Medieval fortification refers to medieval military methods that cover the development of fortification construction and use in Europe, roughly from the fall of the Western Roman Empire to the Renaissance. During this millennium, fortifications changed warfare, and in turn were modified to suit new tactics, weapons and siege techniques.
Landscape designers appreciate Corten for more than its warm hue. Generally available in sheet and plate form, its strength and durability combined with minimal thickness (typically 3/16 or 1/4 of an inch) allows it to serve in situations where a concrete wall, for instance, would not fit or would visually overwhelm its surroundings.
- 1Fortification types
- 2Construction
Fortification types[edit]
Archer towers[edit]
Towers of medieval castles were usually made of stone or sometimes (but rarely) wood. Often toward the later part of the era they included battlements and arrow loops. Arrow loops were vertical slits in the wall through which archers inside shot arrows at the attackers, but made it extremely difficult for attackers to get many arrows back through at the defenders.
City walls[edit]
An exact nature of the walls of a medieval town or city would depend on the resources available for building them, the nature of the terrain, and the perceived threat. In northern Europe, early in the period, walls were likely to have been constructed of wood and proofed against small forces. Especially where stone was readily available for building, the wood will have been replaced by stone to a higher or lower standard of security. This would have been the pattern of events in the Five Boroughs of the Danelaw in England.
In many cases, the wall would have had an internal and an external pomoerium. This was a strip of clear ground immediately adjacent the wall. The word is from the late medieval, derived from the classical Latinpost murum ('behind the wall').
An external pomoerium, stripped of bushes and building, gave defenders a clear view of what was happening outside and an unobstructed field of shot. An internal pomoerium gave ready access to the rear of the curtain wall to facilitate movement of the garrison to a point of need. By the end of the sixteenth century, the word had developed further in common use, into pomery.
Also by that time, the medieval walls were no longer secure against a serious threat from an army, as they were not designed to be strong enough to resist cannon fire. They were sometimes rebuilt, as at Berwick on Tweed, or retained for use against thieves and other threats of a lower order. Very elaborate and complex schemes for town defences were developed in the Netherlands and France, but these belong mainly to the post-medieval periods. By 1600, the medieval wall is likely to have been seen more as a platform for displaying hangings and the pomery as a gathering ground for spectators, or as a source of building stone and a site for its use, respectively. However, a few, such as those of Carcassonne and Dubrovnik, survived fairly well and have been restored to a nearly complete state.
Medieval walls that were no longer adequate for defending were succeeded by the star fort. After the invention of the explosive shell, star forts became obsolete as well.
Harbours[edit]
Harbours or some sort of water access was often essential to the construction of medieval fortification. It was a direct route for trading and fortification. Having direct access to a body of water provided a route for resupply in times of war, an additional method of transportation in times of peace, and potential drinking water for a besieged castle or fortification. The concept of rivers or harbours coming directly up to the walls of fortifications was especially used by the English as they constructed castles throughout Wales.There is evidence that harbours were fortified, with wooden structures in the water creating a semi-circle around the harbour, or jetties, as seen in an artists reconstruction of Hedeby, in Denmark, with an opening for ships to access the land. Usually, these wooden structures would have small bases at either end, creating a 'watch' and defense platform.
Churches and monasteries[edit]
Religion was a central part of the lives of medieval soldiers, and churches, chapels, monasteries, and other buildings of religious function were often included within the walls of any fortification, be it temporary or permanent. A place to conduct religious services was usually essential to the morale of the soldiers.
Mottes and baileys[edit]
Motte-and-bailey was the prevalent form of castle during 11th and 12th centuries. A courtyard (called a bailey) was protected by a ditch and a palisade (strong timber fence). Often the entrance was protected by a lifting bridge, a drawbridge or a timber gate tower. Inside the bailey were stables, workshops, and a chapel.
