Fortification Friday: Mahan’s American Blockhouse

One of the themes I’ve worked in this series of posts is a comparison between pre-war and post-war manuals. In small ways, the comparison points to the influence of wartime experience in the practice of military science… at least in the American army.  Another such example is the use of blockhouses.  Pre-war, Mahan discussed blockhouses in relation to field fortifications as a safety redoubt or keep.  That was not, of course, to say such was the exclusive use of blockhouses.  But Mahan simply offered less thought about the use of stand-alone blockhouses. Rather the emphasis was upon structures which would be used in conjunction with standard, conventional field fortifications to meet the traditional needs of an army on campaign. Again, this is not to say Mahan didn’t agree with stand alone blockhouses, but rather that in instructions to his students he put all the emphasis on blockhouses used as keeps within field fortifications.  So this is a “how they were taught” consideration instead of a “this is the only way they would use it” declaration. Keep that fine point in mind.

On the other hand, the American military experience, drawn out even more so by the Civil War, included the need for fortifications guarding rear areas.  In particular, protection of railroad lines, bridges, and other such infrastructure was rather important. And, as we know from ample examples from official records, photographs, and other sources, the blockhouse became the preferred fortification for that need.  And Mahan identified that in his post-war writings, declaring an American-ism in such employment:

American Block-House. In the more recent block-houses erected in our service for the protection of bridges, railroad stations, etc, the sides and roof … are constructed with a double thickness of logs eighteen inches in diameter, hewn to a face of eight inches where they are in contact.  The inner logs are placed upright, the outward horizontal. A space is left in the outward casing sufficient for the fire from the loop-holes made through the inner. The horizontal logs above the loop-holes are held up by short uprights, mortised into them and into those just below. The ceiling is covered with earth, as shown in the section, three feet thick at the ridge and sloping towards the eaves to about six or nine inches, where it is confined by a pole plate. The earth is protected from the weather by a board roofing.  Tin or sheet iron ventilators are made through the roofing and ceiling, and a brick flue to receive the pipe of the stove used in cold weather.

The loop-holes are nearly of the same form and dimensions of those already given.

Mahan gave us two sets of illustrations supporting this passage.  The first demonstrated the blockhouse particulars mentioned above:

MahanFig48_49_50

We see a double layer of timbers – the inner most placed vertical and the outer laid horizontally. In both cases, the logs are flattened on the sides in contact to close up any gaps.

Also notice the provisions for loopholes.  The dimensions remained the same, generally. But instead of simply carving out loopholes, Mahan suggested a more elaborate arrangement.  I’d describe this as a set of small columns between a gap in the outer, horizontal timbers.  Traditional loophole cuts are made in the vertical timbers of the interior. So there is still the double row of protection and the minimum opening required for the musket barrel.

I say a picture is worth a thousand words:

02147r

This is a blockhouse protecting the railroad between Nashville and Chattanooga in Tennessee.

Continuing, Mahan offered more elaborate plans for these blockhouses:

Some of these structures are built in the form of a cross, consisting of a square central chamber, twenty-four feet on a side, and of four wings of the same form and dimensions when the block-house is for cannon. An embrasure is pierced in each of the three sides of each wing to serve a single gun.  The cheeks of the embrasures are faced with logs, and the mouth is secured by a musket-proof shutter with a loop-hole in it.  The embrasures are below the level of the loop-hole, allowing these to be used whenever necessary.

Though not exclusively for the employment of artillery, the implication is that guns required more space within the fort and thus more elaborate arrangements were needed.  The addition of shutters is noteworthy and speaks to the need for crew protection in the era of muzzle-loading artillery.

But where you have artillery, you must also have a magazine:

Arrangements for magazines and store-rooms are made under the floor of the block-house in the most secure parts.

Ah… OK… answers that question.

And what about the entrance:

The entrance to the block-house may be either through a postern, the bottom of which is on the level of that of the ditch, a ramp leading from this level outwards, a door properly secured, and steps, forming the inner communications; or it may be arranged as shown in Fig. 51, 52, with a plank thrown across the ditch on the same level as the natural ground, the entrance to the door being masked by a double stoccade, leaving the same passage-way as that of the doorway.  Loop-holes in the door and sides of the building sweep this passage.

