A bent in American English is a transverse rigid frame. Bents were a common way of making a timber frame; the term is used for the cross-ways support structures in a trestle. In British English this assembly is called a "cross frame"; the term bent is an archaic past tense of the verb to bind, referring to the way the timbers of a bent are joined together. The Dutch word is bint, the West Frisian is bynt, the German is bind. Compare this with the term bend for a class of knots. Bents are the building blocks that define the overall character of a structure, they do not have any sort of pre-defined configuration in the way. Rather, bents are cross-sectional templates of structural members, i.e. rafters, posts, etc. that repeat on parallel planes along the length of the structure. The term bent is not restricted to any particular material. Bents may be formed of wooden piles, timber framing, steel framing, or concrete. Bents are pre-assembled, either at the timber framing company's shop or at the construction site.
After the basic post and beam structure of the frame has been set in place, the bents are lifted and dropped into place one by one by the crane. Next, the workers bring in additional members, which tie them together and give the frame a more rigid structure; this process is safe and efficient, as it allows a crew to assemble a large portion of the frame without stepping off the ground. This, in turn, minimizes the amount of time that the crew must spend several stories in the air clambering along beams not much wider than their own feet. Barn raising Pike pole Gin pole Timber roof truss Tau Beta Pi — engineering society whose seal incorporates a bent Benson, Ted. Building the Timber Frame House: The Revival of a Forgotten Art. Simon and Schuster. ISBN 978-1-4391-0707-2. Glossary Useful terms in the timber framing trade. Blog Entry Some good pictures and a short narrative about raising bents
Pennsylvania the Commonwealth of Pennsylvania, is a state located in the northeastern and Mid-Atlantic regions of the United States. The Appalachian Mountains run through its middle; the Commonwealth is bordered by Delaware to the southeast, Maryland to the south, West Virginia to the southwest, Ohio to the west, Lake Erie and the Canadian province of Ontario to the northwest, New York to the north, New Jersey to the east. Pennsylvania is the 33rd-largest state by area, the 6th-most populous state according to the most recent official U. S. Census count in 2010, it is the 9th-most densely populated of the 50 states. Pennsylvania's two most populous cities are Philadelphia, Pittsburgh; the state capital and its 10th largest city is Harrisburg. Pennsylvania has 140 miles of waterfront along the Delaware Estuary; the state is one of the 13 original founding states of the United States. Part of Pennsylvania, together with the present State of Delaware, had earlier been organized as the Colony of New Sweden.
It was the second state to ratify the United States Constitution, on December 12, 1787. Independence Hall, where the United States Declaration of Independence and United States Constitution were drafted, is located in the state's largest city of Philadelphia. During the American Civil War, the Battle of Gettysburg was fought in the south central region of the state. Valley Forge near Philadelphia was General Washington's headquarters during the bitter winter of 1777–78. Pennsylvania is 170 miles north to south and 283 miles east to west. Of a total 46,055 square miles, 44,817 square miles are land, 490 square miles are inland waters, 749 square miles are waters in Lake Erie, it is the 33rd-largest state in the United States. Pennsylvania has 51 miles of coastline along Lake Erie and 57 miles of shoreline along the Delaware Estuary. Of the original Thirteen Colonies, Pennsylvania is the only state that does not border the Atlantic Ocean; the boundaries of the state are the Mason–Dixon line to the south, the Twelve-Mile Circle on the Pennsylvania-Delaware border, the Delaware River to the east, 80° 31' W to the west and the 42° N to the north, with the exception of a short segment on the western end, where a triangle extends north to Lake Erie.
