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?

that-was-frenchquerida

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:

PlateIVFig28

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:

PlateIVFig28A

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:

PlateIVFig28A_Overlay

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:

PlateIVFig28C

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:

WheelerFig71

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:
PlateIVFig28B

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.)

Fortification Friday: Accessories for the fort… obstacles

Bet you didn’t realize the need to accessorize your fort?  Yes, after digging all those ditches, piling dirt for the parapet, then addressing revetments and such, there was still work to do.  This work was to supplement the defensive character of the fortification by way of obstacles or other impediments in the path of the attacker.  Mahan called these “accessories, or secondary means of defense.”

The means employed as accessory usually consist of artificial obstacles, so arranged as to detain the enemy in a position where he will be greatly cut up by the fire of the work.

This is a great “sound byte” that holds true for military application even today – slow down the enemy and make him pay for showing his face in front of your works!

From this premise, Mahan discussed the nature and need for accessories:

Anything may be regarded as an obstacle to the enemy by which his attention is diverted from the assailed to his own situation; but no obstacle will be of much service to the assailed which is not within good striking distance of his weapons. the proper disposition therefore, of obstacles, is in advance of the ditch within short musket range.

Marshes, water courses, wet ditches, precipices, &c., may be regarded as obstacles, if they are sufficient in themselves to stop the enemy’s progress.  But, however strong, they are not solely to be relied on, as the strongest natural position may be carried if not vigilantly guarded.

In placing the ground around a work in a defensive attitude, every means should be taken to reduce the smallest possible number of the points by which the enemy may approach; so that, by accumulating the troops on the weak points, a more vigorous defense may be made. In making this arrangement, equal care should be given to everything, affording a shelter to the enemy, would enable him to approach the work unexposed to its fires. To prevent this, all hollow roads, or dry ditches, which are not enfiladed by the principal works, should be filled up, or else be watched by a detachment, covered by an advanced work. All trees, underwood, hedges, enclosures, and houses, within cannon range, should be cut down and leveled, and no stumps be allowed higher than two feet. Trees beyond cannon range should not be felled; or, if felled, they should be burnt, to prevent the enemy’s movements being concealed.

The military “truism” here – an obstacle not under the watchful eyes of a defender is not an obstacle… and may even be an avenue.

The comment about trees beyond cannon range should be placed in some context, I think. Mahan’s notion of tree lines might be that of a wood-lot or such where the undergrowth was minimal (ah… the days of free ranging livestock).  Regardless his advice was to reduce any cover given the enemy, even if that meant clearing a wide view-shed around the fortification.  Common sense at play again.  However, practical application of such translated to a lot of tree cutting… and in some cases displacing civilians and removing their dwellings.

But there are some things that might lay within viewing distance of the fort which should not be removed:

If there are approaches, such as permanent bridges, fords, and roads, which may be equally serviceable to the assailed and to the enemy, they should be guarded with peculiar care; and be exposed to the enfilading fire of a work especially erected for their defense.

We are thus back to “greatly cut up by the fire of the works.”  And often we see in the plan for Civil War fortifications the provision of additional bastions or works just to provide fire on a crossing point.  Redoubt Brannan, in the sprawling Fortress Rosecrans at Murfreesboro, served such a purpose.  On the south side of the fort, Lunette Thomas covered a railroad and turnpike approach to the fortress. And circling back to the question of dwellings in view, many homes and buildings in front of the works in Murfreesboro proper were left in place.  But Federal officers had orders to shell and burn the buildings should any Confederate provocation arise.  None did, but it is interesting how sort of a “risk based decision” was made in that regard.

But let us direct this discussion of obstacles to something specific and detailed.  There were types of “accessories” which the defender might select to construct as obstacles….

The principal artificial obstacles are trous-de-loup, or military pits; abattis; palisades; fraises; stockades; chevaux-de-frise; small pickets; entanglements; crows-feet; inundations; and mines.

So, next up…. Trous-de-loup and abattis!

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 43-4.)

Fortification Friday: Timber for Scarp Revetments

Thus far we’ve looked at Mahan’s advice for revetments, taking each type in detail – sod, pisa, fascine, hurdle, gabion, plank, and sandbag. Mahan offered one more type for use with field fortifications, calling it the scarp revetment.  The name implies use specifically on the scarp within the ditch.  However, it was also referred to as a timber revetment by other authorities, indicating such could be used at other places within the fortification.  But as we focus on Mahan’s instruction here, let’s proceed with the notion of revetting the scarp.

