Fortification Friday: Mahan’s suggestions to improve blockhouses

When it came to fortifications, Professor Mahan held to the “you can never do enough” principle. As we’ve discussed in preceding posts, in a field fortification the blockhouse was the keep, or last point of defense for the garrison.  It was to be outfitted in such a way that the garrison could cover the parapet with fires.  And interior arrangements would grant ample space for handling weapons.  But more could be done to improve the blockhouse.  First and foremost, one should surround the structure with a ditch:

The block-house is surrounded by a ditch, similar to one used for a defensive stoccade. A strong door is made in one of the re-entering angles, and a slight bridge leads from it across the ditch.

We saw this arrangement along a profile line in the illustrations:


Somewhat analogous to the ditch-parapet in profile, with the blockhouse structure itself being the parapet.  Notice the glacis in front of the ditch.  And also the palisade in the ditch to the left. And we have those heavy doorways, conforming to the structure prescribed for outlets.  It’s all coming together here for the keep.

But we were still not finished making the blockhouse unassailable. There was more that could be done.  Perhaps a structure on top from which the garrison could fire down onto the attackers?

It has been proposed to place a slight parapet of earth on top of the block-house.  It is thought that this accumulation of earth would be too heavy for the timbers, independently of leaving but little space for the defense.  Perhaps a better arrangement might be made on top, similar to a defensive stoccade, the uprights being secured at bottom, between two pieces resting on the top pieces, and held firmly by an arrangement of riband pieces and braces.

Mahan’s method would provide a lightweight structure, sufficient to stop musketry.  Artillery, though, might turn that blockhouse parapet to splinters. So, any suggestions to counter that?

It has also been proposed to place the the interior and exterior rows of uprights three feet apart, and fill in between them with closely packed earth, for a defense against artillery.  This method has been tried, and was found to be less solid than the one here laid down, independently of being more difficult to construct.

Recall, the guidance stated earlier was to use two thicknesses of twelve inch timbers for defense against field artillery. This was seen as more sustainable, with less physical footprint, as what would amount to packed earth at steeper than a natural slope.

Other techniques to improve the blockhouse included methods to make the enemy’s closure even more difficult.  “The top pieces should in no case project more than twelve inches beyond the sides, to admit of logs, &c., being rolled over on the enemy.”

And if that was not enough, one could stack up a second floor (somewhat as we saw from the colonial era Fort King George):

The block-house is sometimes arranged with two stories, the corners or the sides of the upper story projecting over the sides of the lower. Either of these methods is sufficient for the defense of the lower story; but the first is the best to procure a fire in the direction of the angles.  It can only be used, however, as a defense against infantry.

When artillery cannot be brought to bear against the top of the block-house, it may be constructed like an ordinary floor, and be covered with nine or twelve inches of earth to guard against fire.

Of course, where artillery might be brought to bear against the blockhouse… well let us just say a lower profile was preferred.

But, Mahan was tapping on this point about “places where artillery will not be” for a purpose.  He was looking toward the “west” of that era:

The application of wood to the purposes of defense is one of paramount importance in our country.  A block-house, surrounded by a defensive stoccade, is impregnable to the attack of infantry if properly defended, and is therefore peculiarly suitable to either wooded or mountainous positions, where a train of artillery cannot be taken without great labor, owing to the impediments that may be thrown in its way, by rendering the roads impassible from obstructions easily obtained.

Of course, just couple of decades after Mahan’s writing armies backed with steam power (rail and river) were able to overcome many of the natural impediments of the Western Theater.  Still, the suggestion held some merit further to the west, where most potential adversary possessed only light artillery. Oh, and the mountains were much higher.

And Mahan also saw an application for the blockhouse in the east, where pre-war thinking was focused on attackers that arrived by ship:

In positions covered by extensive earthen works, such as those that would be required for the defence of the towns on our sea-board, and which would be occupied during a war, a defensive arrangement of the barracks for the troops, so that they might serve, in case of the main works being force, as rallying points, under cover of which the main body of troops may retreat with safety, is a subject that commends itself to serious attention of the engineer.  From the details already entered into, an efficient combination for this purpose will suggest itself to the reader, without entering farther into particulars.

Such arrangements might have been of use for Confederates defending the Carolinas, Georgia, Florida, and the Gulf Coast.  We might contend the inner defensive lines at Charleston served the purposes of a keep in this regard.  But here again technology had rendered the blockhouse less useful.  By 1863, Federals could bring to bear rifled artillery of the largest caliber wherever they might encounter a blockhouse.

Yet, while we can say rifled artillery and means to transport such weapons rendered the blockhouse less desirable, we still can point to widespread use of such structures throughout the Civil War.  In particular along lines of communication.  In those rear areas, raiders were not likely to bring more than a handful of artillery pieces.  And that experience lead to some shifts in the instruction about blockhouses as field fortifications… which we shall discuss next.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 64-6.)


