The iron beams ceiling is the sign that the underground structure is not sufficently deep or, more likely, that the whole defensive system is made from concrete.

The main dwelling room with vault ceiling of a deep underground work. After the consolidating of the rough rocky walls and ceiling, inner walls and the vault were built.

One could think that his concrete work was left when under construction; it reveals the concrete roof  (at top) and the concrete inner walls which would sustain the reinforcing iron beams. More likely they made a very precise extraction of the steel beams. 

A destruction attempt suffered by this underground room has caused the vaulted ceiling to collapse. This reveals the thickness of the concrete vault (top center): about 40 cm.

The entrance was to be protected by a light weapon whose embrasure  peeps out at the end of the corridor.

A good example of ceiling reinforced by iron beams. The upward stairs lead to an observatory on the surface.

To center number 2. This writing on a tunnel wall indicates the direction towards the second  resistance center constituting the overall work is.

To center number 1. Written inside the center number 2, this indicates the direction towards center number 1.

This writing states that the picture was taken inside the consitituing center number 1. It in addition says that one has to turn left to get the two positions or to turn right to reach the main recovering room.

Some more details about the structure of medium and large size defensive works

The local centers of a medium size center of resistance were almost completely independent from each others. Infact, having  its autonomous supporting underground structure along with a local ventilation system, each of them could get a certain degree of self sufficiency and independance from the rest of the medium size work. In this case, the medium size work of the first type may then be considered as made up by a series of smaller units (which were in turn small resistance centers) completely self sufficient so as not to compromise the overall firing capacity in case one center had completely been put out of order.

Anyway, these separate centers hadn't often the total independance from the others since many a center (generally two) had to share at least the same electrical power generator (maybe the ventilation system was considered essential, while defenders relied much on the capacity of the accumulators used to power the pumps).

A typical situation in which a medium size work was made up by smaller independent resistance centers is the protection of a road crossing an alpine pass: one center is placed on one side of the road and a second center is placed at the other side of the road. A gallery beneath the road links the two centers.

The second type of medium size work is the following: the whole structure is not made up by far and well distinguishable centers, thereafter it is impossible to observe any distinct base units making up the whole work. The structure may be complex and extremely variable. Infact, if we consider by definition the base unit having an independent ventilation system, we can easily detect all the ventilation pumps of a medium size work, but it's hard (and somehow useless) attributing that or this position to that or this possible center. The problem doesn't of course sussist if the constituting centers are well separated (first type structure).

Medium size fortifications can be seen on type A or B defensive systems.

Large size defensive works 

The same distinction can be observed in largest defensive works. First type: works whose structure is constituted by many properly organized independent base units which are in turn gathered to make up either well separated centers of resistance or the whole work.

Second type: large works whose combat blocks and logistical systems are organized in so complex a structure that it is impossible to observe any self sufficient base unit.

It can sometimes be observed that a defensive work of the first type may be actually formed by two or more large defensive sub-works connected by long galleries. Each of these large sub-works is in turn formed by as many as two or more smaller sectors each of them is actually constitued, as seen above, by some independent base units.  The overall result is that a certain number of the largest works is made up by many smaller centers having an autonomous logistical part serving one or more positions.  In these cases each unit  may be considered as a branch of the whole opera

The second type is somehow more common; infact most of   largest works  have not so well organized a structure. The adopted solutions can be so varied that both types seems to coexist: for instance, a large center is subdivided in two well distinguishable centers, but each of them has in turn no base units.

It seems obvious that the whole bunch of weapons of a large work was hierarchically divided in groups due to reasons attributable to the military organization: the work commander was over the two or three commanders of the two or three large sub sectors constituting the overall work and each of these sub-commander had in turn the responsability over the smaller sub subsectors (a battery for instance) by which their sub sector was made up and so on up to the commander of a couple of positions. The question is whether these subsectors can be in turn considered actual smaller resistance centers having a certain degree of self sufficency (first type) or they cannot be considered as stand alone centers  (second type). As mentioned before, the answer is uncertain for a few largest centers and it is no for most of them. Infact, a large work could be made up just by a large group of positions without any intermediate structure or clearly separated subsectors having each an easily attributable logistical part. In some cases, the structure of a large work is so complex that it is difficult to attribute any positions or observatory  to any smaller sub-sectors being themselves in turn difficult to point out.