The motte was the final refuge in this type of castle. It was a raised earth mound, and varied considerably, with these mounds being 3 metres to 30 metres in height (10 feet to 100 feet), and from 30 to 90 metres (98 to 295 ft) in diameter.[1] There was a tower on top of the motte. In most cases, the tower was made of timber, though some were also made of stones. Stone towers were found in natural mounds, as artificial ones were not strong enough to support stone towers. Larger mottes had towers with many rooms, including the great hall. Smaller ones had only a watch tower.
Construction[edit]
Construction could sometimes take decades. The string of Welsh castles Edward I of England had built were an exception in that he focused much of the resources of his kingdom on their speedy construction. In addition to paid workers, forced levies of labourers put thousands of men on each site and shortened construction to a few years.
Location[edit]
Nature could provide very effective natural defense for the castle. For this reason many castles were built on larger hills, cliffs, close to rivers, lakes or even caves.
Materials[edit]
Materials that were used in the building of castles varied through history. Wood was used for most castles until 1066. They were cheap and were quick to construct. The reason wood fell into disuse as a material is that it is quite flammable. Soon stone became more popular.
Stone castles took years to construct depending on the overall size of the castle. Stone was stronger and of course much more expensive than wood. Most stone had to be quarried miles away, and then brought to the building site. But with the invention of the cannon and gunpowder, castles soon lost their power.
Costs[edit]
Costs for the walls depended on the material used. Wood would cost very little and was quick to build, but was weak. Stone was strong but very expensive and time-consuming to construct.
Manpower[edit]
Manpower in the medieval era in Europe consisted mainly of serfs.
Walls[edit]
The height of walls varied widely by castle, but were often 2.5–6 m (8.2–19.7 ft) thick. They were usually topped with crenellation or parapets that offered protection to defenders. Some also featured machicolations (from the French machicoulis, approximately 'neck-crusher') which consisted of openings between a wall and a parapet, formed by corbelling out the latter, allowing defenders to throw stones, boiling water, and so forth, upon assailants below. Some castles featured additional inner walls, as additional fortifications from which to mount a defense if outer walls were breached.
Gates[edit]
Any entrance through a wall, being an opening, forms an obvious weak point. To be practical, the entryway would have to accommodate supplies being brought through, yet difficult for attackers to breach. For example, passage over ditches or moats would have to be withdrawn to deny attackers. The use of multiple walls or ditches around an entrance would also make it difficult for defenders to use the entrance practically, necessitating better methods of control. Gates came in many forms, from the simple stone buttress and timber blocks,[2] to the massive and imposing stone archways and thick wooden doors most associated with medieval citadels.
Killing fields[edit]
A killing field was an area between the main wall and a secondary wall, so when the first wall was breached the attackers would run into the killing field to be confronted by another wall from which soldiers bombarded them. Soldiers would be positioned atop the second wall and armed with any variety of weapons, ranging from bows to crossbows to simple rocks.
Moats[edit]
A moat was a common addition to medieval fortifications, and the principal purpose was to simply increase the effective height of the walls and to prevent digging under the walls. In many instances, natural water paths were used as moats, and often extended through ditches to surround as much of the fortification as possible. Provided this was not so unnaturally contrived as to allow an attacker to drain the system, it served two defensive purposes. It made approaching the curtain wall of the castle more difficult and the undermining of the wall virtually impossible. To position a castle on a small island was very favorable from a defensive point of view, although it made deliveries of supplies and building materials more cumbersome and expensive.
Keeps[edit]
A keep is a strong central tower which normally forms the heart of a castle. Often the keep is the most defended area of a castle, and as such may form the main habitation area for a noble or lord, or contain important stores such as the armoury or the main well.
Stairs[edit]
Stairs were also constructed to contain trick or stumble steps. These were steps that had different rise height or tread depth from the rest and would cause anyone running up the stairs to stumble or fall, so slowing down the attackers' progress.
Doors[edit]
A typical exterior wooden door might be made out of two layers of oak planks. The grain of the wood would run vertically on the front layer and horizontally on the back, like a simple form of plywood. The two layers would be held together by iron studs, and the structure might be strengthened and stiffened with iron bands.
