Belgian fortifications overview and Aubin fortress case study

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Renaud Mayers
Renaud Mayers
Currently working on behalf of the Belgian Ministry of Defence, thanks to my knowledge in WWII and other areas. Working in two WWII era fortresses still belonging to the Army.

Belgian fortifications overview and Aubin fortress case study. Belgian fortifications, their origins and their role.


Belgium gained its independence in 1830 after a short war of secession against the Netherlands.
Only 40 years later, in 1870, A newly unified Germany fought a short war against France and won a thorough victory. As a result, Germany annexed the French Alsace and Lorraine provinces. And Belgium found itself surrounded by angry people. The Germans, having won an easy victory started dreaming about more conquests, more victories. The French on the other hand only thought about revenge. It was only a question of time before those two countries would be at war with each other again and Belgium was right in between. The Belgian king tasked General Brialmont, a fortification expert, to come up with a way of securing the borders of this little kingdom. Brialmont’s approach was innovative. He knew modern artillery could very easily reduce conventional forts to rubble.


The conventional fortification design with bastions were simply too vulnerable in the face of modern artillery.


The Brialmont design

Brialmont therefore came up with a model for one of the very first modern semi-underground fortification system in the world. The Brialmont design was the first to make use of concrete for its construction, the outer walls and roof of the structure were covered in sloped earth that would absorb the impact of enemy shells while most guns would be placed in revolving armoured turrets. Throughout the 1880’s, he built a ring of 12 of those forts around Liege to protect against a German invasion and he built a similar ring of 9 forts around the city of Namur, facing the French border (We knew there would be a war, nobody knew who would start it!). The system was simple enough: A crown of forts, 3 to 4 kilometres apart, surrounding a city. The forts were close enough that each fort could receive artillery support from its 2 flanking neighbours, forcing the attacker to attack 3 forts simultaneously to have a chance to go through the line. And they had been built to resist shelling from 210mm artillery systems which were the biggest “horse transportable” artillery system in use at the time. In 1890, those forts were completed and were formidable indeed… But they were never upgraded while artillery made strides forward in terms of performance. So when Germany violated Belgian neutrality and invaded in August 1914, the Brialmont forts were totally outdated. World War One had just started for Belgium.


Linear / Brialmont
Brialmont recognised that a linear semi-underground design with sloped earth protecting the outer walls and roof might be the solution against modern artillery.


The city of Liege and her crown of 12 Brialmont type forts protecting her


Emines fort
Typical Brialmont fort


Brialmont design


Evegnee 1919
Evegnee fort, 1919


The Belgian forts during World War one

Liege resisted 11 days against all odds. The Germans tried to storm the forts with infantry assaults first but were repulsed with heavy losses. They therefore brought in their siege artillery to bear and pounded the forts into submission one after the other. The German’s most formidable weapon against Belgian forts were the Dicke Bertha (Big Bertha) super heavy 420mm howitzers. Those monsters could fire a 900 kilos 420mm shell at a range of 12.5km which was outside the range of the forts own guns. One by one, the forts surrendered or fell, sustaining horrific casualties. The best known example being the Loncin fort: On the 15th of August 1914, the 25th Big Bertha shell landing on the fort went straight through the roof of the fort’s powder magazine which still contained 12 tons of powder. The resulting explosion created a huge crater, pulverised the central part of the fort and instantly killed 350 out of the 550 men garrisoning the place. Having learned their lessons, the Germans did not even bother with infantry assault on the forts of Namur, shelling them into submission straight away. Yet, several of those forts resisted to the bitter end and still managed to inflict losses to the invaders. But, for all their shortcomings and obsolescence, the 12 forts in Liege delayed the German advance by 11 days. Just long enough to give the time to the British and French forces to establish a defensive line in the Marne region and save Paris.


Loncin fortress. The crater that stands where the right powder magazine was before the explosion.


