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On the Trace of Field Fortifications

A Brief Introduction To The Elements And Use Of Field Fortifications

Field Fortifications Defined

Defined in their best nineteenth century sense, field fortifications were temporary works of all kinds employed to fortify points of  more or less transient importance to the conduct of an army's operations, necessary to provide security for its lines of communication, and required for the protection of its base of operations and other significant strategic or tactical points not otherwise covered by fortifications of a more permanent character. Field fortifications were generally constructed using local and immediately available non-durable materials, such as earth and wood, that could be manipulated easily and rapidly with a minimum of technical expertise and specialized tools. This definition necessarily encompasses a wide sweep of very divergent forms of fortification that range from simple loop-holes hacked roughly through house walls to magnificently elaborate semi-permanent fortifications. Fortifications forms are just the beginning; the subject also includes all of the skills necessary for the design and construction of effective fortifications along with the manner in which basic forms were adapted to fulfill the fortification requirements of specific sites.

Basic Applications Of Field Fortifications

Field Fortifications were used to accomplish a number of significant objectives necessary to the operations of an army in the field. First, and perhaps foremost, field fortifications were employed to strengthen tactical positions and provide the means necessary to allow an inferior army to withstand and repel an attack by a stronger or more numerous enemy force. A position could be fortified prior to its occupation by an army or the army could throw up lines of hastily constructed entrenchments after assuming its position. As the Civil War dragged on attacking armies tended to fortify their positions either to maintain close contact with the defending army or gain some measure of security against unexpected counter attacks. Prepared tactical positions could be fortified using both major field works intended as both cover and obstacle and minor field works; those fortified by an army after assuming a position were generally composed of minor field works such as field batteries, skirmish pits, and rifle trenches that could be constructed in a very short time and were primarily designed to provide cover. If a position was occupied for a prolonged period of time these hasty entrenchments could be developed into very strong substantial lines of major field works.

As the Civil War progressed and Federal armies penetrated deeper into the Southern States they found it necessary to cover important points along their lines of communications using a variety of both major and minor field works. An important point along an army's line of communication could be defined as any location where traffic back and forth from the base of operations could be disrupted or any point intermediate point where an army's supplies were collected prior to shipment to the army. This included trestles, bridges, towns, and defiles along railroads, commanding points overlooking watercourses and fords, and population centers that required occupation to intimidate hostile local civilians into submission. Blockhouses and stockades were generally used to protect railroad bridges, trestles, and river fords. Larger redoubts and other enclosed major field works were employed as garrison citadels or rallying points for blockhouse garrisons at occupied towns and villages.

Bases of operations or significant strategic points were generally covered by extensive systems of detached semi-permanent fortifications connected by prepared lines of rifle trenches and field batteries or any of the various types of fortified lines. For the most part these were some of the most elaborate and highly developed field works constructed during the war. Many of the forts constructed to protect Washington, D. C. were redesigned, remodelled, or rebuilt several times during the course of the war until some approached a standard of physical adaptation little short of permanent fortifications. Detached forts were generally designed specifically for an armament of heavy artillery; collateral works could support each other with their fire and defend distant approaches that an enemy would use to reach the defended perimeter. Some, like Fort Negley at Nashville, Tennessee, became so elaborate that they were never really quite finished. Normally held by garrisons just large enough to secure the main works against raids and surprises, a defeated field army could retire within the defended barrier to recover its strength and reinforce the perimeter defenses by occupying previous prepared trenches and batteries connecting the detached forts.

A particular species of field fortifications, called siege works, was also used in the attack of well developed fortifications. One of the objectives of a protective barrier of field works was to compel an attacker to settle down to the long process of reducing the fortified place using an attack by regular approaches. Sieges generally followed a set pattern of activity that involved the use of particular types of siege works to approach the fortification and dismantle the defenses. Once the defenses were dismantled, the defender's artillery silenced, and the work breached, it was generally assumed the the defender would capitulate. This may have applied well enough to the small masonry castles that dotted the sea coasts of the Southern States, but sieges against lines of field works were usually terminated either by an assault that determined final possession of the place or the garrison was more or less starved into surrender.

