By the PICA Corp Engineering Team  |  Updated July 2026  |  Est. reading time: 9 min

Most utilities know which of their force mains worry them. Ask an operations manager to name the three lines that keep him up at night and the answer comes fast. Ask him to justify that ranking to a finance committee, in writing, with a defensible method behind it, and the conversation slows down considerably.

That gap is what a force main risk assessment closes. It turns institutional memory into a ranked list that survives a budget process, and it usually corrects one thing operations instinct gets wrong: the pipe most likely to fail is often not the pipe you should inspect first.

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What is a force main risk assessment?

A force main risk assessment scores every pressurized sewer line in a collection system on two axes and multiplies the results. The first axis is likelihood of failure, which asks how close the pipe is to the end of its structural life. The second is consequence of failure, which asks what happens to the utility, the public, and the receiving environment if the pipe lets go.

Neither score means much alone. A 50-year-old cast iron line in corrosive soil has a high likelihood score, but if it serves a small subdivision and the system can bypass around it in an afternoon, the consequence score is modest. A 15-year-old steel main under a navigable waterway may have a low likelihood score and still belong at the top of the inspection program, because there is no acceptable version of that pipe failing.

The multiplication is the point. It stops utilities from spending their entire assessment budget on the oldest pipe in the system while a newer, far more consequential line goes uninspected.

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Why force mains resist the standard condition assessment playbook

Gravity sewers have a mature inspection routine. Force mains do not, and the reasons are structural rather than a matter of industry laziness.

They run under pressure, so failures arrive without warning

A gravity sewer degrades visibly. Cracks widen, joints offset, infiltration climbs, and a utility gets years of warning through flow monitoring and routine video. A force main under 60 psi does not leak politely. Wall loss progresses invisibly until the remaining wall can no longer hold operating pressure, and then the line ruptures, usually with the pump station running at peak.

Access points are scarce and far apart

Force mains were built to move sewage from a pump station to a discharge point with as few interruptions as possible. That design intent produced pipelines with thousands of feet between air release valves and often no maintenance holes at all. Conventional gravity sewer crawlers have nothing to launch from and nothing to drive on.

The damage is inside the wall, not on the surface

This is the part that catches utilities out. CCTV, laser and LiDAR inspection records what the inside surface of a pipe looks like, and on gravity sewers that record is usually enough. On a force main, the deterioration that matters is loss of wall thickness, and a camera cannot measure it. A pipe with 40 percent of its wall gone to external soil corrosion looks entirely serviceable on video. CCTV of a force main also means taking the line out of service and dewatering it, which is a serious operation for a pipe with no redundancy.

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Scoring likelihood of failure

Likelihood scoring is where most risk models are built, and where most of them quietly go wrong. The inputs below are the ones that carry real predictive weight.

Material and internal environment

Hydrogen sulfide is the dominant internal failure driver in wastewater force mains and belongs at the top of any scoring model. Anaerobic sulfate-reducing bacteria generate H2S, which oxidizes to sulfuric acid at the pipe surface and attacks the crown. Long detention times, warm climates, and low-velocity operation all raise the score. After H2S, look at internal tuberculation in ductile iron, pitting corrosion in steel, and external corrosion driven by soil chemistry and stray current.

Age, but with a caveat

Installation date is the easiest field to populate and the most overweighted variable in the industry. Design life for most force main materials is 50 to 100 years. Observed life in aggressive service is frequently half that. Use age as a sorting field, not as a proxy for condition.

Operating and surge history

Pump station cycling matters more than most models account for. Every start and stop sends a pressure transient down the line, and those transients fatigue joints, fittings, and thrust restraints. A station cycling forty times a day is accumulating damage a steady-flow line is not. Break history on the line and on its siblings, meaning pipes of the same material and vintage installed by the same contractor, is the most useful historical input available.

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Scoring consequence of failure

Consequence scoring gets less attention than it deserves, largely because it requires the utility to write down uncomfortable things.

What sits downstream of the break

Map every force main against what a spill would reach. Sub-aqueous crossings, lines near drinking water sources, pipes above shellfish waters or recreational beaches, and alignments through dense residential areas all carry consequence scores that dwarf the pipe’s replacement value. The 2018 NASTT No-Dig paper on a force main beneath a drinking water reservoir is this problem at its most extreme: twin 16-inch lines under an active reservoir, never inspected in more than four decades of service.

Redundancy, and the honest version of it

Ask whether the system can actually run without this line, and for how long. Many utilities carry a nominal bypass plan that has never been executed and depends on equipment that would take days to mobilize. A force main with a theoretical bypass and a real mobilization time of 72 hours is a single point of failure, and the risk register should say so. Regulatory exposure follows from this. A Sanitary Sewer Overflow is a reportable event, and consent decrees issued after a significant spill routinely commit a utility to programs far larger than the one it declined to fund voluntarily.

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Turning the score into an inspection sequence

A finished risk register usually sorts a force main inventory into three practical bands.

The top band is small and gets a full pipeline condition assessment with wall thickness measurement. These are the high-consequence lines where the utility cannot tolerate being wrong, and the assessment is justified regardless of what the likelihood score says.

The middle band gets screened first. Pre-screening with the NAVIGATOR multi-sensor acoustic sphere runs in-service at pressures up to 300 psi in lines from 6 to 78 inches, identifying leak locations and H2S gas pocket locations along with a pressure profile. It is cheap relative to a full assessment and tells a utility which segments deserve the expensive tool.

