Purpose of this method statement is to outline the sequences and methods of works intended to be used for for laying underground 33 kV power and fiber optic cables including the excavation of trench and backfilling.
The electrical contractor has the right to adapt the methods of execution as contained in this document from time to time and modify as necessary according to specific site conditions.
The scope of works comprises Manual site survey, Trail pit excavation, Trenching, Cable pulling, Cable jointing, Cable termination, Backfilling, Installation of Protection tiles, Installation of Warning tape, Reinstatement, Installation of route markers / Joint markers, Cable testing, Horizontal Directional Drilling / Open-cut for Road Crossings, and Site clearance.
References & Standards
The method statement shall be read in conjunction with the following documents:
- Project Electrical Specifications
- Approved cable route drawings
- Existing underground installations
- Any other directions issued by the local civil authorities at the site
Necessary Tools & Equipment’s
The following tools & tackles will be arranged as necessary depending upon the site conditions:
- Cable Pulling Winch
- Trailer
- Cable Trolleys
- Cable Drum Stand and Shaft
- Cable Rollers
- Pulling Wire or Rope
- Pulling Eye
- Chain Block
- Shackle / Swivel
- KV Insulation (Megger) Resistance Tester
- HV Test Kit
- Multi Meter (Digital)
- HV Hand Gloves
- HV Grounding Stick
- Hand Tools
- Trucks
- Crane
- Excavator
- Water Tanker
- 3.5 Ton Truck for material movement.
- Plate Compactor
- Survey Instrument
- Low Bed Trailer
- Directional Boring Machine
Sequence of Work for Cable Laying
Obtain route approval as required, prior to start of work. Conduct Site survey, establish cable route, and demarcate the cable trench at cable laying site.
Trial Pits (Exploratory Excavation)
Obtain work permit/NOC from various authorities & “As Built” drawings showing all existing services.
Set up site, marked areas to be explored at 100 meters interval or as may be required and also near the existing identified services, and fix with warning tape / safety cones.
Intimate the concerned departments prior to exposing their related services where necessary.
Start manual digging using pick-axe without sharp edges and hand shovel.
Look in for any warning signs / warning tape / cable tiles that might prop up during excavation and take necessary precautions while digging.
Expose existing known buried service.
Record details of existing services.
Protect the services as per specified recommendations.
Road Crossing
As per the requirement of applicable standards and specifications, at crossing points of highways & roads the contractor shall provide separate ducts for each cable circuit plus one spare circuit in the required position.
The duct shall consists of PVC pipes with minimum internal diameter of 200 mm & a wall thickness of 5 mm.
Crossing for future roads shall be done either by HDD method or open cut duct concrete bank as per approved procedures.
Horizontal Directional Drilling (GDD works)
Road crossing by Directional drilling (HDD) shall be done as per method statement of approved subcontractor for the HDD works, the number of ducts shall be as per requirements.
Bentonite filling in the pipes shall be done as required in the road works specification.
Trench Excavation Works
Set out and mark the exact line of cable route as per approved drawings and the obtain NOC/ Permission to Work.
Excavation is carried out using a mechanical excavator.
In areas where usage of excavator is unsafe / not possible, the trench is made through manual digging.
As per requirement of project specifications, where sand dunes or any other obstruction exist in the proposed cable routes, they shall be cleared to the normal ground level.
A designated charge hand / supervisor will be assigned to each machine.
The machine operator will take the directions from the Charge hand or his Construction Supervisor / Project Engineer.
Minimum depth & width of excavated trench and placement of power cable, 100mm dia UPVC pipe and FO cable, sand filling, protection tiles, warning tape etc. shall be generally as per the approved drawings.
Safety cones will be provided with warning tape along the top edge of the trench.
Any existing services, which are uncovered as a result of excavation, will be protected with temporary measures, as required.
All excavated loose material at the top level of the trench will be removed from the edge of excavation.
Bottom of excavation / trench shall be trimmed off and free from sharp objects i.e. stones etc. prior to sand bedding.
Sand Bedding: A minimum of 150 mm sand bedding on trench be provided for cable cushions.
METHOD OF CABLE PULLING & LAYING
Ensure that areas / trench to be pulled / laid with cables are inspected and approved.
Drums shall be placed on a pair of stand and shaft to provide smooth rolling while pulling.
The direction of rotation of cable drum for moving of cable drum or laying of cable will be strictly as per the arrows marked on the cable drums.
Set Winch properly and securely fix to the required distance.
