Penetration, Solid Shot

An Overview

Simply defined, penetration is the distance a solid projectile will travel into a given material before the inertial force of the material saps the projectile of its kinetic energy and brings it to a halt. One of the basic principles in the design of field fortifications was to construct parapets of sufficient resilience to absorb the greatest weight of shot that might be thrown at a field work and thereby secure its garrison and armament from penetrating shot piercing through the parapet. The primary means of preventing projectiles from piercing through a parapet was to make that parapet thick enough to allow a projectile to penetrate to its full expected depth into the body of the parapet and adding extra strength through increased thickness to insure that the parapet could withstand multiple projectile strikes without being broken down by extreme displacement of the soil composing the parapet. A secondary means of preventing piercing was to give the interior slope of the parapet a stout and durable revetment that could withstand a low velocity impact from a projectile already slowed by the resistance of the soil of the parapet and stop it either by further absorption of force by the revetment material or by breaking the projectile into fragments and deflecting them back into the body of the parapet.

The overall protective value of any particular parapet depended on a number of significant conditions other than its simple thickness such as its trace, defilading, and command. A failure of any one of these conditions could render a parapet positively hazardous to its garrison. For the purposes of exposition of the importance of giving a parapet its proper this note will be confined those considerations that bear directly on a parapet's capacity for withstanding the effects of direct strikes by solid round and elongated shot. This limitation does not allow a discussion of the effects of shell bursts within the body of parapets. An established method for breaking a parapet down was to chew the earth embankment up and soften it with solid shot and follow this with howitzer shells to displace the soil and throw it into the ditch which reduced the garrison's cover and created a sort of ramp up the scarp of the ditch. In effect, this could only be adequately accomplished through a prolonged bombardment by heavy artillery. As happened many times throughout the Civil War, at places like Vicksburg, Fort Wagner, and Petersburg, darkness generally fell before the bombarded parapet had been sufficiently broken down to allow a successful assault. Under cover of darkness the garrison was usually able to make effective repairs that would restore both the protective value of their parapet and its value as an obstacle to an assault.

It is important to understand that this note does not present a complete picture of the methods that could be used to both increase the strength of and destroy parapets. Rather, it attempts to impart some basic information on the way solid shot would normally damage an earth embankment, briefly discusses the methods in use at the time of the Civil War for determining how much damage a solid shot would case, that is, how deeply it would penetrate into the body of a parapet, and how parapets could be designed with sufficient resilience to serve their intended protective function. Finally, it attempts to check the theory of resilience by listing the thicknesses of a few parapets actually constructed during the Civil War.

Damage By Solid Shot

It is important to understand that an earth embankment, such as a parapet, does not perform its protective function by repelling or fragmenting solid projectiles hurled against it, rather, the compact soil composing the embankment acts as a sponge to gradually absorb the kinetic energy of the projectile and bring it to a halt. When a solid projectile strikes an earth embankment it causes two distinct types of damage: first, the concussion of its impact on the surface of the embankment throws debris outward from the parapet and creates a crater that reduces the thickness of the parapet. Second, after the initial impact the solid projectile penetrates into the body of the parapet it creates a path of disturbed and loosened soil along its course until all of its momentum is sapped and it comes to a halt. A shell, bursting in the body of a parapet, would cause a third type of damage by by vaporizing and breaking down the soil in its immediate vicinity and creating a cavity, which, depending on the composition and compactness of the soil, would either remain void within the parapet or cause soil above it to collapse into the cavity.

