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Part 2 - Chapter Two
A Geo-technical Map of the City of Naples
under the direction of Profs. of Engineering P. Nicotera and P. Lucini

after the Premise there are these subheadings that you may link to individually:
                                     GENERAL,
                                     EASTERN URBAN AREA,
                                     WESTERN URBAN AREA,
                                     OBSERVATIONS ON THE POSILLIPO HILL,
                                     CONCLUSIONS


PREMISE     

Collecting, examining, reworking and translating the geo-technical graphics having to do with the subsoil of Naples was commissioned by the Institute of Applied Geology of the Engineering Department. The work was divided into three parts and was coordinated by those appointed by the city (geologist, Dr. G. Vincenzi and surveyor, B. Davide, as well as others from the Institute).

The first order of business was to reorder and select from among the materials already in the possession of the Institute of Applied Geology, either material already referenced bibliographically or unedited samples and data (details of stratigraphic sections, geological surveys, stratigraphy of core samples, surveys of cavities and tunnels, studies of cave-ins, etc.). Other materials came from other groups set up by the Commission —surveys, general notes and, above all, stratigraphy and samples of underground cavities. We prepared geological charts (on a scale of 1:1,000 or 1:4,000) of some of the urban areas not particularly well-known but that we considered important in trying to reconstruct the geological make-up of the city. Also, we charted those areas known to be problematic in terms of stability and safety in the urban area.

The materials were collected, evaluated and then charted (at 1:1,000 or 1:4,000) with emphasis on the areas that are the most densely built up (urbanized) and where, obviously, studies of the subsoil take on a special importance and urgency. In any event, to convey an idea of the size of the area under consideration, the charts (at the 1:4,000 scale) of the area of immediate emphasis filled 6 pages (60cm x 80cm) of a total of 29 pages covering the entire urban area; at the 1:1,000 scale, there were 83 50cm x 70xm pages of immediate emphasis out of a total of 400 pages that covered the entire urban area.

As far as the particular problem of underground cavities is concerned, we made a list of all the cavities known to us and noted what we already knew about them. We kept that list up-to-date, adding newly discovered cavities and expanding on the information we already had about the known ones (more precise location, acquisition of better graphics, more detailed surveys, etc.). That list, updated through 30 September 1967, comprises 366 cavities and may be found in the third part of this Report. In addition to these 366, we might have added 85 others, but information about those cavities was the result of work done after 30 September. Those cavities are not in the report because we had to stop making additions after that date in order to allow ourselves time to edit the material we already had.

Further, we dedicated a single file to each cavity containing all notes, reports on samples and surveys pertaining to that particular cavity. All of the cavities (both the old ones we knew about as well as new ones discovered on behalf of those who commissioned this study) were prepared as detailed graphics (on scales of 1:100. 1:200; or 1:250) on glossy paper; topographic relief for particularly significant areas was reduced to 1:1,000 (in relation to the overlying urban topography). We included all data that might help to provide an exhaustive schematic overview and description of the cavities. Some examples of these illustrative graphics may be found in the third part of this Report.

The idea behind the preparation of these graphics was to put as much material as possible at the disposal of anyone directly interested in a particular cavity. The files were prepared on glossy paper to make it easier to make copies.

Obviously, we prepared schematic illustrations only for those cavities that we had sufficient information about; that is, ones for which we had an accurate, detailed and verifiable survey. The detailed topographic survey for those cavities may be found in the third part of this Report. Through 30 September 1967, there are 78 cavities with detailed topographic surveys covering a total area of 150,000 m2.

In the course of surveying these cavities, we found cases of more serious shifting or movement in the terrain (lesions in the crowns or abutments, columns that had given way, loosened detachments fallen from the roof, other cave-ins, etc.) as well as indications of water loss and seepage with erosion of loose ground above. Of the 78 cavities with detailed surveys, 41 had some problems of that nature. In those cases, we notified the Uffico Sicurezza del Comune [Municipal Safety Board] and provided appropriate graphic documentation. The cavities that were in the greatest need of immediate attention were: no. 8 (vico Carcere Sanfelice 7), no. 30 (via Vergini 19), no. 35 (via Carlo Carafa 14), no. 44 (via Duomo 142), no. 60 (the cavity complex at the salita dello Scudillo), and no. 92-200-201 (series of linked cavities with access from via. S. Nicola da Tolentino and Corso Vitt. Emanuele at the Gerli construction site).

All of these cavities under discussion are formed of yellow lapideous tuff with the exception of some access wells that pass through overlying loose terrain. These are, in most cases, collapsed. There arealso a few rare cases of ancient aqueduct shafts through loose pyroclastic materials; they are walled with brick.

It is, however, well known that the subsoil of Naples also contains cavities in the blanket of loose pyroclastic material above the yellow tuff. These are, as noted in chapter 1, shafts dug into pumices and lapilli for the illegitimate extraction of construction stone, shafts with no scaffolding or retaining structures. These shafts are often much more dangerous than those dug in yellow tuff, and trying to locate them individually was particularly fruitless. We have, however, noted some general areas of the city where they are found: the areas around Piazza Carlo III, Piazza Muzi (Arenella), via Miano-Agnano, along the Pigna-Camaldolilli road, the area between Viale Maddalena and Via Nouva del Campo (Doganella).

A got a lot of valuable information about the geological make-up of the Neapolitan subsoil was from stratigraphy of the subsoil done for a great variety of reasons in the urban area. We has access to these or the corresponding bibliographic references through cooperation of a number of specialized agencies that we would like to thank and here name: SAF - the Naples Fire Department Training Corps, SACIF - Societa' Ambrosiana Costruzioni in Ferro - and SAMCEF. Other data about recent core samples of the subsoil in the hill areas cane toed to us by the municipal commission that was instituted to oversee and check retaining walls in those areas.

We examined and evaluated stratigraphy of about 700 core sample. About 500 of these did not prove useful for our purposes, either because they were imprecise as to exact location or because they contained unverifiable data or because the data were too narrow to be useful (i.e. they were done for the purposes of setting pylons to support buildings; thus much of the stratigraphy had identical readings). Some were also outside of the areas under consideration.

In total, we wound up using 196 core samples (56 of which were from the bibliography of Guadagno) and previously unedited samples from the above-mentioned firms. The stratigraphy of these core samples were collected and reproduced in glossy format in order to make them easier to copy for those interested in the nature of the terrain in the city. The stratigraphy in part three of this Report are from that batch.

Where possible, the same samples were  reproduced a 1:1,000 on the map of the urban area. That is, these data: cavities of which we have reliable surveys and which have secure access points from the surface, core samples with precise locations, and terrain outcroppings of particular interest (essentially, outcroppings of lapideous tuff). By way of example, among the graphics in the third part of this Report, we have reproduced an excerpt of the detailed Vili chart; the excerpt corresponds to p. 250 and part of p. 251 of the 1:1,000 map of the urban area. We chose this area because it is among the most significant areas in the city and one that we have relatively complete information about. It is precisely in this area that we find a dense and numerous collection of cavities, some of which are also among those in the worst condition of the ones we looked at. This area also has a number of railway and cable-car tunnels that we sampled. There are also some outcroppings of lapideous tuff. The great amount of data at our disposal allowed us to reconstruct the contour of the roof of lapideous yellow tuff (the substratum of the area) by marking the curve every two meters in relation to sea-level. This detailed reconstruction of the contour of the roof of lapideous yellow tuff is an exceptional case, made possible only by the wealth of data at our disposal, but it does indicate the usefulness of such data and the possibility of expanding the method to the rest of the urban area.

We couldn’t place all samples and cavities on the 1:1,000 urban map. Given the topographic scale, we were able to place only those sample with precise locations and cavities the contours of which could be precisely related to the surface terrain. Thus, the 1:1000 scale map lacks some important and valuable elements that are useful to a more thorough understanding of the urban subsoil. We did, however, incorporate those elements into a broader and more synthetic overview of the entire urban area. Even some of what we included in the 1:1,000 urban map is difficult to fit into a global vision of the area, though it might indeed provide insight into particular morphological and tectonic situations.

