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OFC Duct Installation by Open -Cut Method

Posted By: technopediasite

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Precautions to be taken while Trenching
These parameters of trenching and HDPE installation not applicable universally but maximum countries  work on this parameters.It is a general parameters used for OFC Duct Installation and trenching.
➤ It is necessary from safety reasons that the dugout trenches are not kept open for long. Activities of trenching, laying of ducts and backfilling shall run in parallel as far as possible.

➤ Caution boards, plastic warning tapes, barricades, night lamps (flicker lamp, lantern with glass painted red) and watch man shall be deployed by contractor at his own cost to caution against the danger and to prevent any accident to pedestrians, vehicles or animals.

➤ Caution boards shall be set up at a height of 1.25 meters above ground level so that they are visible from a minimum distance of 25 meters. Boards shall have yellow and black background with writing in bold letters with red fluorescent paint. Boards shall be displayed at start and finish point of area in which work is under progress. Additional display boards shall be placed on either side of the trench. Board markings shall have the name of trenching authority, name of Engineer/ Supervisor in charge of work with contact phone number and planned date of opening and closing trench.

➤ The Contractor shall be responsible for any mishap or accident due to negligence or proper protection of open trenches and all claims arising due to such accidents shall be settled by the Contractor without any liabilities to Company.

➤ Whenever any protective covers / messages of other services or operators are encountered during excavation, earth around them shall be gently removed so as not to cause damage to existing facility. If the underground facilities of others agencies are exposed during excavation, adequate protection shall be provided at suitable intervals along the runs of these facilities and concerned agencies / authorities shall be informed.

➤ Near foundations and boundary walls, excavation shall be taken up carried out in consultation with and in presence of owners of such facilities.

➤ Any valuable material of cultural/ historical/ archaeological importance, if found while trenching, shall be immediately brought to the notice of Company and the concerned authority. Further trenching at the location shall be stopped till further instructions are received from the Company.


➤ In city limits of intra-city routes as well as in built up areas along inter-city routes, Contractor shall use manual labour only, and shall ensure that no damage is caused to any underground or surface installations belonging to other public utility services and/or private party. Cross country trenching can be done using trenching machines/excavators.

➤ The Contractor shall clear, prepare and grade the Right of Way to facilitate marking of the alignment of the trench. Contractor shall remove all bushes, undergrowth, stems, rocks and other obstacles etc. ensuring minimum amount of bushes and shrubs are removed to clear way and contractor shall give all consideration to preservation of trees within right of way. Machines can be used for clearing small bushes along the route; however, trees shall not be cut or uprooted for purpose of movement of excavating machines. Where such necessity arises, permission from competent authorities to partially cut such trees shall be obtained in writing. Compensation to cut such trees for movement of machines shall be paid by the Contractor.

➤ Width of trench shall be kept to a minimum depending on trenching technique used and number of ducts to be installed. For manual trenching, width shall be minimum 450mm at top and 300mm at the bottom for ease of installation. For intra-city routes where number ducts will vary along various routes, the width of trench shall be decided on the basis of number of ducts to be installed when the ducts are placed in single layer. Multiple layers of ducts shall be avoided to an extent feasible. Specific approval will be obtained for such areas where multi-layer construction is to be used.

➤ Rock excavation shall be done by chiselling and/or controlled blasting where permitted by concerned authority. In general controlled blasting is not to be carried out for intra-city works. For inter-city works, controlled blasting may be possible subject to the Contractor having license for carrying out rock blasting and after obtaining necessary permissions from the authorities concerned. Blasting shall only be carried out after all precautions are taken to
ensure no damage to public property (both below and above ground) and safety of public and staff involved in the work. The Contractor shall employ sufficient manpower for this with caution boards, flags, sign writing etc. Any consequences due to lack of precaution or otherwise, on this shall be sole responsibility of the Contractor.

➤ All necessary arrangement shall be made to ensure stability of trenching. Where poor / unstable soil conditions exist, the Contractor shall take adequate precautionary measures to prevent caving in of the trenches while excavation. At such locations, width of the trenches shall be kept adequate and necessary arrangement shall be made for safe working within trenches. Where high water table exists, the Contractor shall make necessary arrangements for pumping out sub-soil / underground water from trench.

