Excavated caves such as those at Dunhuang and Yungang were created in relatively soft sandstone deposits, which allowed easy excavation but whose surface is easily eroded by water or wind. Sandstones form when sand is consolidated under uniform triaxial pressure, typically in a marine environment. Pressure, or the removal of pressure, is a major consideration in the deterioration of grotto structures, as the face of a sandstone cliff is subjected to pressure on all sides except for the exposed cliff face. This lack of surficial pressure causes the compacted sandstone to expand laterally in the direction of the cliff face, thereby promoting surficial
Delamination and/or the creation of weakened planes parallel to, but behind, the cliff face. Once a weakened plane has been created, it is soon further weakened by the intrusion of water until a block is formed. Surficial erosion and cliff recession are caused by these two related events, generally identified as surface deterioration and block failure. The rate of cliff recession, though not uniform, given the episodic occurrence of block failure, is fairly predictable. At the Mogao grottoes near Dunhuang, a reasonable estimate of this rate is about 1 cm per year based on an observed rate over the last twelve hundred years. Anomalies, either extant or introduced, can significantly accelerate the process. This will depend to a large extent on the nature and amount of the erosion, whether by wind and sand abrasion or by water. Consequently, any cliff recession abatement program must strive to (a) restore lateral pressure and thereby maintain the internal integrity of the sandstone formation, and (b) reduce the impact of abrasives on the surface of the cliff.
At the Mogao grottoes, the surficial deterioration is largely the result of wind and sand abrasion and clearly illustrates how a historically predictable rate of cliff recession can accelerate when uncontrolled. Understanding the deteriorative process at Dunhuang also suggests how it may be most effectively controlled. Sand deposits (dunes) above and behind the cliff face are continually being moved by the wind down the back slope above and over the cliff face itself. This action is, at least in part, responsible for the cliff's uniform recession. Accelerated erosion has occurred near the main site. Here, an anomaly has caused the sand flow to concentrate over a grotto, collapsing the grotto roof and creating an overrecessed condition. Well-intended attempts to control or eliminate surficial delamination have been undertaken in the past. The placement of a Ming-period pagoda and construction of a rock facing in the 1950s have succeeded in eliminating the recession of the cliff face at those locations. These remedies, however, have not stopped the recession of the back slope; and this recession has tended to channel the sand flow, causing accelerated erosion to areas where the cliff face mediation has not been undertaken. Attempts have also been made to control the rate of deterioration of the back slope by covering it with a cementitious material. These interventions have been entirely ineffective, however, because the hardened surface has broken down, allowing slabs of cemented sand to slide down the back slope.
The cliff face at Yungang has also receded. Here the recession is wind - and water-related, and block failures appear to have been common. Ming-epoch interventions to control cliff recession have been, and should continue to be, more effective than at Dunhuang, because the amount and effect of abrasive agents flowing over the adjoining cliff face at Yungang appear to be less than at Dunhuang.
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