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Duct Planning and Design Guidelines

Duct Planning and Design Guidelines : Everyday questions are asked, what is the process of duct laying for optical fiber network, what will be duct planning and design procedure, what are the main things to be taken care of in duct laying. This article will explain about duct planning and design related to optical network, not about electrical or other sector ducts.

We all know that optical fiber cables are installed in two ways, first process is inside the duct and other process is buried. Now we will discuss only a few important points about the duct.Underground is laid to install optical fiber cable within the duct. This means that in this article we will only talk about the characteristics of the underground duct, we will discuss how it is planned and designed.
Duct Planning and Design Guidelines
Duct planning and Design for FOC

Duct Planning and Design

Underground duct is one of the major cost items of the network. It is important that new duct or duct additions be well planned. Insufficient duct will result in early additional cost; too large a duct structure will represent uneconomical investment of money and material.

This very important article related to Installation of HDPE Ducts at Crossings :

Techniques for Installation of HDPE Ducts at Crossings

If, underground duct structure will be very costly, like in mountainous places, another alternatives can be studied to be used as aerial network.

For identification of ducts number and dimensions, the primary network cables maximum requirements should be taken into consideration beside the usage of the new technologies to reduce the ducts size.

If copper cables route then the maximum length of 
duct between tow manholes in the same route is 295m. If the fiber cables route then the maximum length of duct is 500m. It is depend upon the road crosses that the duct or its branch outs pass through, or curves impacting cable tension.

The main and important factor affecting the identification of the ducts number is the ducts annual usage rate derived from the cables forecasted requirements.

The network designer should refer to the current O.P. network plan when preparing for the projects in general and particularly the ducts network.

An updated Fundamental Duct Plan should be maintained; this plan should show the main duct routes, approximate manhole locations, alternate routes, proposed Trunk and Junction cable routes.

A duct and manhole design should not be attempted until all the facts that might influence the final design known. Consideration must be given to provision of ducts required on a one time only basis.

Exchange building tend to be located in the central part of built up areas where subsurface congestion is most likely to occur and future duct placement may be very difficult or costly. Consideration should be given to placing the estimated
duct requirement from the building to a point where future problems are not anticipated.

For example a new exchange with an ultimate capacity of 10,000 lines is planned in a new area. The ultimate number of pairs required can be calculated as follows:

Ultimate Exchange Capacity x 1,5 = Ultimate primary pairs.
10,000 x 1.5 = 15,000
Projected number of cables assuming 1800 pairs of cable will be placed:
15000/1800 = 9, which requires 9 conduits.
.junction and Trunk Cables will use optical fiber cables in sub-ducts and can be estimated to require 2 conduits.
Add 50% for unforeseen requirement = 5.
Total conduits required = 9+2+5 = 16.
Adjust to nearest standard size configuration, in this case = 16.


Certain locations such as bridges, railways and major road way crossings will require consideration of over sizing to meet expected ultimate needs. These cases are created similarly to over sizing near exchange buildings. Over sizing consideration should be at least 50% of the required minimum number of ducts.


Duct routes should conform to the fundamental plan for the Exchange. They should be triangular in design. That is to say there should be one main collector system with branches normally running at 90 degree from the collector. Parallel routes should be avoided.



The economies of alternate locations must be compared relative to permanence, safety and utilization. The routes should be selected to avoid any future relocation. Locations should be chosen that are free of danger and also provide a safe working environment. There will be times when the safety aspect alone will have the greatest bearing upon the selection of one route over another.



Advantages of the triangular design from the Exchange to the subscriber are:
➤ Short cable lengths
➤ Smallest cable conductor gauge
➤ Least money per cable pair invested.



Locate the trench line so that it agrees with the Ministry of Municipal and Rural Affairs. Agreement. If that location is already occupied by another utility locate the trench line so that the duct structure will be free as possible from disturbance of other under-ground users. In so doing the following should be considered.

Duct Planning and Design in Cities

Under the sidewalk, especially if the construction precedes the laying of the sidewalk, is usually an excellent 1ocation.

Provides a generally cluttered space between the pavement and the building line.

The best location is not between the footpath and the curb due to tree planting or possible road widening.

In the street near the curb; if this location is used every effort should be made to locate the manholes under the sidewalk.

In parkways, duct may be placed along the edge of the park if permission can be obtained.

