Guide / Geology

Pittsburgh landslides and retaining walls: why this ground moves

Published by Allegheny Wall Works. Last reviewed July 2026.

Landslide concentration

Southwestern Pennsylvania has by far the highest concentration of landslides in the state

Source: PA DCNR, Geologic Hazards: Landslides

Failure-prone bedrock

Conemaugh Group claystones, including the Pittsburgh red beds: highly plastic rock that loses strength when wet

Source: USGS Professional Paper 1229

2018 city landslide spending

Roughly $12M spent by the City of Pittsburgh against a budget of about $1M to $2.25M

Source: Pittsburgh Post-Gazette, April 2018

Every hilly city has retaining walls. Pittsburgh has retaining walls the way coastal cities have seawalls: as working infrastructure standing between private property and a natural process that never stops. This guide explains the process, because once you understand what the ground here is doing, everything else about local walls, permits, and repair urgency makes sense.

The short answer, before the detail: the rock and the soil set the stage, but water is the trigger, and water is the one part of the equation a homeowner can actually change. Keep that order in mind. It is why drainage, not rock type, is where a good wall project starts.

The rock: Pittsburgh red beds

The bedrock under most of the metro belongs to the Conemaugh Group, layered sedimentary rock laid down about 300 million years ago. The troublemaker within it is a set of layers geologists call the Pittsburgh red beds: reddish claystones and shales that are, in engineering terms, highly plastic. They behave like rock while dry and like stiff clay when soaked, losing a large share of their strength exactly when the hillside above them is heaviest with water. The US Geological Survey documented the pattern in detail in Professional Paper 1229, Landslides in the Greater Pittsburgh Region, which remains the reference work on why these slopes fail.

The red beds do not need to be visible in your yard to matter. They form weak layers within the hillsides themselves, planes that overlying material can slide along once enough water reaches them.

The blanket: colluvium

Sitting on top of that bedrock, on nearly every slope in the region, is colluvium: a loose blanket of soil and rock fragments that gravity has been dragging downhill since the last ice age. Some of it is old landslide debris that never left. Colluvium is weak, drains poorly, and on many slopes is still creeping a little every wet season. The Pennsylvania DCNR notes that southwestern Pennsylvania has by far the highest concentration of landslides in the state, and most of them are exactly this: colluvium moving on wet red-bed slopes, sometimes slowly, sometimes all at once.

This is the material most residential retaining walls in the metro are actually holding back. Not engineered fill. Not stable cut rock. A creeping blanket of old slide debris with a slippery claystone floor underneath it.

The trigger: water

Dry colluvium mostly stays put. The failures come with water: saturated ground is heavier, pore pressure pushes the grains apart, and the claystone below gets slick. That is why local landslides and wall failures cluster in late winter and spring, after snowmelt and long rains, and why the region's 30 to 45 freeze-thaw cycles a year keep working walls and slopes open so more water can get in. For a retaining wall, drainage is not a detail. It is the difference between holding soil and holding a slow-motion mudflow.

The thresholds are measurable, not just descriptive. A US Geological Survey study of the February to April 2018 storms found that deep-seated ground movement occurred mostly once groundwater in a bedrock observation well rose to within about 1.7 meters of the surface, and that shallow colluvium began to slide with far less rain once the soil passed roughly 32 percent water content (Ashland, 2021, in the journal Landslides). The rock decides which slopes are vulnerable. The water decides when they move, which is exactly why managing it is the part that pays.

2018: the year the budget broke

The reference year for what this geology costs is 2018. After record rainfall, slides came down across the city: streets closed, houses condemned, hillsides moving in neighborhoods that had been quiet for decades. On Mount Washington, a home on Greenleaf Street in Duquesne Heights was destroyed as, in the city's own words, earth poured over a retaining wall, and across Allegheny County 131 properties were reported damaged. The City of Pittsburgh, which budgets roughly $1M to $2.25M for landslide response in a normal year, faced a projected remediation bill of roughly $12 million, and reporting by PublicSource has tracked the problem, and the region's exposure to wetter years, since.

The public numbers only count public ground. The same rains loaded thousands of private hillsides and the residential walls that hold them, which is where most of the damage lands quietly: a wall that leaned a little more, a slope with a new crack above it, a backyard that did not quite come through the spring level.

A century of moving ground

2018 was severe, not new. The region has been sliding for as long as it has been built on. A 1920 slide at Bigelow Boulevard moved more than 200,000 cubic yards of ground and did about $800,000 in damage, and a 1941 rock slide at the Brilliant Cut displaced railroad track and derailed a train (R.J. Hamel, Four Famous Historic Landslides of the Pittsburgh Area). The University of Pittsburgh's IRISE consortium has since inventoried more than 7,400 landslides across southwestern Pennsylvania. The wall holding your street or your backyard is part of that same long record, which is the honest reason to treat movement early rather than wait.

What this means for your retaining wall

Walls here carry real loads. A backyard wall in this metro is often doing slope-stabilization work that would be an engineering project anywhere. Pennsylvania's building code recognizes this: any wall over 24 inches tall supporting a slope, driveway, or structure requires an engineered design under IRC R404.4, sized against water uplift with a 1.5 safety factor against sliding and overturning. The permit guide covers the details by municipality.
Drainage failures become structural failures. Most local wall failures start as water problems. The drainage page explains the common setups and fixes.
Movement is progressive. A lean or bulge means the resisting forces are already losing. The warning signs guide covers what to watch, and the failing wall page covers what a contractor assesses.
Slopes can move without a wall at all. If ground is creeping where no wall exists, that is a hillside stabilization question, and it is better asked early.

Check your own ground

Two public map tools show how the mapped hazard relates to your parcel:

The Allegheny County Landslide Portal, built by the county's Landslide Task Force, maps landslide susceptibility and historic slides county-wide.
The City of Pittsburgh's Landslide-Prone Areas layer (via the Western Pennsylvania Regional Data Center) shows the areas where the city's Landslide-Prone Overlay applies.

Both tools are informational, not a verdict on any specific property. Ground truth comes from looking at the actual slope, which is what a free assessment is for.

Typical wall section, colluvium over red beds AWW-01

Illustrative only, not for construction