And here’s the referenced figures:

MahanFig51_52

Also note in this set of figures, and that above, the berm built up against the blockhouse.  As discussed earlier, this improved the defensive quality of the work, particularly against artillery.  Furthermore it prevented the enemy from hiding under the loopholes.  We don’t see that in the wartime photo above.  I’m of a mind we are seeing a blockhouse in the final stages of construction, rather than some flaw in the engineer’s design.  Other wartime photos show earth banked up against the blockhouse:

02151r

This is another Tennessee blockhouse.  Wonderful details to consider – the loopholes and the entrance stand out nicely.  And consider the figure offered by Mahan (post-war) to illustrate the two story blockhouse:

MahanPage65Fig1_2

Can you find a better match? That’s great stuff!

Another illustration Mahan provided in the post-war manual detailed arrangements for the loopholes.  So while he “said” such was “already given,” teaching after the war he saw need to elaborate.  We’ll look at that next.

(Citation from Mahan, An Elementary Course of Military Engineering: Part 1: Field Fortifications, Military Mining, and Siege Operations, New York: John Wiley & Son, 1870, page 62-3.)

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Fortification Friday: Loopholes and Vents for the blockhouse

Last week, we discussed the layout and arrangements for the blockhouse, when used as part of a keep in the interior of a fortification.  An important requirement, if the keep was to function as intended, was the ability of the defenders to fire out of the blockhouse.  Just as with building the banquette, embrasures, and other arrangements on the parapet, such arrangements within the blockhouse necessitated attention to details. And those details come in the form of loopholes and vents, as Mahan would write:

The loop-holes are three feet apart; their interior dimensions are twelve inches in height; and eight inches in width for sides twelve inches thick; and twelve inches square for sides two feet thick. The width on the exterior, for the same thicknesses, will be two-and-a-half and four inches.  The height of the loop-hole on the exterior will depend on the points being defended; it should admit of the musket being fired under an elevation and depression. The height of the loop-hole above the exterior ground is six feet.

The visual you should have in mind is that of an aperture which is small on the exterior but larger for the interior.  This would allow the defender to train the musket across a wide arc, as well as providing for elevation and declination.  I don’t like mixing field fortifications with permanent fortifications, but in this case the application is along the same lines.  So consider the loop-holes here at Fort Pulaski, to the right of an embrasure:

Ft Pulaski 3 Aug 11 1346

In this case, there was need for the muskets to cover two zones.  So we see two loopholes incorporated as a pair.  Note the placement of stone slab above and below to strengthen the loophole structurally. Something not needed within the wooden blockhouse – simple cut outs within the timbers usually sufficed.

But the major difference between the blockhouse and brick fortification’s loopholes is the height.  Mahan specified only twelve inches for the blockhouse in a field fortification.  Those at Fort Pulaski are two feet or so.

Another aspect to keep in mind is the depth of the wall.  As the wall became thicker, the loophole’s lateral dimensions, particularly interior, increased.  Geometry at play here, as the musket would need more clearance on the interior as depth increased.

Mahan did not directly discuss interior arrangements for the artillery’s embrasures.  Partly, I think, as such an allocation would have pulled valuable cannons off the fort’s primary defensive line to that of the secondary or even tertiary defenses. But, we can deduce such arrangements would match those described for embrasures through the parapet.  In short, a larger loophole… which is what we see to the left of the photo above.

All this is good thinking.  But we also have to keep in mind the by-product of firing any weapon.  In order to push the projectile out of the barrel, firing of the powder creates gasses. That foul air is not an issue out in the open or on the parapet.  But in the enclosed space of the blockhouse, there is need to expel the gasses:

Vents for the escape of the smoke are made over each loop-hole, between the cap-sill and the top pieces.

Moving to another location in Fort Pulaski, we see a vent above one of the other embrasures:

Ft Pulaski 3 Aug 11 1388

See the weathering on the paint?

