Why Colorado Manholes Fail: Engineering Solutions That Work

Why do Colorado manholes fail?

Age, acid, frost and leaking joints, often all at once. Each one opens the door for the next.

Aging concrete and brick. Hydrogen sulfide attacks concrete manholes and the mortar in brick ones, according to EPA's 2010 report on rehabilitating wastewater collection systems. A leaking manhole also pulls soil fines in, which leaves voids and settlement around the structure.

Hydrogen sulfide. Wastewater that runs out of oxygen makes hydrogen sulfide, and turbulence releases it. Flow dropping into a manhole from the side is a common source. Bacteria on the moist walls above the flow turn the gas into sulfuric acid, which eats the cement, as EPA's 1991 report to Congress on hydrogen sulfide corrosion explains.

Discharge manholes below lift stations deserve a close look. A force main runs full under pressure with no chance to pick up oxygen, a problem covered in our post on force main failures and CIPP lining.

Frost heave. In cold climates, frost in the soil can lift the top of a manhole, fracture it and open a path for groundwater, the EPA 2010 report notes. That upper portion, the frame, chimney and cone, sits closest to the frozen ground.

Mountain towns get frost earlier than the Front Range. The Colorado Climate Center's 2024 look at first fall freezes found that freeze timing tracks elevation: the high country freezes first, and very high terrain can drop below freezing in any month.

Joint separation and infiltration. A manhole is a stack of parts: cover, frame, chimney, cone, wall, bench, channel and pipe connections. Each joint between them can open up, and groundwater comes in whenever it rises above the opening.

How do you find the manholes that need work?

Inspect each one the same way, score it, and compare the results with flow data. NASSCO's Manhole Assessment Certification Program (MACP) gives inspectors shared condition and defect codes for each of those components, so results from different crews and years can be compared.

Flow data tells you where to look first. EPA's long-standing I/I benchmarks in 40 CFR 35.2120, written for its construction grants program, call for a closer study when flow at the treatment plant tops 120 gallons per capita per day in high groundwater, or rainfall-driven total flow tops 275 gallons per capita per day during storms.

Those numbers are screening tools, not permit limits. Our Colorado inflow and infiltration services page covers how flow monitoring and inspection fit together.

When is chemical grouting the right fix?

When the manhole is structurally sound and the problem is water coming in. Crews drill through the wall and inject grout until it stops taking more or a seal forms. ASTM F2414 covers the practice for brick, block, cast-in-place concrete, precast concrete and fiberglass manholes.

Grouting is not a structural repair, EPA's 2010 report says. How long it lasts depends on the grout, the crew and the groundwater. The report notes that F2414 calls for an inspection at 12 months, with resealing where leaks return.

For a field view, listen to our podcast episode on grouting and imaging in sewer lines and manholes with CUES.

When should you line a manhole instead?

When acid or cracking is taking the wall, not just letting water through. EPA's 2010 report describes three main lining families:

• Spray or spin-cast cementitious liners: rigid and able to serve as a structural liner, with moderate corrosion protection. Calcium aluminate and microsilica mixes resist corrosion better.
• Spray or spin-cast polymer coatings such as epoxy, polyurethane and polyurea: high corrosion resistance when applied properly. Thin coats depend on their bond to the wall.
• Cured-in-place manhole liners: a resin-saturated fabric liner inflated against the wall and cured, for full structural repair of deep manholes

ASTM F2551 covers a prepackaged cementitious or geopolymer mortar liner, applied from the bottom of the frame down through the bench and channel, as a full-depth restoration or a partial-depth repair.

Surface preparation is where many liners fail. EPA's report says many field failures of coatings and bonded liners trace back to poor surface preparation.

Other sewer structures with flat walls need their own design. EPA notes that there a liner cannot rely on ring compression to resist groundwater pressure.

Inflow at the top needs its own fix. EPA lists chimney seals, sealed or replaced lids and inflow pans as separate repairs. They target water entering at the frame and chimney.

When is replacing the manhole the better call?

When the structure is too badly deteriorated to rehabilitate. EPA's 2010 report treats full replacement as an option for those structures, and says open-cut replacement of the whole manhole is the best choice under some conditions.

Replacement can also make sense when the manhole has to change anyway, for a new depth, a larger pipe or a new connection. A liner keeps the old shape and adds thickness inside it.

How does manhole rehabilitation fit an I&I program?

Fix manholes where the flow data says water gets in, together with the pipes in the same basin. Water stopped at one joint may find the next weak one. Sealing a basin's manholes, mains and service connections as one job gives the flow monitoring a fair test.

Plan the field season around frost. In the high country the first freeze comes early, so book excavation and lining for the warmer months.

Is your manhole program ready to bid?

If you cannot answer yes to the first three items, inspect before you design.

1. Has every manhole in the target basin been inspected and coded the same way?
2. Do you know which defects are leaks, which are corrosion and which are structural?
3. Do flow data show which basins carry the most infiltration and inflow?
4. Have you checked the discharge manholes below force mains for acid damage?
5. Are frame and chimney problems listed apart from wall defects?
6. Does the plan say which structures get grout, which get a liner and which get replaced?
7. Is the work scheduled around your town's freeze season?

Then ask your engineer how each fix will be measured afterward, and which basins come next if flows do not drop.

A note on limits: this is general education from EPA reports, ASTM summaries and the Colorado Climate Center, current as of October 2026. Product choice and design depend on each structure's depth, groundwater and condition, which only an inspection can tell you.

Frequently asked questions

What causes concrete manholes to corrode?

Mostly hydrogen sulfide. Bacteria on the moist walls turn it into sulfuric acid, which attacks the cement in concrete and the mortar in brick.

Is chemical grouting a structural repair?

No. EPA's 2010 report says grouting is not a structural solution. It stops leaks in a sound structure.

How is a new precast manhole tested for leaks?

A common method is the vacuum test in ASTM C1244, run on precast concrete manhole sections before backfill. ASTM notes that the result applies only to that manhole at the time of the test.

I'm the public works director for a Colorado mountain town, and our manhole chimneys crack every spring. Should we grout them or rebuild the tops?

Find out why they crack first. Frost heave can lift and fracture the top of a manhole, and grout alone does not fix a structure that moves. Chimney seals and frame repairs are made for that zone. If the cone or chimney is badly cracked, rebuilding that section may be the better fix.

I manage a small Front Range sanitation district with brick manholes that leak when groundwater is high. Can we rehabilitate them, or do brick manholes have to be replaced?

Many can be rehabilitated. ASTM F2414 covers grout sealing of brick manholes, and cementitious liners can add structure where the mortar has worn. Replace only the ones too deteriorated to line, after an inspection tells you which those are.

Related resources

• I&I Analysis: Finding and Fixing Your Worst Sections
• Key takeaways from EP11: how inflow and infiltration affect your collection system and plant
• Optimizing Collection Systems: Advanced Solutions for I&I Challenges in Rural Communities
• Understanding Your Community's Wastewater Collection System

Not sure which manholes to fix first?

Send us your sewer map, inspection records and plant flows. We will help you sort the grout, liner and replacement candidates into a program you can fund and build.

We specialize in:

• Inflow and infiltration studies
• Collection system design
• Lift station and force main design
• Funding applications and engineering reports
• Construction administration

Modern Engineering Solutions, McKinney, Texas. Contact: (214) 833-6748 or mod-eng.com