A bollard is a sturdy, vertical post. The term referred to a post on a ship or quay used principally for mooring boats, but is now used to refer to posts installed to control road traffic and posts designed to prevent ram raiding and car ramming attacks; the term is related to bole, meaning a tree trunk. The earliest citation given by the Oxford English Dictionary dates from 1844, although a reference in the Caledonian Mercury in 1817 describes bollards as huge posts. Simpler terms such as "post" appear to have been used; the Norman-French name boulard and Dutch bolder may be related. From the 17th and 18th centuries, old cannon were used as bollards on quaysides to help moor ships alongside; the cannon would be buried in the ground muzzle-first to half or two-thirds of their length, leaving the breech projecting above ground for attaching ropes. Such cannon can still be found. Bollards from the 19th century were purpose-made, but inherited a similar "cannon" shape. Wooden posts were used for basic traffic management from at least the beginning of the 18th century.
An early well-documented case is that of the "two oak-posts" set up next to the medieval Eleanor cross at Waltham Cross, Hertfordshire, in 1721, at the expense of the Society of Antiquaries of London, "to secure Waltham Cross from injury by Carriages". Similar posts can be seen in engravings. In the Netherlands, the Amsterdammertjes of Amsterdam were first erected in the 19th century, they became popular symbols of the city, but they are now being removed and replaced with elevated sidewalks. In the maritime contexts in which the term originates, a bollard is either a wooden or iron post found as a deck-fitting on a ship or boat, used to secure ropes for towing and other purposes; the Sailor's Word-Book of 1867 defines a bollard in a more specific context as "a thick piece of wood on the head of a whale-boat, round which the harpooner gives the line a turn, in order to veer it and check the animal's velocity". Mooring bollards are exactly cylindrical, but have a larger diameter near the top to discourage mooring warps from coming loose.
Single bollards sometimes include a cross rod to allow the mooring lines to be bent into a figure eight. Small mushroom-bollards are found on lock approaches for advancing boats waiting for lock access. A conventional measure of the pulling or towing power of a watercraft is known as bollard pull, is defined as the force exerted by a vessel under full power on a shore-mounted bollard through a tow-line. Bollards can be used either to control traffic intake size by limiting movements, or to control traffic speed by narrowing the available space. Israel's Transportation Research Institute found that putting bollards at highway exits to control traffic reduced accidents. Permanent bollards intended for traffic-control purposes may be mounted near enough to each other that they block ordinary cars, for instance, but spaced enough to permit special-purpose vehicles and bicycles to pass through. Bollards may be used to enclose car-free zones. Bollards and other street furniture are used to control overspill parking onto verges.
Tall slim fluorescent red or orange plastic bollards with reflective tape and removable heavy rubber bases are used in road traffic control where traffic cones would be inappropriate due to their width and ease of movement. Referred to as "delineators", the bases are made from recycled rubber, can be glued to the road surface to resist movement following minor impacts from passing traffic; the term "T-top bollards" refers to the T-bar moulded into the top for tying tape. Bollards are regarded as an economical and safe delineation system for motorways and busy arterial roads. Traffic bollards used in the US are similar to devices found throughout the UK, with the following exceptions: The traffic bollard shell displays the MUTCD "Keep Right" symbol. In addition, the traffic bollard has a yellow diamond below the "Keep Right" symbol instead of a yellow shield. Unlike many existing traffic bollards found in the UK, most new modern traffic bollards installed along roadways today are made of materials that make them collapsible.
