A few technical notes about eastern Vallo Alpino

Chapter III


Three parts armors

Minimum thickness plate

Metal machine gun casemates

Metal Turrets

Metal active observatories

What has this hole been dug for?

The machine-gun emplacement of this casemate is protected by a typical armor whose main part being fully buried into the massive wall, was surely set on place during the construction of the concrete casemate itself. The forged writing at right states: "FONDERIA MILANESE DI ACCIAIO VANZETTI 1931".

From inside the firing chamber, the main steel plate (center) is only partially visible as it is completely buried into the massive concrete frontal wall. It supposedly must cover a vertical surface as large as the other two parts and nearly as high as the whole frontal wall. The other two parts whose junction lines are clearly visible (right and left) make up a shield like protection for gunners.  

From outside the casemate too, is the main plate of a three parts armor only very partially visible. In this picture case, much more plate than usual can be seen: usually the concrete wall covers all the steel armor around the weapon opening.

The steel plate is placed about 45 cm from the  concrete wall inner surface (bottom, at right). At the firing opening , it reveals a thickness of about 35 cm.


Both in underground and surface weapon chambers, where the thickness of the foremost wall was considered not sufficient with regard to the shots it was supposed to stand, a special tempered steel plate was used. Depending on the weapon, the kind of action and the level of protection required, there were a number of possible steel plates reinforcing the frontal part of the firing chamber. The heaviest plate was as thick as 20 cm and most of armors were made up of many a steel part assembled at the firing chambers and held together by reciprocal fittings and strong bolts.

Apart from simple plain plates, there were two main steel plates types used to enhance the frontal protection: the first was referred to as the three parts armor while the second type of plate is called minimum thickness armor. They are both very common in eastern Vallo.


Three parts armors

This type consists in three parts: frontal or main part, ceiling and basement. The frontal part is the strongest of the three: it is a vertically set plate having the weapon opening and it is almost completey buried in the concrete. What is left fully visible is infact only a rectangular plate which seems to be the thickest part of the central plate.

I have never been so far given the possibility to completely see the shape of the actual frontal plate, nor to verify the thickness of its remaining biggest part being completely buried in the concrete frontal wall. Anyway, I guess the plate must be much wider and higher than it can be seen. The central rectangular plate is completely visible from the chamber and partially from outside. Although the rectangle is as thick as about 35 cm, the thickness of the invisible part is supposed to be smaller.

The second and the third element of the armor are joined so as to constitue a shell-like shield protecting the gunners and they are visible only from inside. While these two other parts could be assembled once the combat block was finished, the central steel plate having so big a weight and volume, required to be put in its deep position during the construction of the block itself.   

Where the three parts armor was adopted, the thickness of the concrete wall can reach as many as 3 and half metres. More precisely, the most common noted measures are: the concrete thickness of the casemate frontal wall ranges in 210-230 cm. To this thickness further 120-150 cm are to be added due to  the camouflaging concrete layers. Thereafter from the external side of the plate to the outermost opening, the overall thickness is within 330-380 cm. The biggest the casemate or block, the more likely its chambers are protected by three parts armors.


The minimum thickness plate is bolted on the wall which is in turn reinforced by underlying horizontal and side steel beams.


This armor was given the name of minimum thickness armor with regard to the thickness of the outer concrete wall which in this case might have been thinner than the foremost wall of the three parts armor chambers. Infact the 20 cm armor was considered sufficient to stand heavy and medium calibre shots and this massive steel plate allowed the outer concrete wall to be only 1 and half metres thick. The plate thickess  was then exploited where the overall disposition of the defensive work did't allow engineers to adopt the usual bigness casemates or when the terrain was not suitable to hide such structures.  

Small works defending roads and having embrasures quite on  road sides  are usually equipped with minimum plates. Chambers of twin blocks are three parts armored and i sincerely have never seen blocks with minimum plates which seem to be mounted just in stand alone casemate. But, as usual, these personal observations may be confuted by any further visits.

The plan of the firing chamber depends on the type of adopted armor; infact if the armor is a three part one, the chamber plan is rectangular (about 80 cm x 300), but if the armor is a medium thickness the chamber is smaller and trapezoidal (back side: about 150 cm, foremost armored wall: 110 cm, depth: about 115 cm).


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Apart from higher bush which grows all around, the picture shows how the embrasure of a metal casemate should have appeared during operative times.

