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Dutch casemates in May 1940

Introduction

This article has not the intention to inform the reader about the finest details of the Dutch casemates in May 1940. Such an article would be too extended and go into too much technical details. We have also chosen to leave out the special casemates in the (many) fortresses, for these were practically all specific for their task and besides hardly seen battle. The only two fortresses that really came in the line of fire were Kornwerderzand and Westervoort. Both are addressed in quite some detail in the articles describing the events of 10-14 May 1940.

This article is a comprehensive summary of the typical Dutch casemates that were in use in May 1940 in the reinforced field positions of the Dutch army.

Casemate, bunker, pill-box

Many synonyms are used all over the world to indicate steel or concrete reinforcements in defensive positions. The German word "bunker" is conceived as the most popular indication of these constructions.

The British used the word pillbox. But today even British people recognize the word 'bunker' as a leading term for these kinds of defensive constructions. The word 'bunker' was also used in the Dutch language (before the war) as an indication of a large storage for black coal. It was derived from the old Germanic word 'bunk' for a box-shaped room in which one could sleep.

The Dutch used the name "kazemat" which was derived from the Italian "casamatta", which in itself came from Byzantine 'chasmata'. The latter literally means "crack" or "split", in modern Greec 'chasma'. In the ancient history it was a name given to a defence structure with loop-holes from where projectiles could be launched.

In order to keep things simple we shall only use the word casemate in this article.

The introduction of the casemates

When they were introduced, casemates were (more or less) a logical next step in modern defence warfare. In the early times people had built castles, redoubts, bastions and other permanent means of fortification. Up to the point where the powerful breech loaded guns were introduced [end of the 19th century], fortifications had usually been constructed of stone with a sand wall in front of it that was intended to burry a bullet or grenade or to break the blast of detonation. Modern guns proved these fortifications outdated. During the second half of the 19th century concrete had been introduced as a construction material, soon followed by Ferro concrete [steel reinforced concrete].

The introduction of the very powerful powders and the breech loaded artillery, forced defence specialists to redesign the existing means of passive defence. The revolution in steel milling had made the manufacture possible of strong steel structures with high yields. Not only the breech loaded guns were a result of that new milling process, but also the new design of tougher steel [higher yields]. As such modern fortifications were fitted with the first products of (some sort of) armoured steel. It was used both for bullet protection [infantry] and for gun turrets. Also concrete was introduced, although still homogenous and not reinforced. Both new materials boosted the strength of new fortifications.

In the 19th century - and the first two decades of the 20th - fixed defences and fortifications were anchors in every defensive strategy in Europe. When we focus on the Dutch situation, we see that the country had a dense structure of fortresses, redoubts and bastions. Two major lines were recognized: the new Dutch Waterline [the old was also still present] and the Fortress Amsterdam. These two strategic defence-lines were both completely shaped around a shackled line of fortresses and other permanent strong-holds. All of these were made of stone or concrete [sometimes with steel plate reinforcements], and the far majority gave room to plenty of troops, supplies and guns.

Although WWI would eventually end up in a fixed trench-war, it warned the clever few for the future warfare. Should the Plan von Schlieffen have succeeded it would have proven all fixed fortifications void and worthless. The Germans showed that mobile warfare was the new feature of modern war, and that fixed fortifications were about to lose their value. The fact that the German advance was stalled just in time for the Allies to be able to regroup, saved the French of a definite defeat. But it also prevented the world to realise that the future of defences would not lie in vast and fixed fortified lines, but in the strength of mobile forces and mobile fire power.

When the German advance in Belgium and France had been stopped, the trench warfare started. A dramatic stalemate situation of two sides storming and shelling each others fixed positions. It would last for over three years, and show the inventiveness of military engineers and designers.

The British were the first to introduce pre-fab steel structures that could be placed in the trenches in order to provide for protected machinegun positions. They called them pill-boxes. They were soon followed by the other belligerents.

The huge fortresses in WWI, especially those in the defence-lines in Belgium, had proven that their value had decreased dramatically. Not only were modern armies capable of avoiding these fixed positions, but also it had become obvious that modern artillery was very capable of destroying these constructions. Heavier and more powerful guns were capable of penetrating virtually every existing fortress wall, and the increased range of artillery made it possible for the besieger to bombard the fortress beyond the range of the fortress guns. It was a warning of history that was missed by many ...

WWI made it perfectly clear - for those who wanted to see - that the traditional reinforced position had lost its value. Defences could no longer rely on these conservative means of defence. Yet the besieged allies of WWI didn't quite learn these lessons well enough. Still many defence-designers would rely on fixed fortifications during the interbellum. The French Maginot-line was the ultimate example of that naïve and stubborn believe.

WWII would show a totally different perception of warfare. The introduction of highly capable assault planes and heavy bombers, the tank, powerful long range artillery, high velocity AT guns and the highly mobile and dynamic use of infantry would proof that the times of fixed reinforcements were over, although all belligerents - Germany included - would at some point still invest major efforts in constructing fixed reinforced defence-lines. All of these efforts would be in vain.

