Pressure Loss at a Main Break: Advisory Decision and a Trench That Is Not Safe
A 12-inch cast iron main under a four-lane arterial fails at 02:15 on a Saturday. The crew isolates it, but the valves hold poorly: pressure in the affected zone - 420 connections including a 90-bed hospital and two restaurants - drops to about 8 psi and stays there roughly four hours before isolation is complete. There are no confirmed bacteriological samples from the zone in the past 30 days. The excavation is 7 feet deep in sandy soil with the spoil pile 2 feet from the edge. Three workers are on site: one plans to be in the trench setting a repair clamp, one handing down tools, and one operating the backhoe 12 feet away. Groundwater is seeping in. A system intertie could restore pressure in 20 minutes but would push water through a section with a history of low residuals.
1. Do you issue a boil water advisory? Justify the decision and give your first-hour actions.
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Yes - issue a precautionary boil water advisory for the zone now, and open the intertie.
Why. Pressure at 8 psi is below the 20 psi threshold that is used to judge whether a distribution system can keep contamination out. Four hours of sub-20 psi pressure on a cast iron main is an open invitation to intrusion at joints, service connections, packings and any unprotected cross-connection, and the system has no recent bacteriological data from the zone to argue the water is clean. The intertie's low-residual history is a chronic water-quality problem you can flush and sample your way out of; a depressurized main feeding a hospital is not.
First hour. (1) Open the intertie to restore pressure and confirm positive pressure throughout the zone. (2) Notify EGLE by phone now and follow with written notice within 24 hours. (3) Directly contact the hospital, the restaurants and the school - the hospital needs the advisory for ice, food service, patient water and dialysis. (4) Release the public notice through media, door hangers and the website, with the standard boil or bottled water instructions. (5) Start flushing once the repair holds and pressure is stable. (6) Log times, pressures, phone calls and decisions; the timeline is the record.
2. What has to happen before the advisory can be lifted, and what could force you to restart the clock?
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Three things, in sequence.
- Physical repair verified: the main is repaired, backfilled properly, disinfected in accordance with standard practice, and pressure is stable across the zone, including at the ends.
- Flushing and residual: flush until turbidity at the extremities is back to baseline and free chlorine is measurable throughout - the intertie route needs particular attention given its residual history.
- Two clean rounds of bacteriology: total coliform/E. coli samples from representative sites across the affected zone, including the ends and the highest-risk services, with a second set at least 24 hours later. All must be absent for total coliform.
Rescission is with EGLE concurrence once those conditions are met - not on the crew's judgement alone.
What restarts the clock: any positive coliform result, a residual that disappears at a dead end, a failed repair, or a second pressure loss. A single positive means investigate and resample; you do not explain it away.
Publish the rescission the same way the advisory was published, and tell customers to flush building plumbing, ice machines, water heaters and filters.
3. Assess the excavation plan as described. What must change before anyone enters the trench?
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Nothing about that plan is acceptable - no one enters the trench as described.
- Depth: 7 feet is a permit-required excavation. Anything 5 feet or deeper needs a protective system: a trench box or shoring, or sloping/benching to at least 1.5:1 for Type C sandy soil. Sandy soil plus groundwater is the worst case for a cave-in.
- Competent person: inspections before entry, at least daily, and after every change - rain, seepage, vibration, a new load at the edge. Groundwater requires dewatering and re-inspection before each entry.
- Spoil pile: 2 feet from the edge is the minimum, not a goal. Move it back; its weight is a surcharge on the very wall that will fail.
- Equipment: the backhoe 12 feet away transmits vibration and load to the trench wall and must not operate over or alongside the trench while anyone is in it. Do not swap the operator for a spotter on a four-lane arterial - set traffic control and a dedicated flagger.
- Access: a ladder within 25 feet of lateral travel for every worker.
- Utilities: an emergency locate ticket to MISS DIG before the bucket goes in the ground.
The crew is three people doing four jobs. Call in relief, or stop the repair and manage the advisory while the excavation is made safe - a hospital without water for a few more hours is survivable; a buried worker is not.