When struck by a vehicle at low or high speed, the traffic bollard shell reverts to its original position with minimal to no damage to the unit. Internally illuminated traffic bollards have been in existence throughout the United Kingdom and Ireland since the 1930s, although the term "bollard" only seems to have been in common use since the late 1940s. An illuminated bollard has a recessed base light unit in the foundation which illuminates the traffic bollard from all angles; the main components are housed below the pedestrian surface. Therefore, if a vehicle strikes the traffic bollard, the units below the surface are not damaged, they are used at roundabout intersections within the splitter islands and at the ends of pedestrian refuge islands located at mid-block pedestrian crosswalks. Illuminated bollards are used in Hong Kong, a former British colony. Illuminated bollards are used
A cement is a binder, a substance used for construction that sets and adheres to other materials to bind them together. Cement is used on its own, but rather to bind sand and gravel together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Cement is the most used material in existence and is only behind water as the planet's most-consumed resource. Cements used in construction are inorganic lime or calcium silicate based, can be characterized as either hydraulic or non-hydraulic, depending on the ability of the cement to set in the presence of water. Non-hydraulic cement does not set under water. Rather, it sets as it reacts with carbon dioxide in the air, it is resistant to attack by chemicals after setting. Hydraulic cements set and become adhesive due to a chemical reaction between the dry ingredients and water; the chemical reaction results in mineral hydrates that are not water-soluble and so are quite durable in water and safe from chemical attack.
This allows setting in wet conditions or under water and further protects the hardened material from chemical attack. The chemical process for hydraulic cement found by ancient Romans used volcanic ash with added lime; the word "cement" can be traced back to the Roman term opus caementicium, used to describe masonry resembling modern concrete, made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick supplements that were added to the burnt lime, to obtain a hydraulic binder, were referred to as cementum, cimentum, cäment, cement. In modern times, organic polymers are sometimes used as cements in concrete. Non-hydraulic cement, such as slaked lime, hardens by carbonation in the presence of carbon dioxide, present in the air. First calcium oxide is produced from calcium carbonate by calcination at temperatures above 825 °C for about 10 hours at atmospheric pressure: CaCO3 → CaO + CO2The calcium oxide is spent mixing it with water to make slaked lime: CaO + H2O → Ca2Once the excess water is evaporated, the carbonation starts: Ca2 + CO2 → CaCO3 + H2OThis reaction takes time, because the partial pressure of carbon dioxide in the air is low.
The carbonation reaction requires that the dry cement be exposed to air, so the slaked lime is a non-hydraulic cement and cannot be used under water. This process is called the lime cycle. Conversely, hydraulic cement hardens by hydration. Hydraulic cements are made of a mixture of silicates and oxides, the four main components being: Belite; the silicates are responsible for the cement's mechanical properties—the tricalcium aluminate and brownmillerite are essential for formation of the liquid phase during the kiln sintering. The chemistry of these reactions is not clear and is still the object of research; the earliest known occurrence of cement is from twelve million years ago. A deposit of cement was formed after an occurrence of oil shale located adjacent to a bed of limestone burned due to natural causes; these ancient deposits were investigated in the 1970s. Cement, chemically speaking, is a product that includes lime as the primary curing ingredient, but is far from the first material used for cementation.
The Babylonians and Assyrians used bitumen to bind together burnt alabaster slabs. In Egypt stone blocks were cemented together with a mortar made of sand and burnt gypsum, which contained calcium carbonate. Lime was used by the ancient Greeks. There is evidence that the Minoans of Crete used crushed potshards as an artificial pozzolan for hydraulic cement. Nobody knows who first discovered that a combination of hydrated non-hydraulic lime and a pozzolan produces a hydraulic mixture —but such concrete was used by the Ancient Macedonians, three centuries on a large scale by Roman engineers. There is... a kind of powder. It is found in the neighborhood of Baiae and in the country belonging to the towns round about Mt. Vesuvius; this substance when mixed with lime and rubble not only lends strength to buildings of other kinds, but when piers of it are constructed in the sea, they set hard under water. The Greeks used volcanic tuff from the island of Thera as their pozzolan and the Romans used crushed volcanic ash with lime.