Defence plan 1900-1935

The developments in Europe in the period 1830-1871 laid the basis for the new Dutch defence strategy of 1874. This strategy focussed on one main point of departure: any future threat of Dutch national safety would most likely come from the east. A subsidiary basis for the new strategy was that Holland could not be defended in its entirety. This was mainly a result of two components which were derived from one awareness. The first was the fact that the country would never be able to mobilize enough forces to defend its border-areas as well as the "hinterland" sufficiently enough. The second was the geo-strategical analysis of the country, with its long stretched borders, narrow centre and plenty main rivers. As such the emphasis of the new defence strategy [which did not substantially differentiate from the ancient plans!] was the defence of the western provinces, the "beating heart of the country": the provinces Utrecht, Zuid-Holland and Noord-Holland. In fact no more than one-third of the national territory, but home to half the Dutch population and virtually all important facilities and organisations of the country.

Neutrality was a key-word, a magic word. Already when the Prussian-French war had broken out the Dutch were very expeditious to claim absolute neutrality. Although Europe feared the war to escalate, it didn't. It did however cause a mass evolution in weapons development. When WWI broke out, again the Dutch successfully claimed neutrality, and although at some point the Germans seriously considered to cross the southern territory of Holland, they never actually acted accordingly. The Dutch were quite pleased and content with the proclaimed neutrality "weapon". When after 1933 the German power rose again, the Dutch thought for a very long time that again their strict impartial foreign politics would save the day.

Next to the emphasis on the defence of the three aforementioned provinces, another argument was of importance. At the beginning of the 20th century the Dutch parliament had chosen for a small standing army in combination with a conscription system that would form the backbone of the army in case of crisis or war. Since the Dutch considered a large army a non-desirable asset, the standing army [professional military plus the drafted levies under arms] was kept small. Of any actual levy batch only one third was actually selected. As such not only a relatively small army would have to be housed and paid for, but the fact that only limited numbers were actually trained in the military trade meant that also the reservists were (relatively) small in number. Army strategic planners had to deal with these results of democratically formed legislation and realized that as a consequence they were quite restraint in their operational plans. After all, a small army could only be assigned limited tasks.

The limited strength in numbers was not the only restriction the military planners had to deal with. The defence budgets were trimmed to such low levels that ample funds for new weapons and gear were hardly made available. WWI had briefly boosted the funding but the end of the war and especially the new League of Nations were grist on the mill of parliament. After all - what were the changes of war now this giant monitor was in place?

Up to the first decade of the twentieth century - and during WWI - the Dutch army had still thrived from the revolution in new weaponry. When WWI broke out a rather small but fairly modern army was available. But in fact the country was financially in dire straits. Obviously WWI didn't help to improve the financial position of Holland, and as a result the money tap was quickly shut after the Versailles Treaty and League of Nations had become a fact. Two decades of steep decline followed. The army was even further decreased in quantity and quality.

Halfway the thirties the raise en ambitions of Adolf Hitler woke-up the Dutch too. Gradually new funding was made available for larger drafts and more and modern equipment and material. The evolution of the airplane and the mechanisation of armed forces required large funding and since the Dutch had totally neglected the armed forces for the last decades, a back-log of about twenty years had to be compensated. Obviously that was a mission impossible.

The Dutch military planners, that were obviously - peacetime or crisis - re-evaluating the defence of the country time and again, always had to coop with insufficient numbers of soldiers and inadequate material and equipment. These factors were of major importance when the defence strategy of the country had to evaluated and modified. All the aforementioned factors (of limitation) constantly played in the back of the heads of the planners. In fact it left them with but one option: defence of the country by making use of static defence-lines and fixed defended positions.

The Dutch defence plan during the interbellum was quite plain and simple. A small portion of the mobilised army would be stationed in outer defences, along the border area. These formations were part of the first mobilisation batch, for they had to insure ample time for the entire mobilisation process. The balance of the army - that would be mobilized whilst the enemy was slowed down by the outer defences - would man the main defence-lines.

These main defence-lines were actually divided in two portions. The first screen was the outer-screen, meant to withhold an invader long enough the fully man the defences that were positioned around the Fortress Holland. The outer defences were mainly positioned on the west-bank of the rivers Yssel and Maas [Meuse]. The northern provinces Friesland, Groningen and Drente were not defended at all [prior to the construction of the Afsluitdijk]. The Yssel River ran through the central eastern provinces of Overijssel and Gelderland, whilst the Meuse was defended in Noord-Brabant and Limburg, the south-eastern provinces.

Behind these outer defences the main-defences of Fortress Holland were situated. The Fortress was divided in four sectors: North [from Ymuiden to the Zuiderzee or Ysselmeer], West [the coastline from Ymuiden to south of Hook of Holland], South [from Hook of Holland to Gorinchem] and East [from Gorinchem via Utrecht to Muiden, east of Amsterdam]. Also the navy main sea-port at Den Helder, in the far north, was designated as an individual fortified strong-hold.

All these lines were fortified. Partially with existing fortresses, bastions and other permanent reinforcements from the older days, and as far as the outer defences were concerned with few fortresses, some casemates [as off 1929] and many reinforced trenches.

Since the emphasis of this article is the prelude to WWII we shall not go into further detail about the quality of these older fortifications. It was simply our intention to shape a rough picture of the status of the existing fortifications prior to our elaboration on the new defence structures of the late thirties.

Defence plan 1935-1940

France and Belgium had already started constructing new defences along their borders with Germany during the twenties. Already in 1932 most of these projects had been finished. The Dutch - late as usual - were still investigating how they should improve their defences in conjunction with their southern neighbour.

In 1935 a carefully thought through revised defence-plan was filed at the department of defence. This plan incorporated a main defence-line in Noord-Brabant, between the swampy region in the south [the Peel] and the small river Raam. It was designated the Peel-Raamline. In combination with the introduction of the Peel-Raamline, the plan introduced far more significance to actual reinforced defence-lines along the Yssel and Maas rivers.

