Conservation of Ancient Sites on the Silk Road

147 KEYNOTE ADDRESS Geotechnical Issues in the Conservation of Sites Robert E. Englekirk have been reshaping our planet for mil- lions of years. Humankind has been reasonably successful, at least Tlocally, in attempts to control the impact of this process where a concerted effort can result in a decided benefit. The cost and effectiveness of constraining natural geotechnical processes are important considera- tions in the development of new structures, since permanent solutions may not be possible regardless of cost. Time and cost are also of concern in efforts to preserve ancient grottoes; an added factor here, however, is the impact of the control methodology on aesthetics. Accordingly, the objective becomes the appropriate balance between effectiveness, cost, and aesthetics. What constitutes a “balanced” program will not be decided by the geotechnical consultant. Accordingly, those involved in the decision- making process must understand the geotechnical issues at hand and the mitigation alternatives available, which together are the focus of this chapter. Site soils and geology are fundamental to every grotto preservation effort. Geotechnical Issues Two problems are exclusively geotechnical in nature: deterioration of the grotto structure and water intrusion. The grottoes at Dunhuang and Yungang in China are cases in point. Geological instability of grotto structure 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 considera- tion 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 148 Englekirk 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 rea- sonable 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 intro- duced, 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 over- recessed 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 suc- ceeded 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 hard- ened 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. Water intrusion Grottoes located in regions where rainfall is considerable and groundwater tables are likely to fluctuate will experience variations in the moisture con- G I C S 149 Figure 1 Schematic cross section through the cliff face at Yungang. tent of the rock that constitute the grotto structure. At Yungang, water reportedly flows from the base of grotto walls during the rainy season. The natural geotechnical process that originally formed the cliff site selected for grotto excavation is both the main source of the problem and a resource for possible solutions. A sandstone cliff face, as shown in cross section in Figure 1, is usually created by the formation of a river valley. Water will flow through the ground in the direction of the valley floor, especially if the bedding planes slope in that direction. The toe of the cliff will usually coincide with a change in the character of the sedimentary material near the valley floor, which is often less permeable than that of the cliff face. Water will descend to this less permeable stratum and then flow toward the river valley. Before the grotto was excavated, water would, because of the change in overburden pressure, tend to flow out at the intersection of the valley floor and the cliff face, contributing significantly to its recession. With the excavation of the grotto and the associated change in overburden soil pressure inside the grotto, the water intrusion point moved back to the intersection of the grotto floor and wall. Understanding this water-flow pattern suggests two mitigation strategies: (1) remove or reduce the source of underground water, and/or (2) install an intercept that provides an alternative escape path for the sub- surface water. Cliff recession at Dunhuang Alternatives for Cliff Recession Abatement Cliff recession at Dunhuang is caused almost entirely by the abrasive flow of sand over the surface of the cliff face and back slope. The effectiveness of any abatement effort will depend on the quantity of abrasives it must resist. Accordingly, wind-tunnel studies and field experimentation directed toward reducing the flow of the sand should be pursued. Surficial cliff protection alternatives should be installed when the experienced “average” recession rate cannot be tolerated for long or when accelerated recession is occurring or might reasonably be anticipated. 150 Englekirk Figure 2 Rock bolting a fissured cliff face. Where cliff recession rates can be tolerated but block failures seem likely, mitigation measures may be undertaken. Rock bolting, in conjunction with crack or fissure repair, may proceed in anticipation of the future addi- tion of a surficial cliff-protection device. One means of accomplishing this staged abatement process is described in Figure 2. Any slope or cliff face will recede if entirely unprotected. When a slope, such as the back slope at Dunhuang, is uniform, the rate of recession may be tolerable if the flow of abrasive agents over the surface is minimized. Cohesionless material will tend to erode unevenly, even on a regular slope, and this tendency will increase as surface irregularities develop. Erosion control of otherwise stable slopes is a problem commonly encountered in highway construction, especially in areas where rainfall is heavy. Durability, cost-effectiveness, and maintainability are the principle variables with which abatement alternatives are measured. In many cases, the surfaces of stable slopes are irregular, as they are at Mogao, and this irregularity impacts the cost and effectiveness of erosion-control devices. Surficial back-slope treatments should be attempted at Dunhuang. Back- slope stabilization will not be easily accomplished here because the back slope contains topographic irregularities that will make the development of a uniform mitigation methodology difficult. Slope erosion control devices may be broadly categorized accord- ing to the following objectives: 1. limiting the impact of erosive agents; 2. protecting or isolating the surface material with a covering device; and G I C S 151 3. improving the ability of the surface material to resist erosive agents. Usually only one of these approaches is adopted and applied. At Dunhuang, the most effective solution will probably be to combine all three. Before describing how these methodologies might be effectively combined, the following summarizes the usual implementation of each method and its effectiveness. Limiting the impact of the erosive agent is the method most com- Figure 3 monly used to control erosion by water, since water will not significantly Erosion control by controlled-velocity vertical erode a surface if both the quantity flowing over the surface and the veloc- channels and gradual swales at the Getty ity of flow are controlled. This is accomplished by reducing the slope Center, Los Angeles. length and steepness and increasing its roughness. Time-tested methods include terracing and roughening the surface by plowing it across the slope. Channeling the flow of the erosive material is also a commonly adopted methodology.

Load more