In the course of working on all the graphic material that concern the subsoil of Naples, the need for synthesis was evident. We have tried to present that synthesis in the form of a 1:1,000 geo-technical map of the urban area, which we shall illustrate and comment upon in what follows.

POINT OF DEPARTURE AND GENERAL CONSIDERATIONS 

To make geo-technical map of the urban area of Naples, we used a topographic scale of 1:10,000 with contour lines 10 meters apart. That choice was dictated by various factors, primary of which was the size of the area under consideration. Indeed, for purposes of this study, what we call “urban area” is that area where the density of buildings and roads impedes almost totally direct observation of the terrain at the surface. It is, thus, an area that can best be studied, from a geological point of view, through large-scale investigations of perforations [ trans note: “perforations,” in the context of this report, refers to all, man-made holes in the ground —that is: cavities, tunnels, shafts and wells]. As we noted earlier, most of the perforations in Naples are, in fact, located in such densely urbanized areas as described above. A scale of 1:10,000 gives us an ample overview of the area without the end-product becoming unwieldy. On the other had, the precision of the datA we are using, particularly with regard to the perforations, doesn’t justify a median altimetric contour of less than 10 meters, a contour that fits our scale of1:10,000 quite well.

image 23

images 23-24                           construction against                      and excavated into a tuff bank,                     in the zone of vico ai Ventagliere image 24

Further, we needed to use a scale that would give a complete representation of buildings and roads with the necessary number of points of reference sufficiently accurate to make the final product useful for practical purposes. The most modern topographic representations of the urban area of Naples are those of the 1956 aerial surveys of the Military Geography [MGI] Institute and those commissioned by the city of Naples anddone between 1960 and 1962. The charts of the Military Geography Institute, however, are at a scale not sufficient for our purposes; that is, they are at a scale of 1:25,000 with contour lines, even in inhabited areas, set at 25 meters apart. The maps commissioned by the city, however, are at different scales: 1:10,000; 1:4,000; and 1:1,000 with equidistant contour lines at two meters apart at the first and third scales and one meter apart at the second scale. Unfortunately, the contour lines were drawn only in areas where there were no buildings at the time the survey was done. The reference points within built-up areas were not close enoughtogether to permit us to extend the contours by extrapolation with a precision that matched the scale.

At first we used photographic enlargements of the MGI 1:25,000 maps to bring them up to 1:10,000. After that, we used the above mentioned city map by moving altimetric data from the 1:4,000 (and, depending on the
case, from the 1:1,000) maps to the 1:10,000 map until we had a sufficiently dense display of reference points to let us trace within the inhabited areas equidistant contour lines 10 meters apart; that is, trace accurate extensions of the contour lines already shownon the 1:10,000 for uninhabited areas. In order to show buildings and roads clearly, we had to redraw and simplify how they were displayed on the 1:4,000 city maps; thus, we displayed only the building outlines. Those simplified drawings were then reduced photographically to 1:10,000. For geological purposes, it was particularly helpful to us to have a display of the sea bottom that faces the urban area. For that, we used bathymetric data furnished by the Naval Hydrographic Office, which let us trace isobaths [underwater points] equidistant at 5 meters apart.

Such a complicated procedure is naturally not free from error, but we wantted to have a display that was as clear and complete as possible (and, in any event, indispensable) even at the risk of losing some degree of precision that, given the fact that the product is somewhat of a summary, would not have been necessary in the first place. We should point out that within the inhabited areas, elevation data are linked to the layout of the roads as points of reference; the surface topography traced on the basis of our data really represent the enveloping network of urban roads. Also, there are some points on the map that don’t show contour lines or elevation indications, which may have led us to overlook some morphological features that might have been useful in the present study.

In transferring our bibliographic data to the 1:1,000 scale map, particularly the core samples, we often had serious
difficulties because the bibliographic values did not always match those on the map. Working with the 1:1,000 and 1:4,000 city maps we often noticed large and puzzling discrepancies that led to some uncertainty in trying to reconstruct surface contour. These original errors obviously influenced all subsequent work on data regarding the subsoil (primarily the curve of the yellow tuff roof) and underscored the need for more up-to-date, correct maps of the urban area in future studies of this kind. We alsonote that much of our work with the 1:1,000 and 1:4,000 city maps was done with often illegible photocopies because the originals were no longer available. We need a new, up-to-date, and correct edition of these maps, printed in sufficient numbers and available and affordable.

There is a substratum of lithoid volcanic tuff present everywhere in the Neapolitan urban area except in the extreme northeast. In large part, that substratum is covered by a blanket of loose materials, most of it to some degree disturbed [trans. note: i.e. moved from somewhereelse]. They are of volcanic origin and, at lower levels, also of marine and lacustrine origin. The tuffaceous substratum can be notably thick in parts, although, as already
noted in the discussion of  the formation of yellow tuff, that may vary from to spot to spot in the urban area (see, for example, core samples no. 1, Piazza Leopardi, more than 183 meters; no. 11, Piazza Sannazzaro, 22 meters; no. 97, Royal Palace, 86 meters; no.150, Piazza S. Maria La fede, 50 meters). The tuffaceous substratum rests on loose pyroclastic materials, on lava, and on marine sediments that for the most part are coastal terrigenous deposits and are at the deepest levels reached by various core samples.

We have learned from core sampling in the extreme north-eastern part of the city that the tuff substratum moves deeper as it moves east, but that the thickness of the substratum, itself, decreases. Indeed, perforations do not find the substratum at all except at great depths in the north-east along the Ponti Rossi-Carcere di Poggioreale line (see
samples 170, 173, 182 and 183). It is probably the case that the substratum becomes discontinuous here as the chaotic yellow tuff is replaced laterally by pozzolana of the same age. In the blanket of loose materials that we generally find above the yellow tuff, it is generally possible to distinguish an upper, a middle,and a bottom part, though they may not all be present all the time. The upper part is formed of loose material, disturbed either naturally
or by man; it reaches its greatest thickness in the lowest parts of the city, at the levels of ancient river beds. The thickness varies but can reach more than 10 meters.

The middle part is formed of loose pyroclastic materials in situ (or partially disturbed only by natural causes). It is relatively stratified and can be quite diverse (pozzolana, pumices, lapilli, etc.). These materials are the products of the most recent period of Flegrean eruptions. The thickness of this middle part is about 16-18 meters.The lower part is formed of loose volcanic material that we don’t know too much about —neither the composition nor the stratigraphy. The thickness seems to vary a great deal, sometimes reaching 10 meters or more. Immediately above the yellow tuff, we often find the“mappamonte,” material not too coherent and related to the yellow tuffthat it blends into as it moves down. In the lower parts of the city, as well, marine coastal sediments are also present at various levels, and there are river and lake sediments in the lower eastern parts of the city. On the enclosed geo-technical map, large lines indicate surface soils or surface immediately below buildings and roads.

In distinguishing these soils, we have followed the criteria adopted and explained in chapter one, that is, in the material dealing with the genesis and composition of the subsoil of Naples. Because chaotic lithoid yellow tuff (“yellow Neapolitan tuff”) is a specific point of reference in the urban territory, we have also emphasized the stratigraphic and genetic differences between it and all the other kinds of terrain.

Thus, besides distinguishing among loose, disturbed materials (floods, marine and lake deposits, reclaimed land with artifacts, etc.) and primarily undisturbed loose pyroclastic materials in situ, this latter group was in turn broken down into various kinds and considered individually: that is, loose pyroclastic materials formed after yellow tuff (and thus on top of it), loose pyroclastic materials the origins of which are directly linked to the formation of yellow tuff (which might be on top, on the sides, or laterally within the yellow tuff), and more or lesscoherent pyroclastic materials beneath the yellow tuff formation or layered into it by other eruptions. (That would include those sections
that we can attribute to the formation of stratified yellow tuff before the formation of typical chaotic yellow tuff.) Finally, in a separate group, we have included all those types of exceptional or episodic terrain that are largely related among themselves, such as piperno, pipernoid tuff, volcanic breccia, etc.