Mahan does not provide much information on constructing vents for the blockhouse. These could be vents between the ceiling and wall. Or vents incorporated in the wall itself.  To maintain integrity of the structure, in terms of defense, those vents were best created using an interior angle.  That would allow gasses to vent.  But water… or things the enemy might want to push inside… would be restricted.

From there, Mahan gave brief descriptions of the camp bed (which we noted served as the banquette inside the blockhouse), racks, and other storage arrangements.  But with that he left the interior arrangements.  Instead he turned to an external details.  We’ll look at those next week.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 64.)

Fortification Friday: “a redan or a lunette, is thrown up on the exterior to cover the outlet”

Before we close the discussion of openings for forts (see what I did there?), let me circle back to compare Mahan and Wheeler in regard to one of the fine points considered.  That being the use of a detached redan or lunette in advance of the outlet.  Recall that in pre-war writing, Mahan suggested:

In very frequented passages, a redan or a lunette, is thrown up on the exterior to cover the outlet, and thus ensure its safety in case of surprise.

And Wheeler, in the post-war, mentioned a similar arrangement, but perhaps narrowed the application to those larger outlets, for sorties, where simple interior traverses would not be practical.

Mahan offered two figures that illustrated the redan to the front of an outlet:

PlateVIIFig48_49

Figure 48 offers a wide redan in front of an outlet, which is further covered and flanked by by the “horns” of the larger work.  A very well protected outlet, we might say.  Mahan considered this a Redan Line.

On Figure 49, we see much more complexity.  Particularly with the defensive lines of fire.  The outlet is nested within a redan of a larger line.  On both sides are faces within redans of differing angles. This is considered a Tenaille Line – a proper definition we will discuss later.  But the point being the covering redan, to the front of the outlet, was absolutely necessary here in order to protect that weak spot.  The covering redan is somewhat off center of the outlet, perhaps to limit exposure at the expense of accessibility.

Wheeler, as you may recall, gave us only a simple rendition of the covering redan:

WheelerFig51

The question I have in regard to these advanced, detached “parts” covering openings is… just how often were these employed during the Civil War?

When examining surviving earthworks, we often find the area around the outlets obliterated.  Sometimes, due to necessity, that is done to facilitate visitor access.  But more often, just a case where the structures around the outlets were the most susceptible to erosion.

And when examining wartime plans, we see some use of these redans… but more often not.  Consider Fortress Rosecrans outside Murfreesboro:

FortressRosecrans

This was, some have said, the largest fort built during the war.  And in this plan we see examples of many features suggested by Mahan.  Specific to the outlets, we see up near the top that Battery Cruft was a detached lunette (maybe a “half lunette”) covering an outlet.  Elsewhere, such as next to Lunette McCook at the bottom right, we see an outlet (an existing road) without a covering redan or traverse.  Though we do see obstacles erected to the right of Lunette McCook.  And certainly that named work was positioned to dominate the approaches to the outlet.  Furthermore, what you don’t see in my “snip” are works in advance of the fortress that covered the railroad and road.  Though those were oriented south and not regarded as covering the outlet in question.

Another plan to consider is from Virginia, at Deep Bottom:

DeepBottomSnip

Here we see five road crossings at the main line of the works.  One of those is blocked entirely by a redan.  The other four (including one that appears to be a path cut just to clear a redan) have no traverses or covering works.  Just obstacles placed in front.

If we are assessing the protection of outlets, with Mahan’s suggestions in mind, we find a mixed application of those covering redans.  Seems to me the use of that sort of feature was based on the engineering assessment of need.

Now considering such use under Wheeler’s suggested implementation, let’s look to the location of a few large scale sorties.  First, how about the works were the Crater assault was mounted:

CraterSector

And further around the lines, and further forward in the historical timeline, to the sector around Fort Mahone:

FortMahoneSector

And to the left of that sector near where the Federal Sixth Corps mounted their sortie:

FortWelchSector

Now the scale of these maps mean these are not so much “plans” as operational maps.  So we know there are structures that escaped the pen here.  But what stands out, with double underlines, is the use of something far more elaborate than Mahan and Wheeler discussed.  We see entire sections of works advanced in a manner to provide staging grounds for those formations preparing for the assaults. Major assaults, mind you, involving whole divisions.  These were, you see, works built for the offensive.  Grand offensives!  In that light, might we say the entire Federal line was one large “covering work” in front of an array of staging areas and supply depots?