The studs themselves were pointed on the front so that attackers would damage their weapons (swords, axes, etc.) while trying to break through.
Transition to modern fortification[edit]
From the mid-15th century onwards, the power of cannons grew and medieval walls became obsolete as they were too thin to offer any realistic protection against prolonged bombardment.[3] As a consequence of this, medieval walls were often upgraded with the addition of artillery platforms or bastions, and battlements were replaced by thick parapets with embrasures. In many cases, the medieval walls were dismantled and their stonework, which was still valuable as construction material, was reused in the construction of the new fortifications.[4] The resulting space is often seen in old city centers of Europe even to this day, as broader streets often outline where the old wall once stood (evident for example in Prague and Florence, Italy).
The transition between medieval and early modern fortification can be seen in the fortifications of Rhodes in Greece and the fortifications of Famagusta in Cyprus.[5]
Defensive obstacles[edit]
Just as modern military engineers enhance field fortifications with obstacles such as barbed wire, medieval engineers used a number of obstacle types including abatis, caltrops, cheval de frise, and trou de loup.
Siegecraft[edit]
See also[edit]
- Guédelon Castle - from 1996 to 2020 they will build a 13th-century castle exclusively using methods of that time. A lot of information regarding castrametation and castellology had already surfaced thanks to this project.
- Star fort replaced medieval fortifications.
References[edit]
- ^Toy, p.52.
- ^Avery, Michael (October 1986). World Archeology. Vol. 18, No. 2. pp. 216–230.
- ^Pañeda Ruiz, José Manuel. 'Evolution of siege techniques: From the Catholic Monarchs to Vauban'(PDF). MilitaryArchitecture.com. pp. 6–7. Retrieved 22 October 2015.
- ^Spiteri, Stephen C. (2009). 'A Cubete Artillero at Mdina? – Gunpowder fortifications in Late Medieval Malta'(PDF). Proceedings of History Week: 150–159. Archived from the original(PDF) on 14 April 2016.
- ^Walsh, Michael J. K.; Coureas, Nicholas; Edbury, Peter W., eds. (2012). Medieval and Renaissance Famagusta: Studies in Architecture, Art and History. Ashgate Publishing, Ltd. p. 191. ISBN9781409435570.
Bibliography[edit]
- Toy, Sidney. (1985) Castles: Their Construction and History.ISBN978-0-486-24898-1.
Retaining walls are relatively rigid walls used for supporting the soil mass laterally so that the soil can be retained at different levels on the two sides.Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to (typically a steep, near-vertical or vertical slope). They are used to bound soils between two different elevations often in areas of terrain possessing undesirable slopes or in areas where the landscape needs to be shaped severely and engineered for more specific purposes like hillside farming or roadway overpasses. A retaining wall that retains soil on the backside and water on the frontside is called a seawall or a bulkhead.
- 2Types
- 3Alternative retaining techniques
- 3.2Soil-strengthened
Definition[edit]
A retaining wall is a structure designed and constructed to resist the lateral pressure of soil, when there is a desired change in ground elevation that exceeds the angle of repose of the soil.[1]
A basement wall is thus one kind of retaining wall. But the term usually refers to a cantilever retaining wall, which is a freestanding structure without lateral support at its top.[2] These are cantilevered from a footing and rise above the grade on one side to retain a higher level grade on the opposite side. The walls must resist the lateral pressures generated by loose soils or, in some cases, water pressures.[3]
Every retaining wall supports a 'wedge' of soil. The wedge is defined as the soil which extends beyond the failure plane of the soil type present at the wall site, and can be calculated once the soil friction angle is known. As the setback of the wall increases, the size of the sliding wedge is reduced. This reduction lowers the pressure on the retaining wall.[4]
The most important consideration in proper design and installation of retaining walls is to recognize and counteract the tendency of the retained material to move downslope due to gravity. This creates lateral earth pressure behind the wall which depends on the angle of internal friction (phi) and the cohesive strength (c) of the retained material, as well as the direction and magnitude of movement the retaining structure undergoes.