For general technical purposes in dealing with the formations in urban subsoil, it is important to distinguish the lithoid tuffs of the stratum from the loose and more or less incoherent materials in the blanket above it. That should also lead to a chart of the contour that shows the distinguishes between substratum and blanket; that is, one
that shows the contour of the upper surface “roof” of lithoid tuffs for the purposes of finding out, before anything else, how much loose material is between it and the surface. We reconstructed the contour of the lithoid tuff roof and set the contour lines equidistantly at 10 meters apart. The idea of representing the tuff roof in Naples and environs with contour lines goes back to M. Guadagno, who tried it in 1928 and gave up, considering the number of core samples at his disposal (93) to be insufficient.

image 25

image 26 Cavity in zone dei Ventaglieri converted for use for vehicle parking and storage. Street entrance crew (left) and inside view (right)

As F. Penta rightly noted in 1960 in on the same topic, the contour of the yellow tuff roof in the urban area is highly irregular because of all the excavations over the centuries by man. We add that other irregularities, on a different scale, are also due to erosion and probably volcanic-tectonic phenomena after the deposition of yellow tuff. We can add to these difficulties those that come from uncertainty over some of the data at out our disposal, particularly having to do with altitude and planimetric markings.

In spite of the difficulties, we went ahead with a first attempt at reconstructing the evolution of the lithoid tuff roof. Even an approximate reconstruction seemed useful, not just for the immediate problems at hand, but so we could get a real idea of the procedures we will have to use in the future. We used data only from those core samples that were reliable and precisely located.

We marked topographically, as noted earlier, the absolute values for the tuff roof as shown by various core samples or those points where the sampling was stopped without having found the tuffaceous substratum. Those markings are given in relation to the mouth of the opening if known, or, if not, in relation to the topographic base once we located the core sample. That is he we got the first series of locator points for the lithoid tuff roof. We got a second series of such locators at those points where the lithoid tuff roof actually cropped out on the surface.

Finally, we traced from point to point to get the contour of the lithoid tuff roof; where there was no outcropping of tuff, we showed the curves of the roof as on top of the of contour curves of the surrounding surface topography.
We then corrected hose curves on the basis of data from the most recent perforations (which, in the meantime, we had acquire). In some cases, we also used date from ongoing surveys of cavities where we could actually see the transition from the lithoid tuff to the overlying loose materials. This process, as described, of reconstructing the contour of the lithoid tuff roof of the urban area of Naples was affected by imprecision in our topographic base; much of the data was imprecise,and there were mistakes in many of the procedures that hadto follow.

Indeed, excessive values for the tuff roof result in cases where the contour curves of the roof —if the tuff is not outcropped on the surface— have to be marked as overlapping the contour of the surrounding surface terrain. That is because you tend to follow the curve of the surface terrain without “evening out” the tops of a convex slope. On the other hand, it is also possible that some core samples struck tuff in artificially filled cavities or ancient hidden river beds. In those cases,and lacking other data, the values for the tuff roof will not be accurate.We can definitely say that the larger discontinuities in the lithoid tuff roof (craters, volcanic-tectonic dislocations, large rivers beds) are attenuated, while the smaller and more numerous ones (small natural cuts, artificial excavations) cannot be accurate defined.

Keeping in mind this possible imprecision, the average error should not amount to more than few meters (on average, about five) if we exclude a few special individual case such as well cavities that have been filled or ancient hidden river beds the precise locations of which are not well-known. Indeed, we have been able to verify the reliability of our reconstruction by comparing the predicted data (deduced from maps) with data from new core samplings done in the meantime at various points in the urban area. From a practical point of view, our reconstruction can thus provide
valuable information for large-scale public works and forgeneral  urban planning. Furthermore, even in cases where some data are not extremely precise, they are useful in the planning of single buildings. As a concept, we see our reconstruction necessary fir further studies.

For a visual display of more immediate use, we also traced, using the contour lines of the lithoid tuff roof, an
equal-width (isopach) chart of the loose materials above the yellow tuff at thicknesses of 5, 10, 20 and 30 meters. Those curves thus define the areas in which the thickness of the covering is from 0 to 5 meters, 5 to 10 meters, 10 to 20 meters, 20 to 30 meters, or more than 30 meters. In the areas where the thickness of the loose material exceeds 30 meters, lithoid tuff (as already noted) might be missing altogether, as, for example, in the eastern part of the urban area.

It goes without saying that this kind of isopach chart is not just academically interesting; it can be of immediate use. It can serve to help answer the much-debated question of underground viability of the subsoil for an underground train line, one that would run along the coast and connect to the crucial nerve centers in the city. Also, the chart lets us generalize about the types of foundations most suited for the city (i.e. direct foundation, pillar foundation, suspension foundation, etc.). Thereconstruction of the contour of the lithoid tuff, together with a study of external morphology, lets us offer some general observations.

The dominant morphology of urban Naples is dictated by the presence of level areas, some of which are along the coast (Fuorigrotta-Bagnoli, Riviera di Chiaia, the area adjacent to the port and the eastern zone of the city). Other level areas are plateaus at various elevations in the city (high Posillipo, Arenella and Vomero, the areas of the Cardarelli and Principe di Piemonte hospitals, Capodimonte, Mt. Echia, etc.). The edges of the plateaus are scarps—steep or very steep slopes— covering large differences in elevation. The configuration of the roof of
the tuffaceous substratum of the city generally has the same features as the surface morphology.

It has been tradtional to explain the origins and contours of the major scarps in the city as the effects of continental or marine erosion or by claiming that the scarps are what is left from earlier crater rims and volcanic-tectonic caldera collapses. Those theories, however —even if plausible—do not explain the existence of level surfaces at widely different elevations not joined by a continuous surface. The final surface areas of the various blankets of chaotic yellow tuff, except where they actually come up against more recent craters in the tuff, should be relatively continuous since eruptions (especially like those in the most recent studies) tend to fill in preexisting discontinuities.

It is, thus, at least very probable that the original continuous surface was remodeled or broken apart by phenomena after the deposition of yellow tuff, which we feel cannot have happened more than 10,000years ago. Consequently, we exclude that the current configuration of the city isdue only to the action of the sea, since in such a recent and brief time span there have not been changes in the general sea-level that would account for the various elevations of the plateaus. It is thus probable that other kinds of dislocation broke up the original continuous surface, although we do exclude the effect of the sea insome cases. We do not, however, find dislocation of this type happening so recently on a larger regional scale. We conclude, therefore, that the event or events in question must have been local; that is, connected to local volcanism.

The existence of dislocations even with linear craters does not exclude —indeed, is quite consist with— craters and calderas along the very dislocation, itself. On the other hand, dislocations of that size in recent times, explains the intense erosion that we see in some parts of the city. Both phenomena may have shared in at least partially erasing the discontinuities caused by [ancient] dislocations, as a result of which it is difficult to recognize their contours just on the basis of their morphology.

In effect, the external morphology (surface and underwater) and the contour of the lithoid tuff roof reveal with certainty one single dislocation, and that is along the S. Maria Apparente-Castel
dell’Ovo line; the data on other possible dislocations(for example, Albergo dei Poveri-Sanità-Arenella and one along the Riviera di Chiaia) are debatable.

The same morphological data tell us that there isprobably a crater complex in the area of
Fuoirigrotta (already noted) within the urban area and another in the western part of the Chiaia quarter. No observations allow us to confirm the claims of some authors that there are craters elsewhere in the city. If they do exist, they must have been present before the eruptions of
chaotic yellow tuff.

Morphological data in the urban area also show with various incisions with many linesthat form natural groundwater runoff catchment channels. You can follow these lines from where they originate, normally not in areas affected by urban development; their paths through inhabited areas generally follow those of ancient road beds almost to the sea. You can also define the basins created by these lines.

