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.
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 addition 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 according to the following objectives: 10
Figure 3
Erosion control by controlled-velocity vertical channels and gradual swales at the Getty Center, Los Angeles.
Figure 4
Steel reinforcing laid on the surface in preparation for application of surficial concrete as an erosion-control measure, Yulin grottoes.
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 commonly used to control erosion by water, since water will not significantly erode a surface if both the quantity flowing over the surface and the velocity of flow are controlled. This is accomplished by reducing the slope 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. The construction of channels controls the velocity, direction, and location of flow. The introduction of gradual swales and controlled-velocity vertical channels is shown in Figure 3. Clearly, the aesthetic impact is significant. Swales and channels must be continuously maintained, as the concentrated flow, if allowed to deviate from protected paths, will cause local failures that may be of significant proportion. Treatment between swales, usually vegetation, must also be maintained in a manner consistent with the mitigation program.
Protecting or isolating the surface material from the erosive agent by installing a layer of concrete is a method commonly used to protect highways in Japan. The basic features of this technique include rock bolts that extend through loose surficial deposits and are anchored into firmer substrata; a mat of ferrous reinforcement placed over the surface material; and, finally, concrete applied over the existing surface. An appropriately designed and installed application, if maintained, should virtually eliminate longevity concerns. Unfortunately, concrete is not easily applied to difficult surfaces and volume changes in the material itself will cause cracks to form, especially where the surface is irregular and material thickness is not uniform. Cracks will allow water to penetrate, causing the reinforcement to rust and creating flow channels or piping in foundation material below the concrete. Thus, the protective device may itself become a significant problem. For example, the installation of a reinforced concrete surficial device at the Yulin grottoes near Anxi was in progress in 1991, and some of the difficulties described above are apparent in Figure 4. Given the extremely irregular nature of the exposed cliff face at Yulin, this type of surface protection is probably the appropriate solution, but its maintenance will undoubtedly be a problem.
Improving the ability of the surface material to resist erosion is another alternative. Bare-earth erosion control is most frequently used on oversteepened construction slopes. The process involves spraying the exposed surface with a chemical, such as potassium silicate, that will bind the particles. The procedure is most effective in cohesionless materials such as sand, which is easily penetrated and readily absorbs a fluid. The aesthetic advantages over the previously described alternatives are obvious, as is the impact on cost. Longevity then becomes the issue, which may be significantly improved in the design of a program that minimizes the impact of secondary actions, and through the introduction of nonmetallic fiber reinforcement. Periodic retreatment of the surface must take place, or the resulting problem may be worse than the original one.
The erosion of the back slope at Dunhuang is probably best controlled by integrating a bare-earth treatment with localized enhancements, provided topographic irregularities and anomalies—such as excavated grottoes—are considered and carefully incorporated into the program.
The development of a bare-earth treatment program must consider how maintenance is accomplished and recognize that any surface hardening is likely to create a weakened plane below the zone of hardened material, which may result in a slide. These two considerations can be included in a general solution that maintains the aesthetics of the slope. The tendency of the hardened material to slide can be controlled by the introduction of either horizontal or vertical channels that are rock bolted into the firm underlying sandstone strata. These reinforced areas can then be tied to the surface strata by introducing nonferrous fiber reinforcement into the upper sands before the bare-earth treatment is applied. Reinforced areas should also provide access for construction and maintenance.
The treatment of anomalies will undoubtedly require a combination of solution methodologies. One example is the condition that now exists at caves 272 and 460. A cross section through this portion of the cliff (Fig. 5) graphically illustrates the problems. Major issues that must be addressed by any solution include the cracking and differential settlement in the roof of Cave 272, the expanding hole in the roof of Cave 460, and the accelerated erosion caused by the “river of sand” flowing over Cave 460.
The elements of the solution illustrated in Figure 5 include
• the reinforcement of the cliff face by the installation of reinforced concrete buttresses;
• the installation of upward-sloping rock anchors, which secure the buttresses to the grotto facade and relieve the vertical load imposed on the roof of Cave 272;
• the integration of a new concrete roof structure supported by the grotto walls and new buttresses supporting the existing grotto roof structure; and
• the integration of reinforced concrete sand-diversion channels into the surficial treatment above the grottoes in the back slope; an alternative here would be a gunite-reinforced surface applied locally.
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