The following techniques were designed to combine roof-support and retaining-wall measures. Depending on the condition of the cliff face, support systems fall into three general categories: post-and-lintel, pillar, and retaining-wall structures. Post-and-lintel structures act only as apex supports. The base is built directly on the rock floor, and the lintel stone slabs are placed in close contact with the overhanging bedrock. When pillars are used, they are placed away from the antechamber walls to protect wall paintings. The retaining wall is used when stabilization of a fairly large area of the grotto cliff face is necessary To satisfy various mechanical requirements, the retaining-wall structure should have sufficient mass and strength, and yet leave room for grotto entrances. It should be broader at the base and gradually decrease in cross-sectional area from base to top, forming what is essentially a pyramid-shaped stairway on which walkways can be built on different levels (Fig. 8).
In the process of stabilization, an effort was made to build all the supporting structures on bedrock. In some individual sections with overhanging rocks, however, the area requiring support was inevitably rather wide, and the support structure needed to be enlarged accordingly. If all such foundations had been built entirely on bedrock, it would have been necessary, in some cases, to construct foundations tens of meters thick. To solve this problem, an enlarged base area would be laid down first and observations made as to its degree of subsidence into the compressible, sandy soil below Then the support structure would be constructed to a corresponding distance below the roof rock to be supported. After a period of gradual settling and establishment of relative stability, the space between support and roof would be filled. At that point, a tight support of the bedrock roof could be achieved. This mode of construction was used to support the roof of the antechamber of Cave 171.
When a support structure needed to be wide enough for part of it to stand on bedrock and the other part on compressible soil, two separate foundations were constructed with settlement joints between them (Fig. 9). Since the outer parts of a structure may also settle to a certain extent, those areas of the foundation should be established on the same soil to maintain a uniform degree of subsidence for the entire structure. If a fairly large area and variable foundations were required, a horizontal deformation joint should be constructed every 20-30 m to serve as both the expansion and settlement joints.
In reference to the Mogao grotto stabilization project, the cultural relics administration stated:
The principles of engineering design should give priority to ensuring the safety and stability of the grottoes and, at the same time, take into account
Figure 9
Section showing the foundation of a retaining or buttress wall. The foundation of the retaining wall outside the settlement joint is on compressible sandy soil. Reduction or rebating of the unstable, cracked section was carried out to remove dangerous rock and to reduce load.
The question of aesthetic style, so that major changes will not be made to the appearance of the Mogao grottoes. Therefore, it is essential to bring the structures of the stabilization project into line with the original style of the grottoes and make the utmost effort to preserve their original appearance.
On 9 August 1963, the late architect Liang Sicheng noted in his Comments on the Dunhuang Conservation Project Program (1963:239):
Our main focus today is to build walls and basically preserve the appearance of the grottoes, which is the best we can do. Therefore, I endorse this general principle. . . . As far as I can see, everything of major value is housed inside the grottoes. . . . Our objective is to protect what is inside the grottoes by stabilizing them from the outside, and this objective of ours should be thoroughly understood.
World History