In the eastern part of the urban area, you can see the Ponti Rossi valley that starts in the north and continues by Piazza Carlo III, then along via Arenaccia and Corso Novara, ending at Piazza Garibaldi. The valley that catches the runoff water flows to the east of the line of the Capodimonte Palace and adjacent astronomical observatory-via Rossaroll-Piazza Garibaldi.

To the west of that line, in the northern part of the urban area, comprising viali dei Colli Aminei, the Cardarelli hospital, Piazza Due Porte all’Arenella, the Materdei quarter, on to the National Museum all the way to Piazza Mercato, the waters are caught by various runoff channels that merge at the Sanità bridge into a single runoff catchment channel that follows along via Vergini and via S. Giovanni a Carbonara to Piazza Capuana and along via Lavinaio.

Various drainage channels collect the waters of the area bounded by San Martino, Pizzofalcone and the central point of via Medina. Finally, a number of catchment channels reach the Riviera di Chiaia descending along the southern slope of the Vomero hill below a line from S. Martino, to via Scarlatti, via Belvedere, and parco Comolo Ricci. Other channels descend to Mergellina from via Manzone.

In general, these watershed channels correspond to incisions in the lithoid tuff roof; the channels have a similar contour and are more pronounced. That leads us to think that the hydrography was already stabilized after the deposition of yellow tuff and whatever dislocations might have ensued, but before the beginning of the various eruptions of the last Flegrean eruptive cycle. Each of those eruptions deposited modest thicknesses of mostly incoherent material. Those materials would have eroded and been washed away easily and would not have
substantially changed the preexisting hydrorgraphy or erosion pathways and processes already underway. Only in the level areas, especially in the lower parts of the city, do we find overflowing of river beds, which man probably contributed to, at least partially.

                image 27- Entrance to the San Gennaro Catacombs
                   
                                                                                              
                                                                                             image 28 -San Gennaro catacombs, interior detail
   
With that we have covered the main points of morphology and geology. They are the points that stand out on a 1:10,000 scale geo-technical map of the city of Naples, and they indicate the technical consequences in given situations.

We can move on to particular situations that we found in the course of geological surveying in the urban area and during the collection and elaboration of data on the nature and geological make-up of the subsoil. These observations are more detailed and are not immediately evident from looking at the geotechnical map; they may be useful in furthering our understanding of the subsoil.

OBSERVATIONS ON THE EASTERN URBAN AREA

The eastern side of the area of Naples can be divided into an upper part (the three plateaus of Vomero, Arenella and Capodimonte), a middle part (the eastern slopes of Mt. Echia and Vomero, the Gerolomini valley basin, and the southern side of Capodimonte) and a lower part (the area bounded by via Roma, via Pessina, via Foria, and the sea).

The upper part of the eastern urban area is characterized by slight slopes in the terrain and by relatively high elevation. Between Vomero and Arenella, the terrain forms a large saddle that rises gently towards San Martino from one side and, from the other side, more abruptly up towards the plateau where the Cardarelli hospital sits at higher
elevation. The summit of the Capodimonte hill is practically level and is really the continuation to the south of the Capodichino plateau that risesgradually towards the summit of the Camaldoli hill.

At Vomero and Arenella, the terrain is loose pyroclastic materials, largely in situ, the most continuous and complete succession of products from the most recent Flegrean eruptive cycle. The yellow tuff beneath these materials has outcrops mostly outside of the area under our consideration, but within that area there are outcroppings near San Martino and Castel Sant’Elmo and along via Salvator Rosa near the crossing of via Suarez. In the level central area not all of the perforations [core samples] actually struck yellow tuff beneath the loose materials since the width of these materials exceeds 20 meters. Between Piazza Medaglia d’Oro and via Salvator Rosa, the samples showed a rather marked incision in the tuffaceous substratum corresponding to a shallower runoff channel on the surface that starts here and continues down intothe one at via F.S. Correrà.

Along the upper stretch of the Pedimentina of San Martino, to the crossing at via Pedimentina and along that road, all of the older and smaller buildings are damaged, dangerous and, in large part, abandoned. In this area and the one (under cultivation) above viaPedimentina, there are no outcroppings of yellow tuff and there seem to be disturbed loose materials on the surface. Thesematerials and the steep terrain are, without a doubt, unstabile. The damage to buildings, however, is also due to the type of construction, to their great age, and to lack of maintenance, etc. Even the sewer lines must be in semi-abandoned condition, which, given the general instability of the area, will have consequences.

Stratigraphy of the summit of the Capodimonte hill is similar to that of Vomero and Arenella: however, the loose pyroclastic materials of the last Flegrean eruptive period are not as thick here, and the yellow tuff that outcrops mostly on the southern flank of the hill does not go down more than 20 meters. The width of the yellow tuff, however, tends to get smaller and in the eastern part of the hill is replaced laterally by pozzolana. The plateaus of the Cardarelli hospital and the Principi di Piemonte show similar features.

The central area, the one between those plateaus and the lower part of the city, is often characterized by very notable slopes; there are some exceptions (Pizzofalcone, Stella and other smaller areas) that have the same morphology on a smaller scale as the plateaus found at higher elevations in the city. The steep terrain has favored erosion, which has often laid bare the yellow tuff and let it be extracted more easily. There are, in fact, a great number of cavities in the area dug for that purpose, and, on the surface, there are many exposed tuff sections that are walls of such cavities, ancient as well as more recent ones.

Where the terrain is steeper, there will generally be more yellow tuff outcroppings or, at least, it will not be far below the surface. On partsof the slope that are not as steep, there will be a notable thickness of loose pyroclastic materials often disturbed both by nature and by man. Special mention should be made of the Pizzofalcone hill; it is formed of
chaotic yellow tuff and is stratified. On the slope that descends towards Piazza Plebisicito, it is covered with loose pyroclastic materials often of notable thickness.

The hill has a number of large, artificial cavities. Even the trench at via Chiaia is probably, at least partially, man-made. In another area we find the particular feature of deep gully erosion in the steep Gerolomini valley with its eroded tributary channels that all run into the one at the Sanità bridge. Erosion is quite lively and, probably, the entire web of gullies and channels goes back to before the most recent Flegrean eruptive cycle. There are yellow tuff
outcroppings in very many places, and tuff has been extracted both on the surface and in the many underground cavities.

The lower part of the eastern urban area corresponds to the Greco-Roman and Medieval city. It is characterized by slight slopes in the terrain and relatively low elevation. Behind the Incurabili [hospital], S. Marcellino, and Castelnuovo, however, heights rise from the practically uniform surface. Yellow tuff appears practically nowhere on the surface (even if it does appear in the basements and cellars of many buildings) but it is present everywhere at depth.

The tuff is covered by loose pyroclastic materials, most of which is disturbed either naturally or by man. It is also covered by detritus from the demolition of ancient buildings, etc. Below via Depretis and Corso Umberto, in addition to these materials (most of which have been disturbed by the action of the sea) we also find marine sediments from the ancient beaches. The thickness and depth of materials that cover the yellow tuff vary to a great degree. The minimum thickness is at the heights of the Incurabili and of San Marcellino. The most elevated ones (more than
20 meters) are in the areas of Piazza Dante, Montesanto, via Roma, Guantai, in the area between the Polyclinic hospital and Piazza Nicola Amore, and the area bounded by via Settembrini, via S. Giovanni a Carbonara and via Rossaroll. These areas heavily covered by yellow tuff correspond to the run-off lines in the tuff roof, itself, and have
been only partially erased by products of later volcanic eruptions, floods and human activities.

To the east of Piazza Garibaldi, the depth of the yellow tuff and the width of the covering increase gradually. In some zones, as noted, there is no yellow tuff at all and for many tens of meters there is nothing but loose materials, largely marine or flood accumulations.


