A Few Important Rules for Installing Underground Conduits

Last Updated on April 18, 2023 by Admin

Construction and installation can be tricky business at any time, but things become exponentially complicated when it involves underground or underwater construction. Keeping cable and wiring conduit installations in mind, here are a few important rules for installing them underground.

The 3-Step Rule for Trenches

After digging a well-measured trench to lay your conduits, follow the three-step rule as detailed next.

1. Remove anything from the trench’s surface that’s hard, jagged, or sharp such as rocks, stones, pebbles, glass, plastic, etc.
2. Compact the surface soil and then smooth it up to create a level, tight surface for the conduits.
3. When needed, dewater the location until at least 24 – 30 inches of distance is created in between the trench bed and the groundwater level.

Use the Right Type of Conduit to Use Underground

Every time someone chooses the wrong conduit for a job, they are making a very expensive mistake. Just think about the cost of digging everything back up, replacing the damaged conduits and wiring/cable inside, and then burying everything again. There’s PVC, RMC, FMC, EMT, steel, aluminum, phenolic, fiberglass, and more to choose from nowadays, so it’s easy to make a mistake. However, if you learn the type of conduit to use underground first, you can easily avoid taking such a costly misstep.

Gaps and Spacers: The Standard Rules

Modifications to the values can be made by experienced engineers to better suit the project’s requirements, but these are the general standards or rules to follow for multi-conduit installation projects:

• Every conduit must be separated by a gap of 3-inches to minimize the chances of distortions and depressions.
• Conduit spacers are to be installed at every six feet for vertical stacks.
• Power conduits must never be subjected to electrical currents more powerful than their rated capacity.

As electrical conduits require more precision, knowledge, and understanding of electrical engineering, it is strongly suggested that you go through federal guidelines for power projects.

Backfilling: The Different Layers

Once you are done with installing the conduits, this space must be backfilled in separate layers. While there is scope for making modifications to suit conditions, the following standards are applicable in most instances.

Bedding & Middle Layer

The bedding layer will fill up the trench and come up to the same exact level as your conduits. The NEC recommends only using backfill material that can go through (100%) a ¾ inch standard sieve. If other sieve sizes are used, the following US standards are to be maintained:

• Number 4 Sieve: At least 35% of the bedding material should be able to pass through.
• Number 30 Sieve: At least 20% of the bedding material should be able to pass through.
• Number 40 Sieve: At least 20% of the bedding material should be able to pass through.

This same backfill material is to be used for laying down a middle or intermediate layer over the conduit bed for 12 vertical inches. Since the recommended coefficient of uniformity should not be less than 3 and preferably higher, sand and gravel are good options.

Final Layer

The final layer will start after the 12-inches and continue till the trench has been completely leveled. Standard codes for the final layer of backfill are as follows:

• There should not be any stone or rock larger than 3-inches in dimension.
• Any material with a malleability index of 8 or higher is to be avoided.
• The total percentage of organic content in the final backfill layer should not be more than 3% – 4%.

Keep in mind that if the conduits are to be placed under sub-zero conditions, excavation depths will vary and adequate adjustments will need to be made to accommodate for frost-related expansion, contraction, and water accumulation factors.

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