The Pull That Should Not Have Been Started
A 48-fibre cable is being pulled into an existing 40/33 mm HDPE duct over a 720 m route to a new apartment block. The duct already carries one 96-fibre cable and has two 90 degree bends of about 1.2 m radius, plus a 3 m rise near the building entry. Cable mass is 190 kg/km, the manufacturer's maximum allowable tension is 2,700 N, and the crew is using a tension meter on the winch. Readings: 620 N at 200 m, 1,150 N at 400 m and 1,780 N at 500 m. Ambient temperature is 42 degrees C and the exposed duct surface is noticeably hot. A blue water-main marker sits about 8 m from the proposed chamber position, and the duct route crosses a road shoulder used by two-wheelers. The crew wants to continue pulling the remaining 220 m in one go to finish before the light fails.
1. Interpret the tension readings and decide whether the remaining 220 m can be pulled. Support the answer with numbers rather than a general caution.
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The trend, not just the absolute value, decides this.
- 0-200 m: 620 N, about 3.1 N/m average, consistent with friction on a fairly straight duct.
- 200-400 m: adds 530 N over 200 m, about 2.7 N/m.
- 400-500 m: adds 630 N over only 100 m, about 6.3 N/m. The rate has more than doubled, which is the signature of a bend and the rise now dominating, not of ordinary friction.
Extrapolating the last segment over the remaining 220 m gives roughly 1,300-1,400 N additional, so the finishing tension would be around 3,100-3,200 N, well above the 2,700 N maximum. Even that understates the risk, because a bend at the far end raises tension exponentially rather than linearly.
Working practice is to stay at or below about 60-70 percent of the rated maximum during a routine pull, so the practical ceiling here is about 1,900 N, and 1,780 N is already at the edge. Decision: stop the pull. Do not attempt the remaining 220 m in one go, and inform the site supervisor before the crew tries to force it. Also inspect the cable at the pulling eye and along the last 100 m for sheath ridging, necking or a kink before any decision about re-using this drum.
2. Give the viable options for completing the installation, with the engineering and cost consequences of each, and recommend one.
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Three options are realistic.
- Intermediate chamber at about 400 m, pull two shorter sections from both ends. Adds one joint closure and 48 fusion splices at roughly 0.1 dB each, plus excavation, a chamber and a permit. It converts an unmanageable single pull into two pulls of about 400 m and 320 m that should stay near 1,000-1,300 N. This is the safest and most permanent solution.
- Pull from both ends simultaneously with a figure-of-eight at mid-point. No extra splices and no extra chamber, but it needs 20-25 m of clear working space for the figure-eight, two coordinated winches, a person watching the bend at all times, and strict control of the cable bend radius while coiling. On a road shoulder at 42 degrees C with light failing, this is operationally risky.
- Reduce friction along the existing route. Lubricant compatible with the cable sheath, reaming and rodding the duct, and using a correctly sized pulling stocking. This helps but cannot remove the bend and rise losses, and it will not buy 1,000 N.
Recommendation: option 1. Option 2 would be acceptable only on a longer programme with daylight, more crew and a properly barricaded figure-eight area. Option 3 alone is not sufficient and should be used in addition, not instead.
3. Before the crew restarts, list the safety and statutory controls you would insist on at this site and the records you would keep.
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Service and excavation controls:
- Treat the water-main marker as a live service. Hand-dig trial pits to locate it, no machine excavation within 1 m of a marked service, and call the utility locator before breaking ground. Obtain the road-cutting and excavation permit for the shoulder.
- Keep the chamber position clear of the water main by the utility's specified offset, and record the measured clearance.
Traffic and public safety:
- Barricade and cone the shoulder crossing, with reflective signage and a banksman facing approaching two-wheeler traffic; high-visibility clothing is mandatory.
- Cap open duct ends and guard the excavation when unattended; no open pit overnight without covers and lighting.
Heat and manual handling:
- At 42 degrees C, schedule work-rest cycles, provide drinking water and shade, and watch for heat-stress symptoms in the crew. The decision to push on before dark is precisely the pressure that causes accidents.
- Two-person lifting for the drum and cable coils, safety shoes, gloves and helmets; keep the pulling rope and winch away from any overhead electrical line.
Records: the tension log with distance and reading, the stop decision and who authorised it, the utility clearance measurements, permit numbers, and photographs of the barricading. Hand these to the supervisor with the revised method statement for the intermediate chamber.