OBSERVATIONS ON THE WESTERN URBAN AREA

The terrain that forms the large semi-circle extending from Castel dell’Ovo to Mergellina is essentially classic yellow Neapolitan tuff covered by a discontinuous blanket. The thickness of the blanket varies and is formed of loose pyroclastic materials, some of which is disturbed. The yellow tuff of the western urban area (and thus of the contiguous Posillipo hill) does not come from a single volcano but is the product of various volcanoes and various eruptions separated even by great intervals of time.

A typical section rises from the Riviera di Chaia to the Vomero, starting at the Circolo della Stampa, then past Piazza Amedeo and the Chiaia cable-car station to the [street named] Corso Vittorio Emanuele. In that section we find all of the volcanic formations that make up the western urban area of Naples. Along this path, there are three units of yellow tuff separated one from the other, first by a thick layer of humus and a pile of stratified products (outcropped at the Corso Vittorio Emanuele station of the Chiaia cable-car) and, second by a bank of piperno together with stratified products and visible, eroded surfaces (outcropped along the extension of via Palizzi). These three units of tuff thus result from three different eruptions.

On the first formation, the one at the base, there is not enough information to determine its origin. It is not even clear if it is really classic chaotic yellow tuff and not yellow stratified tuff. Even the outcroppings are of no help in making that determination. This first unit of yellow tuff crops out in only a few places. The unit is the base of the semi-circle and is mostly buried under incoherent pyroclastic products of the more recent eruptions. It is also hidden by the works of man. From recent core samples done at Piazza Vittoria and along the Riviera di Chiaia, we can say that this formation of yellow tuff certainly extends down to more than 50 meters below sea-level. The core sample at Piazza Vittoria, indeed reached the base of the formation. The materials lying beneath it are incoherent pyroclastic
products alternating with banks of disturbed sand, materials that are often rich in shells and other remains of marine life. These pyroclastic products below the first yellow tuff formation certainly go down to more than 150 meters below sea level.

There is a special facies at the base of the formation that we call attention to: the tuff changes in color from a typical yellow to the greenish or grey-green that we have already mentioned. Nothing more can be said about the volcano that produced all this: its products are the oldest in the western urban area of Naples. Not even the location is certain. We don’t exclude —indeed, we hope— that further investigations will shed some light on that problem.

We can now discuss the second formation of yellow tuff. The complete change, in all its facets, from the first unit of tuff to the one above it is visible at only one point, and that is the area of Parco Margherita up to the Chiaia cable-car station on the Corso Vittorio Emanuele. As you go into that station, there is a high and deep wall on the left
that has cut into the hill for many meters and revealed a section of pyroclastic materials, stratified and with well established diagenesis. That section (right near the entrance) starts at the base with a thick layer of humus. You can run your eye up that section for more than 10 meters and watch the materials change gradually until they lose all
stratification and blend into the common yellow chaotic tuff that you see cropping out above. Along the entire section of exposed surface, the strata remain parallel, at a constant thickness, and are angled aabout 25° and oriented to the north-northwest.

These strata alternate between very fine cineritic materials and pumices mixed with lapideous lapilli and scoria diagenesis to the point where they reach the consistency of common yellow tuff. The materials are, however, distinguishable from common yellow tuff by the extreme uniformity of the grain, either much finer or markedly coarse. Part of the same series of stratified materials can also be seen at Parco Margherita where they gradually transition downward into the formation of lower yellow tuff. Studying this stratified series has provided us with some very useful data.

One bit of chronological information tells us that there must have been a long period of quiescence between the two eruptions of yellow tuff. That is revealed by the thick layer of humus that separates the two formations. We can also deduce something else from the way these perfectly stratified pyroclastic materials after the humus stratum gradually
transition to chaotic common tuff. The transition is so subtle that you can’t see where the stratified materials stop and the yellow tuff starts.

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San Gennaro Catacombs, room particulars

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On the exposed wall at the Chiaia cable-car station, you can follow the succession of strata centimeter by centimeter. It is the only outcropping where the transition from humus to stratified tuff tochaotic yellow tuff is visible without interruption. That tells us, primarily, that the stratified tuff and the chaotic yellow tuff are products of a single eruption; it also gives us valuable data for reconstructing the mechanism of an eruption of yellow tuff. That mechanism was amply covered in Chapter 1 in the description of the genesis of yellow tuff based on Rittmann’s classic treatment of the topic.

Also, the wall shows perfectly parallel alternating strata, the materialsand granulometry of which vary from stratum to stratum but are uniform within each single stratum. There are also large pisoliths with concentric strata in some of the cineritic strata; all of this points to a surface origin [as opposed to marine] of these materials and, consequently, also of the yellow tuff that follows it with no interruption. Finally, it is interesting to note that the chaotic yellow tuff
makes the transition to stratified materials not just on the sides, but at the roof. That is seen in the largely cineritic strata that crop out, as noted, at Parco Margherita, and gradually change as they move down to the lower formation of yellow tuff. In various other outcroppings, moving upward toward the roof of the formation, we see the gradual
change from yellow tuff to stratified materials.

A very similar series of stratified materials was found some years ago during work to restore and resurface the large retaining wall at Mt. Echia, at the corner between via Chiatamone and via Santa Lucia. The mass of stratified materials at this point can be even thicker than at the Chiaia cable-car station. Here, too, the materials are pyroclastic products with varying granulometry from stratum to stratum but very uniform within each single stratum. Here, too, we also notice strata rich in large pisoliths and, finally, these stratified materials (which display a high degree of diagenesis) make a gradual transition, step by step, into the chaotic yellow tuff of Mt. Echia and Pizzofalcone. The basic level of humus is not visible here, and the stratified materials continue below street level. This is why there is a difference in elevation of more than 50 meters between the series at the Corso Vittorio Emanuele and this one at Chiatamone.

That brings us to another conclusion, one that has to do with the location of that volcano that produced the stratified material that we find at the Corso and Mt. Echia and the yellow tuff immediately abovethose materials. The placement and contour of the strata clearly indicate that the mouth of the crater must have been located roughly in the center of, or slightly to the northwest of center, of what is currently the vast amphitheater that extends from the Castel dell’Ovo to Mergellina. There are other data that confirm what is so clear just from the morphology. For example, in the tuffaceous rock face of the Castel dell’Ovo, we find signs that the stratification there can be joined to the materials at the series higher up at Mt. Echia. Even if the rock of the Castel dell’Ovo is a single chunk that might have ceded due to tectonic movement or abrasion of the sea, it clearly indicates what must have been the contour of the crater rim.

Other data that indicate the same thing are found in reports and even memories passed down that deal with man-made works in the terrain of this area, things such as tunnels, grottos, sewage lines, elevator shafts, core samples, etc. Of all such works, the one that has given us the greatest amount of important information is, without a doubt, the train line called the “Direttissima,” which connects Fuorigrotta to Piazza Garibaldi. It passes through the hill of Posillipo, Vomero and Sant’Elmo for a distance of about 7,500 meters, most of which is in a tunnel. In this tunnel, at between 700 and 900 meters from the mouth of the entrance beneath the Corso Vittorio Emanuele (that is, almost at the point where via Tasso starts its climb to Vomero) well-stratified tender tough was found, after which there was chaotic yellow tough once again.

Unfortunately, not many particulars were gathered concerning this latest series of stratified tuff, but it seems to us that it shouldn’t be difficult to see that the stratifications are aligned parallel to those ofthe other series of stratified products at the base of the second formation of yellow tuff. The volcano that produced that second formation and the stratified materials connected to it and that crop out at Corso Vittorio Emanuele and Mt. Echia, has been named the
“Chiaia Volcano,” conserving the name already adopted by earlier authors such as Breislak, Gunther and others. They were the ones who saw for the first time in the vast amphitheater between the Castel dell’Ovo and Mergellina the remains of a great crater, enlarged andpartially crumbled by the destructive power of the sea, by erosion and landslides.

