Size – A Matter of Perspective

Is that stick really holding it up??

That’s one big BOULDER!  To find more pictures like this one, try a quick Internet search for images matching “large glacial erratic”.  You’ll pull up more amazing pictures of huge rocks resting in strange places.  Here are a couple more examples:

Madison Boulder in New Hampshire: 23 ft high and measuring 83 ft long and 37 ft wide, estimated to weigh 5,000 tons.

Or the massive quartzite 16,500 ton Okotoks boulders in Alberta, purportedly the large erratic in the world.

Glacial erratics are pieces of rock (usually boulders – larger than 12″ in diameter per USCS) that have been transported and deposited by glacial action and are dissimilar to the soil and rock around them.  According to Wikipedia, the term erratic comes from the Latin for “to wander, roam, or ramble.”  These rocks have wandered and roamed via glaciers to their new homes.

The size of these monsters illustrates one of the amazing things about glacial soils: the HUGE range in particle sizes that ice can transported.  Erratics can be found resting on or in a matrix of gravel, sand, and even clay particles, which are many, many times smaller. Madison Boulder, for example, has an equivalent diameter of about 51 ft.  Compare that with a common clay particle dimension of 0.002 mm.  The boulder is almost 10 million times bigger!!!  Yet both are the result of glaciation.

But are the glacial erratics the biggest in every way?  Consider the specific surface area of the boulders.  Assuming Madison Boulder is a rectangular prism, it’s SSA is a puny 2.4×10-7 m2/g.  For comparison, the clay particles in our class example problem have a SSA of 37 m2/g – almost 200 billion times larger than Madison Boulder!!  Putting it another way, only 34 g (just over an ounce) of our clay are required to have the same surface area as the boulder.

So the way we look at particle size is in many ways a matter of perspective.  Whether something is big or small depends on what property we’re measuring.  Whatever the case, the astounding range of particle sizes encountered in soils is one of the reasons their properties, such as strength, compressibility, and permeability, vary so much.

Ignoring Geology – A Slippery Slope

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.