Cities include Philadelphia, Reading and Lancaster in the southeast, Pittsburgh in the southwest, the tri-cities of Allentown and Easton in the central east. The northeast includes the former anthracite coal mining cities of Scranton, Wilkes-Barre and Hazleton. Erie is located in the northwest. State College serves the central region while Williamsport serves the commonwealth's north-central region as does Chambersburg the south-central region, with York and the state capital Harrisburg on the Susquehanna River in the east-central region of the Commonwealth and Altoona and Johnstown in the west-central region; the state has five geographical regions, namely the Allegheny Plateau and Valley, Atlantic Coastal Plain and the Erie Plain. New York Ontario Maryland Delaware West Virginia New Jersey Ohio Pennsylvania's diverse topography produces a variety of climates, though the entire state experiences cold winters and humid summers. Straddling two major zones, the majority of the state, with the exception of the southeastern corner, has a humid continental climate.
The southern portion of the state has a humid subtropical climate. The largest city, has some characteristics of the humid subtropical climate that covers much of Delaware and Maryland to the south. Summers are hot and humid. Moving toward the mountainous interior of the state, the winter climate becomes colder, the number of cloudy days increases, snowfall amounts are greater. Western areas of the state locations near Lake Erie, can receive over 100 inches of snowfall annually, the entire state receives plentiful precipitation throughout the year; the state may be subject to severe weather from spring through summer into fall. Tornadoes occur annually in the state, sometimes in large numbers, such as 30 recorded tornadoes in 2011; as of 1600, the tribes living in Pennsylvania were the Algonquian Lenape, the Iroquoian Susquehannock & Petun and the Siouan Monongahela Culture, who may have been the same as a little known tribe called the Calicua, or Cali. Other tribes who entered the region during the colonial era were the Trockwae, Saponi, Nanticoke, Conoy Piscataway, Iroquois Confederacy—possibly among others.
Other tribes, like the Erie, may have once held some land in Pennsylvania, but no longer did so by the year 1600. Both the Dutch and the English claimed both sides of the Delaware River as part of their colonial lands in America; the Dutch were the first to take possession. By June 3, 1631, the Dutch had begun settling the Delmarva Peninsula by establishing the Zwaanendael Colony on the site of present-day Lewes, Delaware. In 1638, Sweden established the New Sweden Colony, in the region of Fort Christina, on the site of present-day Wilmington, Delaware. New Sweden claimed and, for the most part, controlled the lower Delaware River region (parts of present-day Delaware, New Jersey, Pe
Prestressed concrete is a form of concrete used in construction that while under construction is "prestressed" in the areas that will be subjected to tensile forces while in service to strengthen it against these forces. This compression is produced by the tensioning of high-strength "tendons" located within or adjacent to the concrete volume and is done to improve the performance of the concrete in service. Tendons may consist of single wires, multi-wire strands or threaded bars that are most made from high-tensile steels, carbon fiber or aramid fiber; the essence of prestressed concrete is that once the initial compression has been applied, the resulting material has the characteristics of high-strength concrete when subject to any subsequent compression forces and of ductile high-strength steel when subject to tension forces. This can result in improved structural capacity and/or serviceability compared with conventionally reinforced concrete in many situations. In a prestressed concrete member, the internal stresses are introduced in a planned manner so that the stresses resulting from the superimposed loads are counteracted to the desired degree.
Prestressed concrete is used in a wide range of building and civil structures where its improved performance can allow for longer spans, reduced structural thicknesses, material savings compared with simple reinforced concrete. Typical applications include high-rise buildings, residential slabs, foundation systems and dam structures and tanks, industrial pavements and nuclear containment structures. First used in the late-nineteenth century, prestressed concrete has developed beyond pre-tensioning to include post-tensioning, which occurs after the concrete is cast. Tensioning systems may be classed as either monostrand, where each tendon's strand or wire is stressed individually, or multi-strand, where all strands or wires in a tendon are stressed simultaneously. Tendons may be located either wholly outside of it. While pre-tensioned concrete uses tendons directly bonded to the concrete, post-tensioned concrete can use either bonded or unbonded tendons. Pre-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned prior to the concrete being cast.