Post war investigation

After the end of WWI in 1918, Belgium started looking into the failures of its fortification system.
The main results of the investigation were pretty clear:
– The forts were not built deep enough
– The lack of adequate ventilation meant that the smoke and gasses resulting from a fort firing its guns would stay inside the building, incapacitating the garrison.
– The fort’s gun turrets were too close to each other, making them all vulnerable to enemy fire.
– The communication between forts had not been good or reliable.
– Once the forts had been surrounded and patrols could not go out on reconnaissance, the forts’ guns had no longer clear targets to fire at, counter battery fire was almost impossible, also.
– The “all under one roof” solution was not a good one as men having fought / worked for 8 hours at a time had to sleep in the same structure, therefore being exposed to enemy shells falling a mere 6 metres overhead and the forts’ own guns firing from within the structure while the men from that shift were running and screaming in the corridors, doing their job.. Sleep was impossible.


Lantin fortress. One of the problem with the Brialmont design is that the gun turrets were too close to each other, making them an easy target for enemy artillery.


Inter war period: Old foe, new threat

In the 1920’s, it was already clear that yet another war was coming. At the end of that decade, Belgium was already refurbishing 8 of its old forts, digging new structures below them to store the ammunition and shelter the men deeper., injecting more concrete inside the galleries, using reinforced concrete inside and out.

A fort below the fort. Gallery dug below the original Embourg fort so as to provide better protection to ammunition stores and shelter to men. Use of reinforced concrete was made to strengthen the walls and ceilings.


In the 1930’s, Belgium started building a new line of 4 modern forts near the German border, ahead of the older modernised forts in Liege, creating a second line of defence. The aim was once again having structures that could cover each other. Aubin-Neufchateau fortress was one of those 4 modern fortresses. Designed along the same Brialmont principles that had worked in 1914, but integrating solutions to the problems encountered by the design in WWI. The other 3 modern forts were Eben-Emael, Battice and Tancremont.


The 8 old refurbished and modernised Brialmont type forts on the blue line and the 4 new modern ones along the pink line, closer to the German border.

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How the modern forts covered each other. Green represent the 120mm gun turrets field of fire (Eben Emael and Battice forts), red represents the field of fire of the 75mm gun turrets (All 4 modern forts). Black represents the field of fire of the 81mm mortars equipping Aubin and Tancremont fortresses. One could add the field of fire provided by the 75mm, 105mm and 150mm gun turrets of the second line of the older forts, covering the newer ones as well as the intervals.

Case study : Aubin Neufchateau fortress. Aubin was a modern “interval” fort or small fort wedged between two larger ones (Between Eben-Emael and Battice). Let’s visit the fort and see how it was built, how it was equipped and how life was inside.

Aubin-Neufchateau fortress: Conception

Triangular in shape, with 1 peace time entrance, 1 war time entrance, 2 air intake stations for ventilation and wooden peacetime barracks outside.
Triangular in shape, with 1 peace time entrance, 1 war time entrance, 2 air intake stations for ventilation and wooden peacetime barracks outside.


Aubin equipped
Aubin equipped

Aubin scale model
Aubin scale model

Aubin is a small “interval” fort situated between two big ones (Eben-Emael to the North and Battice to the South) it was built much deeper than the original Brialmont forts. While its combat blocs are visible from the surface, all of its 3 kilometres of galleries, the barracks, living quarters, machine rooms, kitchen, hospital and ammunition reserves were all built 37 metres underground.
The 3 main combat blocs are 300 metres apart, so enemy artillery cannot target all of them at once. The whole fort is built out of reinforced concrete made to resist shelling of up to 420mm in calibre.

160 steps to go down inside the fort 37 metres underground


37 metres underground, a network of 3 km of galleries and rooms. All the rooms under the fort (triangle on the right) are dedicated to command and control, technical areas and weapon and ammunition storage. The maze like structure in the middle is the underground barracks which is an autonomous town. Bloc description starting top left and going clockwise -BO (Top left) : Observation post and auxiliary air intake. Secret emergency escape -CI : Single coffer protecting one side of the moat / triangle. -BI : Gun turret bloc equipped with one 75mm gun turret and 2 machine gun cupolas. -BII : Similar to BI in all aspects. -CII : Head coffer or double coffer, protects two sides of the moat / triangle. -BIII : Peacetime / Main entrance. -CIII : Anti tank bloc equipped with two 47mm AT guns under cupola, covering the main road on each side. -BP : War time entrance, also main air intake station. -BM (middle of the triangle) : Mortar bloc.


