YONAS SYABAB'S INFORMATION CENTER

The construction of qanats is in the hands of experts (muqanni) and the secrets of the profession are largely handed down by word of mouth from father to son. The termination of the qanat, either farmland to be irrigated or a community to be provided with potable water, or both, will be known in advance, as will the general location of likely aquifers. One of the main skills of the muqanni lies in determining, by examining the alluvial fans for traces of seepage and hardly noticeable changes in vegetation, precisely where the trial well is to be dug. When the excavators reach the impermeable layer the well is left for a few days while the muqanni estimates the potential yield of the well by hoisting up measured quantities of water and at the same time observing any fall in the water level. If necessary, further wells are sunk to ensure that genuine groundwater has been reached; the shaft with the highest yield is chosen as the ‘mother well’. The next step is for the surveyor – or senior muqanni – to determine the route, gradient and precise outlet of the qanat. The route will be selected according to considerations of terrain and, in some cases, questions of ownership. To start the survey, a long rope is let down into the mother well until it touches the surface of the water, and a mark is made on it at ground level. The surveyor then selects a spot on the route 30 to 50 yards from the mother well for the first ventilation shaft. A staff is held on this spot by a labourer, and the surveyor measures the fall with a level. Nowadays a modern surveying level may be used but in earlier times one of the instruments described in the section below on surveying was used. A second mark is made on the rope coinciding with the measurement on the staff; the distance from this mark to the lower end of the rope will be the depth of the first ventilation shaft. He continues to level in this way along the route, marking the rope at the location of each shaft, until he reaches the end of the rope. He has then reached a point on the ground at the same level as the surface of the water in the mother well. For the mouth of theqanat he now chooses a place below this level, but higher than the fields, and divides the drop from the level point to the mouth by the number of proposed ventilation shafts and adds this amount to the previously surveyed depth of each shaft. In this way be determines the gradient of the conduit, which is usually from 1 : 1000 to 1 : 500.

Fig. 1 – The qanat

Dams are required in most hydraulic systems, whatever their purpose, but the functions of dams vary. In wadi irrigation, as we have seen, they are used to trap the floodwaters that result from heavy but infrequent downpours, so that the water-level is raised above that of the surrounding fields, to which it can then be conducted under gravity. For perennial irrigation dams are used to divert water from streams or rivers into the canal network. The impounding of rivers behind dams gives more control over the supply throughout the year. As with wadi irrigation, it also allows the water in the reservoir to be gravity-fed into irrigation and town supply systems. A further advantage, if the water is to be used for hydro-power, is that there is a high, fairly constant head of water, which would not be the case if the river were unregulated.

There are two basic types of dam – gravity and arch. The first, as its name implies, relies upon the weight of the dam to withstand the pressure of the water. For additional strength, buttresses are sometimes added to the downstream wall. As with all hydraulic structures, good foundations are of the utmost importance, since failure can occur if the scouring action of the water undermines the foundations. Arch dams are designed to resist the force of the water and silt by horizontal arch action and are adaptable only to those sites where the length is small in comparison to the height and the sides of the valley are composed of good rock to resist the arch thrust at the haunches. With very rare exceptions true arch dams were not built before modern times.

The selection of the materials of construction was influenced partly by the design of the dam and partly by availability. Earth dams were common and are still in widespread use today. They are perfectly satisfactory for certain kinds of service, provided they have a core of clay or other impermeable material and plenty of overflow capacity, but they are not really suitable for high dams. In certain areas, notably lower Iraq, earth dams were almost universal; they were (and are) quite adequate for diverting rivers into canal systems and, in any case, the cost of transporting large quantities of stone would have been prohibitive. In other areas, where higher dams were needed, some form of masonry construction was necessary. This could be of dressed stone, mortared or not, random rubble or concrete. Quite frequently, dams were constructed by building two masonry walls with a gap between them, and then filling the gap with cheaper material such as earth or rubble. If the dam was designed to discharge overflow water from its crest, this had to be of stone or concrete, since earth would quickly have been worn away by the action of the water.

Roman and Sasanid dams were carefully maintained, but the demands for irrigation water and power was so great that in the more populous provinces dams became more numerous than they had been in pre-Islamic times. Many new dams were necessary as part of the extensions to the hydraulic systems in Iraq.