The concrete bonds to the tendons as it cures, following which the end-anchoring of the tendons is released, the tendon tension forces are transferred to the concrete as compression by static friction. Pre-tensioning is a common prefabrication technique, where the resulting concrete element is manufactured remotely from the final structure location and transported to site once cured, it requires stable end-anchorage points between which the tendons are stretched. These anchorages form the ends of a "casting bed" which may be many times the length of the concrete element being fabricated; this allows multiple elements to be constructed end-on-end in the one pre-tensioning operation, allowing significant productivity benefits and economies of scale to be realized for this method of construction. The amount of bond achievable between the freshly set concrete and the surface of the tendons is critical to the pre-tensioning process, as it determines when the tendon anchorages can be safely released.
Higher bond strength in early-age concrete allows more economical fabrication as it speeds production. To promote this, pre-tensioned tendons are composed of isolated single wires or strands, as this provides a greater surface area for bond action than bundled strand tendons. Unlike those of post-tensioned concrete, the tendons of pre-tensioned concrete elements form straight lines between end-anchorages. Where "profiled" or "harped" tendons are required, one or more intermediate deviators are located between the ends of the tendon to hold the tendon to the desired non-linear alignment during tensioning; such deviators act against substantial forces, hence require a robust casting bed foundation system. Straight tendons are used in "linear" precast elements such as shallow beams, hollow-core planks and slabs, whereas profiled tendons are more found in deeper precast bridge beams and girders. Pre-tensioned concrete is most used for the fabrication of structural beams, floor slabs, hollow-core planks, lintels, driven piles, water tanks and concrete pipes.
Post-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned after the surrounding concrete structure has been cast. The tendons are not placed in direct contact with the concrete, but are encapsulated within a protective sleeve or duct, either cast into the concrete structure or placed adjacent to it. At each end of a tendon is an anchorage assembly fixed to the surrounding concrete. Once the concrete has been cast and set, the tendons are tensioned by pulling the tendon ends through the anchorages while pressing against the concrete; the large forces required to tension the tendons result in a significant permanent compression being applied to the concrete once the tendon is "locked-off" at the anchorage. The method of locking the tendon-ends to the anchorage is dependent upon the tendon composition, with the most common systems being "button-head" anchoring, split-wedge anchoring, threaded anchoring. Tendon encapsulation systems are constructed from plastic or galvanised steel materials, are classified into two main types: those where the tendon element is subsequently bonded to the surrounding concrete by internal grouting of the duct after stressing.
Cornwall Railway viaducts
The Cornwall Railway company constructed a railway line between Plymouth and Truro, opening in 1859, extended it to Falmouth in 1863. The topography of Cornwall is such that the route, east-west, cuts across numerous deep river valleys that run north-south. At the time of construction of the line, money was in short supply due to the collapse in confidence following the railway mania, the company sought ways of reducing expenditure. On the advice of the Victorian railway engineer Isambard Kingdom Brunel, they constructed the river crossings in the form of wooden viaducts, 42 in total, consisting of timber deck spans supported by fans of timber bracing built on masonry piers; this unusual method of construction reduced the first cost of construction compared to an all-masonry structure, but at the cost of more expensive maintenance. Replacement of the timber viaducts by all-masonry structures began in the 1870s but a few remained in service until the 1930s; the Cornwall Railway linked Plymouth with Falmouth.
The section from Plymouth to Truro was opened on 4 May 1859, the remainder to Falmouth on 24 August 1863. Although the line had been designed by Brunel, this was after his death and the construction under the supervision of R P Brereton, it was built as a single-track 7 ft broad gauge line. The 70 miles of railway crossed deep valleys. Of these 43 were spanned by viaducts of various types built or from timber. Workshops were established at Lostwithiel where timber could arrive on barges to be preserved and cut to size; the offcuts from the timbers used for the viaducts and track were used for the construction of the railway's buildings. The choice of timber was made to keep initial costs down, but Brunel had warned that this meant more expensive maintenance—running to £10,000 annually. Replacement of the viaducts started in 1875 but led to a dispute in 1884 between the Cornwall Railway and the Great Western Railway, leasing the line; the lease precluded the conversion of the line to 4 ft 8 1⁄2 in standard gauge, the Cornwall Railway refused to pay for the widening of the viaducts during rebuilding sufficient to accommodate a double line of standard gauge tracks.