Even if it were always possible to detect a constituting base unit, the question would be about how independent from the others each unit would be and which would be the minimum set of elements necessarily present in each of them. I would answer saying that each base unit has to have an independent logistical part with at least a ventilation system and an observatory (i recall that im looking for elements usefull to find a physical structure corresponding to the hierarchical division of the work and an observation post means a commander of the unit).

Sometimes, besides the shared power generator, two distinct centers had to share the same security exit and this prevented them to be two completely independent units (rare indeed situation).

To sum up what can be actually seen in the field: a part of medium size works are well structured and made up by separate smaller units. In most of larger works it is impossible to distinguish any independent base unit.

Enough of the thesis about the independent base units.

A bit of confusion rises when a medium size center of the first type was not called and considered as such if the constituting smaller centers were far from each other: in this case the two centers  linking gallery was very long and the whole defensive work was indeed considered  being two separate centers (unless the document referring to that defensive work goes back to a time prior to the construction of the linking tunnel). In my opinion, when two or more centers are linked by a tunnel, the overall structure becomes that of a single defensive work. Although made up, in this case, by two well separated and far independent centers, the overall system had only one electrical power generator .

What can be in addition said as an observation fact is that the smallest defensive works are constituted by a single center of resistance with one ventilation system and one electrical power generator. Many medium size works have two or more autonomous ventilation systems and generally only one electrical power generator. Largest works have many, more or less independent, logistical structures having anyway each of them its own ventilation system; the overall work has a certain numer of electrical power generators which generally are not so many as ventilators.

This last sentence could be explained saying that being ventilation pumps all the same (to be verified!), the number of  served positions and compartments are similar; while, being electrical power engines able to be coupled, the served area can be bigger.   

Anti tank gun and positions number 4 and 5 are reachable turning to the left.

This position is hidden and protected by a very solid concrete box built against the slope.

Weapon position number 1. Lots of original inside walls writings are in so good a condition that it seems as they were refreshed last week. Black dots are cricket like insects enjoying the fresh darkness of the underground work.

This concrete house was the base used to build a stronghold on a mountain highland area . At the left (just below the hill shape), the reader's keen eyes could point out a work entrance; other machine-gun emplacements lie on the right slope. The picture itself was taken from where there are three other outposts.

Heavy machine-gun position hidden inside a huge concrete structure whose inner core is in turn hidden by an additional concrete layer used for camouflaging   .

When steel and iron resources were to be diverted from the Vallo and began taking the way towards the Navy, cement was to be used even more deeply. This machine-gun emplacement lies inside a light casemate on a mountain area.

This is not the embrasure of an underground position since the   firing chamber is not dug into the rock but made by steel and concrete; anyway it looks like as it were underground.

Mount Razor (2601 m) peeps out from the clouds. This is the highest sector cut by the old border: Julian Alps area is where it is much more likely to find underground works and positions.

 
The missing camouflaging layer reveals the concrete core of this casemate. Frontal wall thickness is only 1,5 metres because it is protected by a huge steel armor (not visible from the outside).

This picture shows the inside of the casemate shown on previous photo. The square shaft links the firing chamber (foreground at bottom) to the inner compartments (7 metres below). .

Firing  chamber  armored by a huge steel plate (center, at top). Besides the frontal plate, horizontal girders below it and vertical steel beams (at right) complete the chamber armor.

A typical block in a mountain area. The circular structure is lighter than lower altitudes ones: the wall thickness is 100-120 cm, the concrete is reinforced by iron and the block hosts two positions (in a single chamber). The second opening is hardly visible at right.

An example of a lower altitude casemate. The thickness of the concrete core (from the plate to the outer square of the hopper) is 2 metres to which 2 more metres of concrete are added for camouflaging reasons. Sections of the outward splaying are, in most cases, more smashed rectangles: the outermost section here, on the contrary, is 143 cm (height) x 156 (width).

