Optical cables are laid directly on a prepared bedding layer, typically sand or fine soil, at the bottom of the trench.

Trench Bottom Preparation

Before laying the optical cable, the trench bottom must be flat and free of stones or debris. In stony or semi-stony soil, a 10 cm or more layer of fine soil or sand is placed to cushion the cable and prevent damage from sharp objects or uneven surfaces . This bedding layer ensures the cable rests evenly and maintains its minimum bend radius during installation.

Cable Placement

The optical cable is then laid flat on this bedding layer without arching or twisting. Care is taken to avoid bending the cable below its specified radius, dragging it on the trench bottom, or applying excessive pulling tension . For direct burial installations, the cable may include protective armor and water-blocking layers, but it still rests on the sand or fine soil bedding .

Protective Layers and Backfilling

After the cable is positioned, an additional 100 mm layer of sand is often placed over the cable to distribute point loads and protect the armor from penetration . In some designs, crushed soil or protective boards may be added above this layer before backfilling with the remaining soil. The backfill should be free of stones, bricks, or frozen soil to avoid damaging the cable .

Summary

In summary, the optical cable is always laid on the bottom bedding layer of the trench, which is typically sand or fine soil. Protective layers are added above the cable before backfilling to ensure long-term durability, mechanical protection, and compliance with installation standards . This method is standard for both direct burial and conduit-based underground fiber installations.