Traditional Forms

By the middle of the nineteenth field fortifications had been under more or less persistent and systematic development for a period of several centuries. Most, if not all, of the basic fortification forms used during the Civil War were products of the seventeenth and eighteenth centuries; parapets of major field works and the various accessory defenses built by soldiers and slaves during the Civil War differed very little from those constructed by the soldiers of Louis XIV 150 years before. There was, of course, one basic reason for this apparent stagnation in the art: traditional designs and methods worked, and worked well. This was particularly true since weapons development up to the 1840's had taken place by way of refinement rather than invention; cannon and muskets used in 1840 did pretty much the same thing in pretty much the same way as those used in 1700. More recent weapons were more effective, but they still shot round cannon balls and round muskets out of smoothbore tubes. Finally, the 1840's saw a bit of inventive movement. Both columbiad cannons, which could fire hallow shells, and rifled muskets were developed and refined through the 1850. During the early 1850's rifled artillery was developed.

Engineers' response to these developments was given in volumes of tamped earth. Columbiads, rifled artillery, and rifled muskets could penetrate more deeply into packed soil than older smoothbores. But the ammunition, though delivered with more force and accuracy than that of old smoothbores, still did pretty much the same things; solid shot still tore solidly into solid objects; shells still exploded with a sudden burst in the air, canister still spread its lethal bullets, and spherical case was just as dangerous as it had been during the Napoleonic wars. Ammunition of newer weapons fell with greater force and burrowed more deeply than that of smoothbores, but with no new method of breaking packed soil, thicker parapets designed according to traditional standards could still absorb their impact and reduce the risk of catastrophic penetrations.

Structural Function Of Major Field Works

All fortifications have the fundamental purpose of protecting their garrisons against enemy fire and assault. While reducing the effectiveness of an enemy's fire, a well arranged fortification would permit its garrison to use its own defensive fire in the most efficacious manner possible to repel an attack. If the defenders' fire failed to stop an assault, a major field work was designed to assist the garrison's defense by breaking up the orderliness of an attacking body of troops and wearing away the attacking troops' will and ability to scale the scarp and mount the superior to drive the defenders from the parapet. Major field works were designed to serve as both cover for its garrison and as an obstacle that an enemy would find difficult to surmount. Minor field works, on the other hand, such as rifle pits, rifle trenches, and batteries, were designed primarily to provide cover and will be dealt with separately.

The Trace Of Major Field Works

The quality and intensity of defensive fire that a garrison could lay on the ground immediately in front of a work to resist an attack was primarily a product of a work's trace. A trace was the shape formed by the lines and angles of a field work's parapet, more specifically, by the lines and angles formed by the crest of the interior slope. Tradition provided an array of standard traces that could be employed to produce specific effects; field works were generally classified and described by their traces rather than the strength of their profile. Redans, lunettes, priest caps, star forts, bastioned forts, cremaillere lines, and other such references describe field works by the general characteristics and arrangement of faces and flanks of their traces. A redan, for instance, was a simple field work that was traced with two faces joined to form a central salient angle while a lunette had two faces and two flanks.

Each traditional trace produced areas in front of a work that were covered by the defenders' fire and areas that were not well covered by their fire. It was generally assumed that an individual firing from a work's parapet would fire in a more or less straight line perpendicular to the line followed by the crest of the interior slope. A group of individuals firing from a parapet would produce a column of fire that was as long as the range of rifles and as wide as the section of parapet they happened to be manning. Troops lining the faces of a redan would produce two columns of fire that diverged from the salient along lines perpendicular to the two faces. This left a wide area directly in front of the salient angle without any fire at all, called the sector without fire. Sectors without fire could be covered by placing one or two pieces of artillery at the salient and arranging them to fire into the sector without fire.

A direct column of fire produced by the face of a field work could stop an attack, if the work was well covered by accessory defenses that could hold an attacking body of troops under fire before they reached the ditch in front of the parapet, but once they did reach the ditch, the defenders could not fire down on the attackers from the work's parapet. Once the attackers were in the ditch and secure from the defenders' fire the working party leading the attack could begin to tear down obstacles or undermine the parapet while the storming party reorganized to climb out of the ditch and rush over the parapet. There were, of course, ways to prevent all of this from happening and to make sure that a major field work's ditch served the garrison's defense better than the attackers' assault. Sally ports and sally port brdiges that spanned the ditch were also weak points in the trace of major field works that broke the continuity of the parapet and offered assaulting columns ready made breaches that could be used to avoid the double obstacles presented by the scarp and parapet. Caution usually compelled sally ports to be placed on the least accessible face of unflanked enclosed field works, within re-entrant angles of flanked works, or on curtains, where they could be protected by crossing columns of fire.