This hill is where a small mixed work was got. At center a concrete block with two positions is visible (a truncate green cone) while a casemate shape becomes blurred getting mixed with the hill slope (center, at right).

The additional camouflaging layers make the shape of this combat block completely indefinable.

Here it is the outdoor appearance of a block camouflaged so as to look like limestone rock.

Blocks are very common in eastern Vallo. Nothing in general can be said about their looking: they may have any shape and height depending on the weapons placement and the terrain. Sometimes do they look like huge truncate cones or smashed cylinders: more often, like casemetes, their shape is indefinable. 

Many casemates and blocks underwent damage or destruction by explosive during the years of the cold war or just after the second world war. Jugoslav army took away every steel or iron device and tried to delete every possible sign recalling the italian administration of western Jugoslavia from 1919 to 1943-45. Lighter blocks and casemates have been swept off, but the heaviest ones suffered little damage. The reason why most standed these destruction attempts is not only the impressive hugeness of the blocks themselves, but it could be perhaps explained by their inner structure. The underground system from which vertical linking shafts begin and the upper block are not two separate structures by they come from the same massive concrete layer. So, the block shape one can see on the surface is just the iceberg top because the same shape is kept underground and it deeply goes down to the underground sector of the defensive work. So the block structure is not limited at the surface but it was got by a single concrete layer filling an enormous vertical hole dug in the rock and as deep as the linking shaft.

This picture wittnesses a unique case amid those seen so far: a downward flight of 72 steps leads to weapons based at a level lower than the underground logistical part.

A shaft seen from its base. The upper concrete platform is visible along with the iron poles on which the circular stairs were mounted.The iron girder reinforcing the platform shows, at its central end, the hole into which the central pole of the stairs was inserted.

The circular wall surrounding a shaft is used to get a light weapon embrasure to protect the entrance corridor. The first steps of the circular stairs are exceptionally made from concrete.

An upward square section shaft leading to a combat block. At the picture top two rings are where separate tubes for two positions were inserted. Also visible: on both sides the supports on which two small manual or electrical lifts are supposed to have been working and, at top left, remains of electrical wires. 

An example of the concrete circular stairs leading to upper firing chambers. The stairs take three complete rounds to overcome as many as 7 metres.

Being the logistical part the deepest structure of the work, most of the positions on the surface were reachable by more or less steep and long flights of upward stairs.

In many cases, the combat blocks were directly connected to the inner compartments by a vertical shaft  which may have been as high as required by the depth of the underground part (8-10-12 metres or more). Soldiers had to climb up the shaft to reach the blocks on the surface.

The shafts have approximately square or circular sections. If the shaft is square, its upper mouth is flush with the upper foor and it's open (sometimes with a trap door) just behind a  firing chamber. If the shaft is circular, its upper opening is a vertical door linking the shaft with the corridor between the firing chambers. The passage is protected by the usual steel door sealing off the combat block.

The circular shaft was where circular iron stairs were placed: the iron structure had a central vertical pole on which the steps were welded. The circular stairs were sustained by vertical iron poles which were thrusted on the concrete walls of the shaft. The last and highest step was indeed a reinforced concrete platform. The central iron pole was bent at the platform to make a sort of protective railing. Electrical power lines and the fresh air distributing tubes ran vertically up the inner walls of the shaft. The shaft diameter is about 150 cm. In some cases the whole round stairs and central pivot were made by  reiforced concrete.

If the shaft is square, an inside ladder was got by iron clamps thrusted into the concrete walls. The section is almost square but it is slightly smashed to make the climbing more comfortable (about 80cm (width) x 90-100cm (depth)).

If the position is in a casemate, the shaft is generally square. Blocks with two or more positions generally have a circular section shaft which was somehow more comfortable being broader than square one. There may also be a square shaft serving a block with two positions and emergency exit, but, in this case, the shaft section is bigger than the section of a square shaft connecting a single position. 

Circular stairs steps were low enough to facilitate the upward carring of weapons parts and ammunitions. On the contrary, in case of shafts with square section, remains of rails on which manual lift are likely to have been working, are still visible.