An intact casemate steel cupola protected by the concrete structure.  

Another example of metal casemate: in this case the steel cupola is almost completely buried and so overgrown by bush and trees that it seems impossible to recognize the reinforcing concrete scructure all around.

Although overgrown by weeds and leaves, here it is how the embrasure of a complete metal casemate appears. 

The steel cupola seen from the pipe base. The fitting between the cylinder and the cupola is remarked by the junction line running all around the inner surface of the structure. The weapon opening is hardly visible at the center above the junction. At bottom right, one of the 6 cast bases on which the grating lay is visible.

This steel cupola has been broken open by an explosion which tore half structure off and left the other half still bolted on its side steel pole.The firing opening was at right. Unfortunately this picture gives only a blurred idea of how thick the cupola was (30 cm at right, 22 cm at left). 

This is the overall structure of a common metal casemate for a heavy machine gun. The flat cylinder shape and the overlying cone like concrete structure are clearly distinguishable since the structure was strangely left without any camouflaging measures.

The same casemate of the previous picture is taken frontally. 


Machine gun metal casemates

A metal casemate for a machine gun is a particular concrete casemate whose firing chamber is actually carried out by a few heavy steel cast elements which are strictly fixed together. The most common model of this bomb proof metal box is constituted by two parts: a steel cylinder and a solid steel cupola having the fire opening.

A 3,5-4 m deep shaft dug in the sheer limestone rock is where the cylinder was vertically placed: then it would be top covered by the cupola. The cylinder has not a circular transversal section, but its section has a characteristical rounded trapezoid shape (its inside central axes are: 156 cm (tranversal) and 90 cm (firing direction)). This cylinder is 240 cm high and, at its lower back side it's round shaped so as to form a door like broad opening. Infact, soldiers had to enter the metal cylinder through a passage at its base and reach the firing chamber by internally climbing the pipe by means of a vertical ladder. Gunners standed on a iron grating which was fixed on 6 iron basements: they were welded 62 cm below the upper end of the cylinder. The grating has a trap-door at which the ladder ends. 

The thickness of the steel cylinder is not constant but varies from 15 cm at the foremost side to 8 cm at the back side. The cylinder upper part exactly fits the lower section of the cupola; infact the external side of the cylinder upper end is rounded so as to strictly get the steel cupola on it. Viceversa, being the cupola thicker than the cylinder, the internal surface at its base has a suitable hollow on which the upper end of the cylinder is inserted. When on place, the steel cupola thereafter externally overlays the upper end of the cylinder and the ovelaying is about 30 cm long. So the cupola lies on the cylinder and the two elements are rigidly held together by the described fitting. 

Besides being tightly fitting the cylinder, the cupola was rigidly held in place by strong nuts working on two long iron poles (67 mm diameter) which were thrusted into the rock so as to come out beside the shaft. The cupola was bolted on the two poles by two lateral ear-like rings protruding from its external faces and cast on the cupola structure itself. These two steel poles are oblique. Two other poles were thrusted into the rock beside the cylinder base. Unlike the poles the cupola was inserted on, these two additional poles were vertical. While the distance between the oblique poles lets one suppose that they were properly used to externally fix the cupola, the vertical rods fixed  the cylindric base since the distance of the remaining vertical poles matches the cylinder width. Both oblique and vertical poles can be observable after having each steel element been taken. But  having the vertical stumps different height, it can be argued about how near the rocky bottom they fixed the cylinder base. 

As for the thickness of the steel cupola, it gradually varies from the back side to the front through the lateral faces. The minimum thickness is on the back side (22 cm) and it reaches its maximum at the foremost side (30 cm). Some sources state that the approximate weight of the cupola is 20000 kg and the approximate weight of the cylinder is 10000 kg. If we approximated the cupola as it were half ellissoid whose thickness is equal to the average thickness of the cupola, considering a density of 7,8 kg/liter, a simple calculation would give about 20 ton showing that the given cupola weight is consistent. An easier calculation would show the same for the weight of the steel cylinder.

All the iron structure got by the cylinder and the cupola is completely buried into the concrete so as to just leave the weapon embrasure visible: the solidity of the steel structure along with this reinforcing arrangement guaranteed the possibility to stand heavy calibres shots.