The Peel-Raamline was being continued to the north. The Grebbeline - already an existing defence from the past - was reinstated. It ran from the Rhine to the north, where it ended at the Zuiderzee [Ysselmeer]. In between the Peel-Raamline and the Grebbeline the connecting shackle - between the Rhine in the north and the Waal in the south - was designated as the Betuweline and the Maas-Waalline. The plan was approved early 1936.

The Peel-Raamline would be situated behind a tank-barrier [canal] that would have to be constructed along two-third of the length. In the remaining one-third the Zuid-Willemsvaart - an existing canal that continued into Belgium - would be used as a "natural" tank barrier. The new canal was baptised the Defence-canal, and would be entirely constructed as an unemployment project. Unemployed labourers would be drafted for the job, and as such the budget could be kept at acceptably low levels!

The Peel-Raamline had its achilles-heel in the south, where it ended south of Weert at the border with Belgium. But the Dutch planners relied on the Belgian army to defend this part that connected to their border-defences. In 1933 the Belgian army had constructed quite a number of casemates along the Zuid-Willemsvaart, and still in 1937 their defence plans incorporated this area. Later, as a response to the Germans reoccupation of the "Rhineland", the Belgium defences moved backwards and as a result the Zuid-Willemsvaart defences appeared to be dropped. Their first line of defence would become the Albert Canal and the second the KW- and Dyle line. This resulted in a very unfortunate status for the Peel-Raamline, for Belgian troops would no longer occupy the Zuid-Willemsvaart south of Weert!

The Grebbeline - or Valley-line [its alternate name] - was situated along the particular hilly and woody country-side in the provinces Utrecht and Gelderland that was formed in the last Ice-Age. The hills - and the relatively flat and low land to the east of it - had formed the basis of the popular name "Valley-line". The defences ran from the Zuiderzee [Ysselmeer] to the Rhine at Rhenen. The southern and centre part of the line was indeed very hilly, whilst the northern part was quite flat meadow land that could easily be flooded. In front of the better part of the line ran a small river, called the Grift, which was extended in the north by an inundation canal. The defensive positions were basically all situated behind these waterways. Another projected tank-barrier in the form of a canal would yet have to be constructed in front of these existing water-ways.

The Betuwe-line and the Maas-Waalline were the [about 20 km long] extension of both the Grebbeline and the Peel-Raamline. These two short extensions connected the two main-defences and were strategically aided by the fact that these sections were squeezed in between the three major rivers Rhine, Maas and Waal. They were not easy to approach from the east since the wide Maas-Waal canal connected both rivers north to south.

The outer defence-lines, the Maasline and the Ysselline, were designated as cordon shaped casemate-lines. These positions lacked any depth, and would basically consist of a line of light casemates that were projected along the westbank of the river. At the major bridges additional river-casemates would be constructed.

The main defence-lines of the old defence-plan - at the borders of the Fortress Holland - remained unchanged. The enemy had to be stopped in the main defence-lines [Grebbeline / Peel-Raamline] manned by the Field Army. When these defences would be jeopardized by increasing enemy pressure they were to be evacuated. The Field Army would then retreat on the last stand: the Fortress Holland.

The role of the Field Army, which formed about half the strength of the mobilised force - was designated to man all defence-lines we addressed hereabove with exception of the Fortress Holland. The Fortress Holland was manned by the remainder of the army, mainly stationary forces. The Field Army was therefore no longer an actual "army in the field", designated for dynamic warfare, but dislocated in fixed and prepared positions. Only the cavalry units and the Light Division - a fully mechanised unit - would remain mobile forces for dynamic warfare. After the defences outside the Fortress Holland would have been evacuated the Field Army would fall back on the Fortress and reinforce its defences. Quite a daring adventure for an army that had already caught the first [outer defences] and second [main field defences] blow of the enemy assault!

Change of command

In January 1940 the differences between the Commander-in-Chief [CIC] Reynders and the Minister of Defences about the defence-strategy had grown to such a proportion that a point break had been reached. Reynders resigned. In February the new CIC Winkelman was installed, and although the relation between the latter and the Ministry was very good, the change of command did unarguably stall the further development of the Dutch defences, for not only the CIC was replaced, also the second in command, the Chief of Staff [COS] was new to the job. The army had to await the plans of the new chiefs, which obviously could not be presented instantly. Winkelman and his new COS made a quick-scan of the status of the Dutch defences. They visited the lines, witnessed the state the existing and new defence-lines were in and had to translate their impressions into a feasible strategy.

When the new CIC presented the revised defence-strategy, it enhanced two major modifications. The first was the fact that Winkelman considered the Peel-Raamline no longer suitable for sustained defence, especially not when this would include large quantity of troops. The fact that the line had an open end in the south was the vital element to the CIC's conclusion that its status had to be devaluated from main defence line to forward defence line. The troops to the rear of the line - being the 3rd Army Corps and the Light Division - would be evacuated within 24 hours after an invasion. The defence would be an assignment that had to be borne by the Peel division alone, less than 10,000 men.

Reynders had persisted that the Germans wouldn't mind about the open end and engage the line straight forward, most likely at Mill. Although Reynders was right about Mill [this thought was shared by Winkelman], he was definitely wrong about the German lack of interest for a southern swing around. In the morning of the second day of the war the Germans discovered the void south of Weert, and if it hadn't been for the already successful penetration at Mill, they would certainly have swung around the line. Winkelman was proven right.