Stratigraphically, among the products of the Chiaia volcano and the earlier yellow tuff formation, we find a number of vertical masses of trachytic lava in the shaft of the “Direttissima” tunnel at Villa Lucia (ex-Bertolini) at about 22 meters above sea level. Others are in the shaft of the western run-off collector for the hill, in the same section
but more towards the sea at about 34 meters above sea level. We can’t say precisely if these trachytic masses are parts of flows or domes, but they may belong to an eruption independent of theeruption of the Chiaia volcano or the volcano before Chiaia. We have no evidence for these eruptive events, but that doesn’t mean that they are not probable. Or they might even be part of the activities of Chiaia eruption. According to Rittmann’s hypothesis on the genesis of yellow tuff, at the beginning of an eruption of yellow tuff a lava dome is formed; pushed by the magma mass below, the dome comes apart. The lava that made up the dome is very viscous and relatively cold; it effuses laterally, clearing the way for rising magma, rich in gas, which then goes through a series of rhythmic eruptions culminating in a very violent one that generates a hot settling cloud.

At the roof, the chaotic yellow tuff of the Chiaia volcano has made a gradual transition into materials that are markedly more stratified. We saw that transition very clearly during work to extend via Palizzi with a bend at about the halfway mark that turns parallel to via Luigia Sanfelice and on the Corso Vittorio Emanuele. In the outcropping revealed during the work, stratified materials were revealed that mark the point where the Chiaia yellow tuff formation stops and where the Vomero yellow tuff formation begins. These two formations of yellow tuff are separated not only by their respective stratified products, but by a series of other pyroclastic materials of different origin, among which is a bank of piperno.

This piperno is somewhat different than the classic kind from Soccavo. It is not as diagenetized; it is richer in cineritic components; the flames are smaller, rather like scoria, porous and only rarely, in the larger ones, do we find a compact lava nucleus. Furthermore, in the series of stratified products that accompany the piperno, we don’t find
that collection of various kinds of lava bits known as breccia museo. All of these characteristics no doubt indicate a greater distance from the mouth of an eruption, in respect to the Soccavo piperno. According to Rittman, this piperno has the same origin as that of Soccavo. We dealt with that in Chapter One.

Wealso  found an outcropping of piperno along the rise at Parco Grifeo, a few meters to the west of the cable-car
entrance. It is a few meters above this area along the road that rises to Villa Lucia and in the direction of the Parker Hotel. The outcropping is visible, and the piperno is surrounded by yellowish, partially coherent tuff. Another piperno find was in the above-mentioned Direttissima train tunnel a little before a vertical line that would correspond to Parco Grifeo. The piperno was at only 18 meters above sea level and in the form of blocks as large as a few cubic meters, randomly set in partially coherent tuff.

These data tell us that piperno interspersed like that is almost never in situ. Even if there must have been local movement and ground giving way, we shouldn’t forget that a long time must have passed between the yellow tuff deposit from the Chiaia volcano and the piperno deposit. During that interval, there were not only other deposits from
other eruptions farther away, but erosion affected and shaped the Chiaia crater to a great degree and contributed to the rather bizarre contour of the piperno bank. In any case, the piperno serves to define the boundaries of the underlying formation of yellow tuff of the Chiaia volcano. The yellow tuff of the Chiaia volcano thus appears, rather
clearly, to emerge from beneath the stretch of the Posillipo hill that then connects to the Vomero hill. The yellow tuff of the Posillipo hill thus belongs to another eruption, one that occurred well before the eruption that produced the Soccavo piperno.

We move now to the third yellow tuff formation that we can call “Vomero yellow tuff.” Vomero yellow tuff does not differ in any way from the two we have just described. It is covered almost everywhere by the incoherent
pyroclastic products of more recent eruptions. It crops out only where erosion has removed the overlying blanket. Where it does crop out, it always has the same straw-yellow color, although various building projects have also revealed in this yellow tuff other patches of various shapes and sizes of the green-grey tuff we discussed earlier.

On top of the Vomero yellow tuff, there is a series of incoherent pyroclastic materials (primarily ash and pozzolana interspersed with pumice and lapilli) largely greyish in color where we find products of the more recent eruptions of the Campi Flegrei, those classified by De Lorenzo as “eruptions of the third period” and that now, according to Rittmann’s newer chronology, are included in the recent eruptive cycle of the Campi Flegrei.

The products of these recent eruptions cover the most ancient formations. They are found almost everywhere and constitute the terrain beneath the foundations of old Neapolitan buildings. These products are almost always greatly disturbed, at least for the first few meters. Further, trying to truncate a chunk of these materials to form a wall would not work unless the wall had adequate retaining support.

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San Gennaro Catacombs, particulars

Continue text,

In the western urban area of Naples (as in other parts of the city) the
succession and size of the strata of incoherent products that cover the
most ancient formations are extremely variable from zone to zone and
even from place to place within the same zone.
In the bottommost part of the semi-circle we find mostly alluvial
products and beach sand that rest on the yellow tuff from the volcano
before the Chiaia volcano.
The original stratigraphy, after all, has been radically transformed as
products of various eruptions were later deposited. Indeed, one of the
greatest unknowns in the Neapolitan constructing business is trying to
determine how deep the stable terrain is at the spot where the
foundation for a building is supposed to be built and exactly what
materials make up that terrain.

OBSERVATIONS ON THE POSILLIPO HILL

We did not include the Posillipo hill on the 1:10,000 scale map of the city, though it is important part of urban Naples. On the basis of our limited data there aren't particular problems in reconstructing the contour of the roof of yellow tuff. With the exception of the extreme point of Coroglio and Cala di Trentaremi (both made up of stratified yellow tuff from eruptions before those that produced the chaotic yellow tuff), the entire Posillipo hill is uniform in structure and composition. The great mass of the hill is, in fact, almost entirely composed of common chaotic yellow tuff covered by a mantel of incoherent pyroclastic products from the recent eruptive cycle of the Campi Flegrei.

The more recent products at the roof of the yellow tuff in this zone show notable variation in size of stratification from point to point in relation to the broken upper surface of the roof (notably eroded). Those recent products are really an accumulation of disturbed materials from elsewhere.The roof of yellow tuff slopes notably on both sides. On the side facing the Gulf of Naples, the slope is inclined at greater then 45° in spots; on the Campi Flegrei side, the slope is almost always greater than 60-70°. On the south-eastern slope of the hill, the yellow tuff goes all the way down to below the surface of the sea for an uncertain distance, while on the north-eastern side, we see the base crop out at points. Finding the yellow tuff base at Posillipo was a very import in reconstructing the geological make-up of the hill.
Leaving the south-eastern slope, which is of no particular geological interest, we now describe the north-western slope of the hill, the one that overlooks the Flegrean region.

Near the mouth of the Laziale tunnel in Fuori Grotta, there is a cavity of yellow tuff. Along the road that leads to the cavity and flanks the road leading into the tunnel, itself. There is an outcropping of the yellow tuff base about 30 meters up. The materials at the walls of the yellow tuff crop out at this point at a thickness of about two meters and are made up of alternating strata of grey cineritic materials relatively rich in small pumices. Near the point of contact, these materials are markedly cineritic, very fine, almost without pumice and are diagenetized enough so that they look like grey-clear lapideous rock, finely grained and uniform in structure. The point of contact is angled at NE/SW and slopes to the SE at about 20°. At this point of contact between yellow tuff and materials below it, it was particularly interesting to find as part of the yellow tuff formation a bank of breccia made up of large blocks of blackish fused scoria.

Moving along the base of the hill from the tunnel towards Coroglio, the contact of the base with the yellow tuff disappears beneath detritus and reappears about 500 meters farther on, directed to the SW, approximately below the old Villanova settlement and much higher than the earlier outcropping. Indeed, at this second point, the base of yellow tuff is at about 70 meters, while at the first one it was at about 30 meters. At the second point, you can see the entrances to two older cavities, and along the artificial cuts in the rock, you can follow relatively clearly the sequence of incoherent materials at the base of the yellow tuff. These materials are made up of a series of strata, some only a few centimeters thick, of relatively cineritic pozzolana alternating with strata of mostly small, irregular pumices. Still moving to the SW, there is a spur after a few meters. It is bounded by two deep cuts along which the contact between the tuff and the base materials stays at about 70 meters. Today, this spur is in the area of the Military Artillery Arsenal and has a number of tunnels passing through it.

