Fortification Friday: A Ditch consists of a scarp and counterscarp

It’s Friday, so time for more fort talk.  Let us continue to look at Mahan’s textbook profile and discuss the component architecture:

Plate1Figure1

Last Friday we focused on the Parapet and it’s parts. Time for the Ditch and other structures in front of the fort.  The Ditch served two purposes, as defined by Mahan:

… from its position and proximity to the parapet, subserves the double purpose of increasing the obstacle which the enemy must surmount before reaching the assailed, and of furnishing the earth to form the parapet.

A good Ditch was convenient to the defender and inconvenient to the attacker. The profile of the Ditch introduces several new points and lines to consider:

Ditch

Points of reference within the Ditch include:

  • G – Crest of the Scarp.
  • H – Foot of the Scarp.
  • I – Foot of the Counterscarp.
  • K – Crest of the Counterscarp.

Working from the last point of the Parapet recall we had the Berm, which was between the Foot of the Exterior Slope (F) and the Crest of the Scarp (G).  The Berm served to shift the weight of the Parapet off the Ditch, but was a necessary weak point in the fortification.

The Ditch itself consisted of three lines. The first was the Scarp, defined as G-H on Mahan’s diagram.   The Scarp is simply “the slope of the ditch next to the parapet….”:

Ditch_Scarp

From there, line H-I defines the Bottom of the Ditch:

Ditch_Bottom

And lastly line I-K is the Counterscarp, which is simply the opposite side from the Scarp:

Ditch_CounterScarp

A lot of fancy words to describe a hole in the ground, but keep in mind this was the vocabulary used by engineers who were deriving the particulars for structures to achieve specific effects in the field.  To those “specifics,” Mahan wrote:

The ditch should be regulated to furnish the earth for the parapet.  To determine its dimensions, the following points require attention: its depth should not be less than six feet, and its width less than twenty feet, to present a respectable obstacle to the enemy.  It cannot, without convenience, be made deeper than twelve feet; and its greatest width is regulated by the inclination of the superior slope, which, produced, should not pass below the crest of the counterscarp.

So put yourself in the diagram (stick figure if you want).  The average man is under six feet tall.  So a ditch shallower than that depth might allow an attacker to reach over the ditch.  Double that depth is fine, but any deeper would turn a ditch into a mine, and thus require some support beyond what the simple earthwork might provide. Close attention to what is said about the width being restricted by the angle of the Superior Slope.  See the highlighted line:

Angle_Superior_Slope

For the Ditch to work as an obstacle, any defender reaching it should not be able to fire over the Parapet into the fortification.  The line in Mahan’s illustration points to another external feature that we will address shortly – the Glacis.  But where no Glacis existed, the line from the Superior Slope could not exceed the location of the Crest of the Counterscarp (K).

Wait… there’s more.  How about figuring the angle, or slope, at which the sides of the ditch are dug?

The slopes of the scarp and counterscarp will depend upon the nature of the soil, and the action on it of frost and rain.  The scarp is less steep than the counterscarp, because it has to sustain the weight of the parapet.  It is usual to give the slope of the scarp a base equal to two-thirds of the base of the natural slope of a mound of fresh earth whose altitude is equal to the depth of the ditch; the base of the counterscarp slope is made equal to one-half the same base.

Some sandbox physics at play here.  Note there are no slope requirements that apply to the difficulty imposed upon the attacker.  Rather, the main driving factor was to support the parapet.

Mahan would later offer a “mathematical calculation” to cover the required dimensions.  We’ll go over that later… and it requires some focus.  A general rule of thumb, Mahan offered:

On the field a result may be obtained, approximating sufficiently near the truth for practice, by assuming the depth of the ditch and dividing the surface of the profile of the parapet by it to obtain the width.

Furthermore, in regard to all that excavated dirt:

In excavating the ditch it will be found that more earth will be furnished at the salients than is required there for the parapet, and that the re-enterings will not always furnish enough.  On this account, the width of the ditch should not be uniform, but narrower at the salients than their re-enterings.

Since our discussion has focused on the horizontal plain thus far (the profile), there are some terms applying to the vertical plain that we haven’t discussed in detail.  But to water this down, the part of the fort which stuck out from the line would produce more dirt from the ditch than was needed.  So the width of the ditch was scaled in proportion.   The key part in reading that is going back to why the ditch was there – an obstacle which also provided the materials to make the parapet.

So why not build a bigger, wider ditch than required?  Two points I would add here as an “armchair Mahan.”  First, the more dirt the men have to shovel, the less time devoted to other chores.  Labor is a finite resource, and often the main governing resource on the project.

Second, the wider the ditch, the more “bad guys” can stand in the ditch.  If the attacker could mass more troops in the ditch, under the parapet and somewhat protected from direct fires, the “bad guys” might reorganize for a rush.  Again, the width of the ditch had a direct influence on the angle of the Superior Slope, and in that manner it had an overall impact on the size and shape of the Parapet.  A ditch more than twenty feet wide would require many adjustments (some extreme) to the parapet.

This fortification thing requires a lot more thinking than just “grab a shovel and start digging.”  Indeed, things like sine, cosine, and tangent are part of the instructions.

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

 

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