As might be expected, the amount of damage and depth of penetration primarily depended on the weight, density, diameter, and velocity of a projectile. Larger, heavier, projectiles tended to retain a greater proportion of their initial velocity as they moved along their trajectories than smaller and lighter projectiles and would strike the surface of a parapet with greater force than smaller and lighter projectiles. Striking at higher velocities, larger solid projectiles tended to penetrate deeper into the body of a parapet than smaller or lighter projectiles striking at relatively lower velocities. With their greater diameter larger and heavier projectiles displaced more soil on impact and drilled a wider path of disturbed soil into a parapet. Shells, which were lighter, less dense, and fired at lower initial velocities than solid projectiles would often break up on impact with the surface of a parapet or would lose their momentum much quicker and not penetrate as deeply. These dynamic characteristics of projectiles meant that parapets designed to stand against heavy siege or ship-borne artillery had to be much thicker than parapets designed to withstand the general run of light field artillery that would be found on any battlefield.

A single projectile might not cause significant damage to the protective value of a well constructed parapet, multiple strikes would, if well placed, eventually begin to tear and pierce the body of a parapet. Of two larger projectiles striking close together on the surface of a parapet, the first would crater the surface and create a path of disturbed and therefore less resistant soil. The second would strike a parapet reduced in thickness and would penetrate somewhat deeper into the loosened soil. But a well designed parapet, though weakened, would have been given sufficient reserve strength to prevent the second projectile from piercing through the parapet. This reserve strength was gained by increasing the thickness of the parapet by one-third to one-half the expected depth of penetration of the largest projectile that would be brought to bear against it. Thus the second projectile, already slowed as it passed through the area of disturbed soil, would be brought to a halt by undisturbed compact soil well before it reached the revetment of the interior slope.

Penetration Tables

Penetration Tables were used as guides to assist engineer or other officers assigned the task of constructing a field work in estimating the appropriate thickness to give the work's parapets. Penetration Tables generally showed how deep particular weights of shot and shells would penetrate into specific types of materials and were usually composed from data gained through experimentation, that is, from results obtained by actually firing various weights of projectiles into various types of material at specific ranges. Very few of these tables were available prior to the Civil War; most were based on foreign experiments using foreign artillery firing round shot and all were outdated by the introduction of rifled artillery that fired elongated projectiles. The first edition of Dennis Hart Mahan's Complete Treatise on Field Fortification, published in 1836, used a rather common table lifted from British sources, which was quite reasonable since, at that time, Great Britain was considered the United States' most likely and dangerous opponent. Penetration Tables produced from French experiments conducted at Metz in 1834 were the center-piece of the United States Army's 1850 Ordnance Manual, which also included a few other, less detailed, tables based on American experiments. The French Tables were reproduced in the Army's 1861 and the Confederate 1863 Ordnance Manuals, along with the results of more recent American experiments. A later updated edition of Mahan's Treatise on Field Fortifications published in 1862 added to its original table by including some of the tables already presented in the 1850 Ordnance Manual. John Gibbon's Artillerist's Manual, updated in 1863, simply referred readers to the Tables in the Ordnance Manual.

The French tables were somewhat difficult to use since they listed the penetration of French shot fired from French guns and howitzers that were of a different size and weight than similar artillery used in the American service. The 16 caliber gun given in the table was, according to the Ordnance Manual, roughly similar to an American 18-pounder; the 36 caliber gun was similar to the American 42-pounder. For an American officer to design a parapet that could withstand the fire of an American made 18-pounder he had to base his estimate of the appropriate thickness on the results given for the French 16 caliber gun. A 16 caliber solid shot fired with a charge 1/3 the weight of the shot at 109 yards range would penetrate, according to the table, some 79.6 inches into an embankment composed of half sand and half clay. To withstand the fire of an 18-pounder firing solid shot at about 100 yards a parapet would have to be at least 6.63 feet thick, the depth of penetration, plus another 1/2 that for reserve strength, making the proposed parapet about 10 feet thick. This result does not mix well with other tables; according to the penetration table in the 1836 edition of Mahan's treatise, a thickness between 11 1/2 and 13 feet was needed to withstand the fire of an 18-pounder.