This mixture could set under water. The material was called pozzolana from the town of Pozzuoli, west of Naples where volcanic ash was extracted. In the absence of pozzolanic ash, the Romans used powdered brick or pottery as a substitute and they may have used crushed tiles for this purpose before discovering natural sources near Rome; the huge dome of the Pantheon in Rome and the massive Baths of Caracalla are examples of ancient structures made from these concretes, many of which still stand. The vast system of Roman aqueducts made extensive use of hydraulic cement. Roman concrete was used on the outside of buildings; the normal technique was to use brick facing material as the formwork for an infill of mortar mixed with an aggregate of broken pieces of stone, potsherds, recycled chunks of concrete, or other building ru
Urban warfare is combat conducted in urban areas such as towns and cities. Urban combat is different from combat in the open at both the operational and tactical level. Complicating factors in urban warfare include the presence of civilians and the complexity of the urban terrain. Urban combat operations may be conducted in order to capitalize on the strategic or tactical advantages with which possession or control of a particular urban area gives or to deny these advantages to the enemy. Fighting in urban areas negates the advantages that one side may have over the other in armour, heavy artillery, or air support. Ambushes laid down by small groups of soldiers with handheld anti-tank weapons can destroy entire columns of modern armour, while artillery and air support can be reduced if the'superior' party wants to limit civilian casualties as much as possible, but the defending party does not; some civilians may be difficult to distinguish from combatants such as armed militias and gangs, individuals who are trying to protect their homes from attackers.
Tactics are complicated by a three-dimensional environment, limited fields of view and fire because of buildings, enhanced concealment and cover for defenders, below-ground infrastructure, the ease of placement of booby traps and snipers. The United States Armed Forces term for urban warfare is an abbreviation for urban operations; the used U. S. military term MOUT, an abbreviation for military operations in urban terrain, has been replaced by UO, although the term MOUT Site is still in use. The British armed forces terms are OBUA, FIBUA, or sometimes FISH, or FISH and CHIPS; the term FOFO refers to clearing enemy personnel from narrow and entrenched places like bunkers and strongholds. Israel Defense Forces calls urban warfare לש "a Hebrew acronym for warfare on urban terrain. LASHAB in the IDF includes CQB training for fighting forces. IDF's LASHAB was developed in recent decades, after the 1982 Lebanon War included urban warfare in Beirut and Lebanese villages, was further developed during the Second Intifada in which IDF soldiers entered and fought in Palestinian cities and refugee camps.
The IDF has a special advanced facility for training soldiers and units in urban warfare. Urban military operations in World War II relied on large quantities of artillery bombardment and air support varying from ground attack fighters to heavy bombers. In some vicious urban warfare operations such as Stalingrad and Warsaw, all weapons were used irrespective of their consequences. However, when liberating occupied territory some restraint was applied in urban settings. For example, Canadian operations in both Ortona and Groningen avoided the use of artillery altogether to spare civilians and buildings, during the Battle of Manila in 1945, General MacArthur placed a ban on artillery and air strikes to save civilian lives. Military forces are bound by the laws of war governing military necessity to the amount of force which can be applied when attacking an area where there are known to be civilians; until the 1970s, this was covered by the 1907 Hague Convention IV – The Laws and Customs of War on Land which includes articles 25–27.
This has since been supplemented by the Additional Protocols to the Geneva Conventions of 12 August 1949, relating to the Protection of Victims of International and Non-International Armed Conflicts. Sometimes distinction and proportionality, as in the case of the Canadians in Ortona, causes the attacking force to restrain from using all the force they could when attacking a city. In other cases, such as the Battle of Stalingrad and the Battle of Berlin, both military forces considered evacuating civilians only to find it impractical; when Russian forces attacked Grozny in 1999, large amounts of artillery fire were used. The Russian Army handled the issue of civilian casualties by warning the inhabitants that they were going to launch an all-out assault on Grozny and requested that all civilians leave the city before the start of the artillery bombardment. Fighting in an urban environment can offer some advantages to a weaker defending force or to guerrilla fighters through ambush-induced attrition losses.