Following the amalgamation of the two companies on 1 July 1889 all the remaining viaducts on the main line to Truro were replaced. Most were either rebuilt in situ or by a replacement viaduct built alongside, in the latter case many of the original piers still remain today. Between Saltash and St Germans, a deviation line was built in 1908, eliminating the wooden viaducts on by-passed section of line; those on the Falmouth branch were all replaced between 1923 and 1934. The distinctive timber viaducts were constructed using yellow pine, preserved by Kyanising, or sometimes by Burnettising in the workshops at Lostwithiel; the various timbers were seated in cast iron chairs, wrought iron was used for parts of the bridge which would be in tension, such as tie rods between spans. Five distinct types of viaduct were built to suit local conditions at each site. Peter John Margary, the Cornwall Railway engineer from 1868 to 1891, classified them as classes A to E. Class A: The majority of viaducts were constructed on stone piers that rose to about 34 feet below track level.
From the tops of these radiated three fans of timber struts to support the beams beneath the track. The struts radiated at around 55, 75, 105 and 125 degrees from horizontal, although there were some slight variations; this gave piers spaced at about 65-foot centres. Class B: A stronger support was given to St Pinnock and Ponsanooth viaducts by replacing the central set of fan struts with a pair; these joined the outer fans at the top of the pier and met each other at the top, giving a W form when viewed across the width of the piers. Class C: The soft ground of the tidal valleys at Weston Mill, Forder and Nottar required a lighter structure; this was achieved by dispensing with the stone piers and replacing them with vertical timber trestles built on top of timber piles driven into the mud. There were no fans supporting the track, instead timber trusses were built directly on top of the timber trestles. Class D: Coombe and Moorswater viaducts had only two fans on each pier - one on each side - but the bottoms of these were joined by laminated timber beams between each set of fans, which created a continuous beam along the length of the viaduct above the top of the masonry piers.
Class E: The shallowest valleys at Grove, Draw Wood, Probus were crossed by simple trestles of three parallel fans with two struts each in a V shape. Brunel's designs allowed any defective timber to be replaced; the first decay occurred at the bottom of legs where they were seated in the cast iron chairs, around bolt holes. The other area with significant decay was underneath the decking that carried the track and ballast. After World War I it became difficult to obtain the preferred yellow pine and other timber with shorter life was used instead, although by this time only the viaducts on the Falmouth branch remained in use. Special gangs of men worked together on the viaduct maintenance. To reach timbers below the deck they worked from 2.5 inches manila ropes. It was possible to work on the viaduct without closing the viaduct to trains, but a temporary 10 miles per hour speed restriction was enforced until the replacement was properly in place; the actual movement of the larger timbers was carried out on a Sunday when there were few trains operating.
The United Kingdom the United Kingdom of Great Britain and Northern Ireland, sometimes referred to as Britain, is a sovereign country located off the north-western coast of the European mainland. The United Kingdom includes the island of Great Britain, the north-eastern part of the island of Ireland, many smaller islands. Northern Ireland is the only part of the United Kingdom that shares a land border with another sovereign state, the Republic of Ireland. Apart from this land border, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the south-west, giving it the 12th-longest coastline in the world; the Irish Sea lies between Great Ireland. With an area of 242,500 square kilometres, the United Kingdom is the 78th-largest sovereign state in the world, it is the 22nd-most populous country, with an estimated 66.0 million inhabitants in 2017. The UK is constitutional monarchy; the current monarch is Queen Elizabeth II, who has reigned since 1952, making her the longest-serving current head of state.