Pre-war penetration tables only applied to shot and shell fired from smoothbore artillery. Although some European armies had been experimenting with rifled artillery firing elongated shot for years, this newer type of artillery was just entering the American service when the war began and had not been adequately tested or compared to the penetrative effectiveness of older smoothbore artillery. Observations from practical experience and experiments made during the course of the war slowly filled the information gap, but this information only saw limited distribution and most engineers had to rely on their own good sense to make adequate adjustments to account for the supposed superior penetrative power of elongated shot. Q. A. Gillmore's report of the reduction of Fort Pulaski, which included a penetration table drawn from observations of the effects of various weights of rifled and smoothbore shot impacted on the fort masonry walls, was published in 1862. Gillmore's table revealed the stark and unmistakable penetrative superiority of rifled over smoothbore artillery fired at long ranges. The result of some trials conducted during the war were included in Mahan's textbook An Elementary Course of Military Engineering, Part I, published in 1865. H. L. Abbot also presented the results of experiments conducted in the defenses of Washington in 1863 in his own Siege Artillery in the Campaigns Against Richmond published in 1868. While information from these sources may be useful for gaining an understanding of the penetrative power of elongated shot, the information itself (and reports of the various wartime experiments) was of little service to engineers in the field who did not have access to it, but still had to account for the effect of elongated projectiles in the their field work designs.

Experience showed that elongated shot had something of an advantage in penetrative power over round shot of similar weight and that this advantage was particularly marked in larger sea-coast guns. An 8-inch Parrott rifle firing solid shot weighing 150 pounds was found to have an average penetration of 8 feet in a compact gravel embankment at 1,000 yards while round shot fired from an 8-inch smoothbore Columbiad weighing just 65 pounds was found to penetrate a sand and gravel embankment only some 3 1/3 feet at 1,200 yards. On the other hand, solid shot fired from a 24-pounder smoothbore siege and garrison gun at a range of 1,200 yards would be expected to penetrate almost 3 feet into a sand and gravel embankment while solid shot from a 20 or 30-pounder Parrott rifle would penetrate a gravel embankment about 3 feet at 1,000 yards range.

Elongated solid shot did have greater penetrative force than round shot of similar diameter. This was primarily due to their greater weight; solid shot fired from an 8-inch smoothbore columbiad weighed just 65 pounds while shot from an 8-inch Parrott rifle weighed 150 pounds. Greater weight combined with a more efficient shape decreased the loss of velocity as elongated projectiles moved along their trajectory; although most elongated projectiles were fired at a lower initial velocity than round shot, elongated shot actually struck with greater force due to their greater weight and retention of velocity. But elongated shot of all weights and diameters tended to become unstable and wobble or even tumble end over end as it moved along its trajectory which reduced its velocity and the force of their impact on the surface of a parapet. The spin imparted by rifling could also increase the chance for a non-penetrating deflections off the surface of a parapet composed of a high proportion of sand or gravel. Where round shot tended to bull its way into the body of a parapet along a more or less straight on path, elongated shot could be deflected away from its path by rocks or other hard objects in the body of a parapet before reaching its maximum penetration depth.

Ideal Thickness of Parapets

The ideal thickness of a parapet would have been calculated by first deciding what weight of shot it would have to withstand then referring to a penetration table to check how deep that weight of shot would penetrate into the sort of soil the parapet would be made from and adding at least one-half the penetration depth. This rather simple calculation would produce the ideal thickness of a given parapet. Using this formula and the maximum depths given in Mahan's 1836 penetration table to calculate the ideal thickness of parapets yields thicknesses of 27 inches for protection against musket balls (18 inches + 9 inches;) 6 3/4 feet for protection against 6-pounder shot, 10 1/2 feet for protection against 9-pounder shot, 15 feet for protection against 12-pounder shot, and 19 1/2 feet for protection against 18 and 24-pounder shot. Using the same table John Shortall Macaulay (Treatise on Field Fortification, 2nd. Edition) suggested thicknesses of 3, 6, 9, 14, and 18 feet, respectively, with a maximum practical thickness of 14 feet for field works since 18 and 24-pounders were rarely encountered in the field.

