The cliff recession at the Yungang grottoes appears to be almost exclusively attributable to block failure. The average rate of recession appears to be
Figure 5
Cross section of Caves 111 and 460 at Mogao.
About the same as at Dunhuang, although the rate is not regular. Block failures may be prevented by anchoring the separating block to the base material with rock anchors, as shown in Figure 1. At Yungang, fissure propagation is more rapid than at Dunhuang because rainfall is greater and the surface of the grotto roof tends to drain toward the cliff face. The effectiveness of any rock-anchoring procedure will require a thorough fissure-repair program and may be enhanced by a diversion of the surficial water flow above the grottoes.
Cave 19 at Yungang poses a special problem of cliff face delamination, as the roof over the grotto does not appear to be thick enough to safely accept rock anchors. Figure 6 shows a section of what remains of Cave 19, once the inner chamber of the original shrine. The antechamber no longer exists, as it was the victim of successive surficial block failures. Figure 1 shows an elevation of the extant exterior wall of Cave 19. The left side window once contained two smaller windows similar to those that still exist on the right side. Clearly, the structural deterioration of the facade is in immediate need of abatement. A physical enclosure of the grotto has been proposed, the primary intent being to control pollutant intrusion via fabric filters. The strengthening described in Figures 6 and 1 may be integrated into the facade development. The two major elements of the strengthening program are the provision of a vertical support for the grotto roof over the left window and the construction of a horizontal truss within the roof enclosure. The latter will provide lateral support for
Figure 6
Vertical section of Cave 19 at Yungang grottoes.
Figure 7
Frontal elevation of the extant exterior wall of Cave 19, Yungang.
The sandstone fascia over the door, allowing it to be anchored into the side walls of the grotto instead of the fragile roof of the grotto itself. The only constraints to the aesthetic objectives of this strengthening strategy will be the need to provide vertical support for the overhanging ledge at the left window and the provision of lateral support to the blocks of sandstone over the grotto entrance at the roof line. The latter support must be high enough to reach competent material that can be penetrated with confidence on either side of the grotto.
Planning for Erosion Abatement
Effective erosion-control programs at Mogao and Yungang and other grotto sites will require a significant effort in terms of engineering as well as cost. A variety of solutions do exist, however. Alternatives should be planned to the extent that feasibility is ensured and aesthetic impact and cost may be assessed. This accomplished, the most reasonable program will usually become clear. Creativity on the part of the conservator, engineer, and builder is essential. Easy solutions and “quick fixes” will often create more problems than they solve.
Water-Intrusion Abatement
World History
