This is because the soil is in contact with corrosive agents and exposed to oxygen.įor this reason, the conduit entering the ground shows the most corrosion. Important Note: The worst corrosion spots are usually near the soil surface, particularly where a conduit turns upwards a few inches below ground. Compact with the appropriate compactor (e.g., 1,200-pound compactor).Keep in mind, the backfilling process follows these layered steps: Use one of these mechanical methods to compact the loose backfill material:Ĭompact to the minimum required percentage of maximum dry density, as specified by the Proctor test ASTM D698 Method A. Once you’ve backfilled the soil, compact it to make it denser, reducing air gaps and water infiltration. Then tamping the poured concrete will fill any remaining air gaps. A better option is fine granular materials.Ĭoncrete works as well as it’s fluid, and it’ll flow around conduits filling almost all air gaps. The entire weight of your fill and the above traffic will then fall on your conduits. Because the poured dirt will create air gaps. Important Note: Dumping dirt over conduits while using spacers can cause damage. Clean natural sand and gravel are both good options, and trench excavation or a mix of both can also work, as long as the sieve is no greater than 200. Make sure the coefficient of uniformity is 3 or greater. Go for granular material that’s free of organic matter, with a sand equivalent value of at least 20. This is the layer used to fill the trench from its bottom to the base of your conduit. Let’s dive into the three layers of backfill material to get a better grasp on this. Your native soil is often riddled with small and large rocks that can wreak havoc on your conduit. Selecting the ideal backfill material is key to a successful conduit installation. Spacers enable proper compaction around conduits, preventing them from floating during the concreting process. Flowable fill is a self-compacting cement-like material used as an alternative to compact fill. Use spacers when stacking conduits vertically with flowable fill. Over time, hot spots can wear down your wire’s insulation and cause ground fault issues. You’ll also maintain a consistent impedance across your wires and avoid hot spots. Check out NEC section 310.15(B)(2)(a) for more info. This ensures the current-carrying capacity of your conductors stays intact. These handy little gadgets help prevent sagging and deforming of conduits.įor power conduits, maintaining a 3″ separation between them is best practice. When you’re working with two or more conduits, install molded plastic spacers every six feet. That’s why adding expansion fittings on vertical risers is essential, allowing for settling. Plus, these changes happen more slowly underground.ĭrastic temperature shifts can cause conduit damage from expansion and contraction. You might have a 10☏ change underground compared to a whopping 100☏ above ground. Important Note: Temperature fluctuations are usually milder underground. Add expansion fittings on the vertical riser sections, allowing for settling. This segment is most likely to get damaged. Now, a part of your conduit will eventually rise above ground, exposing it to frost. So, if your frost line is 5-feet below ground, install the 1-inch thick insulation at the 4-foot mark. For instance, using 1-inch rigid insulation is equivalent to about one foot of regular soil depth. This helps stop frost from reaching deeper into the soil. Simply place a layer of rigid insulation above your conduit in the dug-out area. But if you hit a dead-end, don’t worry! I’ve got an awesome trick up my sleeve. If you’re in a seriously cold place, there’s bound to be some advice on the best installation practices. First, consult your local jurisdiction for guidance. However, if your frost depth is 8-feet below ground, installation might not be practical. Doing this prevents water from freezing inside and guards against damage due to thermal expansion and contraction. In super frosty areas, make sure you install your conduit below the frost depth. Important Note: Control groundwater to prevent the bottom of excavations from softening, and ensure your dewatering systems don’t remove natural soils.
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