Techniques for Installation of HDPE Ducts at Crossings - Technopediasite


Monday, July 2, 2018

Techniques for Installation of HDPE Ducts at Crossings

Posted By: technopediasite

Image result for image of HDD crossing of river
Techniques for Installation of HDPE Ducts at Crossings

 HDPE ducts Crossing existing and / or proposed utilities (pipelines, cables, drains etc.), manmade infrastructural facilities (Highways, Railroads, roads, tracks, footpaths, canals, drains, bridges, culverts etc.) and natural obstructions (rivers, streams, nalas, estuaries, caves, gorges, etc.) may be crossed by open-cut method, trenchless techniques or other suitable methods subject to approval by the authorities having jurisdiction over such utilities/infrastructure/areas. It shall be the responsibility of the Contractor to delineate of all such obstructions before starting installation works and plan for most appropriate method of installation of HDPE Ducts.

➤ Whenever underground utilities such as pipelines (water, drainage, gas or oil etc.) or cables (communications or power) are encountered during survey and/or excavation of trial pits or during actual trenching, they shall be cleared with utmost care. Precautions shall be taken to maintain their stability and safety during the works by providing adequate supports.

 When these utilities obstruct the ducts alignment in any way, the owners of the utility shall be informed and requested to be present during execution of work to ensure trouble free execution. It shall be expressly understood that in the event of damage to any existing utility, the same shall be repaired and restored, to operating conditions as it were prior to damage, by the Contractor at no additional cost to the Company.

 Special protective measures shall be taken during installation of crossing to prevent damage to existing facility/utility or HDPE Ducts in future.

➤ Following sections provide minimum special requirements for protection of ducts as well as some suggested techniques for installation of ducts crossing various obstructions.

Crossings of Existing Utilities

➤ Utilities may be crossed by open-cut or trenchless technique as appropriate / approved by the authorities. However, Contractor to note that it is extremely dangerous to attempt crossing of existing utility by trenchless technique without full and complete knowledge of existing utilities.

➤ For crossing by trenchless technique (HDD, Moiling/thrust boring etc.), it is important to know exact position (both horizontally as well as vertically) of exiting utility by either making trial pits or by other means (e.g. electronic pipe/cable locators) or by obtaining correct as-built data from owner of utility. The GI/DWC pipe shall be encased in 100mm thick concrete if it is at a location susceptible to damage by third party .Under hung/side supported crossing need not be encased in concrete.

➤ The crossing profile within utility corridor shall be designed considering laying of HDPE duct in a straight line, parallel to ground, for at least 2m from the edge of extreme utility on both sides of crossing.

➤ For installation by HDD, minimum 0.5m vertical clearance from the nearest utility is required.

➤ For open-cut installation, HDPE ducts shall be laid with a minimum vertical clear separation (outer surface to outer surface) of at least 500mm between the existing utility and HDPE ducts unless agreed/approved by concerned authorities for lesser separation.

➤ In case the existing utility is already laid deep enough so that duct installed at specified depth will result in a clear separation of 500mm or more between top of existing utility and bottom of ducts, then the HDPE Duct may cross above existing utility.

➤ Back filling on public, private roads, and footpaths in city areas shall be performed immediately after laying HDPE ducts. Back filling at such location shall be carried out by dry compaction in layers of 500mm and thoroughly rammed; so as to ensure that original condition is achieved and made safe to traffic. All excess soil/material left out on road/footpath shall be removed.

Highways, Roads and Railroad Crossings

➤ Highways/Roads/railroads may be crossed by open-cut or trenchless technique (HDD/Moiling) as appropriate / approved by the authorities. Moiling method is generally not suitable for crossing width of more than 20 meters. For such crossings only HDD should be used when trenchless technique only is approved by authorities.

➤ GI pipe shall be provided as additional protection to HDPE ducts at these crossing locations as a measure to prevent third party damage. GI pipe shall extend at least 2m beyond highway/road/railroad RoW on each side. The size of GI pipe shall be decided based on the number of ducts to be installed as per Approved for construction drawings. In no case number ducts shall be less than three. At minor roads where open cut method may be accepted by authorities, instead of GI pipe, DWC pipe of suitable size may be used.

➤ Unless agreed otherwise by authorities, GI/DWC pipe shall be installed in a straight line within road RoW. The GI/DWC pipe shall be installed at a minimum depth 1200mm from lowest points within RoW (e.g. drains running parallel to highway/road/railroad) or 3000mm from top of road surface/tracks whichever is higher. In case approving authorizes required higher depth than as specified.

➤ When more than one GI pipe is used, the sections of pipe shall be welded together to form required length. Use of GI couplers is not recommended as the same can impact/damage duct during pulling. Bends in GI pipes shall be avoided. In case it is necessary to use bends, suitable pipe bender shall be used. Bend radius shall be between 20 – 30 times the outside diameter of GI pipe so as to allow easy pulling of ducts through GI pipe. Use of bends of shorter radius shall need specific approval of the Company.

➤ HDPE ducts shall preferably be inserted into GI/DWC pipe string prior to its installation. Else a wire rope of suitable size shall be inserted into GI/DWC pipe for subsequent pulling of HDPE ducts. Rubber bushes shall be used at the both ends of the GI pipes to prevent any damages of HDPE ducts during insertion/pulling.