it has 2 air intake stations capable of pumping 35.000 cubic metres of air an hour inside the fort, ventilating it and providing enough air to actually pressurise each combat bloc: Air tight doors and airlocks would be in place and each bloc would be pressurised up to 1 BAR (about 14PSI). That system had a dual purpose: The air pressure inside de bloc being greater than on the outside, the smoke and gas produced by the fort’s guns and machine guns would have been pushed out of the structure automatically, keeping the air inside the fort clean. The other purpose was that in case of a gas attack, the air pressure inside the fort being higher than on the outside, the gas would have been unable to penetrate inside the fort…


Typical air lock with an air tight door and grooves inside the lock to place a steel beams and sandbags barrage if one wants to condemn the bloc and detonate it.


There was a filter system for the air, a purification system for the water (The fort was built on top of a 60 metres deep water table), a machine room with 4 diesel engines that supplied the fort with electricity, hot water for the showers and the kitchen and hot water for the central heating system.

Water well taping directly into a 60 metres deep water table


The two air filter rooms for the underground barracks


One of the filter room with all the active charcoal filters still in place. The air would be circulated under that floor and pushed through the filters to be purified before being circulated inside the barracks.


engine room near electrical workshop
2 of the 4 Carels diesel engines inside the fort, providing power and hot water for the central heating. Each engine developed 130BHP, ran at around 600-900RPM and drank about 30 litres of Diesel an hour! Two were running continuously, while one was being maintained and one was kept on stand by.


Aubin, like all the other modern forts alongside the older modernised ones was equipped with a telephone exchange. Each fort was linked to all the other forts by underground phone lines. Each fort was also equipped with a radio room (Wireless transmission system). Finally, each fort was surrounded by a network of smaller hidden bunkers overlooking important / strategic places, crossroads, bridges and so on. Those bunkers were linked to the forts by phone lines too and would be the eyes and ears of the forts, calling in artillery fire with accurate coordinates.


Command centre
Command centre of the fort. This is where the commander and his officer took all the decisions during the battle, communicated with the different combat blocs of the fort and with the other forts and bunkers of the area.


command room
One of the command room, this one is the “heavy weapons” one, so the phone boxes on the wall enabled the officers there to communicate firing coordinates to the two gun turrets blocs (There were phone lines to the ammunition attendants on the ground floor, to the machinery room on the intermediate floor, to the loading crew under the turret and to the gun turret itself).


Each combat bloc was supplied with equipment to condemn and detonate it should it be compromised and should the enemy get close to breach it. There are 2 entrance / exits and one hidden emergency exit so secret that only the officers knew about it.
Life inside the fort was far from comfortable. The inside of the fort was damp. Everything was damp. Conditions in that regards were close to the ones inside of a submarine. There was no sewage system. What men needed to do had to be done in buckets called “odourless buckets” (Who said the army has no sense of humour) or in latrines built on top of sceptic tanks… All of which had to be emptied manually (with a big ladle, bon-appetit!) and carried outside to be emptied in a field. Furthermore, while equipping a structure with hot water showers was more than modern and innovating in the 1930’s, there were only 4 shower cubicles for a crew of 500 men… Meaning each man was entitled to one hot water shower per person per week. The rest of the week, they were more than welcome to wash in the washroom which was equipped with sinks and cold water taps only and which was to be used for the laundry too (by hand, of course).


Typical gallery. Not for the claustrophobic… Note the hot water pipes running along the right hand side wall which were providing heating in 1940. The temperature inside the fort with the heating on was about 18 C.


The underground barracks with bedrooms, toilets, washroom, kitchen, hospital and officer’s mess.