Results of experiments conducted by Captain Benet, of the United States Army's Ordnance Corps, given in Mahan's Elementary Course of  Military Engineering, Part I, on the penetration of various Parrott rifles fired at embankment of sand and clay at a range of 24 yards indicated that a 20-pounder elongated shot would penetrate 8 1/2 to 9 feet; a 30-pounder projectile penetrated about 10 feet, and a 100-pounder firing solid shot weighing between 80 and 96 pounds would penetrate between 15 and 18 1/2 feet. These results probably indicated the maximum penetration depths that the three rifles' projectiles were capable of generating. Applying the calculation of ideal parapet thickness to these penetration results would produce parapets 13 1/2 feet thick for protection against a 20-pounder Parrott, 15 feet thick against a 30-pounder, and 27 3/4 thick against a 100-pounder Parrott Rifle.

H. L. Abbot's experiments (Siege Artillery in the Campaigns Against Richmond) on the penetration of rifled projectiles conducted in 1863 indicated that 100-pounder Parrott shot penetrated between 8 and 10 1/2 feet at a range of 1,800 yards; a 4.5 inch Ordnance Rifle firing a Schenkl shell (not a solid projectile) penetrated between 4 1/2 and 9 feet at 1,800 yards while a solid shot projectile fired from a 30-pounder Parrott Rifle at 40 yards range penetrated between 9 1/2 and 11 feet. 24-pounder round shot fired at ranges of 13 and 15 yards penetrated 8 to 9 feet. The targets were a well settled covering mass of a magazine eighteen months old and a very solid natural bank of sandy soil. Abbot concluded that parapets constructed from the type of soil found in Virginia should have a minimum thickness of 16 feet to withstand 100-pounder solid shot and 12 feet to withstand lighter projectiles when well rammed; parapets formed from new earth, unsettled and unrammed, would necessarily have to be much thicker. Again applying the standard calculation to Abbot's results the ideal thickness in each case would be 15 3/4 feet to absorb projectiles from a 100-pounder Parrott, 13 1/2 feet for a 4.5 inch Ordnance Rifle, 16 1/2 feet for a 30-pounder Parrott, and 13.5 feet for a 24-pounder siege and garrison gun.

So the ideal thickness of any given parapet would be based on, as previously stated, the weight and type of projectile that the parapet would have to stand against. Ideally a parapet would be given reserve strength produced by adding 1/2 to 1/3 of the expected penetration of the heaviest projectiles that would most probably be brought to bear against the work This, as may be guessed, entails that there was no single ideal thickness that all parapets could be matched against; rather, the ideal was situational. The ideal thickness for the parapet of a rifle pit covering a few skirmishers would allow the work to absorb small arms fire and be between 3 and 4 feet thick with a good makeshift log revetment. An isolated fortification with the profile of a major work that would only be expected to withstand the fire of light 6 or 12-pounder guns and howitzers could be given a parapet no more than 12 feet thick. Important works constructed to cover critical points within an extended line occupied by a field army that would be expected to withstand the best and heaviest rifled field artillery would have to have thicker parapets, 10 to 18 feet thick, it absorb elongated projectiles. Works situated along navigable waterways that would be subjected to the fire of very heavy ship-borne guns would have to be even thicker, perhaps 20 to 40 feet. The thickness of any parapet was ideal only in the sense that it matched the resources required for its construction with the necessity for providing efficient cover for the work's garrison and armament. In other words, a parapet 20 feet thick would stop almost any weight of solid projectile, but a parapet 20 feet thick covering a position occupied by two or three men who might or might not receive a light small arms fire would be a terrible waste of time, labor, material resources, and doubtless would tax the patience of any troops ordered to construct such a pointlessly strong work.