➤ Underground power cable shall not be moved. Concerned electricity board / authority shall be immediately informed. A minimum of 600 mm horizontal and vertical separation shall be maintained from power cable. As far as possible power cable shall be crossed at right angle.

➤ Temporary foot-bridges shall be provided when trenches cross entrances of building, premises, etc.

➤ Special care shall be taken in digging footpaths with proper protection to avoid accidents and inconvenience to pedestrians.

➤ Roots of big trees should be bypassed to avoid damage while trenching and ensuring safe passage of OFC. Such negotiation shall be with a smooth curve.

➤ Trench shall be located at lowest point of lower area if feasible. Trench must not come over boundary or any heap of soil / garbage dump.

Trench Profile

➤ Bottom of trench shall be uniform and shall follow ground contour/profile.

➤ In areas with steep slope, trench profile shall be such that bottom of trench shall not have more than 23 degree gradient with horizontal (i.e. difference between two adjacent depth readings at a distance of one meter shall not be more than 250 mm).

➤ In areas of high slope, to prevent soil erosion due to free rain water flowing along or inside the trench, slope breaking barriers in the form of vertical walls of flag stones or stone masonry or bags filled with sand and cement mixture shall be installed inside the trench at spacing of 10m centre to centre after laying ducts such as using.

➤ Profile alignment of trench shall be such that HDPE ducts are laid in straight line, both laterally/ horizontally as well as vertically except at locations where it has to necessarily take a bend because of change in alignment or gradient of trench. Minimum radius of two meters shall be maintained, where necessary.

HDPE Duct Installation

➤ Prior to duct installation, the Contractor shall prepare a manhole installation plan based on approved for construction drawings provided by the Company. The Contractor shall also identify locations where special protective measures. Required and make arrangement for all materials required for such protective measures so that after installation of ducts the trenches can be immediately backfilled.

➤ The location of man-holes and hand-holes shall be optimized so as to minimize the wastage of ducts and to avoid unnecessary use of duct couplers. Equally important is to select duct coils of suitable lengths such that minimum cut pieces are left after installation of ducts.

➤ Ducts colour and sequence shall be as indicated in approved for construction drawings. Ducts of different colour shall be laid parallel inside the trench. Duct runs shall be as straight as practicable without kinks and bends. Care shall be taken to prevent twisting of ducts while installing the same inside trench.

➤ Prior to installation of the duct, the trench shall be checked to ensure the minimum depth requirements are met including protective measures, if any. Trench bottom shall be free from any hard materials, rocks or any objects, which may damage or puncture the duct. Soft padding material free from sharp edged stones/hard materials shall be uniformly placed in trench before laying the duct to completely cover the hard surface with a minimum cushion of 50mm thickness over the hard surface. Company shall inspect the trench prior to laying of ducts. Records of all padding and protective measures, if any, shall be maintained for Company review and later incorporation into asbuilt drawing .As-built data related to trenching shall be incorporated into Company provided Approved for construction drawings and shall be made available to the Company at the time of inspection of trench.

➤ The roll of HDPE ducts shall be mounted on a Vertical de-coiler assembly. The duct shall be smoothly unrolled along the trench prior to lowering into the trench. An alternate method should be the use of horizontal duct de-coiler, from which ducts shall be manually unwound and laid along or inside the tranche. Walking on the duct(s) when laid in the trench bottom shall be strictly prohibited. Pictures placed below shows examples of such de-coiling equipment. In case of intra-city works, due to number of ducts to be installed and space limitations, it may not be possible to use de-coilers at all locations. In such areas, the ducts shall be cut into smaller lengths and spread-out in a manner so as not to cause inconvenience to public and/or traffic. Care shall be taken not to twist/kink/buckle the ducts during spreading out.
Image result for image of decoiler used for HDPE installation
➤ When inevitable, sections of ducts shall joined using HDPE duct couplers conforming to the specifications referred under Section 2 of this specification for a locked and air and water sealed assembly suitable for direct burial. The couplers shall be installed in accordance with the Manufacturer’s recommended procedure. Final assembly shall have adequate tensile strength required to prevent opening during pneumatic blowing of OFC as
well as requirements of duct air pressure testing. In no case more than two couplers shall be permitted in one section between a pair of adjacent manholes/ hand-holes when ducts are installed by open trenching. This limitation does not apply to ducts installed by HDD.. Unless specified otherwise in the Contract, duct couplers and equipment, tools and tackles required for installation of couplers shall be procured and provided by the Contractor. The location of each coupler shall be recorded (with coordinates LAT/LONG) for incorporation in as-built drawings and GIS database.
Image result for image of decoiler used for HDPE installation