Metal casemates were adopted where the action was supposed to be frontal and the terrain flatness didn't allow engineers to adopt a conventional casemate solution. The presence of a massive steel armor didn't infact require the huge protecting concrete structure of a conventional casemate which would be almost impossible to hide on a flat terrain. Metal casemates are instead completely buried and well hidden in the flat environment.  Flat terrain means strong difficulties to adopt a casemate and no depth to dig anything: that is why metal casemates were commonly coupled with concrete works.

What can be seen outside a metal casemate is a circular concrete structure slightly protruding from the terrain and having a diameter of about 6 metres. This overall concrete structure indeed looks like a sort of very flat cylinder concentric with the inside vertical hole. This flat concrete cylinder is top covered by a smashed concrete cone which protects the steel cupola. The cone height is less than 2 metres. This flat cone has a missing 60 degrees sector corresponding to the weapon opening. Whatever the radius chosen to get a vertical section through the center, gunners were protected by three metres of concrete and the thickness of the steel cupola corresponding to the chosen radius. If the section is on the firing direction, gunners were protected by the usual foremost concrete wall which is as thick as about 3 metres and the additional 30 cm thick steel plate of the cupola front side. The whole structure shape is of course hidden since all is covered by earth and rocks. This is what can be seen on the surface. The question is about how deep the 6 metres diameter concrete structure is. It seems likely that the concrete structure has a certain depth but the solidity of the whole structure is given by being deeply set in a rocky shaft. The surface concrete structure, though important to hide as much armor as possible, has the aim to give an additional protection by filling the spaces between the steel structure and the sheer rock. For sure (well visible on blown up metal casemates), the shaft is dug into the rock and the circular concrete structure hiding the metal chamber strengthens just the cupola and the upper part of the pipe (i.e. the metal firing chamber).

Constituting steel elements were two or four depending on the model. Military commands advised that the two elements model should have been adopted whenever possible since the four elements model should have been left for cases of actual transport difficulty (hardly reachable points like works on mountain peaks).

Apart from the number of elements, there were some variants of the same model depending on the vertical firing range. Each of these versions could be normally cast from a few steel leagues. The most frequent was a common league of iron and carbon: the carbon steel gave a good tradeoff between hardness, tenacity and weight. Temper procedures were surely carried out but, in spite of the excellent resistance, the lacking of more noble elements caused the structure to presumably soon be subject to a generalized corrosion. Regardless of the vertical firing range and the used steel league, all versions had 60 degrees horizontal action.

The cylinder base is normally connected to the underground logistical part by a flight of downward stairs whose steps number ranges from one or two to many dozen. As any firing chamber, metal casemate were sealed off the rest of the defensive work by an air tight closing door placed near the base of the stairs flight on the linking underground corridor.


This is one of many dome-shape cupolas which lie intact and forgotten in their 70 years long rest. It is interesting to note that what 70 years ago surely was a  superb look-out on a prominent spot, in present times is only a poor clearing of the wood which seems to swallow any sign of the past. Embrasure shields have been obviously removed.

This  lighter model of metal turret has been set free from the surrounding concrete scructure so as to be easily taken away. The lower part of the turret is fully inserted into the base cylinder (Photo: courtesy of Tuttostoria magazine) .

This cylinder was the base on which a turret was inserted.  The reader may note three more things: the base cylinder was broken by an explosion which occurred outsides to get rid of the concrete structure. A groove hollow (for the turret to be fitted into) is visible at the most left part of the steel cylinder and, finally, the usual square shaft inside the structure.

This picture shows an attempt to hide as much steel cupola as possible by means of limestones (Photo: courtesy of V.Tonic).

An embrasure of a turret shows the emplacement for the rotating shield and the weapon.


Turrets are among the most spectacular and easily recognizable metal structures of the Vallo. They were used everywhere: from western Alps sectors to eastern ones including the defensive works in Slovenia, also around Rijeka (Fiume) and Zadar (Zara) in Croatia. These active observatories are constituted by two steel cast elements: a cylinder and the turret itself which is a massive element whose visible part has the distinctive dome-shaped roof.

The turret inside is reachable from the underground rooms by climbing a shaft dug into the rock by means of a ladder. Vertical shafts serving turrets have variable depth depending on the structure and the depth of the underground logistical part. Normally, the shorter the shaft, the longer the walkways with upward stairs connecting the turret chamber to the underground locals. But it is clear that it is impossible to give a general rule as to the shaft depth and the connecting flight of stairs, since it simply does't exist. Anyway, just to give two examples, it sometimes happens to see a square section shaft of just 2-3 metres at the end of a long flight of stairs with as many as 25-45 steps. In other defensive works a 5-7 metres vertical shaft is reachable at the end of a 4-10 steps stairs. The shaft normally has a square 90x90 cm section but in some cases it may have a broader circular section.