Fortification Friday: Fraise and Stockade – variations on palisading

Palisades, as discussed in last week’s installment, were an effective obstacle to place in front of works. There’s something about a wall of pointed fence posts that naturally prompts the attacker to consider alternative routes.  But, being basically a wood fence, the palisade was vulnerable to artillery fire.  A barrage of solid shot and shell could dismantle a palisading from a safe distance. Thus, as Mahan suggested, the best place for a palisade was on the counterscarp, thus keeping the obstacle under shelter from direct fire.  Thus, in Mahan’s view, this obstacle was best employed in conjunction with the ditch.  There were two additional manners in which Mahan felt a palisading could enhance the ditch, which he identified with separate names – fraise and stockade.

About the fraise, Mahan wrote:

Fraise. This obstacle is formed of palisades, placed in juxtaposition, either horizontally, or slightly inclined.  The best position for a fraise is on the berm, or a little below it, so as to be covered by the counterscarp crest.  The part of the fraise under the parapet is termed the tail, and is about five feet long.  To make a fraise, a horizontal piece of four-inch scantling, termed a cushion, is first laid parallel to the berm; each palisade is nailed to this, and a thick riband is nailed on top of the fraise near the end.

Figure 24, provided by Mahan, illustrates a fraise placed on the berm of a fortification, slightly declined (which is the word I think Mahan intended to use) over the ditch:


As you can see, this makes a formidable obstacle for an adversary trying to claw out of the ditch.  But there are some particulars the engineer had to mind when placing a fraise, lest it become less formidable:

The point of the fraise should be at least seven feet above the bottom of the ditch, and should not project beyond the foot of the scarp, so as not to shelter the enemy from logs, stones, &c., rolled from the parapet into the ditch.

Such arrangements ensured the enemy would have to do “overhead” work if the fraise were dismantled by hand; prevent him from simply climbing over the fraise and walking up the parapet; and gaining some lodgement under the fraise.  We see the textbook example demonstrating all those requirements, particularly that a vertical line drawn from the foot of the scarp passes just beyond the point of the fraise.  The attacker would have to find a foothold on the slope of the scarp … which should also be revetted and otherwise made difficult for footholds.

The disadvantages of the fraise was, of course, the need to plan in advance of erection of the parapet and the labor required.

Another variation on this theme was to place posts on the floor of the ditch in a manner to deny any footfall.  In pre-war texts, Mahan called this a stockade:

Stockade. Trunks of small trees from nine to twelve inches in diameter, and twelve feet long, are selected to form a stockade. They are planted in juxtaposition, in a similar manner to a palisading, and are used for the same purposes.

A literal examination of this passage is the palisade used hewn posts while the stockade uses the whole, but smaller, tree.  Of note, in the post-war manuals Mahan used the archaic “stoccade.”  Though the significance of such is lost on me.  Later into the post-war period, Junius Wheeler offered this illustration for the fraise:


More elaborate than Mahan’s illustration, but matched to far less text.  Wheeler’s version has an inclined fraise on the crest of the counterscarp, protected by a glacis.  To clear that counterscrarp fraise, the attacker would stand elevated directly under the opposing flanks or faces… and thus be in a very “hot” zone.  There is one fault we would note from Wheeler’s diagram – the points of the fraise extend well beyond the foot of the scarp and counterscarp, as the case may be.  This is mitigated somewhat by the obstacles on the floor of the ditch.  Those are what Mahan called small pickets, and would have incorporated entanglements. That, however, is a subject we’ll discuss in detail later.  Just be mindful those pickets served to reduce the attacker’s mobility within the ditch, augmenting the other obstacles.

Closing the discussion of stockades, Mahan offered this passage:

The manner of arranging a stockade, which is sometimes termed a picket, as a primary defense, will be described in another chapter.

Readers should sense two irritants in that sentence.  So… a stockade is placed like a palisade but is a picket?  But, as mentioned in relation to Wheeler’s diagram, small pickets are described as something clearly not related, in arrangement, to a palisade. Imagine the student trying to fix in their mind what word should be used for precision.

Secondly, Mahan alludes to use of a stockade in a manner completely removed from the employment mentioned for palisades.  Certainly, the short description brings to mind the old frontier fortifications – stockade forts.  Wheeler offered this illustration for a stockade employed as a primary means of defense:


At once, this is a recognizable defensive feature… to anyone having watched a western movie or two.  And this is a form of defense which is best discussed in detail later. But clearly it is not a palisade.  Thus we have another term which we need to weigh out when using …  or more importantly when we see it used.

Circling back to the point – the point of the fraise that is – with this variation in employment of the palisade, we see, given ample time and resources, the defender could mitigate many of the flaws inherent within a profile.  In particular, the ditch could become a very difficult area to reach and move beyond.

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 46-7.)