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From the hindsight of
2019: This last chapter of Part Two is the least
technical of the nine chapters. Readers
don't have to be geologists or urbanologists
or put up with statistics and equations.
It's a straightforward explanation of what
can happen to streets and buildings of a
city if the right persons don't pay
attention to what is going on beneath the
city. Essentially, there are a few things
that can go wrong, and if they all go wrong
at the same time, you've got problems.
This is Part Two, Chapter 9, the last chapter in
part two of
The Subsoil of Naples
Chapter 9
Causes of Movement
by the member experts of the entire
Commission
named on 29 September 1967
Movement
of varying degrees
in and on the subsoil —from
simple cracks in buildings to full-scale
collapses of buildings and retaining
structures and the opening of sink-holes in
the streets— are unfortunately not a new
phenomenon for the city of Naples. They have
increased over time and affected most areas of
the city, becoming a plague that afflicts the
city and a cause for justified concern and
alarm on the part of the public. We have to interpret the
causes correctly. We have to examine the evolution of soil
movement over the years and see if and how it
is
different from one zone of the city to
another.
 We also have to determine if
and how the environmental situations that might cause
such phenomena have changed with time as we move from
area of the city to another. The Italian term for
such movement is dissesti (plural). By that, we mean
phenomena in the subsoil that affects structures of
all kinds, from buildings to streets, sidewalks,
underground maintenance structures, retaining
walls and man-made structures of all kinds.
[translator's note: I am using "movement" or
"movements" for dissesti. The
Italian word , however, means an abnormal
and sudden shifting, quite different from
the term movimento or movementi
(plural), the general terms for all
movement.]
image, above: Retaining
wall collapse and street cave-in on via
Tasso (June 1966)
Historically,
there has been no lack of studies of soil movement and
attempts to deal with the problem. The
building edict issued by Ferdinand IV of Bourbon
on 3 October 1781 was the first such
provision of a general nature to try to put a stop to the chaotic
building practices of the day, much of which
actually
went back to the ill-considered practices of
the Spanish viceroys in the 16th and 17th
centuries. The edict had 10 articles; number 4
is
particularly interesting. It said that the
cause of so many cases of collapse in
buildings was the practice of digging
"underground caves and grottoes" to extract stone for
building material; the edict forbade henceforth any
such digging on the premises of buildings or outside on the
public streets.
 The edict also restated the
prohibition of 30 May 1588 and 9 October 1615 against any
further excavations on the slopes of the
Vomero hill near Montesanto and above the
Spanish Quarters all the way up to the San Martino
monastery. Other important articles in the Bourbon edict dealt
with the maintenance of buildings in dangerous condition, the
disrupting long-term presence of scaffolding
used to
prop up such buildings (even back then, they
were everywhere in the city); the need for
quick repair and restoration, the need to
check the
professional competence of engineers and
others working in that field, and the need for
quality control of materials used in
construction, particularly the materials used in
the foundations of buildings. The edict of 1781,
then, was a clear testimony to the disordered
state of
the construction industry as well as an
expression of concern over the stability of
buildings in the city.
That edict was
followed by other provisions issued by the
Building
Council of Naples in the first half of the 1800s
in an attempt to get some control of the excavation of
tuff and lapillo (smaller, round
fragments of hardened lava) as well as to map both the surface and
underground areas of the city. More recently, other commissions have
studied the subsoil (see bibliography) and have come to a number
of conclusions that we summarize below:
images:
above
left, Wall
instability and resulting collapse of
roadway at via Catullo (March 1966)
below right, Street cave-in at Corso
Meridional (Oct. 1956)
 1) 1892. A commission
was formed in the Ministry of the Interior to
deal
with the safety of streets and buildings.
Their task was to explore thoroughly and survey the
underground and all of the old cavities dug to get tuff and
lapillo, and, indeed, all other
cavities; further, to repair the public and private sewers
(especially in the sections of S. Ferdinando, Montecalvario,
Avvocata, S. Giuseppe, S. Leonardo, and
Vicaria) such as to prevent infiltration [of
waste] into the subsoil; further, to prohibit
private
parties from dumping materials into cavities,
wells or cisterns, and impose checks and
repairs of wells and other spaces beneath buildings;
further, to oversee the internal
canalization of the waters of the Serino (river);
and to have the Municipal Technical Office
inspect buildings in hazardous condition and file the
results for future reference.