Fortification Friday: Outlets evolved in the post-war instructions

Earlier we considered the pre-war instructions offered by Mahan in regard to outlets, particularly how those would be constructed.  Training cadets before the war, Mahan covered the topic with a few paragraphs.  Later, after the war, he added a section about bridges associated with the outlets.  However, by the 1880s, Junius Wheeler’s instructions to cadets would sprawl over the better part of seven pages.  Maybe this is simply a case where pre-war there was a premium on copy space for printed manuals.  On the other hand, maybe Wheeler felt “his” cadets needed “special help” getting this particular aspect of fort-building in order.  Or perhaps, which I tend to think, wartime experiences prompted the extended emphasis.

The first thing we see with Wheeler’s treatment of the subject is a focus shift.  Before discussing the construction of these outlets, he steps back to discuss what these are used for.  Notably, the section is titled “Communications, barriers, etc.”:

Communications. – The defenders of a closed work must have arrangements made by means of which they can enter or go out of the work when necessary. In the case of continued lines, arrangements should be provided by means of which the defenders can make sorties.

Word choice here.  Mahan’s writing pressed that outlets were important for servicing the works.  Left unstated, implied at best, were other uses for the outlets – communications, logistics, reconnaissance, or simply just getting out to stretch the garrison’s legs. On the other hand, by selecting this sub-heading, Wheeler set different functional requirements… although, much vaguer than Mahan’s.

Wheeler’s second paragraph discussed the basics.  The outlets had to be made through the parapet, and that created a weak point in the work.  To minimize the risk, he insisted the number of outlets be kept to a minimum and built where least exposed.  Specifically, he gave these suggested placements:

In redoubts, the outlets are on the sides least exposed to attack; in half-enclosed works, they are placed near the middle of the gorge; in forts, they are usually placed near the re-entrants.

And that matches well to Mahan’s suggestions from decades before.  But where we see variations is with the size of these outlets, which Wheeler relates based on functional requirements:

A passage for the use of infantry only should not, as a general thing, be less than six feet wide; for artillery, not less than ten feet wide; for sorties, the outlets in continued lines should be at least fifty yards wide.

See what happened here?  Wheeler took Mahan’s two size categories and then added a third, based on an additional use-case.  I think at this passage Wheeler is thinking back to works at Vicksburg or Petersburg or around Atlanta.  And in those campaigns, field works had become part of an offensive operation – be that a very deliberate siege at some levels, but more dynamic than an “According to Vauban” siege.  Again, not to say wide passages for sorties did not exist prior to 1861, but rather to say those took on different emphasis around about 1863-4… at least in the American context.

Demonstrating that the more things change, the more they tend to stay the same, Wheeler offered this diagram for an idea outlet through the parapet with masking traverse:

WheelerFig50

Yes, he ripped off Mahan.  But give him a little credit for adding more notations and giving us that nice profile on the right.  And he offered the lines (c and g, the “crossing” arrows through the middle) which were the enemy’s “extreme lines of fire”.  Those lines, Wheeler instructed, governed the length of the traverse (T).  Note how those lines are drawn off the corner of the superior slope of the parapet.

Toward that, Wheeler did add a lot more to the notion of masking the outlet:

The length of the traverse may be shortened by turning back the interior crest at right angles to its general direction, and extending it as far as the crest of the Banquette.

This “turn back” is indicated by “B” on the figure.  One on each side of the outlet.  Notice how that would send the lines of fire (again, lines c and g) into a tighter intersection, further back in the outlet, and thus reducing the area the enemy might fire upon. Such angles reduced, the traverse need not be so long.  All in all a nice functional flourish to the outlet design.