Whatever the section of the shaft connecting the turret chamber with the inner rooms, a steel 1.2 m high cylinder is placed on a concrete surface so as to protect the surface mouth of the shaft. This cylinder is the base on which the turret was inserted. Infact, the upper external side of the base cylinder is suitably rounded so as to fit a proper slot on the turret. Moreover, the turret is fully inserted on the base cylinder and strictly held on it by a few vertical groove-tongue fittings: grooves are well visible on the outside surface of the base cylinder. The cylinder is in turn rigidly held on place by three long iron poles which are deeply driven into the rock. These long screws are placed at the corners of a 1.67 m side equilateral triangle. They have a diameter of 65 mm and the bolts have 120 mm sides. The external diameter of the cylindrical base is 180 cm and its thikness is 12 cm.

The same way like in metal observatories, shooters standed on a grating being simply placed on the square shaft mouth. If the shaft section is different there may be a concrete platform with an opening instead of the steel grating with the trap-door.

The turret is a single steel cast element with four embrasures and it could be armed by one or two machine guns whose shooters were protected by circular shells covering the embrasures as shields. Infact, turrets were not intended for soldiers to use as mere observatories, but their functionality included a certain fire action. Moreover, having to possibly cope with infantery attacks on the surface, turrets had to have a wide 360 degree action range hence they coudn't be entirely buried by concrete or earth. Having to be left mostly uncovered by concrete or rock, turrets must have been equipped by armors whose average thickness is about 20 cm.

What can be seen from the outside is what we normally call cupola and it is just the uppermost part of the turret. Infact most of the turret is actually buried into a broad concrete cone like structure. This surrounding flat concrete cone, which protects and strengthens the metal turret, is as high as the four embrasures level. As already said, since the turret has to be fully inserted on the base cylinder, its buried part actually hosts a huge cylindric hollow which is suitable to precisely hold the base cylinder inside it. Admitting that the turret is inserted into the base almost down to the concrete surface, the inside observatory chamber is consequently smashed and it was as high as only 170 cm. If we consider that the average thickness of the cupola is 20 cm, the outside overall height of the turret must have presumably been about 190 cm.

Being the whole structure round, gunners standing inside had the same protection on 360 degrees. Following the trajectory of an hypothetical bullet, to reach the gunners it should have perforated the usual 3 or more metres of the outside massive concrete cone, the turret and the base cylinder.

As for the thickness of the turret, it was of course bigger where it had to be left nake on the surface, while it was presumably smaller where the turret was buried and protected by the circular concrete structure .

There were indeed a few different models of turrets depending on the metal league (thereafter on weight) and the thickness of their cupolas. Lighter turrets had cupola thickness of 10 cm; heavier had 20 cm thick cupolas. Models could also vary due to their cupolas shape: a model has, for instance, more rounded lines, another has a more squared roof line. Regardless of these little weight or shape differencies, the whole structure concept, the embrasures number and the overall dimensions were the same. Being the slot fitting, for the base cylinder to be inserted in, standard, interchangeability may have been kept and different thickness turrets may have been inserted into the same base cylinder.

Although being mainly used as observatories to guide the artillery shots while observing the attackers' movements, these turrets could be used for relatively short distance fire being able to protect the surface of the bunker; this is why they were almost always built on the highest part of the defensive work at isolated and prominent spots.

In small strongholds of type C defensive systems, it is not rare to observe small stand alone defensive works constituted only by a turret and its logistical underground structure. It is as Italians had ultimately built these small works to enhance the weak observation functionality of the overall stronghold thus complying the directives of the circular number 15000.

This is the turret of a common armored observatory set on a mountain summit.

The back side of another observatory is here taken. (Photo: courtesy of V.Tonic). A few details are worth noticing: the generalized corrosion due to the simple carbon steel league (unlike the upper elements of the previous picture observatory), two backward looking smaller windows, the camouflaging limestone rocks thrusted into the circular concrete structure  and the junction line between the dome top cover and the proper armor.

The innermost steel cylinder has a door like opening through which the observatory chamber is reachable.