![Medieval Engineers Landscape Stakes Vertical Soil Walls Medieval Engineers Landscape Stakes Vertical Soil Walls](https://www.wikihow.com/images/1/1d/Build-a-Cinder-Block-Wall-Step-23.jpg)
Lateral earth pressures are zero at the top of the wall and – in homogenous ground – increase proportionally to a maximum value at the lowest depth. Earth pressures will push the wall forward or overturn it if not properly addressed. Also, any groundwater behind the wall that is not dissipated by a drainage system causes hydrostatic pressure on the wall. The total pressure or thrust may be assumed to act at one-third from the lowest depth for lengthwise stretches of uniform height.[5]
It is important to have proper drainage behind the wall in order to limit the pressure to the wall's design value. Drainage materials will reduce or eliminate the hydrostatic pressure and improve the stability of the material behind the wall. Drystone retaining walls are normally self-draining.
As an example, the International Building Code requires retaining walls to be designed to ensure stability against overturning, sliding, excessive foundation pressure and water uplift; and that they be designed for a safety factor of 1.5 against lateral sliding and overturning.[6]
Types[edit]
Gravity[edit]
Gravity walls depend on their mass (stone, concrete or other heavy material) to resist pressure from behind and may have a 'batter' setback to improve stability by leaning back toward the retained soil. For short landscaping walls, they are often made from mortarless stone or segmental concrete units (masonry units).[7] Dry-stacked gravity walls are somewhat flexible and do not require a rigid footing.
Earlier in the 20th century, taller retaining walls were often gravity walls made from large masses of concrete or stone. Today, taller retaining walls are increasingly built as composite gravity walls such as: geosynthetics such as geocell cellular confinement earth retention or with precast facing; gabions (stacked steel wire baskets filled with rocks); crib walls (cells built up log cabin style from precast concrete or timber and filled with granular material).[8]
Cantilevered[edit]
Cantilevered retaining walls are made from an internal stem of steel-reinforced, cast-in-place concrete or mortared masonry (often in the shape of an inverted T). These walls cantilever loads (like a beam) to a large, structural footing, converting horizontal pressures from behind the wall to vertical pressures on the ground below. Sometimes cantilevered walls are buttressed on the front, or include a counterfort on the back, to improve their strength resisting high loads. Buttresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below seasonal frost depth. This type of wall uses much less material than a traditional gravity wall.
Sheet piling[edit]
Sheet pile retaining walls are usually used in soft soil and tight spaces. Sheet pile walls are driven into the ground and are composed of a variety of material including steel, vinyl, aluminum, fiberglass or wood planks. For a quick estimate the material is usually driven 1/3 above ground, 2/3 below ground, but this may be altered depending on the environment. Taller sheet pile walls will need a tie-back anchor, or 'dead-man' placed in the soil a distance behind the face of the wall, that is tied to the wall, usually by a cable or a rod. Anchors are then placed behind the potential failure plane in the soil.
Bored pile[edit]
Bored pile retaining walls are built by assembling a sequence of bored piles, proceeded by excavating away the excess soil. Depending on the project, the bored pile retaining wall may include a series of earth anchors, reinforcing beams, soil improvement operations and shotcrete reinforcement layer.This construction technique tends to be employed in scenarios where sheet piling is a valid construction solution, but where the vibration or noise levels generated by a pile driver are not acceptable.
Anchored[edit]
An anchored retaining wall can be constructed in any of the aforementioned styles but also includes additional strength using cables or other stays anchored in the rock or soil behind it. Usually driven into the material with boring, anchors are then expanded at the end of the cable, either by mechanical means or often by injecting pressurized concrete, which expands to form a bulb in the soil. Technically complex, this method is very useful where high loads are expected, or where the wall itself has to be slender and would otherwise be too weak. Soil-nailed walls (soil reinforced in place with steel and concrete rods).