2) 1934. A commission
was appointed by the Engineers Union of
the Province of Naples after a series of collapses
in 1933-34: Their task: survey structures in
the subsoil as well as the basins and channels
of the Bolla and Carmignano aqueducts; regulate
public services that have to do with the subsoil,
particularly regard the sewerage network; regulate the
secondary services; regulate tramways to ensure proper
grounding of electrical current and reduce vibrations in the
subsoil and excessive traffic noise; insure
isolation
of electrical conductors in the subsoil;
document with building permits the assays
conducted; this will help establish stability
and detail how given conditions relate to the construction of
new buildings, especially the foundations;
make a
general inventory of retaining walls, of
filled areas and dwellings and require owners to
show a certificate of stability from an
engineer;
establish provisions for high tuff walls that
border on public and private roads; adjust rain
run-off collectors and effectively maintain the sides
and slopes of covered river-beds; institute in the Fire
Department a construction section under the auspices of
engineers from the City Technical Office to
deal with disasters.
3)
1956. A municipal commission was
formed to pinpoint the causes of the frequent episodes of
static movement of buildings. Because of the limited time at
their disposal (results were delivered after
less than
one month of work) the commission's conclusion
were somewhat summarized, attributing the hazards
to bombardments [from WWII] and declaring the
urban center of the city to be in the greatest
danger,
conditions that could only be corrected, in
their opinion, by demolition and reconstruction.
In particular, the commission said that
disturbed
conditions in the subsoil caused by the
bombing could make it particularly
dangerous for residents the following areas:
a) a 60-meter-wide strip along the axis
of via Formale, via S. Matteo, via Trinità degli
Spagnoli, following almost a contour line to
the
ex-Gradoni [steps] di Chiaia. That strip is
over the path of an ancient Greco-Roman
aqueduct;
b) zones
of lesser interest: halfway up the Stella
rise, via Nuova Capodimonte, and via Cagnazzi;
c)
potential hazardous areas: vico S. Caterina a Formiello, the
street and square of S. Giovanni a Carbonara;
d) the decumani [east-west
streets] of Greek Naples: S. Biagio dei
Librai, via Tribunali, and via Anticaglia;
The next two images on the
right, 142 and 143, respectively, are both
of the collapse of the
retaining wall and resulting collapse of
the roadway on via Catullo (March 1966)
image
142
 Furthermore, the commission
acknowledged that the main cause of the
disturbances to the stability of building was
water in the subsoil made worse by preexisting
empty spaces. They proposed investigating the
flow of such water, taking a census of all the
empty spaces in the subsoil, putting a
sewerage code into effect and collecting
underground waters even at some distance away
in order to prevent infiltration into the
subsoil of the urban center. Two studies
remain to be mentioned: those done by the
newspaper Il Mattino,
specifically:
4) The Report of 1931-32
came after numerous collapses and cave-ins.
The task: collect data to establish the
location and distribution of all such spaces
in the subsoil; keep aqueducts and sewers
under observation; give proper weight to the
importance of road conditions, which are often
precursors of collapses due to flooding of
terrain or to other underground movement in
the soil.
5) The Report of 1961-62 called
for: collecting data on underground cavities;
mapping the subsoil; inspecting all hazardous
buildings; checking the aqueducts and sewers
to verify their efficiency; regulating subsoil
public services; issuing building licenses
only after documentation of studies done in
the foundation terrain.
From that, we see that some causes of
movements in the subsoil are agreed upon and
others are only occasionally cited. It was
generally agreed on that there are
furthercauses, at least as to particulars. In
picking up the threads of this discussion
here, we have used statistical criteria that
lend themselves to distinguishing regular
episodes from occasional ones.
To that end, we have considered the
statistical investigations done: by the
Municipal Administration for 1889-92;
by R. De Stefano, R. Allocca, and F.