The United Kingdom's capital and largest city is London, a global city and financial centre with an urban area population of 10.3 million. Other major urban areas in the UK include Greater Manchester, the West Midlands and West Yorkshire conurbations, Greater Glasgow and the Liverpool Built-up Area; the United Kingdom consists of four constituent countries: England, Scotland and Northern Ireland. Their capitals are London, Edinburgh and Belfast, respectively. Apart from England, the countries have their own devolved governments, each with varying powers, but such power is delegated by the Parliament of the United Kingdom, which may enact laws unilaterally altering or abolishing devolution; the nearby Isle of Man, Bailiwick of Guernsey and Bailiwick of Jersey are not part of the UK, being Crown dependencies with the British Government responsible for defence and international representation. The medieval conquest and subsequent annexation of Wales by the Kingdom of England, followed by the union between England and Scotland in 1707 to form the Kingdom of Great Britain, the union in 1801 of Great Britain with the Kingdom of Ireland created the United Kingdom of Great Britain and Ireland.
Five-sixths of Ireland seceded from the UK in 1922, leaving the present formulation of the United Kingdom of Great Britain and Northern Ireland. There are fourteen British Overseas Territories, the remnants of the British Empire which, at its height in the 1920s, encompassed a quarter of the world's land mass and was the largest empire in history. British influence can be observed in the language and political systems of many of its former colonies; the United Kingdom is a developed country and has the world's fifth-largest economy by nominal GDP and ninth-largest economy by purchasing power parity. It has a high-income economy and has a high Human Development Index rating, ranking 14th in the world, it was the world's first industrialised country and the world's foremost power during the 19th and early 20th centuries. The UK remains a great power, with considerable economic, military and political influence internationally, it is sixth in military expenditure in the world. It has been a permanent member of the United Nations Security Council since its first session in 1946.
It has been a leading member state of the European Union and its predecessor, the European Economic Community, since 1973. The United Kingdom is a member of the Commonwealth of Nations, the Council of Europe, the G7, the G20, NATO, the Organisation for Economic Co-operation and Development and the World Trade Organization; the 1707 Acts of Union declared that the kingdoms of England and Scotland were "United into One Kingdom by the Name of Great Britain". The term "United Kingdom" has been used as a description for the former kingdom of Great Britain, although its official name from 1707 to 1800 was "Great Britain"; the Acts of Union 1800 united the kingdom of Great Britain and the kingdom of Ireland in 1801, forming the United Kingdom of Great Britain and Ireland. Following the partition of Ireland and the independence of the Irish Free State in 1922, which left Northern Ireland as the only part of the island of Ireland within the United Kingdom, the name was changed to the "United Kingdom of Great Britain and Northern Ireland".
Although the United Kingdom is a sovereign country, Scotland and Northern Ireland are widely referred to as countries. The UK Prime Minister's website has used the phrase "countries within a country" to describe the United Kingdom; some statistical summaries, such as those for the twelve NUTS 1 regions of the United Kingdom refer to Scotland and Northern Ireland as "regions". Northern Ireland is referred to as a "province". With regard to Northern Ireland, the descriptive name used "can be controversial, with the choice revealing one's political preferences"; the term "Great Britain" conventionally refers to the island of Great Britain, or politically to England and Wales in combination. However, it is sometimes used as a loose synonym for the United Kingdom as a whole; the term "Britain" is used both as a synonym for Great Britain, as a synonym for the United Kingdom. Usage is mixed, with the BBC preferring to use Britain as shorthand only for Great Britain and the UK Government, while accepting that both terms refer to the United K
Dolgellau is a market town and community in Gwynedd, north-west Wales, lying on the River Wnion, a tributary of the River Mawddach. It is traditionally the county town of the historic county of Merionethshire, which lost its administrative status when Gwynedd was created in 1974. Dolgellau is the main base for climbers of Cadair Idris; the site of Dolgellau was, in the pre-Roman Celtic period, part of the tribal lands of the Ordovices, who were conquered by the Romans in AD 77–78. Although a few Roman coins from the reigns of Emperors Hadrian and Trajan have been found near Dolgellau, the area is marshy and there is no evidence that it was settled during the Roman period. There are, three hill forts in the vicinity of Dolgellau, of uncertain origin. After the Romans left, the area came under the control of a series of Welsh chieftains, although Dolgellau was not inhabited until the late 11th or 12th century, when it was established as a "serf village" by Cadwgan ap Bleddyn — it remained a serf village until the reign of Henry Tudor.