Although the tunnels are covered, we did get some information about them. The entrances vary in height at about 50-60 meters above sea level and all set squarely in the formation below the yellow tuff. Though some of them penetrate into the hill for more than 130 meters, they never strike yellow tuff. At the beginning, where the digging starts, you find materials with a lapideous consistency similar to tuff except that they are grey-clear in color with a more cineritic and uniform grain. After a few days of exposure to the open air, these materials degrade rapidly and become practically incoherent, which is why the tunnels had to be quickly recovered.

The materials at the base of the yellow tuff are a bit more to the SW, approximately below the old Villa Monte di Dio, still at about 70 meters. Here the materials below the yellow tuff are exposed at a thickness of more than 10 meters and you can follow the strata and see that the materials are perfectly analogous to the two earlier outcroppings described. From this point all the way to Coroglio, the base of the yellow tuff is no longer visible.

These cliff walls mark this stretch of the Posillipo hill in a NE/SW direction. Almost at the peak, there are only outcroppings of yellow tuff, often severely eroded, hollowed out and shifted, covered at the base by a thick layer of debris and deposits from ancient landslides. The contact is probably concealed by this debris, but we can say that contour of the line of contact moves up and down starting from the entrance to the Fuorigrotta tunnel all the way to
Coroglio. At Fuorigrotta the point of starts at 30 meters, is at 70 meters beneath Villanova, remains there until Villa Monte di Dio, and then drops to below 40 meters for the rest of the hill.

Many authors have interpreted the steep northwest walls of the hill as the result of the lining up of different crater rims partially dismembered by the force of the sea that once invaded the Fuorigrotta plain. More accurate investigation of the terrain, however, has revealed the remains of but a single crater. It is termed, simply, the
“Fuorigrotta volcano” by various authors and forms a great semi-circle embracing all of Fuorigrotta from “La Torre” [the tower] to the spur at Villa Monte di Dio. It comes after the Chiaia volcano in time, and its products are largely yellow tuff with stratified pyroclastic materials at the sides and roof, which have also covered the analogous materials of the Chiaia volcano. The overlapping of two units of yellow tuff interspersed with pyroclastic
materials that are lapidified only to slight degree is at the heart of various problems that have occurred in underground construction in this part of the hill.

From Guadagno, who for many years was involved in the study of the subsoil, we learn that seven such projects in this section of the Posillipo hill showed evidence of notable shifting and movement in the terrain. These are: (1) the ancient Roman tunnel; (2 & 3) the two old and adjacent tram tunnels (called the Grande and Piccola, respectively) that were then unified into a single larger tunnel; (4) the Direttissima tunnel; (5) the Laziale tunnel; (6) the two run-off “collectors” for Cuma and Coroglio; and, (7) the large Posillipo sewage line. Various explanations for these subsoil problems have been put forward, from bradisisms to mass movements in the earth to underwater causes, etc. Guadagno was one of those who formulated the hypothesis of a different make-up and structure of the tuff in the
various strata of the hill. Dainelli attributed the differences in resistance in the hill to different degrees of imbibition [trans. note: the displacement of one fluid by another], causing rainwater to keep the entire hill in a state of permanent saturation, thus reducing cohesion in the terrain.

The real cause is the fact the projects mentioned above pass through that part of the hill where the yellow tuff formation of the Chiaia volcano (on the bottom) meets the formation of the Fuorigrotta volcano (on top). Various considerations lead us to that conclusion. First, we note that the only tunnels having such problems are those that pass through the section where the Posillipo hill joins the Vomero hill, the point where they are “grafted together,” if you will. The Cumana tunnel, for example, which passes through the Posillipo hill more to the north, has shown, as far as we know, no such signs of earth movement or shifting. Nor have there been such problems in those structures that pass through the tuffaceous Posillipo cliff more to the southwest, such as the Seiano grotto or the Coroglio collector, mentioned above. Unfortunately we have no detailed descriptions of the materials found during the construction of these tunnels, but we have some accounts that may prove useful.

In the Direttissima tunnel, for a stretch of about 200 meters in from the Fuorigrotta entrance and along the axis of the tunnel, a series of greyish, stratified incoherent materials was found, angled towards Fuorigrotta (products of the Third Period [trans. note: ref to the De Lorenzo chronology]). Then there are alternating strata of pumice and pozzolana angled in the opposite direction, inclined towards Piedigrotta at about 30°. These materials had earlier been interpreted as incoherent products also belonging to the Third Period. They are, however, without a doubt, from the materials at the base of the yellow tuff produced by the Fuorigrotta volcano. They are entirely similar to the outcroppings at various points along the Flegrean slope of the hill.

After those materials [in the Direttissima tunnel] there are two banks of yellow tuff with a different facies than usual. The first is a porous mass of pumice, rounded and altered with not very coherent cineritic cement. The second bank is of very finely-grained, very compact tuff, clear yellow with signs of stratification and marked by very subtle strata of small pumice. These two banks maintain the slope of about 30° towards Piedigrotta and are the beginning of the yellow tuff formation of the Fuorigrotta volcano. They are, in fact, identical to theabove-mentioned points at which outcroppings of the base of the yellow tuff appear. Following the course of the Direttissima tunnel, you come to the section of yellow tuff that belongs to the Fuorigrotta volcano but which is not far from the underlying yellow tuff of the Chiaia volcano. That formation is about 1300 meters from the Fuorigrotta entrance to the tunnel, at the point where it intersects the line of the Laziale tunnel. At this point, you come to very coarse tuff rich in altered pumice, crumbly and almost like powder, and ochre yellow. It is identical to that found at via Palizzi in the series of stratified
products at the roof the Chiaia yellow tuff.

For the rest of the tunnel until the station at Mergellina, Guadagno’s report (which we have drawn on for the preceding descriptions) speaks of generic yellow tuff without further details. In all probability, after passing through the short stretch of Chiaia yellow tuff, the tunnel again passes through Fuorigrotta yellow tuff until the tunnel exit at
Mergellina. The problem of continuity —that is, going back into the Fuorigrotta tuff— may have gone unnoticed, or explained as one ofthose little offshoots that we frequently find in tuff.

The Laziale [car] tunnel, on the other hand, does not pass through the base of the yellow tuff at the Fuorigrotta entrance (that base at this point is said to be slightly below the tunnel), but for a few hundred meters stays within the zone where the Chiaia tuff has contact with the Fuorigrotta tuff. Indeed, we learn from Guadagno that “...on both faces of the hill, the Fuorigrotta and the Naples side, at the initial surfaces there was good, even optimum quality tuff that could be used in construction. The central part, however —the nucleus— for about 570 meters, provided
such poor and incoherent quality as to be practically useless for commercial purposes. It had to be crushed up and dumped at sea at some cost.” There can no longer be any doubt as to the nature of this incoherent
tuff. It is the same product found elsewhere —at the roof of Chiaia tuff.

35. (left) San Gaudioso Cataconmb     36. (self draining tomb niches)

We note that the particulars referred to by Dainelli about “the lack of, or the extreme attenuation of the external zone of high resistance on the lower slopes of Mergellina (compared to the slope that faces Fuorigrotta)...” are not to be attributed, as the author thinks, to the influence of steepness on the contour of the surfaces of imbibition, but rather to the fact that at some points the semi-coherent materials at the roof of the Chiaia formation almost crop out because the covering blanket of Fuorigrotta yellow tuff has eroded away almost entirely.
As a result, in the Posillipo hill we’re not dealing with a “nucleus” of less resistant yellow tuff, but rather with a mantle of semi-coherent materials interposed between two formations of compact yellow tuff belonging, respectively to the Chiaia volcano and the Fuorigrotta volcano. This mantel has the same contour as the low cone of the old Chiaia crater, which is why man-made structures that pass through the Posillipo hill run into it only in the stretch between Piedigrotta and Fuorigrotta. In the rest of the hill, such structures pass above it.

