Gerd Gudehus and Thomas Neidhart
The dafosi (Great Buddha Temple) Grotto, about 200 km northwest of Xian, was completed approximately fourteen hundred years ago. It was excavated in a sandstone cliff by the extension of a natural cavern to about 35 m in breadth, 15 m in depth and 21 m in height. Inside the grotto, three sandstone statues were carved in high relief from the rock of the walls: a sitting Buddha 17.5 m high and two bodhisattvas, each 12 m high. The statues were covered with clay plaster, then painted. The walls and ceiling of the grotto are decorated with hundreds of figures and ornaments carved into the stone. Openings in the partially remaining cliff wall and in the front temple (which was added much later) permit access of light and visitors. Over seventy years ago, a description of the Dafosi grotto was published by Pelliot (1924).
The grotto is currently in dire need of preservation. Parts of the jointed rock ceiling have collapsed, and other parts appear ready to do so. The necks of two statues are cracked and could topple at any time. Immediate support is needed at some points in the ceiling. Sufficient permanent stabilization—carried out with due respect to the monument— should follow. The lower half of the cave temple is seriously weathered from the infiltration of water and salt. Some areas of the ceiling—as well as some of the statues and walls—must be immediately supported, and further deterioration needs to be reduced by stabilization measures.
The geotechnical problems of this grotto and ones like it are rather uncommon, so commonly used tunneling and mining practices are not necessarily applicable.
Geometry
Neither ordinary photographs nor drawings can yield a representation of the complicated spatial geometry of the Dafosi grotto that is sufficiently precise for the geotechnical analysis needed for conservation purposes. While detailed photogrammetric images of the statues have been made, this technique is not suitable for the grotto as a whole. A project supported by the Geodetic Institute of the University of Karlsruhe, with the significant involvement of Manfred Vogel, was undertaken to generate
Sophisticated computer graphic illustrations to aid in conservation of the site. As a result, the coordinates of about 6,300 points of the grotto have been determined with an electro-optical total station (Leica and Wild T2002, and DI3000s Rangefinder). The computer graphic illustrations were processed from these data by Neidhart. The software packet AutoCAD Release 12 was used for image processing and computer-aided design (CAD).
To determine spatial geometry, eleven observation points were distributed on the floor, so that almost all parts of the grotto could be surveyed (Fig.1). These, together with five marked reference points at the same level and three at the openings of the temple, formed a sufficient base of reference. The surfaces of the grotto and statues were scanned according to a grid of roughly 0.5 m mesh width; clearly visible marks, such as sharp edges or a change in surface texture, were used. Because of the shape of the grotto and the fact that the statues hide some of the side walls, it was not possible to methodically conduct an exhaustive survey. As a result, some patches of the surfaces of the grotto and the statues were repeatedly surveyed from several points of view. Only small areas (at the shoulders of the statues) remained hidden from every point of view. The data for every point measured consisted of its Cartesian coordinates and a number through which four typical surface types (rock, ornaments, masonry, and weathered fill) were coded. These numbers enabled the scientists to differentiate and color a CAD model.
Vertical coordinates were interpreted from the reference points, a process that yielded forty-one sets of elevation data (0.5 m, 1.0 m, 1.5 m, and so on) with a range of 0.25 m. The coordinates of the whole ceiling, with a wider range, were stored in the forty-first set. Figure 2 shows horizontal cross sections in four elevations that were prepared with these data sets. Figure 3 demonstrates how the same can be done for any vertical section. Other sections of this kind can easily be made so a rough impression of shape, size, and relative orientation can be obtained.
Figure 1
Survey reference points. Points 112 and 113 are in the openings on the first floor of the front temple.
Figure 2
Horizontal cross sections at four elevations above the grotto floor with letters indicating the statues, as follows: A = Bodhisattva Avalokiteshvara, B = the Great Buddha,
C = Bodhisattva Mahastamaprapta.
It is much more difficult to generate a complete spatial impression of the grotto with computer graphics. The problem lies in the concavity of the surfaces, the possible loss of specific detail, and the omission of hidden parts that might be necessary to fully model the chosen point of view As standard procedure, the points in every data set were rearranged to unify the sense of rotation (in a right-handed Cartesian coordinate system), which is necessary for the CAD software to generate surfaces with normal vectors extending from the surfaces into the grotto. Finally, every data set was split according to the change of materials, corners, and edges, to prevent the CAD software from producing any smoothing of these significant parts of the grotto. The points of each subset were interpreted by the CAD program as three-dimensional polygons connected with rectangular surface patches.
Figure 4 gives a rather complete impression of the grotto except for the remaining cliff wall and the front temple, which are omitted. The sitting Buddha fills the grotto almost to the ceiling. The two bodhisattvas are leaning toward the walls and are clearly subordinate. A kind of circuit surrounds the statues near their bases. The grotto surface is roughly shaped as one-quarter of an ellipsoid. Convexities of this surface carry decorative parts, such as the Buddha's halo. Concave parts are largely the result of losses from rupture in the upper half of the grotto and erosion in the lower half.
Figure 5 shows another image of this kind. The observer's point of view can be chosen arbitrarily, and various parts of the CAD model can be removed to achieve a better perspective. The images have been enhanced with shadows and colors to strengthen the spatial impression. It is evident that photographs and videos are needed to obtain more detailed pictures, but these can be far better understood with the aid of the CAD images. These graphics form a substantial part of the authors' geotechnical reports; they are also useful in the authors' own work and in collaborating with other scientists.
Upper Half of the Grotto
Figure 3
Two vertical cross sections.