➤ The ducts shall be joined with couplers using duct cutter and proper tools only. Use of hacksaw to cut the duct is not permitted. The duct joint shall be practically airtight to ensure smooth cable blowing operation using cable blowing machines.

➤ All coupler locations shall be covered with red stone (25 mm thick X 300 wide X 1000 mm length) prior to backfilling. Coupler shall not be installed along the bent portion of duct/trench in both horizontal as well as vertical direction.

➤ As far as practical, the duct shall be laid in piece length of 1 km or in multiples of 500m in case of inter-city routes and open trenching installation. Where HDD technique is used, the duct length shall be to suit HDD shot length. In case of intra-city routes, due to space limitations, the length of ducts may be cut to suite spacing between man-hole to man-hole.

➤ Ducts shall be lowered into finally prepared trench bottom in a straight line and without twisting, kinks etc. and in sequence as shown in approved for construction drawings.

➤ Ducts shall be laid in a flat bottom trench free from stones, sharp edged debris. No water should be present in trench, while laying the duct/DWC pipe.

➤ The duct shall as straight as possible when placed inside the trench and shall be laid parallel to one another (without criss-cross).

➤ At horizontal and vertical bends, minimum bending radius 1300mm shall be maintained.

➤ Each duct shall be terminated inside a man-hole/hand-hole and its open ends plugged with air and watertight end caps (end plugs) to prevent the ingress of water, mud, debris or other material during the installation. Any collapsed portion of duct shall be removed before backfilling and duct made continuous by putting couplers.

➤ Company shall inspect the laid ducts prior to backfilling operations.

Stone / RCC Slab Protection

➤  Locations where there is high probability of construction activity in near future, stone slabs (Flag stone) of size 300mm (Wide) x 500mm (Long) x 25 mm (Thick) shall be placed inside the trench after 100mm thick layer of soft padding has been put over ducts. In case stone slabs are not available, RCC slab of size 300mm (Wide) x 500mm (Long) x 40 mm (Thick) may be used. Stretches for this protection shall be identified as part of route survey. This protection is meant for areas where, otherwise, trench depth is adequate. The location and extent of such additional protective measures shall be as per instructions of Engineer-in-Charge.

Backfilling and Restoration

➤  Backfilling shall be done with well compacted excavated material after ensuring soft material padding. Adequate dry compaction shall be done before Crowning. Compaction shall be done in layers of 50 cm each.

➤  A crown of 250mm shall be made at the top of the backfilled trench to cater for soil settlement. Crowning shall be confined over width of trench only. No surplus soil shall be left outside trench. Entire area shall be restored and debris removed and disposed-off in a safe manner and in line with requirements of the authority having jurisdiction over the area.

➤  Back-fill shall be maintained by Contractor against wash-out, settlement below original level, until final completion of work and till NOC from the land owners has been obtained and handed over to Company.

The trench shall be filled up to the required height (Measured from top of ducts as per Trench cross-section drawing issued for construction) and a 0.2mm thick high density polyethylene warning tape of design approved by the Company shall be placed above the ducts prior to further backfilling. Company reserves the right to verify presence of warning tape even after the trench is backfilled at random locations in case of a doubt. Installation of warning tape shall be as per approved for construction drawings issued by Company.

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