The Buwayhids held the real power in Iraq and Iran from 320/932 until 454/1062, and the greatest builder of the dynasty was ‘Adud al-Dawla. Among his works was the impressive dam called the Band-i Amir, built about 349/960 over the River Kurr in the province of Fars between Shiraz and Istakhr. This dam was seen by the geographer al-Muqaddasi shortly after it was constructed. He speaks of it as follows:

Fig 2 – Mestella dam: desilting sluices

Bridges

Suspension bridges with cables made of woven bamboo strips were used in China no later than the first century B.C.E. but there is no record, of its use in western Islam or Europe before the Renaissance. This does not mean that they were not used and indeed it would be strange if this simple and effective method of crossing ravines was unknown in the Zagros, the Taurus and the mountainous regions of Spain and North Africa. Similarly, we have no firm evidence for the use of cantilever bridges in Islam, apart from Afghanistan, where they had been built from, at the latest, the fifth century C.E. onwards. These are also an excellent method of crossing ravines in hilly country, but being made of timber they do not have a long life nor do they leave traces, particularly since in many cases modern bridges may have been built at their sites. Usually a timber substructure is built into a masonry abutment on either bank, and the longitudinal and transverse beams to carry the roadway extend from the top of this supporting structure. At the centre the two cantilever spans support a short beam section (see Figure 3). Large modern steel bridges, such as the Forth railway bridge in Scotland, are built on exactly the same principle. In the fourth/tenth century a bridge over the River Tab in Iran was described briefly by Ibn Hawqal. He says that the river was crossed by a wooden bridge “suspended between the sky and the water, its height above the water about 10 cubits”. He may, of course, have seen a suspension bridge, but the cantilever type seems more likely, especially since he makes no mention of ropes.

Fig. 3 –Types of bridges

There are frequent references to pontoon bridges in the works of Arabic writers. They were very common in Iraq for crossing the two rivers and the major irrigation canals. In the fourth/tenth century there were two pontoon bridges over the Tigris at Baghdad, but only one was in use; the other, having fallen into disrepair, was closed, because few people used it. Ibn Jubayr, writing towards the close of the sixth/twelfth century, described a bridge of large boats over the Euphrates at Hilla. It had chains on either side “like twisted rods” which were secured to wooden anchorages on the banks. He also mentions a similar, but larger, bridge over a canal near Baghdad. There were also pontoon bridges on the rivers of Khuzistan, the Iranian province adjoining Iraq, and on the Helmand river in Sijistan (now western Afghanistan). There seems to have been a pontoon bridge at Fustat (now Old Cairo) in Egypt for many years. In the early part of the fourth/tenth century, al-Istakhri says that one bridge crossed from the city to the island and a second bridge from the island to the far bank of the river. About two centuries later, al-Idrisi describes the same arrangement, adding that there were thirty boats in the first bridge and sixty in the second.

Before the introduction of modern materials the masonry arch provided the best solution for the spanning of watercourses and other obstacles. Although they are relatively expensive to build, well-constructed arch bridges can last for centuries and they do not interfere with river traffic to the same extent as pontoon bridges or the many piers of multiple-span beam bridges. Their durability is proved by the survival of many medieval bridges, intended only for the passage of people and animals, but now sustaining the full load of modern traffic.

Many fine buildings were constructed in the first Islamic century: the Dome of the Rock in Jerusalem, the Great Mosque in Damascus, the second Great Mosques in Kufa and Basra and the desert palaces of the Umayyads – to name only some of the more notable buildings. Once established, the tradition for fine architecture continued to flourish in Islam, as witnessed, for example, by al-Mansur’s Baghdad, the works of Ibn Tulun in Egypt, the Great Mosque of Cordoba and the Alhambra palace in Granada, the complex of splendid buildings in Isfahan and so on. Many well-illustrated books have been devoted to describing Islamic buildings and it is clearly beyond the scope of this chapter to attempt even a brief summary of the Muslim architectural achievement. Instead, we shall confine our attention to the most basic element in any building – the materials of construction.

The Muslim geographers usually tell us which type of material was used to construct a given town or city. This could be unfired bricks (labin or tub), fired bricks (ajurr), stone (hajar) or timber. In medieval times timber was more plentiful than it is today, but even so the practice of constructing the main structures of buildings in this material was not widespread. The city of Bukhara was mostly timber-built and the houses in the town of Siraf on the Gulf were made of teak, and timber was also widely used in parts of Spain. Perhaps the most important example of timber used as a structural material is the Dome of the Rock in Jerusalem, in which the dome itself consists of two independent wooden shells, the outer one covered with lead sheets. In general, however, timber was used in conjunction with other materials where some resistance to tensile stresses was necessary, as in lintels over doors and windows, and roof rafters.

The choice of material to be used in a particular building depends upon a number of factors: the availability of a material locally, cost, time and the purpose of the building. Thus it is noticeable that cut masonry was often preferred for religious buildings whereas other large buildings in the same region might be constructed from cheaper materials. Syria can be said to be the region, par excellence, for fine stonework in ashlar masonry, i.e. masonry in which each block is carefully cut to size, with straight edges and plane surfaces. This tradition has persisted in Syria to the present day; the local limestone weathers to a beautiful amber colour that is very pleasing to the eye. Ashlar work was also common in Spain (doubtless due to Syrian influence), Egypt and parts of North Africa. Sometimes money and time were saved by building the wall in random rubble and facing it with ashlar. The mortar was based upon either lime or gypsum mixed with soft sand.

