The Great Wall of China - Engineering of the Great Wall of China
Engineering such a long wall was no easy task, however, a single simple concept was used, that most sections of the Great Wall were built on strategic locations such as hill tops, making troop movement difficult. Since machinery were not available when the walls were built, the Great Wall was constructed using mainly human labor, more specifically, forced labor. Millions of troops, conscripts and criminals were used to during its construction. Owing to the arduous work and poor conditions both for living and construction, a lot of people died in the course of this project. Some of their remains were even buried in the wall. The materials used for different sections varies greatly and were dependent on what was available near the wall itself. Near Beijing the Great Wall is constructed from quarried limestone blocks. In other locations it may be quarried granite or fired brick. Where such materials are used, two finished walls are erected with packed earth and rubble fill placed in between with a final paving to form a single unit. In some areas the blocks were cemented with a mixture of sticky rice and egg whites. In the extreme western desert locations, where good materials are scarce, the wall was constructed from dirt rammed between rough wood tied together with woven mats. The wall consist mainly of 3 types of structures, namely Beacon towers for defence and solidier accomodation, Walls for fortification and Passes for allowing movements.
Beacon Towers
At strategic points along the Wall, square shaped beacon towers were constructed at different height, Making full advantage of hill tops and other high points along the wall for their location signal towers were built. The higher one is called blockhouse, which was used for defense as well as the living space for the soldiers. The complex signal towers would provide accommodation for soldiers as well as storage for ordinance and even stabling for livestock and horses. The low one is called wall platform, it is about the same height as the wall itself but is sticking out of the wall, there are crenels all around it, and it was the place for patrolling and standing sentry. Blocks of woods were set up at all commanding points, hence the name beacon tower. Whenever the enemy was sighted, bonfires were lit on the towers to signal warning messages. Other means of signalling included the use of flags, clappers, drums and bells.
Walls
The average height of the wall from the base upon which it was built was some 8 meters (26 feet) while it would be some 6.5 meters (21.3 feet) wide at the base tapering to 5.8 meters ( 19 feet) at the top. The material used for the wall itself varies greatly by the location and date. Sections of the Great Wall near Beijing are paved with three or four layers of regular shaped gigantic rocks, some of the slabs of rock reached 2 meters in length and weighed several hundred kilograms. The internal of the wall is filled up with soil and rocks, the ground is fully paved with square bricks and made extremely smooth. 1.7 meters high crenels were built externally, there are observation holes above and firing holes below, the internal wall was built with space walls. Sections of the wall in the remote western desert locations, where good materials are scarce and transportation almost impossible, the wall was constructed from dirt rammed between rough wood tied together with woven mats. The stone and brick construction permitted the builders to add a variety of refinements that gave the defending army cover and other advantages over their enemies. These included steps leading to the top of the ramparts where the outer side of the wall was crenelated to provide cover for archers. Openings enabled the guards to keep a look out for any threat of invasion. At regular intervals of approximately 200 to 300 meters (217 - 860 yards) platforms were set up. There were three different types designed to give the defenders a better viewpoint as well as being a vantage point from which to fire upon the enemy, especially if they were attempting to use scaling ladders.
Passes
There were occasions when it was necessary for troops to go out into the countryside beyond the wall and it was the gate within the passes that gave them access to the outside whether is was to mount a counter attack or merely go on patrol. As a result, passes were built at key positions, usually on trade routes. A straight forward gate would be vulnerable to attack and for this reason complex constructions were necessary. These could vary from a simple double wall to a virtual castle with a maze like format to enable the defending forces to control any attempt at an invasion. An important consideration in design was that the fortifications should always be such that a small number of defenders should have the ability to repel a much larger attacking force.
The ramparts were reinforced with huge bricks and stones with earth and crushed rock as a filler. Great bastions that measured some 10 meters (30 feet) in height and up to 5 meters in width at the top enhanced the protection afforded by the wall. Access to the top of the wall for both horses and men was provided by ramps and ladders within the pass. Outer parapets were battlemented to give cover for archers and a low wall approximately 1 meter high ran along the inner side for the safety of both men and horses.
The gate would be protected by enormous double timber doors secured with huge iron bolts and locking rings. Above each gate there was a tower. Typically these would be two or three storeys high to give the garrison the advantage of a lookout from which it was possible to see the approach of travellers or marauders who could be a threat. These towers were of timber or brick or both. What was known as a weng-cheng, a semicircular or polygonal construction was built outside the gate to provide cover against direct attack. At certain of the more important and therefore more vulnerable gates an additional fortification called a luo-cheng was constructed. This would have the benefit of a tower upon it and so could act as an additional lookout or post from which troops could be directed during any hostilities. The ditches formed by the excavation of soil to fill the walls would be extended to create a protective moat around the entrance to the gate providing a further device to slow down the progress of invaders and thereby giving the defending forces a greater opportunity to repel them.
Jayuguan Pass, located at the west of Jayuguan City, is the best preserved of all Great Wall passes, also famed as a key passageway of the Silk Road.
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