The second modification was the main defence of the Field Army in the central sector. The former CIC had decided that the Field Army would defend the Grebbeline for as long as possible and then fall back to the Fortress Holland to take positions in the east front of the Fortress. In the east front the main-defence would be given shape. Winkelman thought slightly different about this case. He required the main defence to be give shape in the Grebbeline and use the east front of the Fortress as a fall back should the Grebbeline fall.

Before we go into too much detail - considering the topic of this article - it may be of interest to say a few words about the details of this last change. Reynders - the former CIC - had thought that the Grebbeline would be suitable for a long sustained forward defence. After the pressure against it would increase, he planned to take back both Army Corpses from the Grebbeline and prepare them for the main defence in the east front of the Fortress. In fact, Winkelman did not change that much, but the emphasis of the defence changed substantially. The Grebbeline would become the main defence, and in fact that made a lot of sense. In both instances the two armies eventually had to fall back on the east front. But Winkelman considered this a fall back scenario, whilst Reynders thought this to be the desired prime strategy. Winkelman gathered however that when the two Corpses would indeed defend the Grebbeline for some sustained time - and without the (lacking) reserves to replace the front-troops - that should they have to be taken back after being occupied in combat for days (maybe weeks!), their value and organisation would be inflicted to such extend that their redeployment in the east front of the Fortress would almost certainly fail. And he was almost certainly right again.

Winkelman considered the Grebbeline as the prime central defensive positions favourable over the Waterline for two more important reasons. First of all the modern artillery of the besieger would be able to shell the major city of Utrecht when the Waterline would be sieged. Secondly the Grebbeline offered better deployment options for the vast forces of the Field Army. The extended forests directly east of the main defences could provide all artillery batteries elevated and covered positions. Tactical reserves, supporting troops and replenishment stocks could also be easily hidden from curious enemy eyes. The Waterline lacked these advantages completely. Moreover, the Waterline had the huge disadvantage of being low land, well below the sea-level and with high ground-water levels. This caused reinforcements, trenches and gun positions to be constructed above ground-level, thus creating high and large profiles and easier targets. Coverage from forest or even modest woody area's lacked almost completely. On the other hand the Waterline obviously provided a far better protection from much extended inundations. But this one advantage did not outmatch the many advances the Grebbeline offered. Anyway, the Winkelman plan was accepted by the Ministry of Defence.

The fact that Winkelman had selected the Grebbeline as the main defence shifted the emphasis of the application of funds from the east front Fortress Holland to the Grebbeline. Mind you - the Grebbeline had hardly been prepared up to the point where Winkelman decided it to be the main defence-line in the centre.

Casemates [1918-1937]

Already during WWI [1918] a rather large number of (reinforced steel) concrete casemates had been constructed within the Fortress Holland as shelter for troops. The military had realised that many of the existing fortresses on which the defences relied would not provide proper protection against modern artillery fire. The casemates had been provided for this required protection.

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Typical river casemate (may 1940)

During the interbellum and during the period 1929-1936 in particular, a number of concrete weapon positions had been constructed along the defence-structure of the Fortress Holland. These structures were mainly built at strategic points such as the bridges over the Holland's Diep. Improvement of the coastal defences was also taken at hand. At the Afsluitdijk - the huge 30 km long dike that connected Friesland and Noord-Holland to the north - two major fortresses had been constructed. One on the east-side near the Friesland coast and one on the west-side near the other coast. The details of these constructions were addressed elsewhere on this site, when the German assault on the Afsluitdijk was described.

In 1934 and 1935 plans were developed to have all strategic river-crossings reinforced with large river casemates. This plan incorporated 43 large casemates - suitable for both light AT guns and heavy machineguns - that were projected at the bridges over the Yssel, Maas and Zuid-Willemsvaart. These casemates would be permanently manned by professional soldiers.

During the year 1937 many shelter casemates were modified to shelters with weapon points. Loop-holes were drilled out in order to facilitate light machinegun positions.

Casemates [1937-1940]

As off 1937 the national funds made available for military purposes grew rapidly. Also the general interest grew to invest more in the fixed defences. Many studies were done as to how to improve the projected defence-lines. These studies resulted in plenty of new local defence options and casemate designs.

In this article we describe the most common Dutch casemates, being the VIS design casemates, the S-, B-, G-types, the river-casemates and we shall briefly address the types that were part of the Beton-Plan.

The reader should know that at some locations unique casemate design were built. Some local initiatives by commanding field officers were supported by the military command of the Field Army, and as such at a number of locations unique designs evolved. We have chosen not to incorporate those in this article.

VIS design

During the interbellum a technical study team was formed. It had been assigned the task to give shape to a casemate construction template for several duties. The first action of the team - mainly formed by military engineers - was to study the battlefields of WWI.

WWI had obviously not taught its lessons to the Dutch military. The Dutch neutrality had saved the country from the devastation of the Great War, but it had created a serious back-log in practical military knowledge. When the task-force team visited the recent battlefields they were quite stunned to learn the effect of impact of modern artillery-fire on reinforced positions. They realized that this required drastic measures to be taken to the defences at home.

Based on the experience of the engineers and technicians that had studied the battlefield damage in France and Belgium, and embracing the theory that the next war would definitely be another trench-war, the task force would lay the foundation for the new "directives for the creation of field defensive positions" called "VIS" [Voorschriften Inrichten Stellingen].