Alternative retaining techniques[edit]
Soil nailing[edit]
Soil nailing is a technique in which soil slopes, excavations or retaining walls are reinforced by the insertion of relatively slender elements – normally steel reinforcing bars. The bars are usually installed into a pre-drilled hole and then grouted into place or drilled and grouted simultaneously. They are usually installed untensioned at a slight downward inclination. A rigid or flexible facing (often sprayed concrete) or isolated soil nail heads may be used at the surface.
Soil-strengthened[edit]
A number of systems exist that do not consist of just the wall, but reduce the earth pressure acting directly on the wall. These are usually used in combination with one of the other wall types, though some may only use it as facing, i.e., for visual purposes.
Gabion meshes[edit]
This type of soil strengthening, often also used without an outside wall, consists of wiremesh 'boxes', which are filled with roughly cut stone or other material. The mesh cages reduce some internal movement and forces, and also reduce erosive forces. Gabion walls are free-draining retaining structures and as such are often built in locations where ground water is present. However, management and control of the ground water in and around all retaining walls is important.
Mechanical stabilization[edit]
Mechanically stabilized earth, also called MSE, is soil constructed with artificial reinforcing via layered horizontal mats (geosynthetics) fixed at their ends. These mats provide added internal shear resistance beyond that of simple gravity wall structures. Other options include steel straps, also layered. This type of soil strengthening usually needs outer facing walls (S.R.W.'s – Segmental Retaining Walls) to affix the layers to and vice versa.[9]
The wall face is often of precast concrete units[7] that can tolerate some differential movement. The reinforced soil's mass, along with the facing, then acts as an improved gravity wall. The reinforced mass must be built large enough to retain the pressures from the soil behind it. Gravity walls usually must be a minimum of 50 to 60 percent as deep or thick as the height of the wall, and may have to be larger if there is a slope or surcharge on the wall.
Cellular confinement systems (geocells) are also used for steep earth stabilization in gravity and reinforced retaining walls with geogrids. Geocell retaining walls are structurally stable under self- weight and externally imposed loads, while the flexibility of the structure offers very high seismic resistance.[10] The outer fascia cells of the wall can be planted with vegetation to create a green wall.
See also[edit]
References[edit]
- ^Ching, F. D., Faia., R., S., & Winkel, P. (2006). Building Codes Illustrated: A Guide to Understanding the 2006 International Building Code (2 ed.). New York, NY: Wiley.
- ^Ambrose, J. (1991). Simplified Design of Masonry Structures. New York: John Wiley and Sons, Inc. pp. 70–75. ISBN0471179884.
- ^Crosbie, M. & Watson, D. (Eds.). (2005). Time-Saver Standards for Architectural Design. New York, NY: McGraw-Hill.
- ^(2011) Commercial Installation Manual for Allan Block Retaining Walls (p. 13)
- ^Terzaghi, K. (1934). Large Retaining Wall Tests. Engineering News Record Feb. 1, March 8, April 19.
- ^2006 International Building Code Section 1806.1.
- ^ ab'Segmental Retaining Walls'. National Concrete Masonry Association. Archived from the original on 2008-03-04. Retrieved 2008-03-24.
- ^Terzaghi, K. (1943). Theoretical Soil Mechanics. New York: John Wiley and Sons.
- ^JPG image. geostone.com
- ^Leshchinsky, D. (2009). 'Research and Innovation: Seismic Performance of Various Geocell Earth-retention Systems'. Geosysnthetics. 27 (4): 46–52.
Further reading[edit]
Wikimedia Commons has media related to Retaining walls. |
- Bowles, J.,(1968). Foundation Analysis and Design, McGraw-Hill Book Company, New York
- Ching, F. D., Faia., R., S., & Winkel, P. (2006). Building Codes Illustrated: A Guide to Understanding the 2006 International
- Crosbie, M. & Watson, D. (Eds.). (2005). Time-Saver Standards for Architectural Design. New York, NY: McGraw-Hill.