The attacking army must account for three dimensions more and expend greater amounts of manpower in order to secure a myriad of structures, mountains of rubble. Ferroconcrete structures will be ruined by heavy bombardment, but it is difficult to demolish such a building when it is well defended. Soviet forces had to fight room by room, it is difficult to destroy underground or fortified structures such as bunkers and utility tunnels. The characteristics of an average city include tall buildings, narrow alleys, sewage tunnels and a subway system
A bastion or bulwark is a structure projecting outward from the curtain wall of a fortification, most angular in shape and positioned at the corners. The developed bastion consists of two faces and two flanks with fire from the flanks being able to protect the curtain wall and the adjacent bastions, it is one element in the style of fortification dominant from the mid 16th to mid 19th centuries. Bastion fortifications offered a greater degree of passive resistance and more scope for ranged defense in the age of gunpowder artillery compared with the medieval fortifications they replaced. By the middle of the 15th century, artillery pieces had become powerful enough to make the traditional medieval round tower and curtain wall obsolete; this was exemplified by the campaigns of Charles VII of France who reduced the towns and castles held by the English during the latter stages of the Hundred Years War, by the fall of Constantinople in 1453 to the large cannon of the Turkish army. During the Eighty Years War Dutch military engineers developed the concepts further lengthening the faces and shortening the curtain walls of the bastions.
The resulting construction was called a bolwerk. To augment this change they placed v-shaped outworks known as ravelins in front of the bastions and curtain walls to protect them from direct artillery fire; these ideas were further developed and incorporated into the trace italienne forts by Sébastien Le Prestre de Vauban, that remained in use during the Napoleonic Wars. Bastions differ from medieval towers in a number of respects. Bastions are lower than towers and are of similar height to the adjacent curtain wall; the height of towers, although making them difficult to scale made them easy for artillery to destroy. A bastion would have a ditch in front, the opposite side of which would be built up above the natural level slope away gradually; this glacis shielded most of the bastion from the attacker's cannon while the distance from the base of the ditch to the top of the bastion meant it was still difficult to scale. In contrast to typical late medieval towers, bastions were flat sided rather than curved.
This eliminated dead ground making it possible for the defenders to fire upon any point directly in front of the bastion. Bastions cover a larger area than most towers; this allows more cannons to be provided enough space for the crews to operate them. Surviving examples of bastions are faced with masonry. Unlike the wall of a tower this was just a retaining wall; the top of the bastion was exposed to enemy fire, would not be faced with masonry as cannonballs hitting the surface would scatter lethal stone shards among the defenders. If a bastion was stormed, it could provide the attackers with a stronghold from which to launch further attacks; some bastion designs attempted to minimise this problem. This could be achieved by the use of retrenchments in which a trench was dug across the rear of the bastion, isolating it from the main rampart. Various kinds of bastions have been used throughout history. Solid bastions are those that are filled up and have the ground with the height of the rampart, without any empty space towards the centre.
Void or hollow bastions are those that have a rampart, or parapet, only around their flanks and faces, so that a void space is left towards the centre. The ground is so low, that if the rampart is taken, no retrenchment can be made in the centre, but what will lie under the fire of the besieged. A flat bastion is one built in the middle of a curtain, or enclosed court, when the court is too large to be defended by the bastions at its extremes. A cut bastion is that, it was sometimes called bastion with a tenaille. Such bastions were used; the term cut bastion is used for one, cut off from the place by some ditch. A composed bastion is when the two sides of the interior polygon are unequal, which makes the gorges unequal. A regular bastion is that which has proportionate faces and gorges. A deformed or irregular bastion is one. A demi-bastion has flank. To fortify the angle of a place, too acute, they cut the point, place two demi-bastions, which make a tenaille, or re-entry angle, their chief use is before a crownwork.