Beyond that, Wheeler offered other options that would reduce the vulnerability:

Instead of having a road along its entire front, the traverse is sometimes joined to the parapet on one side of the opening, as shown by the dotted lines b d and e f, in Fig. 50.

And such would mean only one “extreme line of fire” need be considered in regard to traverse length.  But that did mean traffic had a nasty chicane to deal with.

This is all good, but how about that fifty-yard wide outlet for sorties?

The method adopted to mask the interior of the work in this latter case, is to place the traverse opposite the outlet on the outside, and beyond the ditch….

The traverse in this case is usually broken, generally a redan in trace, with the profile of a parapet, but commanded by the parapet in rear.

Figure 51 illustrated this manner of masking:

WheelerFig51

Basically, if you have to provide a wide gap in the works, the only viable solution is to build a miniature work in front to cover it.  Keep in mind, such sortie outlets would be placed on less exposed sections of the line.  And of course, these would be covered by strong faces on either side.  Such would reduce the chance the enemy might target the outlet for their own “sortie”.  And the redan covering the front would further reduce the danger of direct artillery fire.  Note, however, you don’t see a traverse in the interior of the works to cover the sortie outlet.  The purpose of this outlet is to allow infantry in wide columns to emerge in quick order. Bad enough, though necessary, to have that redan in the way.  Putting a traverse on the inside would impose yet another choke point for maneuver… and yet another delay in an operation where time was critical.

Before leaving Wheeler’s discussion of outlets, we’ll also examine the evolution of barriers and bridges.  Furthermore, Wheeler introduced the notion of ramps within these outlets.  All interesting facets to consider with respect to the art of military fortifications.

(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 148-51.)

Fortification Friday: Gabionades? Let’s get between the guns!

We’ve spent several installments discussing the evolution of shelters through, and after, the Civil War.  Before the war, Mahan gave the subject just under two pages in the manual.  By the 1880s, Wheeler would allocate a dozen pages to the topic.  And this did not reflect the introduction of any great technical advance in the art of fort-building.  Rather it reflected, in my opinion, specifically changes in the manner artillery was employed… but generally towards changes at the operational level.  But keep in mind, this “change” was more so a heavier allocation of ink in the manuals, which translated to different practices being taught to cadets… in turn translating, when that cadet pinned on lieutenant or captain bars, to the soldiers’ work priorities.

A similar evolution, reflected in the ink of the manuals, occurred with respect to the protection used within the batteries, between the guns. These were technically traverses. The intent was to provide safety to the gunners from enfilading fires. Before the war, Mahan offered:

Traverses. Those which are constructed to cover the flanks of the guns from an enfilade fire, are usually what are termed gabionades. To form a gabionade, gabions are placed in a row, side by side, enclosing a rectangular space of about twelve feet in width from out to out, and about twenty-four feet in length, perpendicularly to the epaulment.  A second row is placed within this and touching it.  The area thus enclosed is filled in with earth, to a level with the top of the gabions.  Four rows of large fascines are next laid on the gabions, to support a second tier consisting of one row. The second tier is filled in like the first, and the earth is heaped on top, making the gabionade nearly eight feet high. The work will be expedited by throwing up the greater part of the earth before placing the second tier.  Splinter proof traverses may be made by placing three thicknesses of gabions side by side filled with earth, with a second tier of two thicknesses on top.

Note here that Mahan described two classes of gabionades in this paragraph.  One was a shot proof and the other splinter proof.  The latter using about a third of the materials of the former.

Mahan offered this figure to illustrate a shot proof gabionade, as a type of traverse, in profile:

PlateVIFig38

As described, we see two sets of gabions on each side (four rows total on each side) on the lower tier.  Atop that fascines provide a platform for the second tier, which was two more sets of gabions (two rows on each side).  Those walls defined, the gabionade contained earth providing the mass protecting the gunners.  The result was a twelve foot wide structure (which would run twenty-four feet from the epaulment (or parapet, if you prefer) across the gun platform, which was atop the tread of the banquette. The traverse stood eight feet high, perhaps a little more.  These dimensions were governed by the height of the gun and required dimensions of the platform.  Recall platforms were supposed to run between fifteen and seventeen feet back of the parapet.