Well designed major field works were well flanked. This means that a trace was used that produced crossing columns of fire over ground immediately in front of the work and allowed the defenders to fire into the ditch in front of one part of the work from another part of the work. Bastioned traces and cremaillere lines were particularly useful in producing a flanked disposition; traces (such as redans and lunettes) that were not capable of mutual defense from with the work itself could be flanked by other nearby works or by the addition of auxiliary flanks that jutted outward from the main flanks. If a fortification was sufficiently important to warrant an intense degree of extra labor, its otherwise unflanked ditch could be flanked by a caponniere or counterscarp gallery. A major field work's ditch was supposed to be an obstacle, but the only way to turn it into an effective obstacle was to cover its full length and all of its angles with defensive fire.

It is of some importance to note that the various traditional traces were primarily used as theoretical models to understand the inherent characteristics produced by certain combinations of faces and flanks. They were not applied with textbook regularity to any and all situations and locations. Rather, the engineer's art was based on an ability to adapt received forms to meet the fortification requirements of specific positions to produce a field work that worked well within the context of its site and the ground immediately surrounding its site.

Elements Of The Profile Of Major Field Works

A profile is a vertical section of a field work's parapet and ditch that shows the altitude, thickness, and attitude of the various elements that make up the parapet and ditch. A major field work's profile contained two basic elements: a raised embankment of tamped earth that provided cover and an exterior ditch that provided soil to construct the embankment and served as an obstacle to an assault. This double purpose of cover and obstacle produced by excavated and packed earth is the thing that distinguishes a major field work from any of the minor and accessory forms of field fortifications. Profiles used to design and construct major field works throughout the American Civil War were based on traditional and received forms; their elements and proportions had been worked out by experience over the preceding 200 years.

A good defense of a major field work required a strong profile. A strong profile was one that raised troops lining the parapet higher than an attacking body of troops coming at the work, prevented an enemy's fire from passing through the body of the parapet, and created an obstacle immediately in front of the parapet that an enemy could not easily pass over to get on top of the parapet. Expressed more formally, the three elements of an effective profile were its command, thickness of the parapet, and its relief. All of these elements were proportional to each other; when one element was adjusted in height or thickness, the other elements had to be correspondingly adjusted to maintain the overall stability of the earth embankment and allow the garrison to use it without undue inconvenience.

An effective profile provided a command that both raised the defenders above the level of an attacking body of troops advancing to assault the work and intercepted both the enemy's view of the interior of a work and his direct fire into the work. A command of eight feet was considered sufficient for a major field work on a level site and unencumbered by surrounding heights; on irregular sites or when the site was itself commanded by nearby hills the height of a parapet (and therefore its command) was determined by a process known as defilading. Defilading was accomplished by taking simple levels from the ground to be fortified to surrounding hills and raising the parapet high enough (but no more than 12 feet) or locating traverses with the work to intercept fire from those heights. This was a practical operation that had to be performed on site after a work's trace had been laid out on the ground.

Thickness of a parapet was determined by the weight of projectiles that the parapet would reasonably be expected to withstand. Heavier shot and shells penetrated more deeply into a mass of packed earth than lighter shot and shells; those fired by rifled guns penetrated more deeply than those fired by smoothbores. A parapet that would only have to withstand the fire of light artillery could be made thinner than one that would have to absorb the fire of heavier siege artillery or naval ordnance. Overall thickness could be reduced by employing stout revetments that supported the weight of the interior slopes and put a final stop to shot and shells that penetrated into and were already slowed down in passing through packed mass of the parapet.