Inside the observatory, the junction on which the main part of the cupola and the 45 cm high cylinder are welded is visible in the foreground (see the text for explanation and details).

The same view as on previous picture but taken below the grating which lies (right) on the step beteween the lower cylinder (bottom) and the upper one. Let the reader note that having both cylinders the same axis in the front-back direction, the grating hasn't any step to lie on along this axis (bottom, left).

The three part constituting the cupola (observatory turret) are here well visible; the core (lower part), one of whose cast rings appears above the concrete surface, is where the outer armor with the openings (middle) and the dome-like top are inserted.

The explosion has torn the cupola of this observatory off its rings one of which is still inserted (center, bottom) into its pole. The 45 cm high cylinder, the grating and a vertical bolt at which the cupola broke (top, left) survived the expolsion.

This picture shows in detail one on the steel poles of the observatory taken in the previous photo. Being the explosion not sufficient to tear the pole out the rock, the cupola broke open at its side rings and vertical pole by which it was bolted on the underlying cylinder.

Metal active observatories

The concept is similar to metal casemates: steel pipes were vertically inserted into a shaft dug in the rock and the outermost cylinder was top covered by a steel cupola. The cupola has 6 openings: three big ones on the front side and three backward looking smaller ones.

Besides binoculars, armored foremost openings, in case of fire action, could offer suitable emplacements for a light machine gun, while smaller, horizontal shaped backward looking openings could be for pistols to be used.

The overall structure of the metal box seems simple to describe, but understanding its constituting elements and how they are fixed one into another is quite complicate either to understand while observing as well as to describe. It seems not unlikely that any details i'm about to give may be slightly refined by future observations. Anyway, the following notes about the cast elements of an armored observatory seem good enough to give a sufficient global idea of how they were built.

The most common model of this armored observatory is constituted by some iron cast elements: an underlying steel pipe and a steel cupola protruding from the surface. The pipe is not a constant section single cylinder but is's actually made up by two steel cylinders with different sections (unless the two parts were originally welded to make up a single piece). The deeper element has a circular section with an inside diameter of 90 cm. This element is 2 metres high and its lower back side is round shaped so as to form a narrow door. Infact, like in metal casemates, soldiers had to enter the metal structure through the door at its base and, to reach the observatory, they had to internally climb the pipe by means of a vertical ladder.

In most cases the lower cylinder is connected to the underground structure by a more or less long flight of stairs. But, it is interesting to note that if the defensive work structure didn't allow the adopting of the linking stairs and if the depth of the logistical part required a shaft deeper than 2 metres, a shaft of proper depth could have been dug. The 2 metres cylinder was anyway used on the highest part of the overall shaft and its narrow door was in this case fully useless. The deeper part of the shaft is normally lined up by concrete so as to make up a circular section shaft whose diameter is the same of the metal pipe. The concrete shaft below the metal pipe is usually overcome by an iron clamps ladder.

The previous description serves as one more example confirming that in spite of being steel elements always the same, they could be normally arranged in very many configurations according to the particularities of the terrain.

As said, the upper part of the 2 metres high cylinder is welded to another metal cylinder whose section is not circular as the lower one but it is rounded trapezoidal: its transversal inside axis is 106 cm while its longitudinal (firing direction) axis is the usual 90 cm. This second cylinder is just 45 cm high and its thickness is 10 cm. Lying this upper cylinder on a smaller section cylinder, the step caused by their transversal axes difference is used as a base to emplace the grating on. The grating is where soldiers stood while being inside the observatory chamber whose access was by means of a trap-door.

The cupola is actually made up by three steel cast elements whose reciprocal fittings are described by the following description. The first and most important part constitutes the core of the cupola: it is a steel part whose inside section has the usual rounded trapezoidal shape (inside: 106 x 90 cm) since its tight lying on the 45 cm high cylinder constitutes the observatory chamber and its innermost armor. If observed from the inside, the cupola infact seems to be directly lying on the 45 cm high cylinder and there seems not to be any additional junction lines. This innermost element has a lower part which presumably is as thick as 10 cm, but though such a thickness could host a slot to fit the underlying cylinder, there seems not to be any and both touching faces (core and 45 cm high cylinder) are flat. More precisely the upper surface of the cylinder has indeed two 7 cm thick blades which guided the core fitting upon it. The cupola core and the cylinder are rigidly held in place by four huge nuts working on four steel rods (60 mm diameter) placed along the main axes.