However, at the time Mahan was considering pre-war field and siege carriages.  During the war, large Parrotts and Columbiads required adjustments to the formula.  And we see that in the photos taken on Morris Island during the war:

03109a_Alt

No doubt here, this is Fort Putnam, built atop what was Battery Gregg on Cummings Point. We see a 10-inch Columbiad on the left and a Parrott (8-inch or 6.4-inch) on the right.  Based on the height of the ammunition crates and grape shot, this traverse appears to be twelve to fifteen feet tall.  The traverse is also much longer than specified by Mahan.  However, other photos in the same area demonstrate these traverses were also being used as magazines and shelters.  Thus the larger footprint was partly due to that functional arrangement.  We also see the surface is sod.  Based on engineers’ reports, these were built with gabions but surfaced with earth and sod to prevent the beach sand from blowing away.

An interior view of the works on Morris Island better illustrates the gabionades, or traverses, where not used in conjunction with shelters:

03113a

Here we see the breech end of a Parrott (looks to be a 6.4-inch) and the transom of its carriage standing out from behind the traverses.  Note the wood beams sticking out from the traverse on our left.  Such implies a stacking of tiers within, hidden behind that sodded surface.  A presumption here, but I’m pretty sure there were tiers of gabions within.  The Ordnance Manual gives the height of the trunnions on a 10-inch Columbiad wrought iron barbette carriage as 79 inches, or roughly 6.5 feet.  Add to that the height of the gun’s breech over the bore’s center line, and we’d have about 7.5 to 8 feet.  The top of the traverse is just above that breech band, so let’s call it eight to ten feet?  In the background a fellow is posing nicely on the side of anther traverse.  Looks to me he’s about four feet above the platform.  Add his height, and we have a second data point to consider. No rush, just go out and find out who the soldier is, consult his service records to obtain the height, and get back with me…. or let’s just call it as six feet more or less.  So a ten foot tall traverse?

I should also mention here the tactical setting for these traverses.  Readers know well those batteries were subject to counter-battery fire from Confederate guns on Sullivan’s Island and James Island.  And that fire was not some paltry 12-pdr or 3-inch projectiles, rather the largest and heaviest stuff available at the time.  We are talking about 7-inch Brookes, 10-inch Columbiads, and 10-inch mortars.  So stout traverses were certainly needed.

These photos provide a nice redirect to Wheeler’s description of such traverses in his 1882 instruction.  Expanding Mahan’s one paragraph, Wheeler offered over four and a half pages!  So we’ll look at that next week.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 59-60.)

Fortification Friday: Bomb Proof shelters, as in “force protection”

Last week I pointed to a change… if we may, an evolution… in military practice based on experiences of the Civil War.  In that case, we saw a new class of “semi-permanent” fortifications introduced.  And beyond that, we saw several improvements and refinements of the facilities constructed for fortifications, be those temporary or semi-permanent.

Now let us look at another evolution, just as subtle.  But one I would submit was a major change in the function of fortifications.  In his 1870 instructions, Mahan moved immediately from discussion of magazines into a similar structure with different function:

Bomb Proof Shelters.  Bomb and splinter proof shelters of wood have also been carefully built in the interior of some of these enclosed works, and in the gorges of those open to the rear; both may be arranged with loop-holes for defense. These are mostly constructed in the manner shown in Fig. 39, bis.

This brings us back to reference the shelter built on Morris Island the shelter built on Morris Island in the summer of 1863:

MahanFig39bis

As related before, this was not necessarily a “new” structure.  Rather this was the use of a type of structure in a different manner, addressing an evolving requirement on the battlefield.  In this case, on Morris Island, the requirement was protection from shells and shrapnel delivered from high-angle or, as Mahan called it, curved, fires (or as I sometimes relate – fires acting on the vertical plane).  Overhead cover, we’d call it today.