Relief of a major field work was measured on a vertical line from the bottom of the ditch to the altitude of the crest of the interior slope. A parapet with a command of eight feet, without a ditch would have a relief of eight feet, which was hardly enough to give an adequate check to an attacking body of troops. The lower part of the relief, measured on a vertical line from the bottom to the top of the ditch was the most important element that produced the profile's effect of as serious obstacle. The ditch itself had to do two things: serve as an effective obstacle and produce sufficient soil to construct the parapet. To be a serious obstacle a ditch had to be at least 12 feet wide at the top and no less than 6 feet deep. This width allowed the defenders to deliver sufficient flanking fire through the ditch to effectively disrupt the serenity of attacking troops attempting to use it as cover. A depth of 6 feet produce an overall relief, when combined with a parapet command of 8 feet, of 14 feet which was quite sufficient to give an attacking body of troops pause before attempting to climb out of the ditch to reach the top of the parapet.

For these three elements of a major field work's profile to be effective, they had to be supported by all of the other elements of the profile. All of the slopes had to proportioned to produce a stable mass that the defenders could actually fire over effectively. The exterior slope had to be given a grade that allowed it to carry the weight of the embankment and prevent the mass of the parapet from sliding into the ditch; the upper slope, called the superior slope had to given an inclination toward the exterior slope that allowed the defenders to point their guns downward to fire along a line just above the outside edge of the ditch (called the crest of the counterscarp). The interior slope had to have a sharp inclination so that the defenders could lean against the parapet to deliver their fire, which usually meant that the interior had to revetted or supported to keep its angle. When the command was over 4 feet a banquette had to be added to raise the defenders high enough to fire over the parapet. A slope was added to the rear of the banquette that allowed the defenders easy access to the banquette from the rear without having to run the full length of the parapet to deploy along it.

Accessory Defenses And Artillery

Accessory defenses were adjuncts to the trace and profile that were not absolutely necessary, but were quite useful to the conduct of a successful defense. Obstacles positioned in the ditch and immediately in front of the ditch were quite useful in breaking up the orderliness and ensemble of an attacking body of troops. Obstacles such as inclined palisadings, belts of small pickets, and abatis were designed to check the momentum of an attacking body of troops and hold them under a close and destructive fire delivered from the parapet. The longer an attacker was under fire, the less likely he was to be able to reach the ditch and be able to reorganize for an attempt on the parapet. This was the point where many assaults on major field works met their decided ends: attacking formations were broken by obstacles and shredded by the defenders' fire. Some few assaulting troops were almost always able to reach the ditch where they again came under fire that prevented them from reorganizing for attempt to scale the scarp and enter the work as body. Some few heroic souls were almost always able to claw their out of the ditch and mount the parapet, where they were almost always shot down. For an attack to have any chance of success at all the attacking troops had to work together and mount the parapet in one leap as a body to overwhelm the defenders lining the parapet; more often then not obstacles in front of the ditch combined with the defenders' direct and flanking fire prevented the attackers from maintaining the ensemble necessary for a successful attack.

Artillery in field fortifications served both defensive and offensive purposes that ranged from close defense of the work itself to projecting fire to prevent the enemy from occupying an area near the fortification or passing a point on a river or harbor channel, and battering enemy defensive fortifications. Some field fortifications, such as the forts composing the defenses of Washington, D. C., were primarily designed to protect a powerful artillery armament. A few special arrangements were necessary to render an artillery armament effective. First, gun positions had to be sited at suitable points along the interior crest and the work had to be prepared to receive its artillery armament. There were two basic ways to mount artillery in a field work; either by raising the armament high enough to fire over the parapet or cutting an opening in the parapet so that the artillery armament could fire through the parapet. The first, called barbette mountings, required the construction of an earthen platform that joined the interior slope. These were useful when a gun or howitzer required a wide field of fire and were primarily positioned in salient angles to cover a field work's sectors without fire. The second, called an embrasure mounting, required a well proportioned and revetted cut through a parapet. While embrasures provided better protection for gun crews than barbette mountings, they also disrupted a parapet's overall effectiveness as a shield against enemy fire and presented so many little breaches that attacking troops could use to enter the work and avoid having to mount the superior slope. In either case artillery had to be placed on wooden gun platforms to stabilize the piece and prevent it from tearing up its mounting. An artillery armament also required a secure structure, a magazine, where its ammunition and powder supplies could be stored without being turned to mush by moisture or exploded by enemy fire. In many cases a separate type of field work, called a battery, was used to protect an artillery armament.

January, 2003