Fortification Friday: Know your banquettes and slopes

Over the last couple of posts in this series, I’ve discussed parapets and their function.  Now let us turn to the parts of a parapet and look at those in detail.  As a refresher, this is Mahan’s profile of the parapet (highlighted line):

Parapet

Mahan defined this profile as the lines between points A-B-C-D-E-F.   It’s important to note that each individual line (defined between the points) also defines a separate component of the parapet.

Between points A and B is the Banquette Slope:

Parapet_Banquette_Slope

Specifically, point A is the Foot of the Banquette Slope and B is the Crest of the Banquette Slope.

Wait… what is a Banquette?  Mahan described the Banquette as:

The banquette is a small terrace on which the soldier stands to deliver his fire ; the top of it is denominated the tread, and the inclined plane by which it is ascended the slope.

So this explains lines A-B and B-C.  The latter being the Tread of the Banquette, and including point C, the Foot of the Interior Slope:

Parapet_Banquette_Thread

From a functional standpoint, the Banquette had to be wide enough to allow a rank or ranks of soldiers to stand in formation and work their musket.  The measure would be different depending on the number of ranks that the defender planned to use.  One rank might get by with two feet of width.  Two ranks required four feet.  So something on the order of 4 ½ to 5 feet would be preferred to allow ease of movement.  The Banquette was also given a slight slope to the interior to allow for drainage.

The Slope of the Banquette (A-B) was structured as the hypotenuse of a right triangle.  The slope would be a compromise providing support for the Tread while offering the lowest slope for the troops to climb.

One other functional requirement to consider about the Banquette is its height above the tere-plein (natural surface level), or interior, and the height of the parapet.  The troops had to be able to stand on the Banquette and shoot with most of their body protected by the other parts of the Parapet.  This governed the overall height of the Parapet somewhat, given average height of soldiers and such.

Moving further down the profile of the Parapet, we come to the Slope portion of the Parapet.  By adding point D, the Interior Crest, we have line C-D, known as the Interior Slope:

Parapet_Interior_Slope

As with the height of the Tread, this line’s length was governed by the need to allow soldiers to fire over the Parapet.  A sharp incline of this line allowed the troops space to move while keeping the mass close to their bodies.  But not being the most efficient structural support angle, that incline required careful maintenance.

Line D-E, with E being the Exterior Crest, is called the Superior Slope:

Parapet_Superior_Slope

The Superior Slope declined outward (towards point E).  This allowed soldiers to depress their weapons to engage targets directly in front of the works.  This also ensure any fires hitting the front of the fort would glance upward and away from the defenders (hopefully).  The angle of the Superior Slope was also a compromise.  Too shallow and the Parapet might be excessive and perhaps not allow enough declination for the muskets.  Too deep and the Parapet’s strength is compromised.

Continuing the same convention, the Exterior Slope is line E-F, where point F is the Foot of the Exterior Slope:

Parapet_Exterior_Slope

This portion of the Parapet had the important mission of stopping projectiles.  The preferred angle was 45º, or the natural slope at which loose dirt will pile.  Structurally, that was the best support angle for the Parapet.  Furthermore, when under fire, any dirt thrown up from the Exterior Slope would naturally fall back to that angle… one would expect.

The Exterior Slope completed the profile of the Parapet.  But there is one other part to consider, although it is not part of the defined parapet – line F-G:

Parapet_Berm

Point G is the crest of the Scarp, part of the Ditch.  Mahan called this the Berm.  The Berm connected the Parapet to the Ditch.

I have a problem with the choice of words here.  In modern context, berm is often a raised mound, almost a Parapet itself.  We spoke of “crossing the berm” in the Gulf Wars as noting a passage through defensive works thrown up in the desert.  Likewise, berms are tall, lengthy mounds built between roads and subdivisions to block noise.  And let’s not forget berms put up in front of raising flood waters.  So you see, a “berm” means some other shape to most modern readers.

But for Mahan, the Berm was a construct that allowed the weight of the Parapet to stand on something other than the back edge of the ditch (the Scarp, which we will discuss later).  Frankly, he wrote:

The berm is a defect in field works, because it yields the enemy a foot-hold to breathe a moment before attempting to ascend the exterior slope. It is useful in the construction of the work for the workmen to stand on; and it throws the weight of the parapet back from the scarp, which might be crushed out by this pressure. In firm soils, the berm may be only from eighteen inches to two feet wide; in other cases, as in marshy soils, it may require a width of six feet. In all cases, it should be six feet below the exterior crest, to prevent the enemy, should he form on it, from firing on the troops on the banquette.

Thus the Berm was a necessary evil.  It was a risk that need mitigation during construction.

These terms become very important when considering the engineering involved to build a fort.  Each component had a function. Those functions determined the measures of the line.  Engineers, being engineers, would compute those measures based on formulas provided by Mahan and others.  In short, the troops didn’t just throw this sort of thing up randomly:

They ENGINEERED it.  And that engineering involved careful study of the task using some of those terms presented above.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 1-3, 22.)

3 thoughts on “Fortification Friday: Know your banquettes and slopes

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