Tagliartela for 1951-1961; and by a
working group under the auspices of the
Italian Geotechnical Association, which
concerned itself with the problems of the
subsoil of Naples for a presentation at the
VIII National Geotechnical Congress. This
working group conducted two separate
investigations, one of which was done using
archival news reports of Il Mattino
for the periods 1930-39 and 1951-61;
the other investigation was on the basis of
material in possession of A.M.A.N. (Azienda
Municipalizzata Acquedotto di Napoli)
[Municipal Aqueduct Authority of Naples] for
the period 1918-65.
Unfortunately, the criteria used in these
studies were not the same; they also used
different sources; thus, the results mean
different things. Particularly, we note that
the first two studies were done, respectively,
(1) on the basis of information provided by
the builders, themselves, and (2) on the basis
of reports in the possession of the Municipal
Technical Office for Public Safety. The latter
take into account even the slightest soil
movement and are useful for showing how the
conditions of buildings change as you move
from one part of the city to another. The
other two investigations were done,
respectively, (1) using archival material from
Il Mattino, and (2) using documentation
from A.M.A.N. Those two studies limit
themselves to episodes of movement that caught
the attention of public opinion or caused
economic damage of a certain importance and,
thus, give some indication that conditions in
the subsoil may actually affect the stability
of buildings on the surface.
We can call the latter grandi dissesti
[large movements] and the former piccoli
dissesti [small movements]. The first
two studies include both but emphasize small
movements; the other two focus on large
movements. If we compare statistics from all
four of the studies with some data from
earlier periods, we can draw some conclusions
about the main causes of movements in the
subsoil and how that has evolved.
image
142
 A first conclusion is how both
kinds of movement have been influenced by
construction at the end of the 1800s of the
Serino Aqueduct and the new city sewerage
system. Comparing relevant statistics from
1889-93 and reports of soil movements before
that time, we conclude that the construction
of the new aqueduct and sewerage system
—although they did cause episodes of small
movements— were not the primary cause since
such episodes had been reported all around the
perimeter of the city well before that period.
The same construction, however, had a greater
influence on the incidence of large movements,
which, although not totally lacking in the
past, must have been much less frequent since
there are only rare reports of them.
Another conclusion from a comparison of the
same statistics from 1889-93 and the years
following show the influence of the Risanamento
[urban renewal, 1885-1915] on episodes of
small soil movement; movements were more
frequent when all the construction was going
on and diminished afterwards. Proof of that is
the fact that in the oldest surveys the
Vicaria section was second only to
Montecalvario in frequency of episodes of
small movement; later surveys show it to be
almost in last place.
There is no doubt that many of the episodes of
small movements in the area of the Risanamento
were due to the great age of some of the
buildings and the type of structures they
were. Continuing to compare the evolution of
small soil movements over time, we note that
such episodes are not reported in the data of
the 1934 Commission
of the Engineers Union of Naples, which blamed
the movements on the disordered and chaotic
secondary services in the subsoil. Nor are
they reported by the Municipal Commission
of 1956, which put most blame on damage
from bombardments in the last war. In
comparing the statistics from 1889-93
with the most recent ones, it does not seem
that episodes of small movement in the subsoil
have increased from then to now.
Also, in comparing pre-WW2 statistics from 1930-39
with those from 1951-61, we have not
found anything that would let us say that
wartime bombings had any notable effect on
episodes of subsequent soil movements, large
or small, in the city. Incidentally, we note
that the statistics compare 1930-39
with 1951-61 and seem to have excluded
willfully the immediate
post-war period. Similarly, we conclude that
just as with wartime bombings, the 1930
earthquake did nothing to alter the frequency
of such movements except perhaps for a short
period of time. In the same fashion, at least
as far as large movement goes, we maintain
that the rapid increase in heavy traffic has
not been a determining cause.
Considering the evolution over time of the
phenomenon, we can say that of all the things
that have changed the environment over the
last 80 years and might be listed as causes of
the movement of houses, streets and secondary
services, the most important one was the
construction of the water distribution system
and the sewerage system. Other causes have
been secondary. That main cause has been
responsible for the increase in small soil
movements and, importantly, the cause of large
ones.
image
135: Street cave-in on via Posillipo
(Nov. 1957)
 Considering now how the
phenomenon of soil movement has spread in the
city, we can compare data gathered at
different times and from different zones and
look at the severity, case by case. The zones
of most recent expansion are in the west. The
above-mentioned statistics do not consider
those areas, and we shall consider them
separately, below. More centrally, however,
the zones in which there has been a greater
frequency of movement, both large and small,
are always the same: the quarters of Stella,
Avvocata, Montecalvario and S. Ferdinando,
which follow one another more or less in a
continuous strip along the slopes of the
Capodimonte and Vomero hills.