Duct Planning and Design in Rural Areas and Highways

The above locations are applicable but the designer should assure himself that the duct is not likely to be involved in street widening.

Avoid placing duct on main highways unless the highway has reached its maximum growth, then there may be a location on the side that would be acceptable.

Obtain all available information relating to the location of existing or proposed structures of other utilities. Special precautions must be taken when work is to be performed in the vicinity of other utilities. In some areas, ministry of municipal and rural affairs may be able to provide information regarding the location of other underground structures.

The minimum recommended separation between foreign structures and the telephone duct systems is covered in below table .In all cases, the Ministry of Municipal and Rural Affairs requirements must be satisfied. Below table explain the separation between underground telephone cable & other utilities.

Duct separation between other utilities
Duct Separation 


Where locations are not reassigned, all avail able street and road plans should be reviewed, including plans concerning future road construction or widening projects.

The soil type and nature should be accurately analyzed to identify whether it is normal or not e.g. to avoid location with ground water or rocks.

In the telecom OSP sector the standard duct structures based on the duct being placed in stable soil with 1000 mm (minimum) depth of cover. These conditions are of particular concern because of their effect on forces which produce stress in the structure.

The main consideration in concrete encased duct structures is the tensile stress resulting from two forms of vertical load. These loads are generally classified as live load and dead load. Live 1oads have greater effect on the structure than dead loads.



Live load is the pressure resulting from a combination of the mass and velocity of an object, such as a motor vehicle, moving over the structure. Live loads are assumed to be transmitted in a conical shaped section, with the point of the cone at the top. Therefore, the greater the depth of the structure, the greater the area over which the load is spread. In the Live loads process it can be neglected at points that are 2 m or more below surface grade.



Dead load results from the combined mass of the structure and the back-fill over the structure. Forces resulting from the dead load will increase with the depth of cover, but this is not as critical as live load considerations.


It is the prime responsibility of designers to be aware of any unusual depth requirements that are installed for underground structures. This particularly applies to crossing main service.

In an area where final grade has not been established, the designer should make every effort to obtain the grade from the proper authorities. to achieve standard cover should be shown on the work plans.

When the available maps, plans and input from outside sources do not yield sufficient data to develop a proposed plan, it may be necessary to undertake a detailed land survey of portions of the route.

Read this article to know further : How to Calculate Pulling Tensions in Duct Section?

Conduit augmentation


On existing routes requiring additional conduits different methods should be used to find the best economical solution. First, estimate the percent (%) growth rate and determine the requirement. For example, 5% growth would require (1.05) power 25 = 3.39 or 4 ducts over 25 years. The growth should be calculated for each section of the route and the resulting number of conduits should be coasted.

Conduit augmentation should only be carried out if alternative 1 and 2 are not suitable. If manhole spacing permit, manhole reconstruction should be minimized in the augmentation of exhausted route.

For example if the existing spacing between 3 manholes is less than 290 m it

might be feasible to bypass one manhole to minimize manhole reconstruction and maximize cable run- See below figure.
Duct bypass through MH
MH bypass 




Once the cost of augmentation of the route has been done, this cost should be compared with the following alternatives such as:



1. Out Posted Remote Switch (RSS) : This application would be very cost efficient as its costs can be compared against both conduit and cable work. In tine RSS application it would be sufficient to find enough space for sub duct for one optical fibber cable.



The impact of new technology will be that conduit routes should be dimensioned to provide sufficient conduits for copper cables for 10 years use plus one conduit for optical fibber cables.


This rule will have the biggest impact on routes more than 3 km from the exchange as the longer the copper cable the more economical will be the use of Pair Gain technology. The new technology will increase the utilization of existing cables and the use Optical Fibber cables will require less conduit space.

2. Compression : This alternative can be used where several small cables can be combined into a bigger cable. Furthermore, it requires one spare conduit to be used. The cost of this alternative should include the cost of transferring of all pairs in the MDF as well as in the cut-over joint in the manhole.

Duct structure locations and considerations

Most of the design factors are the same whether a new or an existing location is chosen. Some additional consideration is necessary when an existing route is selected.

The major decision when using an existing duct route is whether to terminate the new duct in the existing manholes or to build new manholes. This decision can be based on three considerations:

➤Will the existing manholes accommodate the additional racking and equipment space?
 Are the existing manholes in a safe work area?
 Could existing manholes reasonably be enlarged or rebuilt to accommodate the new duct?