A church was built in the 12th century, although Cymer Abbey, founded in 1198 in nearby Llanelltyd, remained the most important religious centre locally. Dolgellau gained in importance from this period onwards, was mentioned in the Survey of Merioneth ordered by Edward I. In 1404 it was the location of a council of chiefs under Owain Glyndŵr. After a visit by George Fox in 1657, many inhabitants of Dolgellau converted to Quakerism. Persecution led a large number of them to emigrate to Pennsylvania in 1686, under the leadership of Rowland Ellis, a local gentleman-farmer; the Pennsylvanian town of Bryn Mawr, home to a prestigious women's liberal arts college, is named after Ellis's farm near Dolgellau. The woollen industry was long of the greatest importance to the town's economy; the industry declined in the first half of the 19th century, owing to the introduction of mechanical looms. Another important contributor to the local economy was tanning, which continued into the 1980s in Dolgellau, though on a much reduced scale.
The town was the centre of a minor gold rush in the 19th century. At one time the local gold mines employed over 500 workers. Clogau St. David's mine in Bontddu and Gwynfynydd mine in Ganllwyd have supplied gold for many royal weddings. Dolgellau was the county town of Merionethshire until 1974 when, following the Local Government Act of 1972, it became the administrative centre of Meirionnydd, a district of the county of Gwynedd; this was abolished in 1996 by the Local Government Act 1994. Today, the economy of Dolgellau relies chiefly on tourism, it is believed that Dolgellau Cricket Club, founded in 1869 by Frederick Temple, is one of the oldest cricket clubs in Wales. For nearly a century Dolgellau was the home of Dr Williams School, a pioneering girls' secondary school; this was funded from the legacy of Daniel Williams the Welsh nonconformist of the 17th/18th century. The name of the town is of uncertain origin, although dôl is Welsh for "meadow" or "dale", gelli means "grove" or "spinney", is common locally in names for farms in sheltered nooks.
This would seem to be the most derivation, giving the translation "Grove Meadow". It has been suggested that the name could derive from the word cell, meaning "cell", translating therefore as "Meadow of cells", but this seems less considering the history of the name; the earliest recorded spelling is "Dolkelew", although a spelling "Dolgethley" dates from 1285. From until the 19th century, most spellings were along the lines of "Dôlgelly" "Dolgelley", "Dolgelly" or "Dolgelli". Thomas Pennant used the form "Dolgelleu" in his Tours of Wales, this was the form used in the Church Registers in 1723, although it never had much currency. In 1825 the Registers had "Dolgellau", which form Robert Vaughan of Hengwrt adopted in 1836. While this form may derive from a false etymology, it became standard in Welsh and is now the standard form in both Welsh and English, it was adopted as the official name by the local rural district council in 1958. Shortly before the closure of the town's railway station it displayed signs reading variously Dolgelly and Dolgellau.
Dolgellau is home to Coleg Meirion-Dwyfor. The site it occupies was home to Dr Williams' School, a direct grant grammar school for girls aged 7–18 established in 1875, it was named after its benefactor Dr Daniel Williams, a Nonconformist minister from Wrexham, who gave his name to Dr Williams's Library in Euston, London. The school closed in 1975. Dolgellau Grammar School, a boys' school, had been established in 1665 by the Rector of Dolgellau, Dr John Ellis, at Pen Bryn, before moving to its present site on the Welshpool road. In 1962, it became a comprehensive school under the name Ysgol y Gader, it has 310 pupils and, according to the latest inspection report by Estyn, it has a GCSE pass rate of 75%, which puts it in joint 11th place in Wales, makes it o
A causeway is a track, road or railway on top of an embankment across "a low, or wet place, or piece of water". It can be constructed of earth, wood, or concrete. One of the earliest known wooden causeways is the Sweet Track in the Somerset Levels, that dates from the Neolithic age. Timber causeways may be described as both boardwalks and bridges; when first used, the word appeared in a form such as "causey way" making clear its derivation from the earlier form "causey". This word seems to have come from the same source by two different routes, it derives from the Latin for heel and most comes from the trampling technique to consolidate earthworks. The construction of a causeway utilised earth, trodden upon to compact and harden it as much as possible, one layer at a time by slaves or flocks of sheep. Today, this work is done by machines; the same technique would have been used for road embankments, raised river banks, sea banks and fortification earthworks. The second derivation route is the hard, trodden surface of a path.