The reasons for movement, large and small, are
thus to be looked for either in features that
might make those zones different from others
in the city or in the fact that only in those
areas do certain circumstances conspire to
work together. Having said that, we have to
look at the environmental factors in those
areas. The areas are, as noted, along the
great tuffaceous mass that is the framework of
the hills. Loose materials cover that mass in
varying degrees of thickness. Surface terrains
and tuff substrata are notably sloped; the
underground aquifer is deep; the many cavities
dug by man for one reason or another break the
continuity of both the materials on top and
the mass below; the buildings are mostly old
and the streets have old paved surfaces of
basalt. None of those factors, however,
individually, can explain the phenomenon of
movement in the areas we are talking about.
The mechanical characteristics of the terrains
—not just of the lapideous tuff formations,
but also of the loose materials on top— are in
themselves favorable and, in any event, no
worse than those of subsoils in other large
cities. We might, however, add that the tuff
mass does have occasional and extended
fractures, and there are cuts in some steep
walls that can, in fact, detach dangerously
and unpredictably. In turn, the loose
materials on top are sensitive to water; that
is, they settle notably from simple imbibition
(or absorption) of water, which can cause
erosion and, in the presence of a notable
hydraulic gradient [a steep slope], set up a
true siphoning effect.
On the other hand, with the methods of
calculation now at our disposal in terrain
mechanics, we see that the loads transmitted
to the terrains by foundations of buildings
—foundations of any sort— are totally
compatible with the mechanical characteristics
of those terrains except where there are
anomalies of imbibition or underground
erosion. Indeed, experience has shown us that
in most cases foundations do not give way
because of the earth beneath them
but rather from defects or material
fatigue in the foundations, themselves
[emphasis added].
The areas we are talking about have the
greatest number of cavities in the tuffaceous
banks (see chapter IV of part 2), but even
that is not in itself a danger from a purely
static point of view. Parts of walls or vaults
may occasionally detach, yes, and that does
reduce the stability of the space. That has
happened only in very exceptional cases
because of a load transmitted from the
surface. Finally, the age of the buildings may
be a contributing factor in the cases of small
movement in the buildings, themselves, but
that alone doesn't account for the increasing
frequency of both small and large movement in
these areas. There are equally old buildings
in the ancient parts of Naples, where the
phenomenon of movement is much less
noticeable. It can thus only be the cumulative
effect of all of the above factors in these
areas that combine to produce this plague of
subsoil movement, collapses and cave-ins.
So on the one hand we have the aquifer at
great depth and, on the other, the
considerable slope of the terrain and the
widespread presence of various forms of
cavities. Together, that allows water to
infiltrate into terrain above the tuff,
terrain that initially will not saturate
because of its great thickness but which may
eventually move because of imbibition in the
terrain and, with notable hydraulic gradients,
can cause serious internal erosion.
We thus have in the terrain continuous
episodes of small movement that, at least for
the most part, are to blame for the
progressive deterioration of older buildings
and infrastructure. If, however, there is a
break in a line from the aqueduct or in a
sewer, or if a sewer line is overloaded, that
increases the amount of water seeping into the
terrain and gives rise to a real siphoning
effect and some of the phenomena described
thus far. That may cause collapses and
cave-ins. In any event, it compromises the
stability of structures on the surface. In
most cases, then, either the infiltration of
water into the subsoil is the primary cause of
cave-ins and collapses, or such mishaps
represent the last in a series of prior
episodes that may not have been immediately
evident on the surface but which created the
conditions for the subsoil to move.
image
144:
street cave-in on via Cortese
all'Arenella
(Sept.
