My Science School

Much of the Great Wall standing today was built 300-600 years ago during the Ming Dynasty — mainly as a defence against the scattered armies of the Mongol Yuan Dynasty, who had fled beyond the wall after being over-thrown by the Ming. Much of it is a massive wall of stone and brick, at its most magnificent between Juyongguan Pass north of Beijing and Shanhaiguan Pass near the east coast.

The construction of each section was organised from one of H fortress towns. From Liaodong in the east to Zhangve in Gansu province in the west. The total garrison numbered nearly 1 million men and even more conscripted labourers. Soldiers guarding the wall wore identity tags, and orders were inscribed on discs, or tallies, for runners to carry between sub-commanders.

Where possible, bricks and lime for building were produced in kilns alongside the site. But they still had to be carried to the high ground — mostly by men, although some donkeys were used. Huge slabs of stone were also used, some weighing about a ton. How they were hauled to the heights no one knows. Some may have been hoisted with windlasses —rope wound round a drum with a cranked handle. Cornerstones were sometimes fixed in place with iron tenons, the molten iron being poured into cut-outs in the stones.

 Almost every section of the wall has an inscribed tablet naming the engineers and construction chiefs. But for the many men who died on the job, the wall itself is the only monument.

 

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General Meng Tian, in charge of constructing the wall for Qin Shi Huang, had 300,000 troops for the job. Sub-commanders were responsible for different sections, and all had to build with local materials. It took nine years to complete the task.

Nearly 1 million labourers were con-scripted, many torn from their wives and families. There were also convicts with shaven heads and iron collars, sentenced to years of hard labour on the wall. Some of the convicts were scholars who had disobeyed an imperial edict forbidding the use of books considered 'unsettling', and some were negligent civil servants. They had to work in difficult terrain in extremes of weather — temperatures plummeted from 95°F (35°C) in summer to —5°F (-21°C) in winter. They were pushed to exhaustion, and often left without food.

Although Meng Tian built a road as a supply route for garrisons and workers. Food often did not reach the outlying places — it was sold or eaten by the carriers on the way_ Thousands of men died. And were buried in the wall foundations_ Traditional poems and folk songs tell of their anguish, and name the wall as the longest graveyard in the world.

Although oxcarts or handcarts could be used on flat ground or gentle slopes, in mountain areas where the wall was built along the top of steep cliffs the building stones had to be carried on men's backs, or on baskets slung from a pole — loads of about 1cwt (50kg) per man. On narrow paths, loads were passed from hand to hand along a human chain. Large stones were manhandled by gangs, who rolled them on logs and pushed with levers to inch them up slopes.

In areas where there were no stones, the wall was made of layers of hammered earth, built up between boards supported by wooden posts. In the sandy areas of the Gobi Desert, it consisted of 8in (200mm) layers of sand and pebbles alternating with tin (50mm) layers of desert grass and tamarisk twigs tied in long bundles.

Because of the difficulty of transporting food, Qin Shi Huang began a policy of growing crops on wasteland beside the wall, a policy continued by successive dynasties who repaired or rebuilt the wall. Peasant farmers who resettled in the area doubled as farmers and militia, standing guard or fighting as required. The garrison soldiers were also allotted small plots.

Among the water conservancy schemes organized to irrigate the crops was the Han Qu Canal, fed from the Yellow River near Yinchuan in the central area of the wall. Farmer-soldiers were obliged to grow not only grain but fruit trees as well, so there were other crops to fall back on if one harvest failed.

 

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The island where Columbus landed in 1492 was called Guanahani by the Indians who lived there. Exactly which island it was has puzzled experts for many years.

Until the 1980s. Wading Island in the Bahamas renamed San Salvador in 1926, had long been the favourite — with three monuments marking the event. Then a group of experts began to re-examine the evidence. They plotted the route across the Atlantic, using computers to take account of wind speed, currents and magnetic variations.

Their conclusion was that Guanahani could only have been one of two places — Wading Island or Samana , miles (100km) to the south-west. They claim that Samana Cay’s landscape and surroundings fit Columbus's description better than San Salvador, but the controversy continues.

 

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In the mountains north of Beijing, the Great Wall of China winds snake-like from mountain crest for upon mile. This is the most spectacular part of the world’s longest man-made wall, which starts in the arid west at the Jiayuguan fortress at the foot of the Qilian Shan Mountains.

Crossing deserts, pastureland, river valleys and forested ridges, the wall reaches to the Yalu River on the Korean border in the east — a stretch of some 2000 miles (3200km) across northern China. But when branches from the main wall are included, its total length is around 4000 miles (6500km).

