Fortification Friday: How to dig a trench and pile dirt

In last Friday’s installment, we looked at Mahan’s instructions as to how an engineer (or leader) outlines the area that would need be excavated in order to build a field fortification.  Great stuff… and finally a place to apply all that geometry you learned in high school.  But even that is well short of the goal, retroactively historically speaking, of actually having the works in place.

To fill that requirement, those on hand had to… well… dig up some dirt and fill the framing described in the first step.  Isn’t that just simply taking a shovel and digging? Well, you see the Army has some written instruction for everything.  Digging is no exception.  Mahan approached this by applying practical, hands-on experience to generate a rule of thumb:

Experience has shown that, in ordinary soils, a man with a pick can furnish employment to two men with shovels; that, not to be in each other’s way, the men should be from four-and-a-half to six feet apart; and finally, that a shovel full of earth can be pitched by a man twelve feet in a horizontal direction, or six feet in a vertical direction.

The limitations on how far someone can fling dirt is important. It frames the recommended assignment of working “lanes” or “zones”, as we might call them today:

To distribute the workmen, the counterscarp crest is divided off into lengths of twelve feet, and the interior crest into lengths of nine feet.  These points might be marked out by pickets numbered one, two, three, &c. In each area, thus marked out, a working party is arranged, consisting of a pick with two shovels placed near the counterscarp, two shovels near the scarp, and one man to spread, and one to ram the earth, for two working parties.

So that paragraph defines two things for us.  First, as alluded to above, the “lane” for work as twelve feet of the proposed ditch, by nine feet of the intended parapet.  Another way to look at it – twelve feet of ditch converts to nine feet of parapet.

Also we see the work party defined.  One pick-man.  Two digging out the ditch.  One man spreading the dirt from the ditch.  One man packing down the dirt of the parapet.  Five men total for the lane defined as twelve feet of excavation going into nine feet of the defined line of works.  A good rule of thumb for us historians to put in the pocket when assessing the labor required to build earthworks.

Mahan continued with details about how this work party should operate:

The pick commences by breaking ground so far from the counterscarp crest that, by digging vertically three feet, he will arrive at the position of the counterscarp.  The excavation is carried on at the same depth of three feet, advancing toward the scarp, where the same precaution is observed as at the counterscarp. The earth is thrown forward, and evenly spread and rammed, in layers of about twelve inches, from the banquette slope to the exterior slope.

This is about as fascinating as hole digging can possibly get!  Seriously, however, this describes the precautions taken to ensure the profile of the ditch is constructed as defined.  Figure 19 is in evidence to illustrate this approach:

PlateIIIFig19

See how the first “dig” is prescribed to produce a three foot deep trench across the width of the ditch, but respects what will become the scarp and counterscarp slopes.  Likewise, another dig layer is defined for the next cut.  And so on to reach the desired depth.

However, as that ditch is excavated deeper, how do workers get in and out of it?

For the facility of entering the ditch, whilst working, the offsets, at the scarp and counterscarp, may be formed into steps with a rise of eighteen inches each; and, if the ditch is deeper than six feet, an offset, about four feet broad, should be left at the scarp, about mid-depth of the ditch, to place a relay of shovels to throw the earth on the berm.  In some cases, a scaffold of plank is raised in the ditch for the same purpose.

Now what happens when the bottom level of the ditch is reached?

When the ditch has been excavated to the bottom, the offsets are cut away, and the proper slopes given to the sides.  The earth furnished by the offsets, if not required to complete the parapet, may be formed into a small glacis.

Now we have our ditch. At least if we are working in “ordinary” soil.  But most “common” soil one is apt to work in has obstructions like rocks in the way.  Does that change the process?

If the soil is stony, the vegetable mould on the surface should be scraped off,and reserved to form the top of the parapet, which should be made of earth of this kind, to the depth of at least eighteen inches, to prevent injury to the troops from the effect of a shot striking the top, and scattering the pebbles in their faces.

Assume “vegetable mould” as what we’d call “sod” today.  So a layer of sod need be laid over rocks and gravel to prevent those objects from becoming secondary projectiles against the defender.

Another concern that need be addressed in construction is the drainage of water off the parapet:

In making the parapet, care should be taken to form a drain, at some suitable point, to carry off the water from the interior to the ditch.  The water from the drain should not be suffered to run down the scarp, as it would soon destroy it.  A gutter, formed of boards, should be made to prevent this.

A good gutter that does not leak, I would add.  Queue up the guy selling that amazing sealing material….

This is all good, to define the process of the work.  But how long should all this take?  After all the general wants his earthwork, like, now!

The time required to throw up a work will depend on the nature of the soil and the expertness of the laborers.  From troops unaccustomed to the use of ditching tools, six cubic yards may be considered a fair day’s work in ordinary soils, when the earth is not thrown higher than six feet; but when a relay is placed on an offset in the ditch, from four to five cubic yards may be taken as the result of a day’s work for each man.  Expert workmen will throw up from eight to ten cubic yards at task-work.

There is much to use with these work-factors, speaking from the perspective of a historian who is “reverse engineering” the works.  In particular, knowing how large a work is (from surveys taken today), one might derive the figures for the number of men needed and how long the work should have taken.

A side note here as we consider the “equation” to derive those figures.  Keep in mind the various types of labor employed at different places during the war.  Of course, we have to consider “green” troops.  Likewise, “veteran” troops who might not be experienced with excavation of this type.  But even more so, we must consider the use of non-military hands at this work… particularly freedmen and slaves, be they paid or impressed for duty.

Those hands working the picks and shovels were connected to larger bodies, you see.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 33-35.)

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