1967)
 We see an easy and convincing
interpretation of such phenomena. In practice,
however, their evolution is quite complicated
as seen from the endless legal battles caused
by these episodes of subsoil movement, battles
that often revolve around trying to
reconstruct after the fact exactly what
happened. It is difficult to say that the
first bit of water that seeped into the soil,
or the first incident of soil settling, was
the result of a break in a pipe from the
aqueduct or in a public or private sewer.
Perhaps those components were initially sound
and never would have leaked at all unless set
upon by water from another source. It is
difficult to say whether the initial cause of
siphoning in the subsoil is a line from the
aqueduct or sewer or even a drain-pipe running
alongside somewhere. Except in exceptional
cases, once siphoning has occurred, the
aqueduct lines, sewer lines and secondary
pipes will all have been so badly damaged or
even destroyed that reconstructing the event
is impossible.
At the most, we can say that small leaks in
the aqueduct or sewers can lead to small
movement; large movement, on the other hand,
will occur in cases where breaks in the
aqueduct or in sewers are not swiftly repaired
or where a run-off sewer overflows and large
amounts of water are allowed to flow through
steep gradients, letting the water actually
carry material away. It is obviously
impossible to keep any water at all from
getting into the subsoil. It is equally
impossible to get rid of the cavities, large
and small, that exist in the subsoil.
Consequently, in order to keep water from
infiltrating from the surface and from moving
through cavities with notable hydraulic
gradients and cause erosion and siphoning. We
can only put the strata in order that are
closest to the surface and create an effective
drainage system between street level and the
sewers such as to intercept and move the water
into the sewers.
Aside from the practical possibility of such a
project, it is evident, without singling out a
particular cause (as was the case with the
1934 commission of the Engineers Union) that
there is extreme disorder beneath the streets.
Various services are at work down there; they
work independently of one another and often
get in each other's way. This, too, is a cause
of movement in the subsoil. Similar phenomena
have occurred elsewhere in the city. Not all
of the contributing causes, however, are
present in those areas the way they are in the
areas along the hill slopes. Those other areas
thus have fewer episodes of movement;
importantly, they have fewer episodes of large
movement.
In those areas where the aquifer is just a few
meters from the surface (S. Giovanni a
Teduccio, Barra, Ponticelli, Poggioreale, S.
Anna alle Paludi, Borgo Loreto, Vasto, Via
Marina and the Rettifilo) there are no
underground cavities. There is thus no
possibility that water seeping into the
subsoil can be absorbed by thick strata of
initially unsaturated terrain, so movement
there is limited and less serious if it does
occur. Damaged buildings can generally be
restored except in the presence of structural
deficiencies such as foundations not resting
on a solid base, or insufficient load-bearing
sections in walls, or material fatigue, etc.
Static* equilibrium can be
reestablished by normal work beneath the
foundation and wall consolidation. [*translator's note: in the
mechanical sense "static" meas acting
through weight only, said of the pressure
exerted by a motionless body or mass. Static is the
opposite of dynamic. Thus, if a
solid wall is resting against or on solid
ground but threatening to topple over,
"restablishing static equilibrium" means you
can "keep it from falling over."]
In that regard, we note that the buildings
with traditional ordinary cement foundations
(Vesuvian scoria, pozzolana del
Campo, hydraulic lime) did not move at all
during the risanamento [urban renewal
1885-1915] along the Rettifilo-via Marina
strip. A few of the foundations in the Vasto
quarter were insufficiently thick or did not
rest on a sold base; they had to be rebuilt,
after 50 years, but still with traditional
methods of restoration.
Finally we note that filled-in river beds
(Vergini, Arenaccia, Lavinaio, Toledo, etc.),
except in some rare and well-defined cases,
have not caused notable mishaps.There have
been serious episodes of soil movement even in
some areas where the aquifer is near the
surface. That happened along the route of the
Metropolitan tunnel between Piazza Cavour and
Piazza Garibaldi as, during construction, the
aquifer dropped artificially, which tallies
with what we said earlier.
Recent urban expansion deserves separate
comment, particularly in those areas on the
slopes of the Vomero and Posillipo hills. The
morphological characteristics and the nature
of the terrain are similar to those described
for Stella, Avvocata, Montecalvario, and S.