Even the well-known case of the ancient Roman tunnel fits this hypothesis of the internal structure of the hill. As long as this tunnel stays within the compact upper yellow tuff Fuorigrotta formation used (out of intuition or just plain chance) by the Romans, and even the Greeks, there are no signs of perturbations in the subsoil. Starting, however, in the 1400s, restoration and expansion led to the lowering of the tunnel bed and dealt a death blow to this structure that had held solid for two-thousand years. Indeed, with the lowering of the road surface, you get ever closer to the semi-coherent tuff  between the two units of resistant tuff. When the supports finally reached this less stable zone, the shifting and cave-ins got worse until the tunnel finally had to be abandoned. The steep walls of the Flegrean slope of the hill are not due to the action of the sea or to a series of craters lined up NE to SW, but are rather the traces of tectonic movement. In those walls we can see the rim of the lowering of the current Fuorigrotta plain. In all probability, it was a volcanic-tectonic lowering connected to the great Archiflegrean caldera collapse.

Other evidence for this tectonic event is the fact the subsoil of the Fuorigrotta plain contains very thick yellow tuff at much lower elevation than the base of the yellow tuff that crops out along the slope of the Posillipo hill. That was demonstrated by core samples placed in the old square of piazza G. Leopardi in Fuorigrotta. The samples found yellow tuff at -67 meters beneath a 100-meter blanket of materials carried there from elsewhere. The thickness of the yellow tuff is unknown, but it is certainly greater than 180 meters. This lowering caused intense fracturing and dislocation of yellow tuff with a subsequent state of instability and danger, and landslides, on the slopes along the steep Flegrean side of the hill. That is why there are frequent cave-ins and landslides. This is one of the most delicate and dangerous zones in the urban area, where any human attempt to intervene and build roads, buildings or other structures, has to be undertaken only with greatest caution so as not to upset irreparably an already precarious balance.

CONCLUSIONS

We now see that geology can make a substantial contribution to dealing with technical problems that have to do
with the subsoil if Naples. First of all, we have seen that in the urban area, there are a number of craters, often overlapping or intersecting, and that we are still not even totally sure precisely where they are or how big they are.
These craters are largely composed of lapideous yellow tuff, but within single units of yellow tuff there are also only slightly coherent pyroclastic materials, patches of pipernoid tuffs, and volcanic breccia, not to mention lava domes and expansions. The various craters are further interesting because of collapses and tectonic dislocations that have faulted and subdivided the yellow tuff (and the other formations within it), creating a dangerous state of disjointedness and fracture along many lines (not all of which are identified and/or located). We have evidence that the yellow tuff, before the deposit of the overlying blanket of loose materials, was profoundly modeled by erosion, which is why the roof of this formation still has an irregular and quirky contour. Finally, we have seen that within the mass of yellow tuff, there are very many cavities created by man over the centuries, either to extract stone for construction material or to make tunnels and shafts for roads, train, cable-cars and elevators. Most of these cavities (some of which are very large and deep) are known to us only from historic and literary tradition, while we remain ignorant of exactly where they are, their size and sometimes even the location of the entrances.

Such a situation obviously creates difficult problems that range from projecting and building urban infrastructure to determining the causes of shifts in the soil and subsoil. There is no doubt that a resolution to the problems depends on an exact knowledge of the geological make-up the area. Geology can be equally helpful in resolving the geo-technical problems of dealing closely with the loose pyroclastic materials that overlie the yellow tuff or replaces it laterally in the eastern part of the urban area. We have seen that the mechanical, physical and granulometric characteristics of these materials can vary greatly, depending on whether we are dealing with disturbed or undisturbed materials. It is, thus, clear that we should determine the nature, the genesis and the environmental state of such deposits by means of an accurate geopetrographic study. Knowing whether the material is disturbed or not
can also help in reconstructing a buried morphology and lead to solving many technical problems. Among these same disturbed materials, a precise description of the surroundings in which sedimentation has taken place can help us plan further geotechnical studies.

Deepening our geological knowledge of the subsoil of Naples is absolutely indispensable for solving these technical problems, and we believe we have shown this in the studies we have carried out for the Commission. Indeed, these investigations, besides letting us determine just how much we know about the geology of our subsoil, have also
provided us with very useful pointers for particular problems such as the earth slides that alarm the citizenry. They are, in point of fact, the very reason the Commission was set up in the first place.

Chapter IX of this report will deal with the cause of slides and cave-ins. Here, below, we shall simply highlight some geological, lithologic and morphological factors that bear on the topic.
These factors can be briefly summarized:
    —notable upwards slopes on the surface in some hill zones in the city with an accentuated inclination of the                tuffaceous substratum;
    —tectonic fractures in the tuffaceous formations at the points where many excavations have been cut into both the        bases and halfway up the hills;

left - image 37            right - image 38

    —extended and often deep blankets of materials disturbed by surface waters and artificial run-off in some of the            particularly steep areas;
    —in both the semi-coherent pyroclastic materials and the lapideous
       tuff formation there are underground cavities that attract water and cause underground erosion;
    —alternating semi-coherent pyroclastic materials in the same
       lapideous tuff formation and tectonic fracturing in that formation as well as fracturing caused by earth slides in        tunnels and other underground structures.


Where those factors come together, they create very dangerous situations that can degenerate rapidly and suddenly. A geo-technical map of the area might help indicate and “flag” such spots as potential slide areas. A complete analysis of the situation, however, requires other, non-geological factors, which we do in Chapter IX of this report.

The geo-technical map that we have edited is a synthesis of all the information we have collected and collated on the urban subsoil. Our work would not have been as complete as it is without the notable amount of earlier data and documentation stored in a number of university institutes, at various agencies, private companies and state offices such as the State Mining Agency (which furnished a census of underground cavities). We can only imagine how much more important and complete our work might have been had it not been for the loss of voluminous amounts of data in the period following WWII, a time during which the city was rebuilt and the number of dwellings increased by half. The data was lost due to the lack of a municipal office that might have undertaken to save and store it.

It should also be remembered that our work covered only the urban area of Naples. This area is the most important in terms of density of dwellings and potential problems in the subsoil, but it represents only 20-25% of the area administered by the city of Naples. The time allotted to us forced us to exclude a large area that has its own subsoil
problems that will become pressing in the near future. The investigations of the subsoil can by no means be considered exhausted with the production of this geo-technical map or any of theother works done at the behest of the Commission.

That is the reason we warmly support the institution of a special section within the City Technical office, staffed by technical personnel with adequate equipment. The special section would collect and coordinate data on the urban subsoil and keep the work started by this Commission up-to-date. But it would also investigate special problems (for example, ascertaining the stability of hill roads, or the stability of quarry surfaces or retaining walls). It would issue its own opinions on whether regulations are being followed in the building of foundations and retaining walls, regulations that this Commission has suggested be adopted for the issuing of building permits.

Naturally, such a special section, as well-intentioned and efficient as it might be, would not be able with its own means and personnel to solve all of the complex problems connected with the subsoil of Naples. We need other instruments and legislation that would draw upon private citizens as well as construction firms and others (public or private) and thus make it possible to collect all of the necessaryinformation towards a complete understanding of the subsoil of Naples.

For particular problems (such as locating and surveying underground cavities or verifying the stability of retaining walls) we urge investigations by outside specialists and experts. This program may seem too chimerical, too futuristic, but we stress that if the problems of the Neapolitan subsoil had been approached in this spirit by the very first of the many commissions set up in the last century and down to the present day, we wouldn’t still be at the beginning of all this. We would have quite different and very valid documentation at our disposal. And we wouldn’t have thrown away enormous amounts of money.

This is the only way, in our opinion, that we shall ever have sufficient knowledge of the subsoil of Naples, knowledge that will help us solve specific problems that range from the stability of individual buildings to the general renewal of the underground infrastructure.


This is the end of Part 2, Chapter 2

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