The VIS directives were issued in a series of instructions. The entire series gave a complete overview of the guidelines for modern warfare and focussed on the Dutch precautions and measures. Part VII represented the directives related to reinforced concrete [Ferro concrete] casemates, and it's this very part that shall be addressed hereunder.

There is one other aspect of the VIS design worth mentioning in particular. The Dutch emphasized on the aspect of constructing the far majority of necessary reinforcements when the forces would be actually mobilized during times of serious crisis or when war had actually broken out. The casemates would therefore have to be built by the troops themselves. The design should therefore be kept simple and straight forward, thus making it feasible to have only modestly trained supervisors able to manage the construction of these casemates everywhere in the country. This explains the very basic square features of the Dutch casemates.

The VIS directives produced no less than ten standards for Ferro concrete casemates. These could all come in certain variations. Variations were usually given in by the geophysical factors [for example: a casemate constructed in a dike would have an elevated entrance due to the high dike body] and the tactic situation [for example: a loop-hole could be positioned in a different angle].

The strength of the constructions was predefined in a scale. This scale indicates the impact resistance ratio of the construction against HE explosives. Three scales were applied: W12-15, W15-21 and W21-28. The W stands for "Weerstand" [Dutch for "resistance ration"], whilst the extension indicates the calibre of impact. The first argument indicates the grenade- calibre of which sustained impact could be resisted; the second argument the calibre of which a few hits could be sustained. In other words, W12-15 stands for "sustained resistance against 12 cm HE grenades, and capable of resisting a few hits of 15 cm HE grenades."

Obviously these scales represented a certain thickness of the construction elements. When we compare three main parameters, the following [minimum sizes] applied:

  W12-15 W15-21 W21-28
Roof: 0.8 metre 1.25 metre 1.5 metre
Front-wall: 1.0 metre 1.50 metre 1.8 metre
Inner-wall: 0.4 metre 0.6 metre 0.8 metre

As said the scales had been based on the impact of HE explosives. The growing importance of aerial bombs and armour-piercing munitions had not been measured. Especially the latter should have been measured, for in 1940 the standard German light infantry gun fired high velocity armoured projectiles. The design of this gun was from 1928, and the Dutch themselves had tested it in the early thirties. Its existence and capability was known. Also the German 88 mm FLAK gun could be used against surface targets and would proof a devastating casemate killer. This (gross) negligence was definitely a serious flaw in the standard design.

Another aspect that was largely argued was the very limited inner-space. The constructions were fitted with very narrow entrances and came with extremely low ceilings. The designers had based these measures on the assumption that casemates would only be manned when the enemy would be assaulting. In other words: part-time occupation of the construction. And reducing the volume would ..... save construction materials and as such save money. It was another flaw in the design, although it didn't substantially decrease the offensive ability.

Casemate types

Hereunder we shall address the common casemate types that were designed and constructed in the period 1935-1940 in the Netherlands.
These individual types themselves knew many variants. Variety of the interior, but moreover variations like different loop-hole location, elevated entrance location and sometimes complete tailor-made designs to fit a certain specific location. We have no intention to address all these variants in this article.

Personnel shelters

The first Ferro concrete casemates had been constructed in 1918 within the Fortress Holland. These constructions were all designated as personnel shelters, and lacked any offensive facilities. It were simply shelters, intended for infantry and other personnel, and designed to sustain artillery bombardments up to 15 cm calibre. In fact these 1918 casemates were no more than quick fixes of out-dated fortresses and related strongholds.

The most well known shelter-casemates - which are still visible at many locations today - are the so called "pyramid shelters". Indeed these casemates resembled pyramid shape constructions. A pyramid shape roof was constructed on top of the square substructure. This roof was purposely constructed in this shape in order to bounce off or deflect projectiles and light bombs. About 700 of these constructions were originally planned to be built within the Fortress Holland. In the end - when the Grebbeline suddenly became the prime line of defence - the construction of these shelters in the Waterline was suspended. It is not exactly known how many were actually built prior to the invasion.

The pyramid shelters were of very modern design. The first ones were built in October 1939. They were intended for first line defensive positions and therefore laid-out in scale W21-28. Their smaller brother, used in secondary defensive positions, were constructed in the W15-21 range. Both designs lacked any weapon point, with exception of a small rifle loop-hole at the entrance for defence against intruders.

The casemates basically came in two sizes. The bigger one was for an entire rifle group [12 men], and the smaller one for 8 men. At some locations entire series of shelters were built in a row, and sometimes they were constructed in pairs [16 men shelter].

Light machinegun casemate [S-type or porcupine]

The casemates for light machineguns were projected in the so called casemate defence-lines, which were the Maas- and Ysselline, the Peel-Raamline and the Grebbeline [and the two connecting lines in between]. In total 2,000 casemates were envisaged in these lines, mainly comprising the light machinegun casemate [S-type, which equipped with a heavy machinegun was designated as Szw-type], flanking light machinegun casemate [B-type] and the heavy machinegun casemate [G-type].

These casemate defence-lines were all meant to slow down the German invasion to such extend that the main defences could be manned and prepared. Mind you, that up until March 1940 the Grebbeline was such a casemate defence-line and intended to slow the invader down in order to be able to prepare the main-defence in the Waterline [Fortress Holland east front].

