September 2006:  500,000 – 600,000 cubic yards of soil and rock slid off a slope in Killbuck Twp. PA and covered PA Route 65 west of Pittsburgh as well as a busy railroad line.  The debris took days to remove from the road and tracks, inconveniencing thousands of people and costing millions of dollars.

The slide originated on an active construction site, which was being developed for a large retail store.  After the road was cleared, my old firm got involved briefly to monitor instrumentation that was installed to measure ongoing slope instability.  Retail development of the site was cancelled and slope stabilization was completed in 2011.

So, what happened??

Simply put, someone forgot to think about geology.  The Pittsburgh red beds are a geologic formation in Western PA well-known to cause landslide hazards.  To quote the state Advisory Committee report on the landslide,

Western Pennsylvania is particularly susceptible to landslides because of two natural geologic characteristics: (1) the bedrock land composition, consisting mainly of incompetent mud rocks such as silty shales and clay stones that weather easily;

As with any failure, blame lies with many parties.  However, had the civil engineers planning the development and their geotechnical consultants remembered their local geology, they would have been on the lookout for these conditions and the landslide would likely never have occurred.