➤ After exiting GI pipe/DWC pipe, the HDPE ducts shall be installed in a smooth transition up to the adjoining duct and/or man-holes provided at each side of the crossing.
Image result for image of single road crossing of HDPE Ducts by moiling
Single Road crossing by Moiling techniques
Image result for image of crossing highway of HDPE Ducts by HDD
Highway Crossing by HDD techniques

Image result for image of railway crossing of HDPE Ducts by HDD
Railway crossing by HDD

Water Body Crossing

➤ Water bodies and other natural obstructions can be crossed by open-cut method, trenchless techniques or using existing infrastructures such as bridges and culverts depending upon the nature of water body (dry/stagnant/flowing) and availability of permissions from concerned authorities for using existing infrastructure.

➤ In all cases, installation technique shall be such that protection of HDPE ducts is ensured. Necessary protection as described herein below shall be provided depending on installation technique used.

Crossings Using Existing Bridges / Culvert

➤ Existing infrastructure for crossings water bodies such as streams / rivers / canals, etc. shall be used wherever possible and permission to use the bridge or culvert is available.

➤ Ducts shall be installed inside GI/DWC pipes (of diameter suitable for specified number of ducts) when duct are laid on/along bridge/culverts at locations approved by authorities. DWC pipe shall be used when the location is not exposed to direct sunlight. For locations exposed to direct sunlight, high winds or heavy rains GI pipes shall be used as protective measure. In exceptional cases, where authority does not permits use of GI pipe, DWC pipe shall be covered with a GI sheet to prevent direct sunlight and rain falling on DWC pipe. As far as practical GI sheet should not touch the DWC pipe. Wooden/other low thermal conductivity material spacers can be used to maintain gap between DWC pipe and GI sheet.

➤ GI Pipe/DWC duct shall extend at least for 10 meter beyond the banks/limits of water body / embankment of bridge on both sides of crossing. Specified protection shall also be extended accordingly. Man-hole/hand-hole shall be located minimum 10 meter further away from end of GI/DWC pipe or as indicated in IFC drawings.

➤ On arch type bridges where depth up to 300 mm is possible to dig, a trench (minimum 150mm wide) shall be made in tar road section of the bridge. DWC pipe then shall be installed inside this trench.Concreting (using concrete mixture of cement, sand and aggregates in 1:2:4 ratio) shall be done over the DWC pipe up to the road level. The ducts shall be pulled into DWC pipe.

➤ Bridges where it is not possible to make trench, GI pipe / DWC pipe (selected as per criteria at given above) shall be installed at safe available place on the bridge designated for the utilities. The pipe shall be fixed in place firmly using clamps and/or encasing in M15 concrete. HDPE ducts then shall be pulled into this pipe.

➤ When GI pipe/duct is installed on the underside of bridge or culverts, additional measures shall be taken to prevent it getting washed out during flooding. The ducts laid on adjacent areas (banks of water body) shall also be protected against washout or settlement of backfilled area.

Crossing by Open-cut method

➤ Open-cut method can be used when installation on bridge / culvert is not available. This method however is more suited to seasonal water bodies (prone to flowing water such as nala/ stream/ river/ canal etc.) which may be completely / partially dry during installation period. This method can also be used for small water bodies where it is possible to divert water for a short period of installation.

➤ In case there is only one Hume pipe up to 600mm dia used as a culvert to equalise the water level on both side of road. The duct can be laid at a min depth of 1.65 m without any protection for more than one 600mm dia and all sizes of Box-culverts.

➤ In Case the water body bed is not rocky (i.e. normal soil is present), a trench 2.0 meter below the bottom of culvert bed shall be dug out and DWC pipe of suitable size shall be shall be installed inside the trench. In case the bed is rocky then minimum depth of trench shall be 1.5 m. A 50mm thick sand padding (covering highest point on the trench bottom surface) shall be provided before installing DWC pipe.

When the water body is full of water and PCC cannot be done

(a) Trench to be filled with sand for a height of 200mm from top of DWC pipe.
(b) 300mm thick layer of crushed rock/stones of size 50-100mm be filled in the trench and         compacted.
(c)  Remaining trench be filled with excavated material and compacted.

When the water body is dry and PCC can be done

(a) Trench to be filled with M15 plane cement concrete (1:2:4 mixture of cement, sand and          aggregate) for at least 300mm above top of DWC pipe.
(b) Remaining trench be filled with excavated material and compacted.
(c) HDPE Ducts shall be pulled through the DWC duct.

Crossing by Trenchless Techniques

➤ Where there is no bridge or culvert on water body and/or open cut method is not feasible due to continuous presence of water that cannot be diverted, crossing of water body can be made by HDD (if crossing length is more than 20m) or by moiling if length is less than to 20m.

➤ The ducts shall be installed inside GI pipe installed by HDD. The size of GI pipe shall be suitable for the number of HDPE ducts to be installed. HDPE ducts can be pre-installed in GI pipe before it is installed.

Image result for image of HDD crossing of river
Image result for image of HDD crossing of river


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