In order to prevent cable damage during pulling operation, cable rollers shall be placed at maximum 2 meters intervals as per requirement of specifications.
The distance between the rollers shall be maintained and well fixed.
Appropriate corner rollers will be provided as required.
Special observance and care will be taken when pulling in areas with corners, through road crossings and through existing ducts.
Check cable ducts for debris and sharp edges and clean the ducts if necessary.
Cables to be applied with sealant or pulling compound for smooth and easy pulling when entering cable ducts if necessary.
Both ends of cable cut to be capped, taped and identification to be provided.
Perform sheath test & meggering on laid cable prior to jointing and back filling as per specification requirement.
Sheath test shall be conducted by injecting 10 kV DC using DC HV Test kit & 5 kV Insulation Tester between armour and the external graphite surface of the sheath for one minute, with all remaining phases solidly grounded and the leakage current recorded on test Form.
Cable Jointing & Termination: Cable jointing shall be done as per the Joint Kit manufacturer’s instructions / instruction manual with the approved jointer in accordance with the best workmanship.
Trench Backfilling & Reinstatement
Soil Thermal resistivity shall be measured as per specification requirements along the route of cable.
The soil Thermal resistivity & temperature of backfilling material shall be measured as per the specifications requirement.
Backfilling material should have a maximum thermal resistivity of 120 cm2 C/W under operational conditions & shall be chemically neutral as per project specification requirements.
Special bedding material shall be used as per project requirements, where the thermal resistivity of natural soil exceeds 120 cm2 C/W.
The Consultant / Owner / Client may conduct inspections prior to sand covering / installation of warning tapes / final back filling / installation of cable route markers to keep the continuity & progress of the work.
Installed cables shall be covered with selected fine sand up to the level of warning tape as per the drawings.
The protection tiles for 33 KV cable trench shall be 500 mm length, 250 mm width & concrete grade is C35 with cement type V (SRC), min cement content 370 kg/m3, all edges to be chamfered & name to be fixed on each protection tile as per the specification.
Protection tiles for MV & LV cables shall be of 500 mm length, 200 mm width & 60 mm thickness.
Balance of the excavated debris, if any, shall be level-spread to reinstate the area to the pre-excavation condition, and if necessary extra earth shall be disposed off.
Permanent reinstatement shall be done in foot paths and tiled areas.
Cable Route Markers:
Cable Route & Joint markers shall be installed for each power cable circuits.
The cable route markers shall be installed on straight route at distances not exceeding 30M and at the points where the cable route makes a turn and both side of road crossings.
Joint marker shall be installed on top of jointing to mark the location of the joint.
All cable route & joint markers shall be made of reinforced concrete and of truncated cone type as per approved specifications.
Cable Testing & Commissioning
Sheath test and insulation resistance measurement of conductors
Notify the concerned parties to witness the test as necessary.
Ensure that cable rating, type, size and cable number conforms to specification and drawings.
Surroundings of the testing area shall be cordoned off and access restricted to authorized persons only.
Ensure that valid & calibrated test equipment’s are available at the test area.
HV hand gloves and HV grounding sticks shall be provided for safe handling and draining of voltage residue.
Make the cable ends at both ends ready for testing by removing 500 mm long graphite coating from the cable.
Shortwave communication system shall be in place prior to commencement of cable testing.
For safety precautions, one designated representative shall guard the cable at the non-testing end.
Cables shall be checked for continuity and for correct phases.
All tests required as per local standards shall be done and the values shall be recorded on test formats.
Sheath Test shall be conducted by injecting 10 kV using HV DC test kit between armour and the external graphite surface of the sheath for one minute, with all remaining phases solidly grounded and the leakage current shall be recorded.
Immediately after testing of each phase, voltage residue shall be drained by using grounding stick which is directly connected to earth ground.
The insulation resistance shall be measured between phase to phase and phase to earth prior to jointing.
Results shall be recorded as per the approved test formats.
HV testing: After completion of all straight through joints and end termination the phasing out/identification shall be done from one end to other end and HV test shall be conducted as per specification and results shall be recorded.
Health & Safety Requirements
All excavation work will be executed in accordance with the project health and safety plan and necessary precautions shall be made to ensure safety of men and protection of environment and materials. Special attention will be given to:
Cordoning off the open pit excavation area.
Making stable slope if required.
Placement of applicable safety warning gadgets and sign boards.
Allow only authorized personnel to have access to the site.
Provision of first aid kit at the site.