep groundwater out of the trench. This can be done with wells, well points, or sump pumps.
Floor Preparation & Padding
Bedding material for trenches should be made from free-flowing material like sand or gravel that is free from rocks over ½” in size. If large rocks are found and removed during construction, the space left needs to be padded with 4-6” of tamped bedding material.
Pipe embedment materials need to provide enough strength and stability to minimize deflection, which is a change to the diameter of the pipe due to pressure. See ASTM D-2321 embedment classifications for details. Embedment materials can be course grained soil like gravel, sand or soil containing silty or clayey sand, or crushed rock. Embedment should be compacted to 85% density or 95% density if under streets.
Embedment particle size for embedment material depends on the pipe size.
2-4” pipe: ½” max
6-8” pipe: 3/4” max
> 8” pipe: 1” max
Pipe must be laid, and NOT dropped, rolled or pushed into the trench. If the pipe is less than 8” OD, and weighs
Backfilling & Compacting
Backfill materials should be free from any large particles, clumps or rocks. The excavated soil can be used or fill from off-site can be used. Backfill should also be compacted to 95% under roadways and 85% for all other applications.
HDD (Horizontal Directional Drilling) or Directional Boring
Horizontal directional drilling and directional boring are trenchless installation methods that allow pipe to be installed underground without having to dig up, or trench, the surface. This is especially useful when installing pipe under roadways, rivers and lakes, rail crossings, existing utility installations, or other obstacles. Other advantages to HDD/directional boring include lower installation cost, achieve deeper installation, and less needed manpower. Numerous types of pipe including HDPE, PVC, steel, and ductile iron can be installed using HDD/directional boring.
- Directional boring normally refers to small diameter bores with a shorter distance.
- HDD normally refers to larger diameter and greater distance bores and require larger equipment.
Process for HDD
Entrance and receiving pits are dug, which allow drilling fluid to reclaimed.
A pilot hole is drilled from one location to the desired endpoint.
The bore (hole drilled as the pilot hole) is enlarged to fit pipeline, otherwise known as “reaming”.
The pipe is pulled into the reamed path.
Entrance and receiving pits filled.
Advantages of HDD and Directional Boring over Trenching
- Faster install times
- Less surface disruption
- Lower installation costs
- Flexibility of location – under a road, river, etc.
- Less soil contamination
- Less permit/licensing issues
Other Tools & Equipment
In addition to the rig and pipe, there are other tools and equipment needed for underground pipe installation. Chapman Electric carries a full line of gear to make sure your project is a success.
These are sometimes called birds, and are used when installing lines into duct. The line is attached to the carrier, then the carrier is inserted into the duct, where blowing is used to move the carrier and line through the duct.
Duct Plugs are used to seal the ends of conduit so water and sediment cannot enter the conduit.
Seal-off kits insert into duct, then twist tight to create a vapor-tight barrier. Seal-offs are installed to prevent explosions where a spark producing devices, like a breaker, could possibly ignite the gasses inside the conduit.
Tracker wire is also called locating wire. When plastic pipe is laid in the ground, tracer wire is installed along the top of the pipe. Once buried, the tracer wire allows pipe location equipment to detect the metal in the tracer wire.
Break-away swivels protect cable during pulling. If the pulling capacity of the cable is exceeded, the swivel “breaks away” so the line isn’t broken.
Pulling grips are used to give installers a way to hold onto cable or lines when pulling through conduit.