About 3000 years ago, the many warring states of northern China built defensive walls round their territories. In 221 BC, the Prince of Qin, one of the states, annexed six other states and called himself China's First Emperor. Qin Shi Huang. He destroyed all but the northern walls, and linked them to protect his territory from the Huns and other nomadic northern tribes.

In the following centuries, other rulers rebuilt and extended the wall, particularly those of the Han Dynasty (206 BC-AD 220) and the Ming Dynasty (AD 1368-1644).

 

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Like most tunnels, the Anglo-French Channel Tunnel is being drilled from both ends. But if one of the tunneling machines digging it should stray even one degree off course, after 15 miles (24km) it would miss the other half of the tunnel by over a quarter of a mile (400m).

To keep the machines on line a laser guidance system has been installed at both ends of the Channel Tunnel. A thin pencil of light shines down the tunnel hitting the target on the back of the boring machine. The guidance system has a computer that is designed to measure how far the machine has gone, whether it has moved up or down and whether it has rolled or turned to the right or left.

All tunneling machines, or 'moles', have a tendency to deviate in one direction or another. As soon as the computer detects any deviation from the correct line it automatically steers the machine back on course. It sends signals to the steering `shoes' which are hydraulic pads that control the machine's direction by pushing off the ends of the tunnel lining. The signals sent to each shoe will adjust the direction of the mole so that the tunnel can be built in a curve or on rising or falling gradients, or straight, as the engineers require.

Since the laser can only operate in a straight line it has to be moved to a new site as the tunnel changes direction. The laser has to be positioned with great accuracy — any mistake will mean that it could be guiding the mole in the wrong direction. Usually the laser is positioned near the top of the tunnel where it is less likely to be disrupted by the clouds of chalk dust caused by tunneling.

 In the past engineers have had to rely on conventional surveying methods. Theodolites are still used to measure horizontal and vertical angles. They are like a small telescope mounted on a secure base. The surveyor looks through the theodolites and focuses on a target farther down the tunnel. He can then calculate by how much the tunnel is rising or falling, or veering to the left or right, and make the appropriate corrections.

 One of the most convincing demonstrations of the effectiveness of these traditional methods was the two tunnels bored by the Canadian Pacific Railroad through the Rockies between 1907 and 1909.

The purpose of the two tunnels was to bypass the steepest stretch of track in North America, the 'Big Hills between Kicking Horse Pass and Mount Stephen, just west of the border between Alberta and British Columbia. It used to take tour engines to haul a train up. Coming down was even worse. Men were posted along the line ready to operate points to divert the trains travelling too fast up specially built spurs running steeply uphill.

 The hill was bypassed by drilling two spiral tunnels with low gradients into the mountains. Both curved steadily round, rising slowly for more than 3000ft (914m), and both were built by drilling from each end. When the two halves of the first tunnel met they were only 2in (50mm) out.

 

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Tunneling underwater was considered impossible until the French-born engineer, Marc Brunel and his son, Isambard, built a tunnel beneath the Thames between Rotherhithe and Wapping in 1841. In the nine years it took to complete, it was flooded five times, killing at least 12 men. Water seeped through the riverbed and poured into the tunnel when the roof gave way.

 The Brunels were successful only after they invented the shield, which was a 1 movable framework that supported the tunnel face and the earth immediately behind it. The workers would excavate a few feet of tunnel, then jack the shield forward against the new face. The newly exposed part of the tunnel was lined with bricks and the process would be repeated until the tunnel was finished.

Lining a tunnel right up to the shield meant that, in theory at least, the only place where a leak could occur was at the face. And shields were designed so they could be made watertight and stem a potential flood. In practice, there were still fatalities.

In 1908 the first attempt to drive the Lotschberg Tunnel through the Swiss Alps, under the Kander River, proved disastrous. Geologists had predicted that the tunnel would be going through safe bedrock, but the roof collapsed near the face and the tunnel quickly filled with soil and water, killing 25 men. The tunnel was rerouted and successfully completed upstream where the bedrock proved sounder.

Underwater tunneling is still one of the most difficult and dangerous feats of engineering. The problems vary enormously depending on the nature of the ground. Water only trickled into the English end of the Channel Tunnel, because the ground is virtually impervious to water, but at the French end engineers had to contend with the full sea pressure.