Aubin Neufchateau fortress underground barracks map


Oh, and  men had to share their beds using the hot bunk system: You had a garrison working the 3X8 shift, as in one shift/team sleeping, one working/fighting and one devoting the other 8 hours between relaxing and the chores (You hold the shit ladle, no, you hold the shit ladle). And shifts would obviously rotate… So as one man was hopping out of a bed ready for his shift, one would jump straight back in for his 8 hours of sleep.
The only people not sharing their bed with anybody else were the fort’s commander, the fort’s priest and the surgeon.
And while officers had a little radiator in their bedroom to cut the damp, there were none equipping the crew’s bedrooms to avoid soldiers fighting to have the bed closer to the nearest radiator… Finally, the 35.000 cubic metres of air being pumped inside the fort hourly were circulated through the galleries. So on top of living in a damp environment, the soldiers lived in a draughty one too.


The weapon systems

Aubin was equipped with two 120 tons gun turrets.
Each of those turrets could rotate over 360 degrees, being raised to expose the guns and fire and then being lowered (eclipsed) again, protecting the guns and its crew under its 36 cm (14 inch) of Nickel-Chrome armour. Each of those two gun turret was equipped with two 75mm guns that had a rate of fire of 12 RPM per gun and a range of 10.1km.


Gun turret raised and ready to fire


Gun turret lowered, only the 36cm tick armoured dome is exposed. the guns and crew members are safe.


Gun turret
Gun turret from Tancremont fortress, similar to the one that once equipped Aubin fortress. The two 75mm guns are visible as well as the central embrasure that was equipped with optics for close quarter grapeshot fire. Note the battle damage in the form of bullet impacts all over the armour.


Those guns would fire 3 types of shells: HE, detonating on impact, Shrapnel, detonating above ground for long range anti-personnel use and Grapeshot which had a range of only 300 metres but was devastating against infantry at close range.


HE 75mm shell


Grapeshot 75mm shell


Both gun turret blocs were also equipped with two machine gun cupola for close defence.


In the background, the gun turret, on the foreground, one of the two machine gun cupola protecting that bloc. The cupola weight around 35 tons and was equipped with a Maxim machine gun.


Double Maxim mount used in all the casemates and cupolas. One machine gun is in battery, ready to fire, the other is as a backup. Should one machine gun jam or overheat, the gun crew could simply flick a lever and rotate the mount 180 degrees. The back up machine gun would therefore find itself in battery and ready to use.



double Maxim machine gun mount explained. One gun is in battery and ready to fire, the other is as a back up. There is a water container above the embrasure. The water circulate through the machine gun sleeve, cooling it down, the water then goes through the little radiator which is above the aiming window. As the combat bloc is pressurised to 1bar, there is a constant flow of air rushing past the radiator, going off through the aiming window, therefore cooling the water down which is then pumped back into the tank above the embrasure. On the side of the mount, a flexible tube catches the spent brass when the machine gun fires, directing them into a bucket that collects them, keeping the floor clean and clear of slipping hazard.


The second main weapon system of the fort was the mortar bloc. It was equipped with three 81mm mortars that had a rate of fire of 8 RPM per tube and a range of 3.6 km. The Mortar bloc was equipped with an observation cupola which unlike a machine gun cupola, gave its crew a 360 degrees view over the roof of the fort.


One of the three mortar pits and the observation cupola atop. the mortar would fire from the inside of the fort through the embrasure (The smile). The pit in front of the structure is to catch any enemy shell or rubble, ensuring that the embrasure stays undamaged and unobstructed.


81mm mortar. The mortar could pivot on its axis but the elevation could not be adjusted. The range was therefore adjusted by a valve letting more or less pressure off the tube. Close the valve, all the pressure generated by the charge stays in the tube and you obtain your maximum range. Open the valve fully and most of the energy of the propelling charge escapes out of the tube, sending the bomb only a short way out. As a Fortress type mortar, it was a breach loader, not a muzzle loader. The muzzle comes flush with the embrasure. Therefore the crew is not exposed to enemy fire.



The base of the Mortar Bloc. The ammunition reserves and the bomb lifts are visible on the picture.