Scarp revetment. This revetment is formed of a framework of heavy timber, and is used only for important field forts. A piece, termed a cap, or cap-sill, is imbedded in a trench made along the line of the berm; other pieces, termed land-ties, are placed in the trenches perpendicular to the cap, with which they are connected by a dove-tail joint; they are about eight or ten feet asunder. Cross pieces are halved into the land-ties about two feet from their extremities, and two square piles, about five feet long, are driven into the angles between the land-ties and cross pieces; inclined pieces, which serve as supports to the cap, are mortised into its under sides at the same points as the land-ties.  These supports usually receive a slope of ten perpendicular to one base; they generally rest on a ground-sill, at the bottom of the ditch, to which they are mortised, this being held firm by square piles. The ground-sill may be omitted by driving the supports below the bottom of the ditch.

Figure 27 of Plate III was offered to illustrate the scarp revetment, in “section” and “plan”:

PlateIIIFig27

You can see how these are matched across the diagram with dotted lines between the section and plan.  For some clarity, I’ve also broken these down into individual cuts for each. Here’s the section:

PlateIIIFig27A

And here’s the plan:

PlateIIIFig27B

The annotations in the diagram follows the description (not in alphabetical order, though):

  • B is the cap-sill.
  • D is the ground-sill.
  • C are the uprights between the cap and ground.
  • A are the land-ties. Notice how those are dovetailed into the cap-sill.
  • E depicts the cross pieces attached to the land-ties.
  • F shows the short piles used to anchor the land-ties leveraging the cross pieces.
  • G are piles anchoring the ground-sill.

Note the wood is connected by way of joints – specified as dovetail or mortise joints.

With this framework constructed, Mahan called for planks over the scarp:

Behind this framework, thick plank, or heavy scantling, is placed side by side, having the same slope as the supports; or else a rabate may be made in the cap and ground-sills, and the scantling be let in between those two pieces serving as a support to the cap. This is the more difficult construction but is the better, since, should the heavy supports be cut away, the cap will still be retained in its place.

Thus the face of the scarp would be covered by a revetment of plank.  And in the best case, those planks added to the support of the framework, using a rabbet (Mahan says rabate.. a rose is a rose…) joint.  Note the uprights (C) and the planks both ran vertical in the preferred construction.  There’s a reason, as Mahan noted next:

Scarp revetments are sometimes formed by laying heavy timber in a horizontal position; but this method is bad, as it enables the enemy to gain a foot-hold by thrusting their bayonets between the joints.

So don’t be lazy and allow the enemy gain a foothold.  Take the time, get out the wood-working tools and make the right joints.

Another bit of advice Mahan offered was, “The length of the land-ties should be at least equal to two-thirds the depth of the ditch.”  Thus the length of “A” had a direct relation to the length of “C”.  Those are 9 and 12 feet, respectively, in the diagrams above. Thus extended, the land-ties would firmly anchor the weight of the revetment with enough strength to resist an enemy’s attempt to pull the structure loose.

Looking to the other side of the ditch, Mahan gave a little attention to the counterscarp:

The counterscarp is seldom reveted. A framework similar to that for the scarp might be used, and thick boards, laid horizontally, be substituted for the inclined scantling.

Observe, the horizontal orientation was fine for the counterscarp.  Who cares if the enemy gets a foothold while trying to retreat… and at the same time, those footholds might prove useful should the defender need to pursue. But above all, Mahan saw little need to waste time with fancy woodworking joints for the counterscarp.

Now this is all good talk about revetment of the scarp.  But we have to consider how it fits into the overall construction process:

When a scarp revetment is made, the excavation of the ditch must be conducted in a different manner from that already explained. In this case, after the cap-sill and land-ties are laid, the excavation is continued to the bottom of the ditch, by removing only earth enough to allow the framework to be put up. A scaffolding of plank is then raised in the ditch on which the earth that remains to be excavated is thrown, and from there to the berm.

Yes, it would be rather difficult to plant those land-ties after the parapet was piled up!  So the scarp revetment needed to be set before digging out the ditch.

To sum up the discussion of revetments, these were supplemental structures within the works.  But were considered necessary improvements to strengthen the works, adding resiliency against erosion and enemy action. We’ve seen a progression here from simple, low-labor cost solutions to more elaborate and labor-intensive options.  Each type of revetment offered different qualities that an engineer could consider within the overall plan.  And he couldn’t just go to some home improvement store to purchase materials!

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 41-42.)