Mahan gave a general description as to the construction of these shelters:

The exterior side is of heavy logs placed in juxtaposition, resting on a ground-sill and capped at top.  Parallel to these is another row, which may also be in juxtaposition or at short intervals apart and capped like in the outside row.  The roofing, consisting of heavy logs laid in juxtaposition and covered by thick jointed boards, rests on the capping.  The back face may be sealed on the inside to obviate the dampness from the earth resting against the back; and some simple method of drainage, by fascines or tiles, is arranged to carry off the water from the earthen covering.  This last should be the same as for the powder magazines.

So, these shelters were “like” magazines, but were not magazines.  The purpose of these shelters was to protect the troops, supplies, and equipment.  We’d call this force protection today.

But there is scant mention of this in the pre-war courses.  Here’s what I think (as in my thoughts… make sure you don’t get the idea I’m putting words in Mahan’s mouth…) – prior to the war, doctrine did not envision a situation where a field army would remain in postion subject to prolonged attriting fires.  Prolonged, as in more than a few hours.  Yes, things like mortars and howitzers existed prior to 1861.  And yes, during battle armies would be subject to fires from those type of weapons.  But, prior to 1861, the general perception was that sort of exposure would be limited, depending on the situation.  Perhaps only in a situation where an army was preparing a large scale assault… or preparing to receive a large scale assault.  So such exposure would be limited to a phase in a particular battle.

However, wartime experience brought with it a new perspective.  Entire campaigns would be fought with soldiers almost constantly in contact and exposed to attriting fires.  For emphasis, let’s just say in 1861, William T. Sherman led his command toward Manassas with the notion his troops would only be under fire (save some minor delaying actions) for a limited period of time on the battlefield.  But in 1864, he led a much larger force through Northern Georgia fully expecting the troops to be under fire every single waking hour.  An army could no longer afford to simply “lay in the open” waiting for the signal to attack. That wait might be for days, if not weeks.  And the artillery, of both sides, would not be idle.  We can offer many reasons this change occurred.  Regardless of cause, the effect was a need for better protection of the force.

Applied to practice of field fortifications, this brought a need for structures that would afford relief to the infantry.  Again, this is not to say such shelters did not exist prior to 1861.  Rather, such shelters were of little concern, prior to the war, for those constructing temporary, field fortifications.  However, by the summer of 1863, witnessed on Morris Island, these structures became necessary.  So much that a significant amount of manpower was diverted from other tasks in order to ensure “force protection” was given as the siege lines advance.  Oh… and so much that the defenders in Battery Wagner spent more time on shelters than on erecting more batteries.

And speaking of the defenders, Mahan added, with respect to the shelters:

It is highly important that these works should be so organized as to afford a retreat for the garrison should the main work be carried.  This might be done in some cases, by masking, with earth, only the lower portion of the side looking on the interior of the work, and covering the exposed timber with iron plating with loop-holes, to sweep all the interior space.

This sounds very much like a safety redoubt or keep…. but “sounds like” is not “the same as” in this case.  In his pre-war writing, Mahan proposed safety redoubts and keeps for field fortifications.  And he retained, those designations, even going further to elaborate on the construction of blockhouses as a form, even when proposing these shelters with arrangements for retreat.  The difference between here is the employment.  The keep was intended for a fortification with large interior capacity and for use as a last line of defense.  The shelters described, in the paragraph above, were employed even in smaller fortifications and were not necessarily the “last line” to defend.  The point being – shelters would protect the soldiers AND offer them a position from which to fight in conjunction with the localized defense within the fortification.  A keep was a shelter from which to rally and reform for the final defense of the entire fortification.

Not quite the trenches of World War I.  But the concept was in the air.

(Citations from Mahan, An Elementary Course of Military Engineering: Part 1: Field Fortifications, Military Mining, and Siege Operations, New York: John Wiley & Son, 1870, page 54-5.)

Fortification Friday: Splinter proof shelter, from the wartime experience

Last week, we split all manner of hairs regarding shelters within fortifications. Some of this hair-splitting had to do with nomenclature – shot proof, shell proof, and splinter proof.  And we saw that post-war writings introduced differences between facilities designated magazines and those designated shelters.  We can read into this a shift in doctrine.  Not only fortification doctrine, but also that of the practice of artillery.  After all, there existed (and still exists) a direct relationship between fortifications and artillery.

