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Technical Specifications for Underground Duct

Underground duct : In this article, I will explain about underground duct. Why underground duct used in telecom sector or in any sector. We all know that it is an expensive project, yet for optical fiber, underground duct is more preferred. Because cable installation in the ground is not subject to heavy snowfall, storm winds and is not damaged by fallen trees. Wind is not prone to dangerous and disturbing electromagnetic effects.

In this article,specially I want to discuss the technical specification of underground duct used in telecom sector for the optical fiber cable. Underground duct is also used in other areas but I will not discuss technical specification of other underground duct. Different types of ducts are used in other areas, but in the telecom sector, generally used the PVC duct.Which I will discuss specifically on this.

Technical Specifications for Underground Duct in telecom sector
Underground Duct

Installation Details for PVC Duct

The line and level of the duct formation shall be kept as straight as possible. Bends will be required for duct formations to be routed around corners at intersecting roads. Keep in mind for safety reasons manholes shall usually be planned and located away from intersections.

Duct laying shall not be done in the absence of the telecom company Supervisor unless prior permission has been received.Check the duct carefully, the duct used for laying must be certified by the company.

Keep in mind the duct laying distance from gas pipelines, electrical routes, trees, and other utilities as instructed by the company.

As far as is practicable, ducts shall be watertight between manholes. A PVC solvent shall be used. Installation methods shall prevent sand and soil from entering the ducts.

At manhole, handhole and vault walls the bond between the outside surface of the duct and the wall shall be watertight, as far as is practicable, to prevent seepage of water into the manhole.

Ducts shall be terminated flush to handhole, manhole and cable vault inside walls. Edges shall be beveled off.

Ducts entering manholes, handholes and cable vaults shall be plugged and watertight. The plugging mechanism or material shall be readily removable to allow for future cable installation.

Ducts shall enter manholes, handholes and cable vaults at 90° ± 10° to the structure's walls. Splaying of the conduits shall be done a minimum 6m from the manhole end wall and the maximum bending angle shall be 6 degrees.

Wherever practicable, ducts shall leave a manhole in a standard formation and enter the subsequent manhole with each duct in the same relative location.

At location between manholes where the duct formation must be modified due to obstructions the formation shall be altered to minimize the movement of each duct.

The duct formation may be curved to approximately 90°. The cold bending radius for PVC duct shall not be less than 12 metres. Preformed bends and/or 5° couplings shall be used where smaller radius is required.

Upon completion of cable installations in new duct systems, at least 50% of the remaining unoccupied bores shall be provided with a draw cord. This cord shall be of sufficient strength to enable the pulling in of the pulling rope.

Only one form of construction shall be utilized between 2 manholes. Where the required depth of cover cannot be achieved, and with the prior approval of the Engineer or Supervisor, concrete slabs may be provided above or integral with the duct structure. The slabs may be reinforced (minimum 6-Kg steel/sq.m). It's depend upon the company specification.

PVC underground duct in concrete 

The method of construction shall ensure that the standard spacing is maintained and that all space between the ducts and a minimum of 75-mm surrounding the ducts is filled with concrete. The distance between the ducts shall be 30-mm. Duct spacers shall be used every 2.0-m along the duct line to position the ducts.

Concrete immersion vibrators ("pokers") shall be of 30-mm maximum OD, and shall only be used in the 75mm plus space between the duct work and the sides of the excavation.In soft ground, reinforced concrete may be required. The Supervisor is required to approve such installations.

In locations where concrete encased PVC ducts are installed with less than a 400-mm depth of cover, as may occur with a road crossing, the concrete formation shall be increased by an additional 100-mm thickness of R/C slab.

PVC underground duct in sand

The dimensions of the PVC-duct, duct spacers etc., must be use as per company specification. Duct spacers shall be used to position ducts in formation and to support the ducts while maintaining adequate duct separation in preparation for subsequent sand embedding.

The sand shall be compressed between each layer and there shall be a minimum covering of 10 cm of sand. Minimum covering above upper duct layer shall be 100-cm.

The separation either longitudinal or perpendicular, to other services, should be 150-mm. Where such is not possible a separating/retaining layer of 50-mm of concrete is required.

The Supervisor shall have the discretionary authority to change the construction method to PVC in concrete or steel ducts, should the circumstances indicate such a requirement.

If ducts up to 6 ways are to be laid, they shall be supported by spacers so that the formation will maintain the standard spacing between ducts throughout the length of the installation. Sand, to the appropriate highway specification, shall be placed to fill all spaces between ducts, and compacted.

The duct formation shall be covered with 100-mm of sand. This is to be followed by approximately 200-mm of suitable backfill and a plastic warning tape. Warning tape shall be in accordance with company instruction. 

Sub-duct technical specification

Sub-duct shall be installed, where possible, as a continuous length between manholes. For economy of material usage one joint may be permitted in a length of sub-duct.

The joint shall be made so as to produce the minimum interference with the installation procedures of both the sub-duct, subsequent cabling, and possible future additions of sub-ducts if the first installation does not consist of the full complement.

The installer shall show evidence that the joint is able to withstand the typical forces applied to the sub-duct during installation and subsequent cable pulling.

The chosen duct bore shall be mandreled and cleaned prior to the sub-duct installation.Sub-duct shall be secured to the main bore by suitable means, so as to prevent movement during subsequent cable installations, and the lifetime of the installation.

After installations, the sub-duct shall be mandrel tested and sealed with end caps. The duct cord shall be tested for necessary strength for pulling of the cable hauling wire.

The individual sub-ducts shall exit the main duct parallel to its axis and be trimmed square to the manhole wall, approximately 50-mm from the duct face.

Mini ducts (50-mm pe duct) shall be used to protect the cables in hazardous areas like wadi crossings or road crossings. Mini ducts may also be used for extra protection at crossings with high voltage cables, or to give additional protection to cables when required by difficult soil condition.

The fabric sub-duct, which consist of multiple inter-cooected fabric cells, shall be used primarily in the underground duct network as a linear for occupied 110mm OD.main duct, when required by difficult duct access / space conditions (e.g. where placement of additional polyethylene sub-ducts in an occupied duct are not longer feasible).

Upon completion of cable installations in new duct system, at least 50% of the remaining unoccupied bores shall be provided with a draw cord. This cord shall be of sufficient strength to enable the pulling in of the pulling rope.

Underground duct in road crossings 

The crossing of roads and highways with Direct Buried Cables, coupled with the requirements of maintenance, possible future cable additions and the inherent problems of re-excavation across road junctions, implies the use of short sections of conduit at such points.

Thrust boring or Directional drilling shall be used, where excavation is prohibited by local authorities. Directional drilling may be preferred where in some circumstances local authorities or road authorities place restrictions with regard to the minimum bore size/distance.

To be noted for the installation of road crossings, is the requirement for the free passage of longitudinally installed utilities below the crossing. Hence the depth of installation may have a lower limit as well as a structural requirement for a minimum depth below the highway surface.

All road crossings shall have the duct ends plugged. The ends of such ‘'unterminated’ ducts, shall be identified by marker plates as per company construction guidelines.

Last Word

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