Fortification Friday: Trous-de-loup… French for pits in the ground

Trous-de-loup!  Oh-la-la! Anything in French just sounds sweeter… dare I say romantic?


Mahan listed Trous-de-loup as a type of obstacle. What is a Trous-de-loup, anyway?  Um… a straight translation would be something like “holes.”  In the context of military fortifications, Mahan described them as pits, but kept the French nomenclature.  Now these were not just random holes in the ground.  Rather these were fashioned in an orderly manner to serve as an obstacle:

Trous-de-loup. These are pits in the form of an inverted truncated cone, or quadrilateral pyramid; their diameter at top is six feet, their depth six feet, and width at bottom eighteen inches.  A stake is, in some cases, planted firmly in the bottom, its top being sharpened, and the point a few inches below the upper circle.

Mahan offered Figure 28 to illustrate Trous-de-loup:


Let us focus on the left side of Figure 28 for a moment where the pits are demonstrated to the dimensions Mahan specified in the text:


As obstacles go, the Trous-de-loup broke up the ground over which the attacker advanced.  And notice the specified dimensions.  At six foot depth, this ensured the attacker could not gain a lodgement which was not dominated from the defender’s parapets. This pit was dug from the surface level, giving no artificial elevation to aid the attacker. Furthermore, the attacker would have to share the eighteen inch bottom with, if the option were exercised, a post or stake.  Certainly not something an attacker would like to deal with while crossing the “beaten zone” to get at a fortification.

Trous-de-loup are generally placed in three rows, in quincunx order, a few yards in front of the ditch.  They are readily laid out by means of an equilateral triangle, formed of cords, the sides of the triangle being eighteen feet; the angular points mark the center of the pits….

Quincunx order?  Yes, a pattern… arrangement, if you will.  Quick, familiar reference – pick up a six sided dice and look at the five side.  Scott Manning in one of his Wednesday Warpaths will likely point out to us that quincunx is Latin.  It derived from a name for the denomination of Roman currency.  The geometric pattern served as a good arrangement orchards.  And the Roman legions sometimes used it as a tactical formation… but that’s Scott’s shtick.

To illustrate Mahan’s suggested placement of trous-de-loup, let us drop some equilateral triangles on the figure:


Regular placement of obstacles forces the attacker to adopt predictable approach methods. This enables the defender to better place larger weapons… like artillery… to achieve the maximum effective damage.  So don’t scoff at Mahan’s triangles.  There’s a reason for the specification and, in tactical parlance, it rhymes.

With the arrangement set, the digging would commence. And that leads to the question – what to do with the removed dirt?

The earth taken from them is spread over the ground between them, and is formed into hillocks to render the passage between them as difficult as possible.

Looking back at the top portion of the figure, we see that illustrated:


Notice how the “hillocks” would serve to force the attacker to scale more elevation and at the same time put the men above the line of sight from the parapet. So if the enemy stayed in the six foot deep hole, he was exposed to fire from the defender.  And if the attacker attempted to advance through (as in skirting around) these pits, he was silhouetted, exposed, and bunched to the fire of the defender.   The word sometimes used in military discussions is “canalized”, as in redirecting the flow of the enemy’s attack into streams.  I know… a tricky use of the word, but this is the profession that derived the term “uncoilation” to describe movement out of an assembly area….

Continuing with the arrangement of pits, these trous-de-loups get better:

If brush wood, or light hurdles, can be procured, the pits may be made narrower, and covered with the hurdles, over which a layer of earth is spread.

So these might be concealed from the attacker’s view, creating a trap of sorts.

Great, trous-de-loup were formidable obstacles.  But the French is difficult to spell and pronounce.  Writing in the 1880s, Major Junius Brutus Wheeler, who taught engineering at West Point, opted to suppress the French terminology while offering a couple variations of the obstacle type:

Military pits. – Excavations made in the ground, conical or pyramidal in form, with small picket driven into the bottom, are called military pits. (French, trous-de-loup.)

They are of two kinds, viz: deep and shallow.

Describing the deep pits, Wheeler wrote:

Deep military pits should not be less than six feet in depth, so that if they fall into the possession of the enemy, they can not be used against the defense.

They are usually made about six feet in diameter at top, and about one foot at the bottom, and are placed so that the centers shall be about ten feet apart.  They should be placed in rows, at least three in number, the pits being in quincunx order. The earth obtained by the excavation, should be heaped up on the ground between the pits.

The deep military pits match directly to those described by Mahan, save the dimension of the bottom and distance measured between pits.   Wheeler offered this figure to illustrate the deep military pits:


As for shallow military pits:

Shallow pits should not be deeper than about two feet, so that the enemy could not obtain shelter by getting into them.

They should cover the ground in a zig-zag arrangement, the upper bases being made square or rectangular in form, and in contact with each other.  The side of the upper base should be made about equal to the depth of the pit.  The earth obtained from the holes is thrown in front of the arrangement, making a glacis.

Wheeler did not offer an illustration to support this description.  However, we can go back to Mahan where the right side of Figure 28 demonstrates just such an arrangement of shallow pits:

Mahan described these as “small pyramidal pits, with pickets.”  Notice to the right of the illustration we see the glacis described by Wheeler.

Closing the discussion of trous-de-loup… er… pits… Mahan suggested other locations for employment of this obstacle:

Trous-de-loup are sometimes placed in the ditch; in this case, their upper circles touch.

This obstacle is principally serviceable against cavalry.

While these military pits look formidable in the diagrams and seem to be an excellent obstacle, there are considerations governing their employment.  As with all obstacles, the trous-de-loup must be “under the guns”, otherwise the attacker would simply navigate through, perhaps only losing a few steps on the march.  Also consider the time and labor required to place the trous-de-loup.  That’s a lot of earth to displace.  The shape of the pit is somewhat demanding for just shovel and pick.

The trous-de-loup worked best when placed in front of the works in the area cross-fired by flanks.  That ground, presumably already cleared by the defender, might not need much augmentation to deter enemy advances.  So one reason we might not see many trous-de-loup in Civil War fortifications is the engineers weighed the effort against benefit.

In that light, Mahan’s last sentence stands out.  Trous-de-loup was rather effective at breaking up fast moving attacks, such as cavalry.  By the time of the Civil War, direct assault of field formations, much less than field fortifications, with cavalry had fallen out of favor.   With that, the engineers found those pits of less importance.

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 44-5; Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 176-7.)

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