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S-casemate

Since these lines had only the limited function of slowing down enemy progress the army planners evaluated the German infantry armament in order to establish the required strength of the constructions. They concluded that since the heaviest infantry guns and primary artillery support was of calibres of 12 cm or less, with only an occasional 15 cm battery available, the strength of the casemates could be set in the W12-15 range.

In April 1939 the design for the S-casemate was issued. The S stands for "stekelvarken", Dutch for "porcupine". This name was related to the steel rods that stuck out of the construction in order to attach all kinds of camouflage at. Its design was almost an exact copy of a German machinegun position from WWI. The front wall and roof was only 0.8 metre thick.

The standard S-type had a 190° field of fire divided over three loopholes with an individual angle of 70°. The front-wall was formed by these three angled positions. The side-walls and back-wall were straight, the roof was flat. The three loop-holes could be covered by a hatch of 20 mm armoured steel. The loop-holes were constructed of heavy steel reinforced profiles, anchored in the concrete of the main-structure.

Usually these S-types were manned by three or four men and one light machinegun. The casemates were constructed with hardly any facilities. The S-types lacked an air exhausting system [for the gun fumes].

The S-type with three loop-holes [S3] became the most popular version of this casemate type, and was built in large numbers. Also a version for the heavy machinegun Schwarzlose was designed [Szw-type], which had a smaller field of fire and smaller loop-holes. This type did get an air exhaust system. Of this variation only few were built.

Later a new variant was added with five or seven loop-holes. Of these only few were constructed.

The huge disadvantage of this casemate - and all its variants - was its large profile and frontal loop-hole(s). It would proof extremely vulnerable to enemy fire.

Many hundreds S-type casemates were built. It was the most seen casemate type. Many of these casemates were thoroughly camouflaged. Sometimes a complex shape-braking construction was attached to the steel rods, but also more sophisticated camouflages were applied. At some locations entire fake houses or barns were built around the casemates, which - from a distance - appeared to be very realistic.

Heavy machinegun casemate [G-type]

Without any doubt the most capable Dutch casemate was the G-type. Although its design was issued in combination with the aforementioned S- and B-type, it had one distinctive difference: the weapon position itself was entirely constructed from armoured steel and shaped in the form of a cupola.

In March 1939 the design was launched. It was based on the design of the steel-casemates in the Fortress Holland, although the cupola was constructed from 10 cm thick steel in stead of 14 or 17 cm. The steel cupola was embedded in a Ferro concrete basement that protected the sides and rear of the weapon point.

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hMG cupola casemate (may 1940)

The VIS design presented no less than 35 options for the G-type, so that it could be fully adjusted to the specifics of the envisaged position it had to defend. The loop-hole could be constructed in five different positions [from the basement]. The G-type was designated for the use of a heavy machinegun [Schwarzloze] with a special spherical armoured sleeve. But also the light machinegun Lewis could be used [which rarely happened]. The field of fire was quite limited however, with only 35°.

The cupola was fitted with an automatic exhaust system, and suitable for maximum three men. But three men was already a crowd in this very narrow position.

The profile of this casemate was extremely low. The concrete basement was often completely sunken into the ground, and only the top and the loop-hole showed. These features made the G-type an excellent object to camouflage and a very small target for enemy counter fire.

Of the G-casemate about 600 were constructed.

Flanking fire machinegun casemate [B-type]

In June 1939 the last of the three designs was launched, the B-type. This was a design for a flanking-fire position suitable for a light or heavy machinegun. The B-type was a heavier construction that the S-type with a frontal wall of 1.25 metre and a roof of 1.0 metre thickness.

The loop-hole could be placed at the left or right side of the front section, and many variants were seen with a different entrance structure. The field of fire varied a lot between the many variants. Often the loop-hole was protected at one side by one of the concrete side-walls, which made it a lot less vulnerable to frontal fire than the S-type.

The B-casemate was mainly used in defence-lines in a position behind the front-defences. Its flanking weapons could spray an enemy that had penetrated the line and as such prevent this enemy of jeopardizing the entire defence-line. Also these casemates were used to produce flanking fire at strategic positions in the front-sector, such as crossings or roads.

The B-casemate was built in far lower quantities than the S- and G-type. Its profile was almost identical to the S-type, but it did not have the vulnerable frontal loop-hole and was fitted with thicker walls.

Only a few hundred were built of this type.

Steel cupola casemate

The steel casemate design [1937] was derived from the French Casemate Pamard design of WWI. The Dutch improved the design and made a version composed of chromium-nickel-steel. It was extensively tested by direct and indirect artillery fire up to 15 cm calibre [at only 25 metres distance!]. It sustained these tests beyond every expectation. The concrete embedding proved vulnerable to heavy impact, but the steel only dented.

The steel casemates were fitted with a cupola of 14 or 17 cm thickness. These versions complied with the toughness of the W15-21 class. The weapon exclusively used in this type was the Schwarzlose heavy machinegun. An air exhaust system was incorporated in all these casemates. About 90 of this type were actually built at strategic locations in the east- and southfront of the Fortress Holland.

The G-type casemate was a light variation of this design, and would become far more popular because of its easier and cheaper design.

Another version of the steel casemate was the version designated for the AT gun Böhler 4.7 cm and the "kanon van 5". In May 1940 the first orders for 160 of these casemates were still in delivery, and as such none was actually commissioned when the invasion occurred.