Ferdinando. The unbridled race to put up new
building as well as new options offered by
recent methods of construction, however, have
led to a terrain architecture that employs
very high terraces and embankments even in the
older sections. Furthermore, private
initiative, acting ahead of public planning,
has prevented the orderly layout of streets
and sewer lines. The sad result of this has
been the rapid deterioration of road surfaces,
the equally rapid deterioration of public and
private sewers, the increasing frequency of
breaks in water lines, and a series of
collapsed streets, caved-in sustaining walls,
and earth slides.
Some of these things, such as detached
fragments of tufo or slides along banks of
pozzolana can be attributed to particular
geological conditions and are not easy to
predict. Other mishaps, such as cracks in and
collapses of old retaining walls halfway up
the slopes have been caused by the increase in
the amount of traffic and also by the nature
of heavier vehicles. Other incidents, just as
they have occurred in older quarters of the
city, have happened here because of the
infiltration of water, made all the more
frequent because new sewers are too small or
old sewers can't handle the increased load. As
a result, they overflow even when rain is not
intense. In most cases, however, ruptured
streets or collapsed walls are man-made and
are traceable to bad
planning or a lack of respect of
ordinary construction norms.
Indeed, except in cases where streets rest on
landfill thrown in helter-skelter, little
effort is made to ensure that even the strata
of properly mined pozzolana beneath roads have
been properly packed down. As a consequence,
in spite of the use of abundant terrain with
good mechanical properties, with the passage
of time and under stress, streets break up and
rupture their otherwise impermeable surfaces.
The phenomenon accelerates from breaks in the
impermeable surface to localized cracks caused
by passing traffic, such that the very sewers
meant to channel water properly away from the
surfaces become damaged themselves and contribute
to the problem of infiltration. The seeping
water looks for a way out, finds it, and moves
towards it, thus increasing erosion and
movement in the subsoil. In some cases, the
street, itself, winds up suspended over a void
and survives only because it has found some
way to stay in place as a kind of "bridge."
All it then takes sometimes is for single
vehicle to pass over it, and it
can collapse. Sometimes the earth slide
is the last in a series of incidents leading
up to true siphoning.
image
145
street
cave-in, via A. Falcone
(Sept.
1967)
 It is quite different if the
water seeping in doesn't find a way out. It
will "imbibe" —be absorbed— into a much
wider area. Where that occurs, the soil
characteristics, themselves, are altered, and
hydrostatic forces arise that were not
foreseen during the original planning. [note: A
standing body of water pushing against the
wall of a damn is an example of
hydrostatic force.] That can cause
retaining walls to collapse. In that kind of
collapse, it is more commonly the case that
even if the filler material in back of the
wall was properly laid, the wall itself was
not sound and might have collapsed anyway. In
most cases, we find that such walls were of
the "build-as-you-go" variety or built to
antiquated standards. Even in modern
construction we haven't found many cases where
a serious investigation was done beforehand,
whether in calculating values for terrain
characteristics or for setting values for
safety standards supposedly based on those
values. Human guilt is increased if we
consider that many of the episodes that have
occurred were preceded by warning signs that
no one paid attention to.
Before concluding, we should consider a
particular kind of mishap —the ones that have
happened in tunnels. A number of tunnels built
recently in Neapolitan yellow tuff, either
during construction or shortly after being
completed, displayed notable signs of
movement, which is why they had to be
reinforced. For example, we note the tunnel
beneath the Posillipo hill, [now] called the Quattro
Giornate Tunnel (formerly the Municipal
Tramway Tunnel). It was opened in 1884 and
closed in 1890 because of cave-ins; also, the
Diretissima Naples-Rome [train]
tunnel, tested in 1917, had problems and was
closed in 1922, was under repairs until 1925
and had more problems in 1931; the Laziale
tunnel, finished in 1926, after numerous
incidents of movement during construction
showed additional damage in 1932. All of these
cases displayed longitudinal lesions along the
crown or crushed support columns. Similar
mishaps have occurred recently in the
Circumflegrean [train] tunnel that passes
through the Vomero hill; it was finished in
1954 and is currently under repair.
