Expansive clay and window wells: why Fort Collins wells heave, flood, and how to build one that lasts

September 21, 2026

Most window well failures in Fort Collins are soil failures, not steel failures. The Colorado Geological Survey’s map of the Fort Collins quadrangle says the ground under this city is Pierre Shale and wind-blown sediment, and that swelling and collapsing soils in those areas are among the most common natural hazards here. A corrugated well set in that clay with no gravel column under it will heave, hold water, and walk away from the foundation. The fix is rarely the well itself. A drainage rebuild runs $350 to $1,100, a full well replacement with a proper gravel column $1,100 to $3,300, and a fitted cover $175 to $650.

This post explains what the clay does, why it does it here, and what a well has to include to survive it.

What expansive soil actually is

The Colorado Geological Survey defines expansive soil and rock as soil and rock that contains clay and expands to a significant degree when it gets wet and shrinks when it dries. The clays doing the work are bentonite and montmorillonite, which the survey describes as weathered volcanic ash that underlies many populated areas of the state, sometimes as soil and sometimes as soft bedrock.

Two numbers from that same page explain almost every heaved well we see. These clays can expand by up to 20 percent by volume when exposed to water. And they can push with a force of up to 30,000 pounds per square foot. A window well is a light steel or composite shell anchored to a concrete wall. It has no chance against that kind of pressure if the soil around it is allowed to cycle between soaked and bone dry.

The survey’s related page on heaving bedrock puts a cost on it: tens of millions of dollars in damage along the Front Range piedmont, which is the strip of foothills-to-plains ground that Fort Collins sits on.

Why Fort Collins in particular

Three sourced facts make this a Fort Collins problem rather than a generic Colorado one.

The bedrock. The survey’s 2025 geologic map of the Fort Collins quadrangle states that the Upper Cretaceous Niobrara and Pierre Shale underlie the quadrangle. The same map names the most common natural hazards in the mapped area as flooding in low-lying areas, especially next to the Cache la Poudre River, and swelling and collapsing soils where the Pierre Shale and eolian sediments are at the surface. That is the city’s own geology, not a regional generalization.

The topsoil. The USDA’s official description of the Nunn soil series, whose type location is in Larimer County near the Harmony School in south Fort Collins, classifies it as a fine, smectitic soil formed in loess and mixed alluvium. Smectite is the family of clay minerals that swells. So the loam on top of the shale is a shrink-swell soil as well.

The water. A 2018 Colorado State University groundwater study found that roughly 10 percent of parcels in the Fort Collins area are at risk of high groundwater, that most parcels along the Cache la Poudre have high groundwater problems, and that the average depth to the water table is about 3.9 meters. A window well bottom sits four to six feet below grade. On the low ground near the river, that puts the bottom of the well close to, or in, the water table for part of the year.

Put those together and you have clay that swells 20 percent when wet, sitting on shale that does the same, with water arriving from above in spring and from below along the river. That is the environment every Fort Collins window well lives in.

The three ways a well fails in this soil

The flooding well and heaving well pages walk through each of these with a five-minute check you can do yourself.

The 2013 flood as a reminder

The extreme case is on record. The U.S. Geological Survey’s report on the September 2013 flood documents 12 to 20 inches of rain in the Front Range foothills between September 9 and 16, more than 31,000 damaged or destroyed structures statewide, and an estimated 3 billion dollars in damage. The Cache la Poudre at the mouth of the canyon peaked on September 13 at 9,730 cubic feet per second, the fourth highest of 131 recorded peaks. The gage at Fort Collins recorded 8,140 cubic feet per second, the highest since that gage was installed in 1975. The National Weather Service lists Larimer as one of the hardest-hit counties.

Nobody designs a window well for a flood like that. But every spring is a small version of the same thing: water arriving faster than clay can absorb it, looking for the lowest point, which is your well.

What a well has to include here

The City of Fort Collins already requires the most important piece. Its emergency escape window guide states that a new window well installation shall be designed for drainage and connected to the building’s foundation drain if one is present. The city’s residential basement finish guide repeats it: drain the well to the existing perimeter drain if present. Those are the rules, and they are the right rules for this soil.

Beyond the code minimum, this is what we build and what you should insist on from any bidder:

1. A real gravel column under the well. Clean washed rock, deep enough to reach the footing drain where the house has one. On a house with no perimeter drain, a deep gravel dry well below the frost line. This is the single line item that separates a 25-year well from a 5-year well in clay.
2. Filter fabric between the rock and the clay. Without it, fines migrate into the gravel and seal it in a decade or two. That is the “silted shut” failure on the flooding-well page.
3. The well anchored to the foundation, not the dirt. The shell has to be bolted to concrete so the swelling clay pushes against the house instead of lifting the well away from it.
4. Backfill graded away from the house. The city’s drainage handout for residential permits asks applicants to show flow arrows directing water away from the building, downspout locations, and window well locations on the site plan. Do the same in the yard.
5. Downspouts away from the wells. Roof water dumped next to a well is the most common preventable cause of a flooded well we see.
6. No irrigated beds against the foundation. Colorado State University Extension’s advice for homes on expansive clays is to limit landscaping along the foundation to non-irrigated mulch and xeric plants, to avoid planting trees next to the foundation, and to direct roof drainage away from it. Extension also notes plainly that much of Colorado has heavy clay soils. A sprinkler zone soaking the clay beside a well is a heave waiting to happen.
7. A fitted cover. Not required by code on every well, but on a clay lot it keeps rain and snowmelt from filling the well from the top while the gravel handles what comes from the sides and below. Covers are $175 to $650 and the covers and drainage page explains the options.

What this means for the price

An honest quote for a window well replacement in Fort Collins includes the excavation below the well, the gravel column, the fabric, the tie-in or dry well, the anchors, the ladder if the well is deeper than 44 inches, and the regrade. That is why the range is $1,100 to $3,300 rather than the cost of the steel. A bid that is half of that has usually left the drainage out, which in this soil is the same as leaving the well out.

If the well you have still holds its shape and the anchors are tight, the drainage rebuild alone at $350 to $1,100 is often the honest answer. The window well calculator prices both paths from the same table we quote from.

When to call

If your well holds water for more than a day after the snow melts, if there is a gap between the well and the wall you can fit fingers into, or if the rim of the well has tilted since last year, the clay has started working on it. The measure is free, the quote is flat, and the first thing we look at is the ground the well is sitting in, not the well.