Thickness of Parapets Constructed During the Civil War

Producing a parapet appropriate to the situation and circumstances of any particular field work had at least as much to do with its designer's creative ingenuity as it did with the application of more or less standard calculations. Thickness, as previously stated, was not the only condition that determined a parapet's protective value; trace, command, and defilading were just as important and all had to be considered in relation to each other. A work, for instance, that required a command of 10 feet might be located in a position that could not produce sufficient soil for its required thickness or there might not be enough time or labor available to construct the work with an adequately effective profile. In this case the conflict between required command and necessary thickness would not be arbitrated by any ideal form, but by the best profile obtainable under the immediate circumstances. Most parapets constructed during the war were the result of this balancing process, so much so, that a discussion of the thickness of parapets becomes impossible outside of a more complete discussion of the general characteristics of particular works' profiles. But if any single trend is discernible it is that over the course of the war engineers tended to increase the thickness of parapets exposed to all types of fire, but this was as much of product of the field armies' tendency to settle down and stay behind fortified lines as it was a matter of necessity to increase the protective value of parapets that had to withstand prolonged fire from rifled artillery.

A summary survey of the thickness of parapets described in the Atlas to Accompany the Official Records displays the wide variety of thicknesses adopted for particular purposes over the course of the war. References in parenthesis give the plate and map number where the various parapets are described. Confederate works at Columbus, Kentucky (Pl. V, No.2) that were not subject to the direct fire of gunboats were given parapets ranging from just over 6 feet to 8 feet thick. Redoubt Number 4, which would have been reached by direct short range fire from gunboats was given parapets 19 feet thick. At Centreville, Virginia (Pl. X, No. 7) Confederate works designed to cover infantry were between 4 and 12 feet thick while battery parapets were 14 feet thick. At Fort Donelson, Tennessee (Pl. XI No.5) the original central fort's parapets were about 11 feet thick while that of the outer line of rifle pits were about 3 feet thick. The bastioned irregular pentagonal fort constructed by the Confederates at Gloucester Point, Virginia (Pl. XV, No. 1,) just across the York River from Yorktown, had parapets 15 to 20 feet thick. Redoubts in the Confederate's Williamsburg Line (Pl. XX, No. 2) were given parapets about 12 feet thick. Federal batteries at Vicksburg (Pl. XXXVI, No. 2) had parapets 8 to 12 feet thick. A Confederate battery at Atlanta (Pl. LI, No. 3) had a parapet 11 feet, 9 inches thick. The sand parapets at Fort Fisher, Federal Point, North Carolina (Pl. LXXV, No. 2) ranged from just over 26 to 42 feet thick, with the sea-side parapets being the thickest. Federal batteries fronting Petersburg, Virginia (Pl. CIV) ranged from 10 to 18 feet thick, with many 13 and 15 feet thick. The Federal forts around Nashville, Tennessee (Pls. CXIII and CXIV) were given parapets ranging from 10 to 16 feet thick. Parapets 8 to 12 feet thick were originally thought to be sufficient for the semi-permanent works of the Washington defenses, but these were later increased to 12 to 18 feet.

Summary

A well designed parapet was tailored to absorb the greatest weight of shot that the enemy would probably be able to bring to bear against a field work. Thickness was determined by estimating the depth of penetration of the weight of shot using a penetration table or other relevant data and adding 1/3 to 1/2 the penetration depth to give a parapet sufficient reserve strength to absorb multiple impacts in a restrict area of the parapet. Elongated projectiles fired from rifled ordnance were found to have slightly greater penetrative power than solid round shot of similar weight. While this did not have a significant impact on the general protective value of parapets of most temporary works, it led to a tendency to build much thicker parapets for works subject to the fire of heavy rifled artillery. For the most part parapets expected to stand against field artillery were between 7 and 12 feet thick, those expected to hold up under the fire of heavier siege and sea-coast artillery were between 12 and 20 feet thick. Parapets of sea coast fortifications could range up to 42 feet thick to absorb heavy naval bombardments.

[This page originally appeared as a Detailed Note Page on the old Civil War Field Fortifications Website.]

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