Fortification Friday: Sandbag and Plank Revetments

Somewhat serendipitous, my pal XBradTC forwarded a link to a lavishly illustrated forum post. Wonderful dioramas depicting British trenches from World War I in cut-away profiles.  Please browse through there, and in particular look at the annotations on the side of the elevations. Since the focus of my late Friday installments has been revetments, let us look at those. In those three selections from the modeler, we see four types of revetments, of the type used in the Great War.  Three in view on this picture:

 

666425

Sandbag, wood, and wicker revetments.  Missing from this profile are corrugated iron revetments (which you will see on the other two dioramas in the post).  And corrugated iron was not something Mahan suggested for Civil War-era engineers.  Corrugated metal was around in mid-19th century America, and could have been used.  But its military applicability seems to have eluded Mahan when he was writing in 1846.

However, as we have seen, Mahan did discuss at length the manner of creating wicker-based revetments, in the form of fascines and gabions. Though the diorama depicts a simpler wicker form, more as a sheet than bundle or basket.

Closer to Mahan’s ideas on revetments are the wood and sandbag revetments.  As for the use of wood planks, Mahan wrote, briefly:

Plank revetment.  This revetment may be made by driving pieces of four-inch scantling about three feet apart, two feet below the tread of the banquette, giving them the same slope as the interior slope. Behind these pieces, boards are nailed to sustain the earth.

And we see something very similar, though not driven two feet below the tread, depicted below the firestep in the diorama.  So the physics of using wood planks to restrain earth did not change in the fifty years from Appomattox to the Somme.  Go figure.  I would point out, before we completely dismiss corrugated iron in context to Civil War revetments, the manner of fixing that type in the Great War period was similar to that described by Mahan for planking:

666429

Another type of revetment mentioned by Mahan which would also be familiar to the Tommies used sandbags:

Sand bags are sometimes used for revetments when other materials cannot be procured; though their object, in most cases, is generally to form a speedy cover for a body of men.  They are usually made of course canvass; the bag, when empty, is two feet eight inches long, and one foot two inches wide; they are three-fourths filled with earth, and the top is loosely tied.  From their perishable nature, they are only used for a temporary purpose, as when troops are disembarked on an enemy’s coast.

Let’s examine Mahan’s emphasis on “temporary” with respect to sandbags.   We know well the Tommies on the Western Front were using sandbags through 1918.  And closer to Mahan’s period, we know that on Morris Island the Federals used sandbags extensively from 1863 through the end of the war.  Though… let us acknowledge that initially the situation fit to a “T” Mahan’s proposed scenario – being on the enemy’s coast. Far from the coast, sandbags were employed at Petersburg … and not in some “temporary” fix.

Allow me to make much about little here.  Mahan’s main objection to the sandbag was the tendency to deteriorate.  Writing in 1863, Major Thomas Brooks indicated he turned to sand bags on Morris Island were gabions had failed to retain the beach sand.  Addressing the deterioration, Brooks observed:

At the end of two months the sand-bags used in revetting the siege works herein described began to show signs of decay; but with careful usage, under favorable circumstances, sand-bags might not require replacing in twice the above time.

 

Brooks went on to say that in time sandbag revetment was often replaced by sod revetments…. when sod was more plentiful for the Federals along the South Carolina coast.

Now in reference to Petersburg, we see another dynamic at work, I think.  Most of the sand bags were used in revetments in battery positions.  Like Brooks earlier, the engineers had issues with sand pouring through the gabions (sand vs. soil at work here).  Furthermore, the Federals at Petersburg had ample hands, as the siege developed, to work filling sandbags to meet needs.  So deterioration was met with replacement.  Likewise, the Western Front of the Great War the density of troops at the front during periods of defensive posture (between offensives and such), left many hands for sandbag detail.  Another aspect addressing deterioration, the fabric used in 1914 was more resilient.  And today we use poly-fibers and other “space age stuff” that ensure sandbags don’t even deteriorate after discarded!

Sandbag revetments offer many advantages, no doubt overlooked by Mahan for brevity.  Already mentioned above, sandbags work better with … as the name implies… sand. Another advantage is that sandbags don’t create splinters when struck by enemy projectiles, which wood, corrugated iron, or even wicker do.  Furthermore, the sandbag offers a relatively uniform construction material over sod and other types that Mahan suggested.  The uniform nature became more appealing in situations with large armies engaged in prolonged siege operations.  Particularly where troops in rear areas might work details to produce large quantities of sandbags for distribution.