Various techniques have been used to reduce the risks of flooding. By filling the tunnel with compressed air, the flow of water can be held back until there is time to erect the lining lithe pressure inside the tunnel equals the pressure of water outside, no water can get in — just like a diving bell. This method was used for many of the tunnels built in the 19th century, but it has drawbacks. A large compressor plant is needed with standby equipment to insure

against a loss of pressure. It can also give workers 'the bends' when they emerge into normal atmospheric pressure.

At a pressure of three atmospheres (441b per square inch) a man can work only one hour a day and has to spend six hours in a decompression chamber. All the tools and spoil have to be passed through a complex system of air locks.

 High-pressure tunneling is avoided whenever possible and other techniques are used. The ground ahead of a tunnel is often strengthened before digging by injecting it with a cement-like compound called grout. Grouting also makes the ground more resistant to water by filling and sealing small fissures.

The biggest aids to modern tunneling are moles — massive machines that not only burrow through the ground but also provide protection for workers by acting as a shield. Moles can also trail an erector arm that lifts heavy precast concrete segments which are used to line most tunnels.

 

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When divers cut through parts of British Petroleum's Magnus platform in one of the deepest parts of the North Sea, they were further in time from the Earth's surface than astronauts on the Moon. They were carrying out repairs at depths up to 660ft (200m), and to avoid getting the 'bends' they had to spend more than eight days in decompression, before they were able to come to the surface.

Underwater cutting is often necessary for building or repairing oil platforms or pipelines. For example, a damaged pipe-line has to be cleanly cut before a new section can be welded to it, otherwise the weld would not hold.

There are two basic methods for cutting metal underwater; one is foxy-arc cutting, the other uses explosives. Oxy-arc cutting is a similar process to arc welding except that a supply of oxygen and a stronger electrical current are used to provide enough heat to cut right through the metal rather than just melt it.

For cutting through large pieces of metal, such as the leg of an oil platform

which is being dismantled, the oxy-arc method is used to make grooves in the metal into which plastic explosives are placed. As long as the right amount of explosive is used and it is positioned correctly, there should be no damage to the leg apart from a clean cut.

 

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Welding is the fastest and cheapest way of making a repair underwater. In the past, damaged oil platforms or pipelines had to be brought to the surface before they could be mended. Now divers can weld at depths up to 1000ft (300m), but it can be dangerous and difficult.

Welding works by generating heat intense enough to fuse metals together. There are several ways of providing the heat to melt the metal, but only one of them — electric arc welding — can be used underwater.

By using a strong electric current an electrical discharge, or arc, can be made to cross a small gap between two electrodes. In the case of welding, the charge travels from an electrode at the end of a cable to the metal which needs repairing and in effect is the other electrode. The heat produced by the charge has to be sufficient to melt the metal.

People have known that welding underwater was possible since 1802 when the British scientist Sir Humphry- Davy discovered that an electric charge would arc underwater. But although the method was used to temporarily repair ships in the Second World War, it has only been widely used since the 1970s, when an increasing number of underwater repairs needed to be carried out on oil platforms and pipelines.

There are two different sorts of underwater welding. The simplest method is wet water welding, for which a generator on the surface provides a large current of about 500amps which is fed with through an insulated cable to the diver who does the welding. To prevent the current leaking into the water and becoming weaker, the electrodes are coated with wax or waterproof paint.

The major problem with wet water cools the metal being welded very quickly, which makes the weld hard but also very brittle. Oxygen with hydrogen gas, produced from, heat of the weld, penetrate the weld when it is hot, which can weaken it. Another limitation of wet water is that it cannot be carried out at depths of more than 300ft (90m), because the pressure of the water becomes too great for the electric charge to arc.

The second method of underwater welding — dry chamber welding — can be used at greater depths and also produces better quality welds, but it is expensive.

 First, the area that needs repairing is surrounded with a tough, transparent plastic jacket or box. Then the water is displaced by blowing compressed gas into the box so that the area of metal around the weld is dry.

The bottom of the chamber is open, so the diver can reach inside to use the welding torch. The gas trapped inside the box prevents the water getting in. Divers sometimes have difficulty seeing how their work is progressing because smoke and steam generated by the weld can obscure their view.

Even more ambitious welds can be carried out by surrounding the whole area with a high-pressure chamber large enough to accommodate the diver as well. This produces better results, comparable with welds above the surface, but it is expensive because the chambers usually have to be specially designed to fit over the area of pipeline or joint which needs repair and costly seals have to be made. At least one support ship is needed, together with a barge crane to raise and lower the chamber.

