Land Movement-Sep.2000

Creep, Slumps, Flows, and Falls

If there is one thing that Mt. Lebanon has, it is hills. And where there are hills, there is mass wasting. Mass wasting is a geologic term that refers to any down-slope movement of soil and/or rock debris. Most people refer to this movement as landslide. The movement of material may be fast or slow, and may occur on very gentle slopes to vertical cliffs. There are several types of landslides that are common in western Pennsylvania.

Creep is the most common, yet slowest type of movement. The motion is nearly imperceptible, occurring at a rate of millimeters per year. The evidence though is unmistakable. Take a look around on the local hillsides for trees that have trunks that curve into the slope, and trees that tilt and overhang a cliff. Also, look for broken or bent retaining walls, tilted poles and posts, and broken water and gas lines. In extreme cases, look out for cracked foundations. One of the contributors to creep is freeze-thaw cycles in the colder months of the year.

Slumps are characterized by the down-slope movement of rock, soil, or fill material as a unit on a curved slip plane. These are commonly large, covering several hundreds to thousands of square feet. The movement is relatively fast, occurring over several hours or days. Slumps can be devastating, destroying buildings and roads. The most recent major slump in the Mt. Lebanon area occurred in 1971 when a large portion of the hillside north of Painters Run slumped into the creek. Several smaller slumps may be seen along the south hillside along Scrubgrass Road.

Flows occur when soil and/or rock debris moves down a slope like a fluid. These landslides are common after periods of continuous rainfall, when the ground is saturated with water. Many of us have experienced small earthflows or mudflows in our backyards after heavy rains.

Falls are characterized by… falling rocks, of course. After a long winter we commonly come across rocks in the road adjacent to roadside cliffs. The rocks were dislodged by a process called frost-wedging. It is also caused by freeze-thaw cycles in the colder months of the year.

Landslides are an inescapable consequence of living in a hilly terrain, however, you can reduce your risk of landslide damage to your property. If you are purchasing property or if you are already a homeowner, look for the following danger signs both on, and near your property:

Loading: Actually overloading. Here, look for houses, garages, pools, etc. built on fill (slag, soil, rock debris, or worse, construction debris) used to extend a backyard on a slope or ridgetop. When a structure is not built on bedrock, it will contribute to the load on the slope, and the slope will fail.

Cutting into a slope: We all like nice big backyards, or we need space for a new business. Cutting deep into a slope to make more flat ground is a common practice as any journey down West Liberty Ave., Banksville Rd., Cochran Rd., or Mt. Lebanon Blvd. will demonstrate. However, this can be disastrous as slopes may fail over time. Expensive retaining walls, such as by Shop ‘n Save may temporarily reduce the risk of landslide, but the potential for landslide will never go away.

Placing fill on slopes: If you must add fill to a slope, or are purchasing a property with fill, make sure that it is/was placed following proper engineering practice. The slump along Painters Run was the result of poor fill emplacement.

Altering water conditions: Adding water to a slope may also result in slope overloading. The water may be added by faulty rainwater drainage systems, over-watering your yard or garden, or leaking water lines. In addition, poorly designed retaining walls will allow water to collect behind them, leading to failure.

Our hills give Mt. Lebanon its unique character and charm. Although we can never escape the potential for mass wasting, we can reduce the risk of damage by being aware of how the mass wasting processes operate. If you have any major concerns, contact a geologic or soils engineering consultant.

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