A double bastion is that which on the plain of the great bastion has another bastion built higher, leaving 4–6 m between the parapet of the lower and the base of the higher. Semi-circular bastions were used in the 16th century, but fell out of favour because of the difficulty of concentrating the fire of guns distributed around a curve. Known as "half-moon" bastions. Circular bastions or roundels evolved in the 15th and early 16th centuries but were superseded by angled bastions. Bastille Battery tower Roundel Whitelaw, A. ed. The popular encyclopedia. P&G, pp. 50–54, ISBN 978-1-906394-07-3 Nossov, Konstantin. H. (19
A Jersey barrier, or Jersey wall, is a modular concrete or plastic barrier employed to separate lanes of traffic. It is designed to minimize vehicle damage in cases of incidental contact while still preventing vehicle crossovers resulting in a head-on collision. Jersey barriers are used to reroute traffic and protect pedestrians and workers during highway construction, as well as temporary and semi-permanent protections against landborne attack such as suicide vehicle bombs. A Jersey barrier is known in the western United States as K-rail, a term borrowed from the California Department of Transportation specification for temporary concrete traffic barriers, or colloquially as a Jersey bump. Plastic water-filled barriers of the same general shape are now called Jersey barriers. Jersey barriers were developed in the 1950s, beginning in the U. S. state of New Jersey as separators between lanes of a highway. Over time, they became more modular. Taller barriers have the added advantage of blocking most oncoming headlights.
Although it is not clear when or where the first concrete median barriers were used, concrete median barriers were used in the mid-1940s on US-99 on the descent from the Tehachapi Mountains in the central valley south of Bakersfield, California. This first generation of concrete barriers was developed to minimize the number of out-of-control trucks penetrating the barrier, eliminate the need for costly and dangerous median barrier maintenance in high-accident locations with narrow medians – concerns that are as valid today as they were 50 years ago; the Jersey barrier called New Jersey wall, was developed in the 1950s, at the Stevens Institute of Technology, New Jersey, United States, under the direction of the New Jersey State Highway Department to divide multiple lanes on a highway. A typical Jersey barrier stands 32 inches tall and is made of steel-reinforced poured concrete or plastic. Many are constructed with the embedded steel reinforcement protruding from each end, allowing them to be incorporated into permanent emplacements when linked to one another by sections of fresh concrete poured on-site.
Their widespread use in road construction has led to wide application as a generic, portable barrier during construction projects and temporary rerouting of traffic into stopgap carpool and rush-hour reversing highway lanes. Most of the original barriers constructed in New Jersey in the 50s and early 60s were not "modular". Many of the first installations were much shorter than the heights discussed here about two feet tall; some dividers on county or local roads may have been lower than that since they replaced a raised concrete rumble strip that would dissuade but not prevent traffic crossing from one lane to another. Route 46 had the rumble strip in many places before the higher barrier was installed; these lower dividers are visible in old photographs. When the Bergen Mall was first opened in Paramus, these rumble strip dividers were extensively used on the roadway that separated the grocery stores from the mall proper; the design of the Jersey barrier was intended to minimize damage in incidental accidents and reduce the likelihood of a car crossing into oncoming lanes in the event of a collision.
In common shallow-angle hits, sheet-metal damage is minimized by allowing the vehicle tires to ride up on the lower sloped face. Head-on vehicle collisions are minimized by lifting the vehicle and pivoting it away from oncoming vehicles and back into traffic heading in its original direction. Modern variations include the F-shape barrier; the F-shape is similar to the Jersey barrier in appearance, but is taller, with somewhat different angles. The UK equivalent is the concrete step barrier. First tested in 1968 by the Department of Highways in Ontario, the Ontario Tall Wall is a variant of the Jersey barrier. Standing at 42 inches, it is 10 inches taller than the standard Jersey barrier. In Ontario, the Ministry of Transportation is replacing guiderails with these barriers on 400-series highways; the New Jersey Turnpike Authority developed and tested a similar, but reinforced, design. This barrier design has been credited with containing and redirecting larger vehicles, including semi-trailer trucks.
The states of New York and New Jersey have adopted the taller barrier for their roads, as compared to the standard 32 inches suggested by the Federal Highway Administration. Designs with two rectangular notches at the bottom allow for forklift-style lifting by front-end loaders. Barriers meant for short-term placement in military and security barrier uses, might include steel rebar loops embedded in the top surface for rapid hook-and-cable system lifting; the 2010 G-20 Toronto summit used modified modular Jersey barriers with wired fencing bolted onto the concrete. The fence used the barrier as sturdy base to prevent protesters from toppling the fence around the security zone at the Metro Toronto Convention Centre; the U. S. military nicknamed the devices as "Qaddafi Blocks" after truck bomb attacks in Beirut in 1983 resulted in more widespread use in military installations. Sometimes they are deployed to form a chicane to slow vehicular traffic arriving at military installations or other secure areas.
