The French WWI trench concept
For the right perspective and reference it is good to elaborate a little more on the defence structure of the tranches and positions in the Grebbeline.
The French concept of trench-lines and defensive perimetres have been addressed before. But during the battle at the Grebbeberg, it is imperative that the reader understands the concept of this doctrine. That's why we shall kick-off with charting the Dutch defences at the Grebbeline.
French WWI defence structure
The French concept was based on a main-defence zone that was given shape by a front-line and a stop-line, as well as a third line in the rear of the main-defence zone.
The front- and stopline were interconnected by linking-trenches every [so many] hundreds of metres. As such the main-defence zone comprised some sort of a box-pattern. These interconnecting trenches had many functional purposes. Reinforcements could be fed to the front-line, casualties and injured men could be transported to the rear and - in case of an enemy penetration is some sector - the linking trenches could be used to seal off the penetration in the flanks. Also the relief of front-units could be executed under full cover. Ammunition could be transported as well as spare arms.
The distance between the lines should not be too short, nor should it be too long. Would it be too short, it would create a hazard to own troops once artillery would be applied in support of the stopline defences after an enemy penetration. Would the distance be too long, the stopline would not be able to suppress a penetrating enemy force, which would provide the enemy too much room to manoeuvre and expand. Generally spoken the distance between the frontline and the stopline was supposed to be in the range of 300-500 metres.
The frontline had to be the most formidable defence for the French doctrine prescribed the imperative value of this main defence. The front-line was not to be lost and any sector penetrated by the enemy had to be retaken soonest. As such the front-line had to have a subsidiairy trench structure with first reserve units, additional supplies and company command posts. The capacity of the front-line had to be large and as such extended dug out shelters had to be part of both the actual forward trenches as well as the subsidiairy line.
Both the 'no man's land' in front of the forward trench-line and the area between the front-line and stop-line had to be packed with obstructions and land-mines. The area between front- and stopline obviously had to be modestly underminded, but the 'no men's land' had to be virtually impregnable.
The stopline defences were supposed to provide the releave of the forward defences. Moreover the battalion command posts and local supply caches had to be incorporated in the structure. The stopline was not extremely fortified like the front-line. It did have to contain the light infantry support guns and mortars though.
Behind the stopline the light and medium field artillery units had to be deployed. These units had to be highly mobile so that they could be applied in a dynamic theatre. In between these positions the rear line was situated, containing second line reserves, bulk caches and regiment command posts. Behind the rear defences the more static artillery was positioned.
The Dutch implementation
The Dutch had obviously cut out all the frippery of the French concept. The interconnecting trenches between the stop- and frontline had not been constructed. The subsidiairy trenches at the front-line were absent. And the extensive obstruction and undermining of the no man's land failed [not in the Peel-Raamline though!]. Moreover the capacity of the trench system was modest. Large forces were as such spread out and concentrations of force were impossible due to lacking shelters and hide-outs.
The frontline itself was the main-defence line with many fortified sailants incorporated in it. These sailants contained machinegun nests which were able to cover at least 120 degrees of the front, as such being able to give both frontal and flanking fire. Also concrete pillboxes were constructed in and just behind the front-line.
The stopline was the rear of the main-defence zone and hosted company CP's, first reserve troops and ammunition stocks. Only occasional weapon-points were prepared in this second line. Infantry guns, AT guns and mortars were positioned in front of and behind the stop-line.
Behind the stopline the artillery positions had been dug out. The artillery had prepared many different fire missions in the sector in front of the main-defence as well as some in front of the stopline. These fires had been accurately measured in [usually by triangular measurement] and field commanders requesting a fire mission could as such suffice by just requesting a certain prepared fire code, possibly with addition of the length and intensity of the fire and the target typicals [e.g.: infantry targets usually required HE / air burst ammunitions with quick fuses, vehicles required slow or impact fuses].
The rear line - which the French applied during WWI - was absent all along. At some locations a thin trench had been dug out, but that was nothing like the French concept prescribed. Basically the entire defences consisted of the front- and stopline.
The front-line was situated behind vast inundated area's, but at two locations near the city of Amersfoort and at the Grebbeberg these inundations had not been set due to the elevated terrain. A concrete mill was under construction that had to flood the area in front of the Grebbeberg defences too. It had not been delivered when war broke out. In the sectors where these inundations were omitted so called forward positions had been prepared. These were isolated trenches - usually containing a licht machinegun squad or entire platoon. The positions were not connected and usually placed in a pattern with some hundred metres of depth. Unfortunately the designers had not developed a hedge-hog shape but a traditional face forward trench that onbviously had a blind spot equalling at least 120 degrees, often more. That feature in combination with the isolated in depth pattern posed the immense liability that positions could be outflanked one by one. These forward defences also lacked fire power. Their task was to prevent the enemy from undertaking reconnaissance missions against the main defences and prevent the positioning of enemy troops close by. Obviously these forward defences would be called back or forced back at some point after which the alarming and preventative tasks would be lost as well.
Probably the two worst mistakes the Dutch had made - cutting back on the proven French concept - were the absence of linking trenches between the two defence-lines and the failing of a genuinely strong front-line with large capacity and intergral subsidiary trench system. Both these flaws had seriously decreased the value of the defence structure as a whole.