81mm mortar bombs


Armoured observation Cupola. Has 6 openings enabling the look out to have a 360 degrees field of view. The embrasure (opening) is here in the opened position, providing a wider field of view. In case of the cupola being targeted, the lookout inside could close it with an armoured cork equipped with armoured glass. The field of view would be restricted but the lookout would be safe.


The main entrance bloc was defended by 3 machine gun cupolas, 2 machine gun guns in casemate and equipped with a searchlight and two grenade ramps.


Main entrance bloc (BIII). The big opening on the left was for the armoured searchlight, the smaller embrasure next to it was for the Maxim machine gun.


To access the main entrance bloc, one had get to the moat through the main postern, facing the Maxim machine gun. The searchlight embrasure is out of view on the left of this picture. It shone a light inside the postern at an angle, therefore the crew manning the machine gun could see inside the postern but assailants could not see and target the searchlight.


Access to the postern. Visible in the background, the Maxim machine gun embrasure


There was a bloc equipped with two 47mm AT guns under cupolas.


The two 45mm anti-tank cupolas. Picture taken in 1941, roughly one year after the battle, hence the battle damage visible

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47mm anti tank shell. Could penetrate 4cm or armour which was more than enough in 1940 to penetrate most German tank. In the case, from top to bottom, you have the case, in the middle, the anti-personnel shell which would fragment on impact and at the bottom, the anti-tank shell with a tungsten core and ballistic cap. Bottom left is the fuse.


The war time entrance was defended by two machine guns and two grenade ramps, it was acting as the main air intake station and was within range of the mortars if needed. It was also equipped with two observation cupolas.


BP or war time entrance. Picture taken by the Germans just after the battle, hence the visible battle damage on the concrete and the two observation cupolas.


War time entrance as it looks today.


The emergency exit bloc also had an observation cupola and was acting as the secondary / back-up air intake station.


The “BO” bloc, with its observation cupola, its air intake (lowered) and its hidden emergency exit.


Finally, the dry moat of the fort was protected by one single and one double coffer (casemate). Each of those coffers was equipped with one machine gun, one grenade ramp, one 47mm AT gun and one searchlight.


Bloc CI, single coffer protecting the moat. The three openings from left to right: 47mm AT gun, searchlight, machine gun. Picture taken by the Germans after the battle, hence the impacts visible all over the concrete.


The fort also had a fake machine gun cupola situated outside its outer wall. That decoy provided a juicy bait for enemy soldiers looking for a weak spot to breach the fort while being within range of the fort’s grapeshot…Just before the German invasion, the fort was equipped in total with four 75mm guns under two armoured turrets, three 81mm mortars, three 47mm anti-tank guns in casemates, two more in cupolas, four Maxim machine guns in casemates, seven more in cupolas, five automatic rifle embrasures, 4 observation cupolas and 5 searchlights. Every single one of the 11 cupolas on the fort were equipped with either a periscope or a flare launcher.

Aubin armament
Aubin armament

The ammunition reserves were as follow:

  • 11.200 x 75mm shells (3.000 shrapnel, 7.000 HE, 1.200 grapeshots shells).
  • 6.000 x 81mm bombs for the mortars.
  • 1.500 x 47mm shells (360 Anti-tank and 1.140 Anti-personnel)
  • 720 Mills grenades (hand grenades)
  • 315.000 x 7.65mm cartridges for Maxim machine guns, automatic rifles and rifles.
  • 27 FN Mle 1930 automatic rifles
  • 433 Mauser rifles

One can add to that medical supplies, food, coal (for the kitchen cookers) and over 8.000 litres of diesel in underground tanks, all of which was in theory enough to enable the fort to hold for one month.The mission of the double line of forts was to delay the German invasion as long as possible to enable the French and British armies to move forward and defeat the German thrust. The forts’ commanders had order to fight until relieved or until the fort was no longer in a position to fight.

Please note not all the pictures detailing Aubin fortress were taken at Aubin fortress : Most of Aubin systems including its weapons were scrapped in the 1950’s. Some of the pictures presented here were taken at Tancremont and Battice fortresses which were using identical systems.

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