Let us focus on the splinter proof shelter for the moment.  Prior to the war, Mahan mentioned splinter proofing as a means to protect the magazine entrance.  But after the war, he introduced a structure called splinter proof shelter:

Splinter proofs for trenches and enclosed works faced with timber from eight to twelve inches in diameter, and covered with a sheeting of thick boards, and from four to six feet of earth, which are supported by uprights at the back; having a board flooring as shown in the figure, have been recently used in our field works and trenches with great benefit in the saving of life.

And the illustration provided demonstrates such as structure:

MahanFig39bis

Note the dimensions of the interior of this splinter proof.  Eight feet tall at the entrance, slanting to six feet.  Shown as 3 ½ feet wide, with a plank floor.  The structure is open to the left, which would be the interior, or rear, of the line of works.  And it is partially sunk into the ground, roughly three feet deep.  The arrangement would protect the occupants from direct fire (from the right of view) and high angle fire (dropping on top).  Being partially sunk down, some protection was afforded against shells bursting behind (to the left) of the structure.  But clearly the solution balance ease of access against protection.

And notice the caption, “Shows a section of Splinter Proof used in the trenches at the Siege of Fort Wagner.”  Yes, we’ve seen this sort of structure before… many times:

MorrisIslandBatteries2

Looking to a handy example, right at the top is the a-a’ profile line, working from one of the splinter proofs forward through Battery Brown to the Howitzer Battery in the Second Parallel. For cross reference, this line runs through the red oval highlighted here:

BatteriesRight2ndParallel_A

A clean look at the profile:

BatteryBrownProfile

Looking to the left, we see a slightly more elaborate splinter proof shelter, with two supporting uprights.  But notice the Battery Brown splinter proof is at surface level, not sunk in.

Something closer to what Mahan illustrated stood just a few yards behind Battery Brown, indicated by profile d-d’:

InfantryTrenches2ndParallelPlan_dd

In profile:

InfantryTrenches2ndParallelProfile_dd

The walk-space is wider than on Mahan’s diagram. But the structure generally matches. We know from reading accounts from the campaign, the intent was to provide shelter for troops staged for work on the parallels.  The orientation of the trench provided protection from Confederate batteries further up on Morris Island, as well as those on James Island. The Confederate fires reaching this point of the Federal lines were typically large caliber weapons fired at higher elevations.  Though not high-angle as used with mortars, which were out of range to hit these Federal trenches, the columbiad shells arrived at an angle which would normally defeat standard parapets.  So a splinter proof provided some overhead protection.

So we see, documented with the maps, diagrams, and accounts from Morris Island, a shift in emphasis for field fortifications.  This is not to say overhead cover was not used prior to the Civil War. Nor is it to say splinter proof shelters did not appear on earlier battlefields.  What it does say is that field experience in the Civil War caused engineers to focus more attention on overhead cover, to the extent that more elaborate shelters were built.  A shift in doctrine, you see.

Keep in mind, these examples come from a field army engaged in a siege.  So field fortifications directed for offensive purposes, as opposed to defensive arrangements.  Certainly these sort of works continued to appear on Morris Island after the fall of Battery Wagner, as the Federal presence shifted more to garrison of the hard-gained foothold in front of Charleston.  But more to the point – field fortifications are “tools” that can be used for either defense or offense as the tactical situation demands.  (And thus we’ll see later “lessons” from Mahan on how to build fortifications in support of siege operations.)

Writing even later, Junius Wheeler would further refine wartime experience to suggest even more elaborate shelters, in particular using wartime experience building the defenses of Washington.  We’ll consult Wheeler’s lessons in turn… before then, we should consider another of those split hairs – shelters vs. magazines.

(Citation from Mahan, An Elementary Course of Military Engineering: Part 1: Field Fortifications, Military Mining, and Siege Operations, New York: John Wiley & Son, 1870, page 52.)