Some of the largest chambers have room for several divers to work and even rest between shifts. They have been used at depths of 1000ft (300m) or more.

With oil companies drilling deeper and deeper, there is a need for underwater welding at anything up to 2000ft (600m). Engineers believe this will soon be possible using the dry chamber technique. But for welding at such great depths it is likely that divers will be replaced by remote-con-trolled robots.

 

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Modern earth-moving equipment has made it easier to move large amounts of material. Making an embankment of mil-lions of tons of rock or earth is often cheaper than building a slender arch of concrete. In 1980 the world's tallest embankment dam, the Nourek Dam, was completed on the Vakhsh River in Tajikistan, USSR. It is 1040ft (317m) high and contains enough Fill to make a cube taller than the Empire State Building.

Arch-gravity dam The Hoover Dam on the Colorado River was completed in 1936. It is 577ft (176m) high and relies on its weight (gravity) and its shape (an arch) to hold 38 billion tons of water.

Embankment dams come in two types: the earth-filled dam (also called an earthen dam or terrain dam) made of compacted earth, and the rock-filled dam. A cross-section of an embankment dam shows a shape like a bank, or hill. Most have a central section or core composed of an impermeable material to stop water from seeping through the dam. The core can be of clay, concrete, or asphalt concrete. This dam type is a good choice for sites with wide valleys. They can be built on hard rock or softer soils.

 

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The design which uses least concrete of all is the overhung or doubly curved arch dam, which uses a three-dimensional arch like part of a dome. The Coolidge Darn built in Arizona in 1929 is an example of a `multiple dome' — three part-domes with buttresses in between.

The whole dam is looks like shell type so, it is also called as shell arch dam.

Because of double curvature, the thickness of arch wall is reduced. But this non-vertical type dams are difficult to construct compared to other three types. They also require stronger foundations. Shell arch dams are more suitable for very narrow valleys.

 

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At sites with steep valleys of strong rock, even greater savings of material can be made by using arch dams, which transfer the pressure of the water to the sides of the alley. Cabora Bassa Dam, completed in 1975, on the Zambezi River in Mozambique, is 525ft (160m) high and used just over a million tons of concrete, which is only about a quarter of the amount needed for a gravity dam in the same position.

Most often, the arch dam is made of concrete and placed in a "V"-shaped valley. The foundation or abutments for an arch dam must be very stable and proportionate to the concrete. There are two basic designs for an arch dam: constant-radius dams, which have constant radius of curvature, and variable-radius dams, which have both upstream and downstream curves that systematically decrease in radius below the crest. A dam that is double-curved in both its horizontal and vertical planes may be called a dome dam. Arch dams with more than one contiguous arch or plane are described as multiple-arch dams.

 

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Another way of economizing on the amount of concrete used is to build the dam as a series of connected buttresses, like the `right-wing dam' at ltaipu. To build solid gravity dams at Itaipu, instead of the hollow and buttress dams, half again as much concrete would have been needed.

Existing dams can also be modified to possess qualities of the buttress dams. An example of this modification would be the Butt Valley dam in Plumas county, California. The dam is located within 0.5 km (0.31 mi) of the Butt Valley Fault zone and within 7 km (4.3 mi) of the Lake Almanor Fault. The location of the dam makes it highly susceptible to seismic failure leading engineers to reinforce it with rock fill buttresses. 

Another example of applying buttresses would be East River from Astoria to the Bronx, where it was necessary to construct a concrete blanket to resist the upward thrust of water under considerable pressure. There was insufficient room for a blanket that would provide the required resistance due to its weight, so the concrete was laid in the form of an inverted arch.

 

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The weight of a gravity dam can be used more effectively by changing its shape and leaving hollow spaces inside its walls. This can save up to a third of the concrete used in a normal gravity dam. The main dam at Itaipu (overleaf) is a hollow gravity dam.

The name "Itaipu" was taken from an isle that existed near the construction site. In the Guarani language, Itaipu means "the sounding stone". The Itaipu Dam's hydroelectric power plant produced the most energy of any in the world as of 2016, setting a new world record of 103,098,366 megawatt hours (MWh), and surpassed the Three Gorges Dam plant in energy production in 2015 and 2016.

The world's largest waterfall by volume, the Guaíra Falls, was drowned by the newly formed Itaipu reservoir. The Brazilian government liquidated the Guaíra Falls National Park, and dynamited the submerged rock face where the falls had been, facilitating safer navigation, thus eliminating the possibility of restoring the falls in the future.

 

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