The bottom band gets a calendar entry and a note to revisit when new data arrives. That is a legitimate outcome. Not every pipe needs inspecting this year, and a risk model that flags everything as urgent gives no more decision support than no model at all.

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Why a desktop risk model alone isn’t enough

Here is the limitation utilities discover late. A desktop risk model is built entirely from proxies. Age is a proxy for deterioration. Material is a proxy for corrosion susceptibility. Soil class is a proxy for external attack. Every input is an inference about a pipe nobody has looked at.

Proxies fail in both directions, and both failures are expensive. A model can rank a pipe as low risk when it has already lost half its wall to a soil condition the mapping missed. It can also condemn a pipe that turns out to be sound, sending a utility into a replacement project it did not need. Across a large inventory the second error is usually the costlier one, because most systems replace far more pipe than condition data would justify. That is the argument behind inspecting before you replace.

Measured data resolves both errors. Remote Field Technology measures actual remaining wall thickness continuously along the full run, through internal liners, scale, and deposits up to about 1 inch (25-30 mm) thick, with no cleaning to bare metal required. PICA’s SeeSnake and Chimera free-swimming RFT tools cover force main diameters from 2 to 36 inches and detect both internal and external wall loss, down to a minimum reported defect of 1 inch by 1 inch at 20 percent wall loss. The tools can run in-service, though flow must be reduced to hold tool speed in a workable range, which in practice often means night shift work or a partial bypass. CCTV can be added where the utility also wants a visual record for out of service inspections.

The result is a risk register where the likelihood column stops being an estimate. See how this plays out on real projects in PICA’s force main inspection case studies.

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What force main assessment costs, and how to budget it

Electromagnetic condition assessment on a force main generally runs $20 to $40 per linear foot. The spread reflects diameter, access complexity, how much progressive cleaning the line needs before a tool can pass, and how much flow management the run requires. Lines needing launch and receive barrels built from scratch sit at the upper end.

Set that against replacement, which for open-cut work on a mid-size force main generally runs several hundred dollars per linear foot before restoration and traffic control. A utility that assesses 20,000 feet of high-consequence force main spends roughly what one short replacement project costs, and gets back the data needed to decide which segments actually require that project.

The deliverable is a wall thickness dataset indexed by linear position, the locations and extent of measured metal loss, findings ranked by risk, and recommendations separated into rehabilitation, monitoring, and replacement. PACP coding does not apply here. That is a gravity sewer CCTV standard with no role in a force main electromagnetic report.

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Frequently asked questions

What is a force main risk assessment?

A force main risk assessment ranks every pressurized sewer line in a collection system by the product of two scores: how likely the pipe is to fail, and how bad the failure would be. Likelihood draws on pipe material, age, internal environment, surge history, and soil conditions. Consequence draws on what sits downstream of a break, whether the system has redundancy, and the regulatory exposure attached to a spill. The output is a ranked list that tells a utility where to spend inspection money first.

Which force mains should a utility inspect first?

Inspect the lines that score high on consequence before the ones that score high on likelihood. A corroded 6-inch force main serving forty homes is a maintenance problem. A 30-inch force main crossing a river or running under a reservoir is a regulatory event waiting to happen, even at moderate deterioration. Sub-aqueous crossings, single-feed lines with no bypass, and pipes discharging near drinking water sources or shellfish beds belong at the top of the list regardless of age.

How do you calculate likelihood of failure for a force main?

Most utilities weight four inputs: pipe material and wall schedule, installation age, internal corrosion exposure driven by hydrogen sulfide, and external soil corrosivity. Pump station cycling and surge history matter as well, because pressure transients fatigue joints and fittings faster than steady operation. These inputs produce an estimate, not a measurement. Two pipes with identical scores can have very different remaining wall thickness, which is why desktop scoring is a screening tool rather than a verdict.

How long does a force main last?

Design life for most force main materials is 50 to 100 years, but observed service life is frequently much shorter. Hydrogen sulfide generated in the wastewater oxidizes to sulfuric acid at the pipe surface and attacks the crown of the pipe, so a force main with long detention times or a warm climate can lose decades of life. Ductile iron and steel force mains in aggressive service have failed in 25 to 40 years. Age alone is a weak predictor without condition data.

What is the difference between a desktop risk assessment and a physical condition assessment?

A desktop risk assessment uses records a utility already holds: as-built drawings, material and installation dates, break history, soil maps, and hydraulic models. It costs little and narrows a large inventory to a shortlist. A physical condition assessment sends a tool into the pipe and measures what is actually left of the wall. The desktop model tells you where to look. Only the physical assessment tells you whether the pipe is in trouble.

Is inspecting a force main cheaper than replacing it?

Electromagnetic condition assessment on a force main typically runs $20 to $40 per linear foot, depending on diameter, access, cleaning requirements, and how much flow management the run needs. Open-cut replacement of the same pipe generally costs several hundred dollars per linear foot before restoration, traffic control, and easement work. Inspection is worth doing when it can defer or retarget replacement, which is usually the case in systems where only a fraction of the pipe is actually degraded.

How often should a force main risk model be updated?

Review the model annually and rebuild it whenever new condition data arrives. Every completed inspection changes the likelihood score for the line that was inspected and often for similar lines installed in the same era with the same material. Utilities that treat the risk register as a static spreadsheet lose most of the value of the inspections they paid for. The register should get more accurate every year as measured wall thickness replaces assumptions.

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