In the Philippines, jersey barriers were used for security and crowd control along the route of the papal co
Structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as biological organisms and chemicals. Abstract structures include data structures in musical form. Types of structure include a hierarchy, a network featuring many-to-many links, or a lattice featuring connections between components that are neighbors in space. Buildings, skeletons, beaver dams and salt domes are all examples of load-bearing structures; the results of construction are divided into buildings and non-building structures, make up the infrastructure of a human society. Built structures are broadly divided by their varying design approaches and standards, into categories including building structures, architectural structures, civil engineering structures and mechanical structures; the effects of loads on physical structures are determined through structural analysis, one of the tasks of structural engineering.
The structural elements can be classified as two-dimensional, or three-dimensional. The latter was the main option available to early structures such as Chichen Itza. A one-dimensional element has one dimension much larger than the other two, so the other dimensions can be neglected in calculations. Two-dimensional elements with a thin third dimension have little of either but can resist biaxial traction; the structure elements are combined in structural systems. The majority of everyday load-bearing structures are section-active structures like frames, which are composed of one-dimensional structures. Other types are Vector-active structures such as trusses, surface-active structures such as shells and folded plates, form-active structures such as cable or membrane structures, hybrid structures. Load-bearing biological structures such as bones, teeth and tendons derive their strength from a multilevel hierarchy of structures employing biominerals and proteins, at the bottom of which are collagen fibrils.
In biology, structures exist at all levels of organization, ranging hierarchically from the atomic and molecular to the cellular, organ, organismic and ecosystem level. A higher-level structure is composed of multiple copies of a lower-level structure. Structural biology is concerned with the biomolecular structure of macromolecules proteins and nucleic acids; the function of these molecules is determined by their shape as well as their composition, their structure has multiple levels. Protein structure has a four-level hierarchy; the primary structure is the sequence of amino acids. It has a peptide backbone made up of a repeated sequence of two carbon atoms; the secondary structure consists of repeated patterns determined by hydrogen bonding. The two basic types are the β-pleated sheet; the tertiary structure is a back and forth bending of the polypeptide chain, the quaternary structure is the way that tertiary units come together and interact. Chemical structure refers to electronic structure.
The structure can be represented by a variety of diagrams called structural formulas. Lewis structures use a dot notation to represent the valence electrons for an atom. Bonds between atoms can be represented by lines with one line for each pair of electrons, shared. In a simplified version of such a diagram, called a skeletal formula, only carbon-carbon bonds and functional groups are shown. Atoms in a crystal have a structure; the atoms can be modeled as points on a lattice, one can explore the effect of symmetry operations that include rotations about a point, reflections about a symmetry planes, translations. Each crystal has a finite group, called the space group, of such operations. By Neumann's law, the symmetry of a crystal determines what physical properties, including piezoelectricity and ferromagnetism, the crystal can have. A large part of numerical analysis involves identifying and interpreting the structure of musical works. Structure can be found at the level of part of the entire work, or a group of works.
Elements of music such as pitch and timbre combine into small elements like motifs and phrases, these in turn combine in larger structures. Not all music has a hierarchical organization, but hierarchy makes it easier for a listener to understand and remember the music. In analogy to linguistic terminology and phrases can be combined to make complete musical ideas such as sentences and phrases. A larger form is known as the period. One such form, used between 1600 and 1900 has two phrases, an antecedent and a consequent, with a half cadence in the middle and a full cadence at the end providing punctuation. On a larger scale are single-movement forms such as the sonata form and the contrapuntal form, multi-movement forms such as the symphony. A social structure is a pattern of relationships, they are social organizations of individuals in various life situations. Structures are applicable to people in how a society is as a system organized by a characteristic pattern of relationships. This
A fortification is a military construction or building designed for the defense of territories in warfare, is used to solidify rule in a region during peacetime. The term is derived from the Latin fortis and facere. From early history to modern times, walls have been necessary for cities to survive in an ever-changing world of invasion and conquest; some settlements in the Indus Valley Civilization were the first small cities to be fortified. In ancient Greece, large stone walls had been built in Mycenaean Greece, such as the ancient site of Mycenae. A Greek phrourion was a fortified collection of buildings used as a military garrison, is the equivalent of the Roman castellum or English fortress; these constructions served the purpose of a watch tower, to guard certain roads and lands that might threaten the kingdom. Though smaller than a real fortress, they acted as a border guard rather than a real strongpoint to watch and maintain the border; the art of setting out a military camp or constructing a fortification traditionally has been called "castrametation" since the time of the Roman legions.