Like for metal casemates, two 45 degrees oblique poles come out from the rear side of the observatory. These two additional  poles are thrusted into the rock. To accomplish its inserting on the poles, the cupola core has two ear-like oblique rings protruding from its external face and cast on its structure itself.  So the core was either inserted on the oblique poles and strictly bolted on the underlying cylinder. Having never found these additional four bolts left after any metal observatory full removal, i deduce that they are not thrusted into the rock and, consequently being not so long as the oblique ones, they work as simple bolts joining the cylinder and the cupola core.

The cupola core makes up the whole inside observatory chamber. Infact the outermost part of the observatory is carried out by a properly shaped cylinder which is inserted into the core by some groove-tongue fittings. This external cylinder constitues the armor of the observatory and it has either the three front bigger embrasures and the other three smaller back openings. Once inserted into the cupola core, the armored cylinder is held in place by some giant saw teeth fittings.

The third and last element forming the cupola is a heavy steel cast dome-like top inserted into both the core element and the armored cylinder. NEWTake a look at this drawing.

As mentioned many a time on previous discussion, being internal dimension 90 x 106 cm (front-back direction x transversal direction) and overall outside dimension about 130 x 146 cm, the thickness of the armored observatory is about 15 cm.

The whole iron structure is buried into the concrete, but the upper part of the cupola which is fully visible. So, the metal structure is reinforced by a surrounding concrete structure which looks like that of a metal casemate: a sort of very flat 6 metres diameter cylinder concentric with the inside vertical shaft hosting the metal pipe on which the cupola is inserted. This flat concrete cylinder is top covered by a smashed concrete cone having on its summit the steel cupola. The concrete cone strengthens the whole metal structure hiding the nuts of both oblique and vertical bolts but partially leaving outside just the armor with the openings and of course the observatory top. This flat cone indeed has a missing front sector corresponding to the front openings. This is what is visible on the surface. The question is about how deep the 6 metres diameter concrete structure is. It seems likely that the concrete structure has a certain depth but the big solidity of the whole structure is given by being deeply set in a rocky shaft. Though important for hiding as much steel as possible, the surface concrete structure has the aim to enhance the solidity by filling the spaces between the steel structure and the sheer rock.

The described metal active observatory has four elements (five if we consider the 45 cm high cylinder as standing alone and not welded on the 2m high cylinder). There was a simpler model with only two elements: a pipe and the cupola, but is wasn't used as often as the described heavier model. The subdivision of the whole metal structure into smaller parts of similar weight made the transport more comfortable. So it was easier for small trucks, cableways or mules to carry these elements where bigger and heavier single parts would have never been carried by any means available in those times. Once on place, all was mounted possibly by highly specialized squads of workers whose excellent work is witnessed by what nowadays still remains.

Steel elements could be normally cast from a few leagues. The most frequent was a common league of iron and carbon: the carbon steel gave a good tradeoff between hardness, tenacity and weight. Temper procedures were surely carried out but, in spite of the excellent resistance, the lacking of more noble elements caused the structure to presumably soon be subject to a generalized corrosion. Another metal league had inside some nichel chrome which not only offered slightly lighter elements with the same or better hardness, but it gave a better guarantee against the corrosion. Being cast elements obviously interchangable in spite of the diffent league they were made from, all parts buried and protected by concrete were simple carbon steel while elements (like armored cylinders with embrasures and rounded tops) which were more exposed to rain, wind and snow, sometimes could be made from the nichel chrome league.

As any other chamber exposed to gas attacks, metal observatories were sealed off the rest of the defensive work by an air tight closing door at the linking underground corridor.

Although similar, there seems to be a few differences between the pipe of a metal casemate and that constituting the armored observatory. A substantial difference is given by the overall dimensions; infact, metal casemates being much bigger and heavier than armored observatories, required broader shafts to be dug. Unlike the cylinder of metal casemate, the thickness of the observatory pipe is constant and it's smaller than the thickness of the metal casemate pipe. Moreover the cylinder of the casemate normally has a standard height, while the observatory pipe elements being thinner could be cut and fitted to a certain range of shaft depths.

Long rock thrusted poles are vertical for a metal casemate to be bolted on but slanting for metal armored observatories.

A little structural difference between turrets, metal casemates and metal observatories is that both turrets and observatories have at least their lowest part having a circular section unlike metal casemate whose pipe has the usual trapezoidal section.