If the above conditions are satisfied, a further decision must be made whether the proposed duct will be placed in the same location with the existing duct or in a new location.

If the conditions of paragraph precedent cannot be met, a new manhole, either a rebuild of an existing manhole or a new manhole in a new location, will be required.

If the decision I s to place the duct in the same trench location as the existing, the designer must decide whether to place the duct on top of or alongside the existing structure. If the duct is to go above the existing, the designer must determine if there will be sufficient depth of cover to provide adequate protection from vehicular loads and still be within allowable limits set by the Municipality for depth of cover.

If the decision is to go alongside the existing structure, the designer must check for municipal and other utility conflicts; also there is a danger of the new excavation work undermining the existing duct. In both cases, the Network Engineer must make sure that the new and existing structures do not conflict with the allowable space permitted by the Government Agreement if this Agreement is enforced in that area.

If the decision is to place the proposed duct some distance away from the existing, then the same general conditions will apply as though it were a new duct route. The designer should realize that additional duct bends will be required to bring the duct into the existing manholes. These bends will increase the pulling tensions and should be taken into consideration when designing the new structure.

When designing duct structures, the designer must work towards orderly cable racking in the manhole. A two wide formation uses single racking; four wide requires double racking and six wide requires triple or guardruple racking for joints on each wall.

Underground obstructions may necessitate the use of a variety of duct formations; however, the ducts must terminate in the manholes or the Local Exchange in an orderly manner.

Duct structures entering manholes generally should be centered in the end wall of the manhole. If the ultimate duct requirements are not being placed on the initial job, an appropriate number of extra ducts should be placed in the manhole wall to avoid future breaking of the wall. Depending on the formation and anticipated difficulty of future access, consideration should be given to extending the extra ducts a short distance from the manhole.

Subsidiary ducts are additional ducts that will be required to house cables that extend from the main duct line to buildings, poles, etc., along the duct line. The accessory ducts may be separate ducts or join the main duct structure. When the accessory ducts are separated but placed at the same time as the main duct structure, they must be located on top.

If the endpoint is known, or can be located approximately, the supporting duct should be extended to that location. When the accessory duct is part of several types of duct structure, the upper corner ducts should be reserved for auxiliary use, as they are more easily accessible. The subsidiary duct should extend from the main duct structure to the termination point, at right angles and in a straight line. 

This results in less excavating costs and keeps undergrounds telephone plant in specific locations, reducing the possibility of accidental cut off. Duct runs should be planned on private property in sufficient space near the property line to avoid digging for buildings and far enough away to avoid fence and wall construction.

When subsidiary duct is planned from the last manhole, and it is expected that the main duct run could be extended in the same direction at a future date, It may be economical to extend the main run when placing the accessory nozzles.

In planning a subsidiary duct it may be economical to place an additional spare duct. This would depend upon the anticipated growth in the area and the proposed size of the original cable placed. In the case of small sized cables feeding buildings or aerial cable leads, the duct may be large enough to place an additional cable, therefore, an additional duct would not be required.

The designer should use the same guidelines for the length of subsidiary duct runs as were used in determining the length of main duct runs. Calculations should be based on the size of the cable that could be placed in the subsidiary duct.

Knowing that the route is acceptable, the designer is then in a position to estimate specific construction costs. If these costs differ appreciably from the original study estimates, the planning and studies personnel should be advised accord- ingly. This might necessitate changes in project requirements or in the OPP and require a re-examination of the duct structure size. In general the economic structure size is dependent upon three factors: growth rates, estimates of construction costs, and limited availability of subsurface space.

All under minted ducts, such as those left below grade for future extension, must be capped. The minimum amount of space required for the duct structure will be determined by the type, size, and number of ducts planned. 

Where no form work or sheeting is required, the width of the trench shall be the width of the formation of the duct as well as the distance required on each side for the working space, back-filling or concrete encasement. The depth of the trench will be the height of the structure and the depth of cover and any top protection or bedding required.

Normal the minimum depth of cover under roadways subject to traffic is 1000 mm. As per location additional cover may be required in some locations for extra security or to meet special or local regulations. It is very important to that where sheeting or shoring is required, width must also include the dimensions of the material used.

The designer must be familiar with the various safety rules and regulations that govern the safety of workers in the area of his responsibility. Some government agencies have strict regulations related to working in trenches or confined spaces.


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