entry May 2010,
revised Oct. 2023
A brief history of
cave-ins, sink-holes & siphons in Naples
—"Did the earth move for you, too, my darling?"
—"It sure did, and I was in the other room!"
The black & white photos on
this page are from the 1960s. I've seen them before and
have even stood personally at a few similar scenes. They
are from a book entitled The Subsoil of Naples,
specifically the chapter on movement in the subsoil that
leads to such episodes.
Actually, the
history of serious collapses is not that ancient. The
Greeks and Romans built old Neapolis
on pretty solid ground, down almost at sea-level and on
ground that was just a few feet above solid tuff rock.
They also dug out a vast array of underground
caverns outside Naples for stone blocks for city
walls and buildings. Note that they dug outside the city and
on the slopes. They did use some of the sites for burial,
but they did not put their dwellings near them, on top of
them, or below them because the real peril in those
caverns is not that they are unstable, but rather that
they collect water. In the case of Greco-Roman Naples,
when water collected and gushed out and ran downhill
towards the city, there was a convenient river running
east-west below the north city walls to take the water.
I think I
have always connected the frequent episodes of
large-scale, disastrous movement in the subsoil with those
caverns; that is, that somehow all the holes in the hills
disrupted the stability of the region. As it turns out,
water is the real culprit —or, better, our inability to
handle the water.
First, we may
speak of small movement in the subsoil, the kind
that may put cracks in the walls of your home. The old
city certainly had some small movement, mostly caused by
erosion in the few feet of loose terrain between their
homes and the bedrock beneath. That may have been caused
by rain run-off or even leaks in aqueducts, but it was
nothing they couldn't handle. Even much later, when the
Spanish started building dwellings in the old city with
material dug from beneath(!)
the very houses they were building, it was not
particularly perilous because it was solid rock. (Most
buildings that have collapsed in that old part of the city
are 400 years old, and that certainly enters into why some
of them have structural problems.)
(The following paragraph was
revised on Oct.22, 2023.)
The kind of damage seen in these
photos, however —full-scale collapses of buildings and
retaining structures and the opening of sink-holes in
streets— is caused by erosion; that is, the action of
water flowing downhill (geo-geeks say "with a high
hydraulic gradient") through soil and taking much of that
soil with it and leaving whatever is above to fall in. It
takes place in the presence of looser terrain, but since
the old city was built on flat, solid rock, there was none
of that going on. That didn't happen until Neapolitans
starting building on the slopes of hills, where there was
often a considerable amount of loose terrain on top of the
underlying tuff. So, you expand onto the slopes,
overbuild, put in a new aqueduct and sewer system
(1880-1915) and then run hitherto unheard of amounts of
sorely needed water beneath all those new homes and hope
that none of it leaks. Boy, are you an optimist. The water
can come from various sources: rainfall and leaking water
pipes, for example. Even a small leak can grow to be
devastating over time. Once water starts to flow through
soil, it's no contest. Erosion always wins; soil
always loses. Cave-ins and landslides always occur.
Stop-gaps, such as filling in the holes with concrete, are
plausible but
very difficult in a densely populated urban area.
Historically speaking, even the
Spanish in the late 1500s forbade digging into the slopes
of hills above the city for construction material. In
order to get to the rock, you had to loosen a lot of
overlying loose terrain that could then wash down onto the
buildings lower down, but, again, there was nothing
particularly unstable about digging holes in solid rock.
In 1781, Ferdinand IV issued an edict to
try to put a stop to the chaotic building practices of the
day. It declared that the cause of so many cases of
collapse of buildings was the practice of digging
"underground caves and grottoes" for the purpose of
extracting stone for building material. He made the same
mistake I did. If there were collapsed buildings in the
middle of town in the 1781, it was because they were very
old and/or not constructed properly. It was not because
the weight of the house was trying to rest on empty space
beneath it. Of course, by that time, the city had already
built itself up the slopes to a considerable degree.
Ferdinand even had a residence all the way up on top of
the Vomero hill and an additional palace on top of the
Capodimonte hill.
In the post-Risanamento
euphoria (1915-onward) of having a new aqueduct and
sewerage system, it isn't clear that the new builders
understood the downside of what they had just built.
Studies done in the 1930s through the 1960s examined a
list of possible causes for the collapses that were
starting to occur. There were peripheral factors, such as
age of buildings, improper construction, increased
vehicular traffic, even earthquakes
and the after-effects of WWII bombing. The fact
remains, however, that virtually all of the episodes of
large subsoil movement and resulting damages have occurred
in the last 100 years (and most of it since 1960) in the
same areas —i.e., the slopes of the hemisphere of hills
around Naples, and the cause has been water leaking into
the subsoil from the aqueduct and sewerage system.
It
then becomes a very laborious and necessary process of
finding leaks and patching them. It is too late to stop
the wild overbuilding on the hills in Naples; that chapter
is, unfortunately, written. BUT you have to insure that
all new construction follows a solid building code that
keeps tabs on foundations, retaining walls, terraced
slopes, pressure in the water and sewer mains (especially that!),
even such things as whether landfill has been sufficiently
packed down. I learned a new expression in Italian while
researching this: alla
rinfusa. It means higgeldy-piggeldy or helter-skelter. We
can't have that.
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