The name by this route came to be applied to a firmly-surfaced road. It is now little-used except in dialect and in the names of roads which were notable for their solidly-made surface; the 1911 Encyclopædia Britannica states "causey, a mound or dam, derived, through the Norman-French caucie, from the late Latin via calciata, a road stamped firm with the feet."The word is comparable in both meanings with the French chaussée, from a form of which it reached English by way of Norman French. The French adjective, chaussée, carries the meaning of having been given a hardened surface, is used to mean either paved or shod; as a noun chaussée is used on the one hand for a metalled carriageway, on the other for an embankment with or without a road. Other languages have a noun with similar dual meaning. In Welsh, it is sarn; the Welsh is relevant here, as it has a verb, meaning to trample. The trampling and ramming technique for consolidating earthworks was used in fortifications and there is a comparable, outmoded form of wall construction technique, used in such work and known as pisé, a word derived not from trampling but from ramming or tamping.
The Welsh word'cawsai' translates directly to the English word'causeway'. A transport corridor, carried instead on a series of arches approaching a bridge, is a viaduct; the distinction between the terms causeway and viaduct becomes blurred when flood-relief culverts are incorporated, though a causeway refers to a roadway supported by earth or stone, while a bridge supports a roadway between piers. Some low causeways across shore waters become inaccessible; the Aztec city-state of Tenochtitlan had causeways supporting aqueducts. One of the oldest engineered roads yet discovered is the Sweet Track in England. Built in 3807 or 3806 BC, the track was a walkway consisting of planks of oak laid end-to-end, supported by crossed pegs of ash and lime, driven into the underlying peat; the modern embankment may be constructed within a cofferdam: two parallel steel sheet pile or concrete retaining walls, anchored to each other with steel cables or rods. This construction may serve as a dyke that keeps two bodies of water apart, such as bodies with a different water level on each side, or with salt water on one side and fresh water on the other.
This may be the primary purpose of a structure, the road providing a hardened crest for the dike, slowing erosion in the event of an overflow. It provides access for maintenance as well as a public service. Notable causeways include those that connect Singapore and Malaysia and Saudi Arabia and Venice to the mainland, all of which carry roadways and railways. In the Netherlands there are a number of prominent dikes which double as causeways, including the Afsluitdijk and Markerwaarddijk. In Louisiana, two long bridges, called the Lake Pontchartrain Causeway, stretch across Lake Pontchartrain for 38 km, making them the world's longest bridges, they are the oldest causeways on the Gulf Coast that have never been put out of commission for an extended period of time following a Hurricane. In the Republic of Panama a causeway connects the islands of Perico and Naos to Panama City on the mainland, it serves as a breakwater for ships entering the Panama Canal. Causeways are common in Florida, where low bridges may connect several man-made islands with a much higher bridge in the middle so that taller boats may pass underneath safely.
Causeways are most used to connect the barrier islands with the mainland. The Churchill Barriers in Orkney are of the most notable sets of causeways in Europe. Constructed in waters up to 18 metres deep, the four barriers link five islands on the eastern side of the natural harbour at Scapa Flow, they were built during World War II as military defences for the harbour, on the orders of Winston Churchill. The Estrada do Istmo connecting the islands of Taipa and Coloane in Macau was built as a causeway; the sea on both sides of the causeway became shallower as a result of silting, mangroves began to conquer the area. Land reclamation took place on both sides of the road and the area has subsequently been named Cotai and b