Fortification is divided into two branches: permanent fortification and field fortification. There is an intermediate branch known as semi-permanent fortification. Castles are fortifications which are regarded as being distinct from the generic fort or fortress in that they are a residence of a monarch or noble and command a specific defensive territory. Roman forts and hill forts were the main antecedents of castles in Europe, which emerged in the 9th century in the Carolingian Empire; the Early Middle Ages saw the creation of some towns built around castles. Medieval-style fortifications were made obsolete by the arrival of cannons in the 14th century. Fortifications in the age of black powder evolved into much lower structures with greater use of ditches and earth ramparts that would absorb and disperse the energy of cannon fire. Walls exposed to direct cannon fire were vulnerable, so the walls were sunk into ditches fronted by earth slopes to improve protection; the arrival of explosive shells in the 19th century led to yet another stage in the evolution of fortification.
Star forts did not fare well against the effects of high explosive, the intricate arrangements of bastions, flanking batteries and the constructed lines of fire for the defending cannon could be disrupted by explosive shells. Steel-and-concrete fortifications were common during the early 20th centuries; however the advances in modern warfare since World War I have made large-scale fortifications obsolete in most situations. Demilitarized zones along borders are arguably another type of fortification, although a passive kind, providing a buffer between hostile militaries. Many US military installations are known as forts. Indeed, during the pioneering era of North America, many outposts on the frontiers non-military outposts, were referred to generically as forts. Larger military installations may be called fortresses; the word fortification can refer to the practice of improving an area's defence with defensive works. City walls are fortifications but are not called fortresses; the art of setting out a military camp or constructing a fortification traditionally has been called castrametation since the time of the Roman legions.
The art/science of laying siege to a fortification and of destroying it is called siegecraft or siege warfare and is formally known as poliorcetics. In some texts this latter term applies to the art of building a fortification. Fortification is divided into two branches: permanent fortification and field fortification. Permanent fortifications are erected at leisure, with all the resources that a state can supply of constructive and mechanical skill, are built of enduring materials. Field fortifications—for example breastworks—and known as fieldworks or earthworks, are extemporized by troops in the field assisted by such local labour and tools as may be procurable and with materials that do not require much preparation, such as earth and light timber, or sandbags. An example of field fortification was the construction of Fort Necessity by George Washington in 1754. There is an intermediate branch known as semi-permanent fortification; this is employed when in the course of a campaign it becomes desirable to protect some locality with the best imitation of permanent defences that can be made in a short time, ample resources and skilled civilian labour being available.
An example of this is the construction of Roman forts in England and in other Roman territories where camps were set up with the intention of staying for some time, but not permanently. Castles are fortifications which are regarded as being distinct from the generic fort or fortress in that it describes a residence of a monarch or noble and commands a specific defensive territory. An example of this is the massive medieval castle of Carcassonne. From early history to modern times, walls have been a necessity for many cities. In Bulgaria, near the town of Provadia a walled fortified settlement today called Solnitsata starting from 4700 BC had a diameter of about 300 feet, was home to 350 people living in two-storey houses, was encircled by a fortified wall; the huge walls around the settlement, which were built tall and with stone blocks which are 6 feet high and 4.5 feet thick, make it one of the earliest walled settlements in Europe but it is younger than the walled town of Sesklo in Greece from 6800 BC.
Uruk in ancient Su