PAG casemate

There were also a number of casemates used for the positioning of Böhler or 6-veld AT guns in the Grebbeline and Peel-Raamline. These had not very heavy constructions, but were more or less shrapnel protective positions. Some were of W12-15 strength. They were specially designed for some locations where the use of AT guns in the first line was regarded imperative, although heavy enemy assault was anticipated. As such it was decided that some sort of permanent protective gun-position had to be designed.

The walls and roof of the shrapnel free shelters were constructed of only 20 mm Ferro concrete. The casemates themselves were simply box shaped, and could house a Böhler AT gun [and some locations a 6-veld gun, which casemates were of W12-15]. Only few were built of which one survivor is still visible at the Grebbeberg in Rhenen.

River casemate

The river casemates were built in 1936 and 1937 as a measure of strategic outer-defence. It was considered in those days that in order to be able to mobilize the army in reaction to an unexpected invasion, a screen of troops along the rivers Maas, Waal and Yssel would have to slow down the enemy to such extend that the mobilisation of the balance of the army could be effectuated. The fixed bridges [there were also floating bridges, so called 'ship-bridges'] over these rivers would be reinforced with two special casemates each. At one location [Nederweert] no less than five of these large casemates were built since this position was a junction of river-arms.

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River Casemate A-type

These casemates - which were designated "river casemates" - would be constructed to the scale W12-15 and be able to house at least one light gun and one heavy machinegun, as well as ample storage facilities for the bridge demolition charges. Also additional concrete obstructions were built at both the bridge ends and an illumination system was added to be able to watch the approaches during dark hours. Therefore one of the two casemates at each bridge had a generator room.

Although the same design concept was used for all river casemates, they were all more or less uniquely constructed. The fact that dikes and bridge ramps were non-identical caused all locations to get casemates of tailor-made dimensions. In the end three main concepts could be identified, designated as types A, B and C.

Type A: this was a concept with one floor and a dual loop-hole for a 5 cm gun and a heavy machinegun. These were built at seven different locations.

Type B: this concept provided two or three floors (especially for location behind a high dike) and also suitable for a 5 cm gun and a heavy machinegun. At nineteen locations these were built.

Type C: this special concept provided room for more than one gun and machinegun, and was actually a combination of two types A or two types B. This version was seen at four different locations.

Except for two locations all casemates were constructed on the westbank [rivers running north to south] or north bank [rivers running east to west]. In total 43 river casemates were built.

The gun that was used in these casemates was a 5 cm AT gun capable of firing HE and AT ammunitions. Originally this gun was a design of the HIH Siderius company, but when this company had gone bankrupt in the early thirties the Dutch Artillerie Inrichtingen had adapted the design and produce a modified 5 cm gun. This was selected for all Dutch river-casemates. Each gun-position had 175 HE and 325 AT grenades in stock. Obviously the main threat was expected to come from armoured targets!

The Schwarzlose was selected as the heavy machinegun for these casemates, like everywhere else.

The river casemates were manned by professional soldiers of the special Police Troops. These units were formed by well selected military personnel. The casemates were manned 24 hours per day, 7 days a week.

Some river casemates are still visible today.

Challenges for the crews under fire

The Dutch casemates were obviously designed in such a way that they wouldn't cost a penny more than strictly necessary. That meant a bare minimum of facilities to its crew. All casemates - with exception of the personnel-shelter and river-casemates - lacked sanitary facilities. There was often no active air filtering device and hardly any storage for f&b. Also the casemate equipment-storage was hardly ever fitted with tools, spare parts or lubricants - let alone replacement weapons. Spare water for the cooling of the weapons was often usually stored on own initiative of the crew or their commander, although some casemates were fitted with rain-recovery systems. At many locations where the going would really get tough, this lack of means and material would cause much trouble.

Also - the far majority of the isolated casemates [they were usually built 200 - 500 metres apart] - could hardly be resupplied with ammo or f&b during combat. Usually the casemates were surrounded by a limited trench-system, but these trenches stretched out for a few meters around the position only. Beyond that usually plain flat country surrounded them. It made the casemates virtually unapproachable while under fire.

There are many known anecdotes about funny events taking place in casemates when above described shortages began to weigh on the shoulders of the occupation during battle. For instance in one casemate where the heavy machinegun started to hamper after extensive and prolonged action, the crew realised that they were completely out of lubricants. They than started picking their noses and greased the mechanical parts with the products they had harvested. It turned out the gun operated on this improvised lubrication too! In another instance the coolant of the machinegun was boiling, and the crew lacked any fresh and cold water, although they were defending the westbank of the Maas; only yards away from the river. One attempt to retrieve a bucket full of river water resulted in exactly one litre of water, whilst received a hail of fire from the German troops on the other side. When no volunteers could be found to repeat that action, the sergeant took out his junior and started watering the gun. This gave just enough "refreshment" to the barrel to sustain another two belts of ammo!

Obviously many machinegun crews were not so inventive or couldn't produce the liquids necessary. They had to deal with failing weapons because of the lack of the above mentioned provisions, and had to give up their positions.

Another experience that was far from pleasant was the confrontation with the (hazardous) fumes from the blazing guns. The machineguns - the heavy ones in particular - produced dense gas-clouds when firing, and these gasses were often caught in the narrow room of the casemate itself. With exception of the G-type and river-casemate, only few casemate types were actually fitted with capable conditioning systems [mechanical air recycling]. This forced many crews - of especially the S-types - to open the rear-door in order to let fresh air in, or to use the annoying gas-mask during operation of the machinegun. When the enemy was shelling the position - which they often did - the option of opening the rear-door was hardly feasible because of the shrapnel danger.