OK… sandbags… I prefer them.  Mahan did not.  Enough said.

The last type of revetment discussed by Mahan was the scarp revetment, which used a framework of timbers.  Since it is more elaborate, and its explanation needs more space, we’ll pick that up next week.  But in closing this installment, I would ask readers to consider the similarities and differences between the Mahanian trenches and those of World War I (and later periods).  Moving earth to make an entrenchment remained a task accomplished by the shovel and pick.  But the intent and practice of the entrenchments changed somewhat with time.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 40-1; OR, Series I, Volume 28, Part I, Serial 46, page 318.)

 

Fortification… Thursday: “Every point of the parapet should be guarded”

With Christmas falling on Friday, I’ll move the regular cycle of posting up a day… you get Fortification Thursday.

Let us close out the “lesson” on simple “intrenchments”, or basic field fortifications, by looking at the last passages from Mahan’s chapter on the subject:

The defense of enclosed works demands that every point of the parapet should be guarded, at the moment of assault, either by cannon or musketry. The troops may be drawn up for the defense either in one, two, or three ranks; and there should, moreover, be a reserve proportioned to the importance attached to the work.  The free interior space, denominated by the terre-parade plein, should be sufficiently great to lodge the troops, with the cannon and its accessories, and will therefore depend on the nature of the defense….

This paragraph takes us back to Mahan’s principles for the defense.  However, in the earlier lessons, Mahan had not discussed the interior structures at length.  Here he cites a feature by name – the terre-parade plein – and gives a requirement for that feature.  Goes to reason that important places, which would require large reserves, would thus need a large interior parade.

Mahan then began to provide some practical application of this advice:

Each man will occupy one yard, linear measure, along the interior crest, and each cannon from five to six yards.  The space requisite to lodge each man is one and a half square yards; and about sixty square yards should be allowed for each gun.

Think of that “frontage” as applied to Mahan’s rule of thumb about fort dimensions. So a thirty yard flank would need 30 men … or five or six cannon… or a combination of those.  Multiply that allocation against the similar sized opposite flank, then add in the manpower needed on the faces and curtain.  Very quickly we have need of a full strength company plus a battery of artillery (at least) to defend a single side of a bastion fort.  So project that math further… four sides, plus a need for reserve.  You see how we ring up the requirement for a full strength regiment plus three or four batteries of artillery.

Well… on the other side of the coin, at least there would be plenty of hands around to do the digging.

Mahan continued with details of the arrangements, and immediately brought up another interior structure for discussion:

Besides this space an allowance must be made for the traverses, which are mounds of earth thrown up in the work to cover an outlet, to screen the troops from a reverse, or an enfilading fire, &c.; and for powder magazines, when they are not placed in the traverses.  The area occupied by a traverse will depend on its dimensions, and cannot be fixed beforehand; that allowed for a magazine for three or four cannon may be estimated at fifteen or twenty square yards.

The traverse will be the focus of another post in time.  But for the moment, consider that Mahan basically instructed us to “bolt on” the traverse after building the fort.  Though he indicated a rough size for planning.

But wait… he said fifteen to twenty square yards for 3 to 4 guns.  Earlier he indicated we needed to factor in sixty square yards for the individual guns.  So for a four gun battery, we’d need 260 square yards.  A six gun battery (say holding back two guns for reserve), would need 380 to 400 square yards. And our math determining how many muskets and cannon were needed for a standard-size, by-the-book Mahan-approved fort was pointing to four of those batteries.  Better figure on at least 1500 square yards just to have space for the artillery… between a quarter and a third of an acre.  And that is just the artillery.  So if you have a fort that really needs to be defended properly, better plan on having a lot of space to dress out that fort.

Hold on… recall we have a preference for 250 yard exterior faces…. or a 62,500 yards square box in which to put the fort (including those areas that would lay outside the structures, mind you!).  Sounds like one would have ample space.  But when you subtract the area that lay outside the faces, flanks, and curtains; factor in the space that the works themselves will take up; and then figure on a regiment of infantry plus all those guns… well one has to work hard to avoid running out of space!  The engineer’s work was not simply drawing polygons on paper.

The last paragraph in this chapter provides the “soundbyte” for the philosophy of the defense:

As a field fort must rely entirely on its own strength, it should be constructed with such care that the enemy will be forced to abandon an attempt to storm it, and be obliged to resort to the method of regular approaches used in the attack of permanent works.  To effect this, all the ground around the fort, within the range of cannon, should offer no shelter to the enemy from its fire; the ditches should be flanked throughout; and the relief be so great as to preclude any attempt at scaling the work.

