June 30, 2012

Ancient Egyptian Pyramids at Dahshur Part 4

Even when compared with Zoser’s Step Pyramid the very much larger Red Pyramid loses by being less steep. Fakhry’s suggestion that the new form of the upper part of the Bent Pyramid ‘must have appealed to the architect’ somehow seems to lack conviction. As we have seen, the lowering of the angle of elevation was purely a safety precaution and it is likely that only for this reason was it chosen for the Red Pyramid. How keen the Egyptian architects were to erect again a steep, imposing building is shown by the fact that they returned at Giza to the original gradient of 4/TT and that they retained this shape for all future pyramids. On the other hand, they were fully aware that this steeper angle had caused the disaster at Meidum and that, in order to avoid a repetition, they would have to introduce structural modifications in any new venture of this kind. Our next step, therefore, must be to look for these modifications in the structure of the great Giza pyramids and find out what was done to overcome the defeatist attitude of the cautious builders we have seen at Dahshur.

Ancient Egyptian Pyramids
The new features introduced at Giza show, like the modifications of the Bent Pyramid, an amazing clarity in the analysis of the stability failure at Meidum. The stability of the Giza pyramids, which has preserved them structurally intact for more than four and a half thousand years, bears witness to the acute understanding of the basic physical and technological problems with which the builders of the Fourth Dynasty approached their task.

Since earlier and later stone pyramids relied on a basic core of buttress walls it is more than likely that the same design was used in the great Giza pyramids. Borchardt has drawn attention to the existence of ‘girdlestones’ in that part of the ascending passage of the Khufu pyramid which was cut through already existing masonry at the first alteration of the interior design. These are large vertical slabs through which the new corridor passes at intervals, and he has taken them as parts of internal buttress walls. This view has been disputed by Clarke and Engelbach, who have pointed out that it would be wholly fortuitous for the passage always to have encountered whole stones. They also maintain rightly, that the walls of this passage are made of fitted stones. Probably both sides are correct. The passage was evidently lined with new masonry and the girdlestones, while not being part of the original buttress walls, were placed to mark their positions. This seems the more likely since the girdlestones are spaced at intervals of 10 cubits (about 5 m.), which is the distance between buttress walls in the Meidum pyramid. This indication of internal buttress } walls shows that no novel features seem to have been introduced in the core structure of the Giza pyramids.

On the other hand, the builders clearly improved the stability of the outer skin in order to make a pyramid with the steep gradient of 4./ir proof against slip. They had evidently recognised that the right way to prevent the appearance of lateral forces was the use of well-squared packing blocks. These blocks are not only carefully shaped but also very large, each weighing approximately 2 1/2 tons.

In addition to the size and good shape of the packing blocks the builders of the Khufu pyramid introduced an additional measure to ensure stability. In each horizontal row of blocks a gentle grading was carried out by which the blocks at the edges were very slightly higher than those in the middle of the face. In this way the corners of each layer of packing blocks was somewhat lifted, making the whole layer slightly concave towards the apex. This method provided an additional inward thrust which further counteracted any tendency of lateral forces to develop.

This last-mentioned safeguard was clearly a laborious and time- consuming device, requiring selection and grading of the blocks before they could be laid. It seems to have been regarded as an unnecessary precaution and was not employed at the next Giza pyramid, that of Khafre. Its place was taken by a new method of preventing slip in the casing. This consisted of fashioning the lowest layer of casing stones out of granite blocks which, owing to their hardness and strength, formed a reliable base for the outer mantle. Granite blocks in the same position were also employed in Djedefre’s pyramid at Abu Roash whose reign immediately followed that of Khufu. At the last of the Giza pyramids, that of Menkaure, the sixteen lowest layers of the casing are of pink granite. This feature had always been regarded as an embellishment. However, since clearing the sand from the base of the Djedefre and Khafre pyramids has revealed single layers of granite, which were hardly visible, it seems justified to assume that their function was structural rather than artistic.

Egyptian Pyramids at Dahshur :


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