The G-type had a similar problem, although these types were fitted with a mechanical air recycling system. The room in which the crew had to operate was much smaller however, especially in comparison to the S-type, and as such the air was very soon saturated, notwithstanding the recycling system. As a consequence also these crews had to wear gas-masks, often in combination with opening the back-door.

Quality casemates under fire

When a German officer witnessed the Dutch casemates during an intelligence mission prior to the invasion, he stated in his report that these Dutch field reinforcements were "human traps" ["Menschenfalle"]. He intended to state that personnel occupying these concrete weapon positions wouldn't stand a change of survival. After the May War the Germans would however express their awe and respect for the quality of the Dutch casemates, for the best part of these constructions which they had found on their path to the west had proven quite hard nuts to crack!

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Casemate special model at Venlo (may 1940)

The only exception to this quality mark was the S-type casemate. And unfortunately this was the most popular of all - with military defence designers that is. The S-type was by far the most often built casemate. Its extreme vulnerability was mainly caused by two aspects. First the fact that the main loop-hole was constructed front and centre of the casemate. This presented the enemy guns a fine window of opportunity. The second disadvantage was the relatively large loop-hole. Both disadvantages contributed to many fatalities inside the S-type bunkers that would come under fire, and it caused many of these casemates to be put out of action within relatively short time.

The major disadvantage of the vulnerable front was effectively punished by both the German 37 mm PAK gun - standard gun of the infantry - and the notorious 88 mm FLAK gun. The latter was only used against the casemates in the south, at the Maas-front. But the 37 mm PAK was applied everywhere in the field against these S-types. Just a few direct hits into the loop-hole - which was only protected by a 20 mm steel plate - were usually enough to kill the crew or destroy the casemate-weapon. In particular along the Maas many S-type casemates were destroyed this way by German AT guns.

The best performance was seen from both the river-casemates and the G-type. The river casemates had quite some fire-power available and were usually hard to hit. The fact that they were not easily fired upon had mainly to do with the circumstances that arose from geo-physical conditions; the fact that dikes prevented the Germans from positioning their AT guns such that direct fire could be given. It was not the strength of the casemate itself that proved it tough to hit, but its position behind the dikes or in the columns of the bridges that made them hard to reach for assault weapons.

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Porcupine casemate at Venlo bridge (may 1940)

The G-type casemate received the best German credits. Many of the German field officers applauded these constructions due to their elusiveness in the field [low profile] and the virtual impossibility to destroy the cupolas. The 100 mm armoured cupola was impregnable by the 37 mm PAK guns, and only a lucky direct hit on the machinegun itself would pay off. Only very few of these constructions were therefore taken out by German doing, most of the time by 88 mm hits.

The casemates along the river defence-lines would almost all be challenged. Especially the casemate line along the Maas was hit virtually along the entire river. At many locations the Germans had plenty of trouble taking out these defences. Still, when the Germans were able to mobilize enough fire-power the casemates - which were mostly of the S-type - were taken out one by one. Many brave crews perished inside these casemates and it were these crews who lived up to the German name of "human traps". The Germans on their part did pay a high price for the Maas-line. The casemate-lines had been intended to slow down the enemy [not hold the enemy] and at many locations they performed well.

The quality of the casemates may have been quite good, their positioning and especially their relatively small number, made them a whole lot weaker than they could have been. Like a barricade in the field - which is undefended - hardly slows down an enemy, a casemate that defends an isolated position and is not supported or covered by adjacent troops and guns will eventually yield. This was the case with many of the Dutch casemate positions; they were isolated and hardly ever supported.

In the Grebbeline the value of the concrete reinforcements proved very effective in the central section of the line, but the quite unwise positioning of the casemates at the Grebbeline at Rhenen showed the relative worthlessness of a cordon-shaped casemate line. Once the enemy penetrates at one point, all the other positions were attacked in the rear. The battle at the Grebbeberg, where plenty of casemates had been constructed, proved that these constructions only paid-off in effective defence if and when combined with coonsiderable depth of the defences and close cooperation with infantry and artillery. That was hardly ever the case at the Grebbeberg, and as such the Germans had little problems with the casemates at that location.

The Beton-Plan

"Beton" is the Dutch translation of "concrete". Early 1940 the Dutch had designed a totally revamped plan to further reinforce the selected main defence lines. The main lines would receive an extensive expansion of ferro-concrete fortifications. The plan incorporated thousands of new casemates. Plenty of weapon-points, but also large concrete command-posts and many large personnel-shelters. The plan was eventually approved in April 1940, but came obviously too late to be executed prior to the invasion. Still we would like to address it briefly.

Four types of new casemates were approved and introduced:
Type UB-80: W12-15 observation post, designated to be placed in all front line sectors in between the machinegun casemates Type S and G.
Type A150: W12-15 Personnel shelter, designated to be placed behind the front-line. Two groups could find shelter here.
Type F150: W12-15 Personnel shelter with flanking machinegun post, designated for both frontline and stopline. It provided space for about 15 men and had nine beds.
Type 2F150: W12-15 Battle position with a flanking and frontal loop-hole for a heavy machinegun, designated for the stopline.

These new designs would be constructed in combination with the existing types. Many hundreds of these new casemates were projected in the Grebbeline and southfront Fortress Holland. They would have been a tremendous quality impulse to the Grebbeline in particular. They would have ...