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:

 

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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:

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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.)

 

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