Anyway, the main difference is the functionality of these metal structures: it'd be sufficient to say that a metal casemate is a casemate while an armored metal observatory is an observatory. It means that one structure protects the emplacement of a heavy machine gun whose frontal action requires a special regard to the foremost side, while covering the observation almost 180 degrees, observatory turrets were to be armored all around; not to mention that the action range of a metal casemate is 60 degrees while that of a turret can be 360 degrees.


This is what remains of an observatory. But was it really a metal observatory or a turret or a metal casemate? If the hole is indeed a shaft surrounded by a ring of spread concrete blocks, the shaft is much likely to have hosted one of the three named metal structures. Further left elements will then solve any doubts (see the text).

The presence of the two steel oblique poles shows that a metal active observatory or a metal casemate was once set into that shaft. Note that, both bolts still having their washers and nuts, at least one nut seems as it had never been loosened; infact the inside explosion must have broken the cupola core at its weakest points: its side ear-like rings.

This writing on an underground corridor reveals the presence of a metal turret at the end of that corridor.

Infact (look at the previous picture), at the end of that corridor there is a short square section shaft and the three typical poles on which the base cylinder was bolted (the third is hidden under the printed date (bottom, right)). Once taken the turret away, it was sufficient to loosen the three nuts to easily take away also the base cylinder without any explosion. Infact the three nuts are never left in these cases.

What has this hole been dug for ?

After the end of WWII, partizans and the jugoslav army normally took away any removable steel parts (doors, engines, ventilation devices, circular stairs, armors etc) and normally used the explosive to take away the fixed steel elements. Then, many casemates, turrets and metal observatories were blown up and what now remains is just the shafts into which these structures were set and a certain bunch of spread around concrete blocks. Having most of the main steel parts normally been taken away, how can i undestand from what of a structure remains if it was a metal casemate or a turret or a metal observatory? It sounds an interesting question. 

If one wanted to understand what a hole was dug for, one should of course first look for any useful writings on the inside walls of the underground part. If a writing and an arrow exist, the problem is of course solved. On the contrary, we have to look for other useful elements around the hole. First thing to observe is the position of the shaft with regard to the work structure and the physical disposition of the terrain. Observatories and turrets are placed on the highest spot of the area. Infact, having turrets a 360 degrees action, they had to be isolated and common metal observatories were also placed on the highest and isolated spots. So if we observe an isolated rough hole without any other useful elements, it's much likely to be a metal active observatory or a turret. Once extablished that the hole was not for a metal casemate, if the hole is indeed a vertical square shaft, it surely was the underground access to a turret. Some doubts can rise if the vertical hole is not square and there are no left elements to judge if it was a turret or and observatory.

If the hole is instead placed under the highest parts of the work, if there are other similar structures beside and, above all, if the position with regard to the target requires a maily frontal action, it is much likely to have hosted a metal casemate. Being perfectly concealable, metal casemates were largely used where the terrain is flat. 

However accurate the removal of any steel element should have been, many interesting iron parts may have left around the hole. Infact, fewer problems can there be if the hardest to remove steel elements are left: this is the case of the poles which are deeply thrusted into the rock. As said, most of turrets have a short square shaft but there are cases in which the vertical square section shaft is substitued by a circular shaft. What should be in any case present regardless of the connecting shaft, is the three vertical steel poles placed at the angles of an equilateral triangle. The couple of oblique steel poles laterally on the hole, witnesses to the presence of a metal observatory or a metal casemate. It may happen that the nuts are still on the poles which are in turn still inserted into the ear-like steel rings which have been torn off the cupola by the explosion. If the distance between the two left oblique poles is more than 2 metres; it was a metal casemate. If it is around 1.5 metres or less, the poles were used to fix an observatory cupola. If, besides the oblique ones, there are also one or two vertical steel stumps and their distance is about 1.7-1.8 metres, there might be no doubts to recognize that hole was where the lower cylinder of a metal casemate was placed.

Speaking of metal casemates, a last note is the following: the terrain in this case is not a steep slope but flat and uniform (a conventional casemate was not suitable on such a terrain). Being in this case the action frontal, the nearby space behind the fire sector might be used for many purposes. Infact, although most of metal casemate are stand alone, in some cases a single metal casemate could be coupled, for instance, with an emergency exit or a small recess for the photophone to constitute a larger block.

To Chapter IV