Tunnel collapses and earth slides are not
always that easy to explain. On the one hand,
we note that theories of homogeneousness and
isotropy* are
at the basis of calculations to build
tunnels, but those theories are not
always verifiable when working with tuff. That
is because of the particular origins of the
material. An average bank of tuff can often
display notable point to point variations in
mechanical characteristics without any change
at all in outward appearance. Even during
excavation, it is practically impossible to
notice the variations. That is not even
considering that fissures and cracks can open
in the material during excavation but stay
hidden beneath the excavated surface, which
can alter considerably the load-bearing
characteristics of the material.
*[homegeneous:
having the same or similar characteristics
or qualities.
isotropy: when plasticity or
conductivity are the same no matter where
you measure.]
But even if the theories of homogeneousness
and isotropy are totally verified, analysis of
stress in the tuff masses in each of the
tunnels just mentioned has shown that the load
on the columns and crown was greater,
respectively, than the resistance to
compression and traction of the tuff. In the
absence of adequate calculations of stress, we
have to look for a cause behind the mishaps
during excavations of tuffaceous rock in the
tunnels.
In that regard, we note that the internal
covering on the surfaces of the tunnels was
put in some time after the completion of the
tunnels and cannot have reduced the tension in
the rock, a phenomenon that is noticeable the
moment the digging is finished. On the other
hand, in the case of some of the tunnels
mentioned, internal covering is almost absent
(for example, the Circumflegrean tunnel was
lined with 40-cm-thick tuff blocks for the
sole purpose of
making the surfaces regular. We thus hold that
in order to avoid analogous mishaps in the
future, tension in the surrounding tuff mass
has to be maintained within permissible
limits.
In summary, both small and large movements in
the subsoil of the city can be attributed to a
variety of causes, only some of which have to
do with the make-up or structure of the
subsoil, itself. More often, causes are
man-made. First, we can say that the loose
terrains in the subsoil are particularly
sensitive to water; the terrains settle
notably through simple imbibition and are
subject to internal erosion as water moves
through them, particularly where hydraulic
gradients are steep. That characteristic [of
sensitivity to water] assumes great importance
when the aquifer is deep and there are many
cavities of various sizes either in the loose
terrains or in the tuff substratum, which is,
in fact, the case in a large part of the urban
territory.
As far as man-made factors go, from
statistical studies we see that these factors
are diverse and of varying degrees of
importance in those parts of the city that go
back to before the recent post-war wave of
urban development. We see that in the oldest
pasts of the city, there have always been
episodes of small movement, while large
movement, with some exceptions, have their
beginning with the construction of the Serino
aqueduct and the modern sewerage system. The
small mishaps have had to do with the fact
that the houses in the Naples of old were not
built to withstand the slightest settling of a
foundation caused even by a minor episode of
water seeping into the subsoil. With time, the
aging buildings had the added stress of such
modifications as add-on rooms and even entire
floors on top of the original roofs, often
with no technical criteria.
Episodes of large movement in the subsoil, on
the other hand, have had to do with
significant and concentrated emissions of
water from breaks in the aqueduct or sewerage
system. The situation is somewhat different in
the areas of recent expansion in the city,
particularly on the Vomero and Posillipo
hills. There the cases of small movement are
relatively rare, which is explained by the
fact the newer buildings are structurally
sounder and built to withstand
even appreciable movement in the subsoil.
More frequent, however, are those cases of
great movement that opens sink-holes in
streets, causes cave-ins, and collapses
retaining walls. From one viewpoint, the
causes are in the infrastructure, which has
been rapidly more and more insufficient in the
face of the speed and intensity of new
construction. As well, though, there has often
been improper planning and excavating for
embankments, terracing, and retaining walls,
which is most important here since it often
has to do with development in the hill areas.
In those areas of recent urban development,
further episodes of small and large movement
in the subsoil are, unfortunately, to be
expected. We can't reasonably expect that they
will not happen, even if the provisions that
this Commission shall recommend to the
Administration are adopted and carried out as
rapidly as possible. It is clear,
however, that if we put it off and don't start
on some of these provisions, perilous
conditions in the city can only get worse.
With that, we are saying that all plans for
urban development and construction in Naples
have to be regulated by a uniform plan for
urban safety.
END OF CHAPTER 9 END OF PART
TWO OF THE SUBSOIL OF NAPLES
Oct. 30, 2019
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