The use of unfired bricks was common in early Antiquity and is still widespread today. The clay which is the main constituent of the bricks is readily available in many parts of the world and houses made from this material are warm in winter and cool in summer. Moreover, its use is not confined to the building of small dwelling houses. Some of the multi-storey houses in southern Arabia are made of unfired bricks, and they can also be used in vaults and domes. They cannot, however, be used in areas with high rainfall, for heavy rains cause severe deterioration to the walls to the point of making them disintegrate. The labinagenerally has a geometric, fairly regular shape, that of a parallel-sided rectangle, whose variable dimensions often have the ratio 4 x 2 x 1 (e.g. length 56 cm, width 28, thickness 14, or 36 x 18 x 9) but in South Arabia 45 x 35 x 5 is usual and in Iran 20 x 20 x 4. To prepare the mixture for the bricks, the loam or clay is thoroughly soaked, mixed with straw and chaff and trodden with the bare feet. It is then carried in baskets to the moulders. Each moulder has a wooden mould, just an open frame. He covers the ground with a thin layer of chaff, puts the moulding frame flat on the ground, throws a quantity of the mud-straw mix into the mould, beats it into the corners with his bare hands, and scrapes off any surplus with a small straight-edge. He lifts the frame with a swift movement, leaving the fresh brick on the ground, and places the frame next to the brick just made. Moulding row after row in this way, he makes about 250 bricks in an hour. Unfired bricks are jointed with a mortar made with a mixture of lime or ash and are usually lined with a coating of earth mixed with lime or plaster.

Burnt bricks were already made in the fourth millenium B.C.E in Babylonia and in Iran. kilns have been unearthed going back into the first millenium B.C.E. They are still in common use in many parts of the Muslim world. They are generally smaller than unfired bricks, and the preparation of the clay for them is more thorough – it has to be slaked and sieved to clean it of impurities, and additives are sometimes included, e.g. grey sand to give the bricks a whitish tinge. After moulding they are left in the open, lying fiat, for 24 hours, and then turned on edge and left to dry for another three days before being stacked in the kiln. The kiln is similar to that of the potters, and consists of a furnace with a firing room on top of it. Buildings made exclusively from burnt bricks are rare. More usually they are combined with other materials. The Qasr al-Hayr al-Gharbi in Syria, for example, built in the first Islamic century, consists of a wall of limestone, fired bricks and unfired bricks at the top. Burnt bricks were (and are) used for certain parts of buildings such as arches, vaults and staircases, and put to good use by architects to vary the decoration of their works. From the sixth/twelfth century, the glazed brick has offered the possibility of obtaining similar effects to those of mosaic.

Fig. 4

For setting out straight lines and for measuring distances ropes were used, with knots at intervals to mark the dimensions. A rotatable alidade, provided with sighting holes and mounted on a plane surface, could also be used for alignment. The astrolabe, which we shall meet in a later section as an instrument for astronomical observation and computation, was also widely used for land surveying. Here we are concerned only with the back of the instrument, which consisted of an alidade turning about a central pivot, its ends moving over a graduated circle, each quadrant of which was divided into 90 degrees. In the lower half of the face was a rectangle, one side of which was divided radially into decimals, the other side into duo-decimals (see Figure 5).

Fig. 5

The instrument was used for alignment and measuring the angles between two points, but a number of Arabic writers also describe the solution of various triangulation problems using the astrolabe. The two matching squares into which the rectangle is divided are used for this purpose. Although the squares are divided into ten and twelve equal parts respectively, the choice of number is purely a matter of convenience. With the astrolabe freely suspended the alidade is adjusted so that a distant object is viewed through the sights simultaneously. When this happens the real right-angled triangle formed by the distance of the object and its height is reproduced on a small scale within the confines of one square on the astrolabe, by an exactly similar right-angled triangle. The real and similar triangles have a common line for hypotenuse. The ratio of the lengths of the sides of the triangle on the astrolabe is the same as the ratio of the height and distance of the object, so that if either of these is known the other may readily be calculated. If neither is known, the observer reads the angle from one station, moves back a measured distance, and again reads the angle. Since the Muslims had perfected the methods of both plane and spherical trigonometry, problems of this sort could easily be solved. For the surveyor in the field, however, it was clearly preferable to use the constructive methods provided by the astrolabe, and manuals for his use were prepared by scientists. Other problems solved by the use of the astrolabe included finding the width of a river, or the distance between two points separated by an impassable obstruction.