You see, if employed successfully by the defender, the fort would buy time for the defender.  That time would allow the defender to adjust to meet the attacker, or at least exact a toll upon him.

Apply this to one of the many episodes from the Civil War.  I think immediately of Morris Island.  After the Federals failed to carry Battery Wagner with a direct assault, they were “obliged” to lay a “regular approach” across a narrow beach in order to gain the Confederate works.  And that bought the Confederates a significant amount of time – in which they re-positioned forces (heavy cannon) out of Fort Sumter and to other locations in the harbor.  The Confederates could not stop the capture of Morris Island, but they could – and did – exact a toll for that prize.  Such was a “by the book” utilization of a field fortification… even if the fort itself were to fall.

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 16-7.)

Fortification Friday: The symbiotic relationship between attack and defense

As we leaf through Mahan’s Treatise on Field Fortifications, the lesson plan offered, after defining components of the profile and trace, an important perspective for planning and evaluating fortifications – a connection between the attack and defense of the works:

The attack and defense of intrenchements bear a necessary relation to each other; and it is upon a knowledge of the course pursued by the assailant, that the principles regulating the defense should be founded.

Grant me some license for an analogy here and call this a symbiotic relationship.  Symbiotic, that is, borrowed from biology and defining two organisms of different species that exhibit a long-term, close interaction.  In the case of military affairs, the only reason a place would be defended with earthworks is because the defender feels an attacker might wish to gain possession.  Furthermore, the defender would build the works specifically to counter (if not deter) the most likely form of attack.  Likewise, the attacker would plan the assault based on knowledge of the layout of the defenses.  In short, one plan will exist only because of the other, counter, plan.  Otherwise, there’s simply no reason for the planned action – be that placing a defensive work or organizing an assault of the position.

Mahan elaborated further, providing the students a generalized example of what an attack looked like:

 An attack is, generally, opened by a fire of the enemy’s artillery, whose objective is to silence the fire of the intenchments, and to drive the assailed from the parapet; when this object is attained, a storming party, which usually consists of a detachment of engineer troops, a column of attack, and a reserve, is sent forward, under the fire of the artillery, to the assault. The detachment of engineer troops proceeds the column of attack, and removes all obstacles that obstruct its passage into the ditch. The line of march is directed upon a salient, through a sector without fire, and on the prolongation of the capital, as this line is least exposed to the fire of the works.

Depicting that approach on Mahan’s figure:

Attack1

See how this approach was designed to take advantage of an inherent flaw of the works? Mahan continued with more exploitation of the fortification flaws:

When the ditch is gained, the party makes its way to a re-entering angle, where, sheltered from the fire of the flanks, the work is entered by the column of attack, either by making a breach in the parapet, or else by means of ladders.  The reserve supports the column of attack in case of need; and if it is driven from the works, covers its retreat.

Again, as that would look on Mahan’s figure:

Attack2

This approach allowed the attacker to pick apart the defense by working under the parapet within the ditch inside the dead space, avoiding the angles of defense.

So how does that look from the defenders side?

The manner of making the defense is with artillery, musketry, the bayonet, and sorties.  The enemy is attained at a distance by the fire of the artillery and musketry, whose effect will chiefly depend upon the length of time that he is kept exposed to it by the ditch, and the obstacles in front of it. The bayonet is resorted to, as soon as the enemy shows himself on the berm; and sorties are made, either when any irresolution or confusion is seen in the enemy’s ranks, or at the moment he is repulsed from the parapet.

Note that Mahan didn’t emphasize here the nature of the parapets, faces, and flanks in order to build the perfect defensive line.  That technical perspective he saved for a more detailed explanation.  Instead he focused on what the defender could do with their weapons.  Implied in the notion of the sortie is that the defender retained high motivation to conduct such a counter-attack.  And with that, Mahan is admitting that flaws would be present in any defense.  To mitigate those flaws, where existing, the defender applied cold steel, hot lead, flesh, and bone.

But you see here how the nature of attack and defense fit against each other.  The attack had to be planned with a mind to exploit the flaws of the defense.  The defense had to be planned to minimize those flaws.  After establishing that symbiotic relationship, Mahan proceeded to lay out nine principles of the defense – some technical, others tactical, and yet others addressing the “spirit” of the defenders.  We’ll take a look at those next week.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 5-6.)

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.)