Hamilton County landslides: what to check on a hillside lot

If you own or are shopping for a house on one of Cincinnati's hillsides, the ground under the lot has a documented record. A 1980 U.S. Geological Survey study found that Hamilton County had the highest annual per-capita cost from landslide damage in the country, according to a summary by the Hamilton County Soil and Water Conservation District. Over the study years, 1973 to 1978, damage topped $5 million a year, about $5.80 per resident annually, more than four times the figure for the nine-county San Francisco Bay region. That total even left out the $22 million repair of the Mt. Adams slide.

The exposure has not gone away. Ohio's 2019 state hazard mitigation plan summarizes a University of Cincinnati landslide assessment from Hamilton County's 2013 plan that listed 1,346 residential buildings, with estimated losses or exposure of about $280 million.

Most hillside homes here are fine. The ones that are not usually gave warning, and the warning signs are things you can check on a Saturday.

Why the hills here move

The bedrock under Hamilton County is Ordovician shale and limestone, roughly 450 million years old, and one unit in it, the Kope Formation, causes most of the trouble. The exposed Kope is about 200 feet thick, rising from roughly the level of the Ohio River and forming the walls of many of the county's deeper valleys. Its shale weathers quickly into a weak, clay-rich soil called colluvium that coats the slopes like a skin.

Ohio's Division of Geological Survey puts it directly: most bedrock slope failures around Cincinnati are in the shale-dominated Kope, with fewer in the Miamitown Shale, and landslides tend to happen in the thick colluvium on those units when excess water pressure builds up in it.

Two more ingredients make this county unusual:

• Lake bed clays. Ice age glaciers dammed the ancient Teays River and its tributaries, forming lakes that left fine, layered clays. Later glaciers buried them. Where they sit beneath a slope, they are exceptionally unstable, even on gentle grades.
• Two centuries of earthwork. Cutting, filling, quarrying, and rerouting drainage on the hillsides. Early limestone quarrying on East Price Hill, and the spoil dumped over its eastern flank, caused slides there that still recur.

The three kinds of movement you might see

A regional guide written by University of Cincinnati and Kentucky Geological Survey geologists, and posted by the City of Cincinnati, sorts local slides into three types:

• Creep. Slow, shallow, and the most widespread. It shows up as tilted poles and fence posts, curved tree trunks, and walls that lean or crack. The guide pictures an old stone retaining wall in Mt. Auburn being broken apart by it.
• Translational slides. Faster and shallow, with a layer of soil sliding off the bedrock along a flat surface. Common on local hillsides, and easier to remediate because the rock itself is not moving.
• Rotational slides. Deep and slow, on a curved slip surface, most often in thick glacial deposits and stream banks. The guide calls this the most damaging type in our area and cites the 1973 Mt. Adams slide, which destroyed two streets and sixteen houses.

A hillside lot checklist

Walk the lot, the slope above it, and the slope below it. Take dated photos. Look for:

• Cracks in the ground running roughly parallel to the slope. These scarps can be an inch tall or several feet.
• Bulging soil at the bottom of a slope, the toe of a moving mass.
• Tilted trees, poles, or posts, especially trunks with a kink or curve near the base.
• Retaining walls that lean, bow, or show open joints.
• Seeps and wet spots on the slope face, or small closed hollows holding cattails or other wetland plants.
• Bumpy, hummocky ground under honeysuckle, which the guide flags as a common look for an old slide.
• Broken or leaking pipes. Many slides are associated with cracked sewer or water lines, so learn where every line on the property runs.
• Signs of old quarries or fill near the crest.
• Inside the house: new stair-step cracks, doors that start to stick, and gaps opening between a porch or garage and the main structure.

One sign alone may mean little. Several together, especially near the top or bottom of a slope, deserve a professional look before you close or before the next wet spring.

What to do, and what not to do

The sources agree on the first line of defense: manage the water. Saturated colluvium is heavier and weaker, and heavy rain is the usual trigger.

• Keep roof and sump water off the slope. Ohio's Geological Survey lists downspout water discharged onto a slope among the practices that invite failure.
• Do not add fill at the top of a slope or dig into its base.
• Do not clear trees and vegetation from a slope; roots and water uptake help hold it.
• Repair leaking water and sewer lines quickly.

Hillside earthwork is regulated. Cincinnati adopted earthwork rules in 1974, and Hamilton County followed in 1990. In general terms, the county rules require a registered geotechnical engineer's involvement when cuts or fills exceed 5 feet, the existing slope is 20 percent or steeper, final slopes are 33 percent or steeper, or the work is in a known landslide area. Check with your city or township before grading on a slope.

What it means for the foundation

When a hillside house is already moving, the structural answer is usually to carry it past the weak material to rock or competent soil, which is where helical piers and push piers come in, paired with drainage that keeps the slope drier. Our earlier piece on why Cincinnati hillsides move explains how piers bypass creeping soil, and our answer on hillside homes covers monitoring when nothing is moving yet.

Two cautions from the Soil and Water Conservation District's geotechnical engineer: landslide damage is generally not covered by homeowners insurance unless an earth-movement policy was in place beforehand, and disaster aid for slow-moving slides is nearly nonexistent. That makes early detection the cheapest protection you have.

If your lot shows more than one of these signs, or a crack inside the house has changed since spring, check our warning signs by severity, then call or text us. We will tell you whether you need a geotechnical engineer, a foundation specialist, or a set of dated photos and another look in April.