Toward the ceiling of the grotto, the rock is almost pure sandstone, rather dry, and as permeable as fine sand. It has an old system of fissures, typical
Figure 6
Cracks at the Buddha’s head with an inclination of approximately 60°.
Figure 5
Unshaded CAD model. Point of view from the north.
Figure 4
Unshaded CAD model. Point of view from the northeast. Letters mark vulnerable parts of the grotto surface and statues: A = head of the Great Buddha, B = eastern bodhisattva,
C = ceiling to the left and in front of the Buddha, D = area of dangerous “coffin lids,” E = foot of the eastern bodhisattva, F = circuit behind the statues, G = base of the Buddha.
Of cliffs, which can be seen from the outside. These cracks have extended and opened as the stress release along the grotto surface has produced stress concentrations close to the cracks. Also, the cracks tend to expand with time as a result of reduction of strength caused by weathering, temperature changes, and occasional dynamic impacts. The rubble on the floor indicates that, over time, substantial parts of the former ceiling collapsed long ago, but it is not possible to determine from below which parts are likely to fall next. A scaffold has therefore been erected; from it, close inspection of the whole grotto surface and the statues was conducted by Zou Yazhou of the University of Hydraulic and Electric Engineering in Wuhan. The inspection revealed a far more dangerous situation than had been expected. Vulnerable portions are labeled in Figure 4.
The Buddha’s head (Fig. 4, area A) has two visible parallel cracks that extend from the back almost to the chest (Fig. 6). They can barely be seen from the floor, and an even closer view does not fully reveal them because of the clay-plaster cover. Measurement of the attenuation of weak
Figure 8
View of the Dafosi grotto, showing rock outcrops (upper right) in ceiling area to the left of the Buddha.
Figure 7a, b
The head of the eastern bodhisattva (a) showing location of a visible crack; and (b) horizontal cross section through the head of the bodhisattva, showing existing cracks and their expected development.
Shock waves reveals that the two cracks pass through the stone horizontally. It can also be seen that part of the cracks appear to be fresh and thus are developing. Indeed, the head will eventually break off when the cracks are sufficiently deep. The eastern bodhisattva, on the right side of the Great Buddha, has a similar weakness: the head is partly separated from wall and body by two cracks (Fig. 4, area B; Fig. 7a). Only one crack is visible from the floor, and only a very close inspection revealed the danger presented by the second one (Fig. 7b). As the head is inclined toward the Buddha, it is likely to fall in this direction.
The ceiling to the left and in front of the Great Buddha (Fig. 4, area C; Fig. 8) contains a few very loose protruding blocks with masses of up to about 50 kg. One of these blocks fell in 1992; this dangerous event led to the decision to erect the scaffold and conduct a close inspection. Other very loose small blocks, previously unseen, were then identified from the vantage point of the scaffold. Because their documentary value appeared low relative to the cost of stabilizing them, they were removed immediately.
Close inspection itself posed a high risk. A very dangerous area was discovered to the right of the Buddha's head (Fig. 4, area D). In that spot, orthogonal patterns of joints almost permit the separation and falling of a series of slabs or plates weighing about 2 t each (Fig. 9). Gentle tapping applied experimentally to these plates caused them to vibrate with a low frequency—an indication that the plates are attached on only one side. The term “coffin lids,” used by miners for such slabs, indicates the danger they pose.
The authors, together with Ge Xiurun of the Academia Sinica in Wuhan, conducted a detailed stability analysis and designed proposals for stabilization. The two bodhisattva heads will be temporarily secured by
Figure 9
Vertical cross section through the ceiling and head of the eastern bodhisattva, showing the dimensions and shape of the “coffin lids.”
Steel brackets traversing the cracks. The use of small-diameter drill holes with interior application of glue is an acceptable intervention, considering the otherwise high risk of loss. Subsequent long-term stabilization will require bolts placed nearly vertically with reference to the cracks; it is possible that filling the cracks with mortar would worsen the situation. Respect for the statues precludes bolting their heads from the front; instead, holes must be drilled from behind or from the sides. There is a narrow cavity behind the Buddha's shoulders that should permit drilling. Holes drilled from the side at appropriate angles can reach the cracks behind the bodhisattva.
For this type of repair, stainless steel or fiberglass bolts will be placed into the drill holes (Fig. 10a). Filling the holes with compacted sand (Fig. 10b) and prestressing the bolts with screw nuts (Fig. 10c) will achieve the necessary static contact (Fig. 10d). This type of rock anchor, which was developed at the Institute of Mining of the Russian Academy of Science in Novosibirsk (Stashevski and Kolymbas 1993), is strong and durable. It is also chemically neutral and therefore reversible from a conservation standpoint. The anchor system was further developed at the Institute of Soil and Rock Mechanics in Karlsruhe and tested on sandstone blocks, including overhead installations. Field tests have also been performed on this anchor system in soil—for example, in the stabilization of retaining structures. The installation of anchor systems combining bolts and sands has been extensively and successfully achieved under various conditions (Gudehus 1994). In Bulgaria, such sand anchors have been used to fix rock blocks in steep slopes in cases where they threaten to destroy historic buildings (Stashevski and Kolymbas 1993).
Figure 10a-d
Schematic drawing, showing installation of a sand anchor in rock.
Before applying rock anchors at the Dafosi grotto, the coffin lids require temporary props; otherwise they cannot be touched. Of course, the floor below them must be closed off. Drill holes with bolts and sand, such as those used for the heads of the statues, have been prepared and will be prestressed so as to carry the entire weight of the rock plates.
Ge Xiurun is analyzing the stability of the upper half of the grotto using a finite element method to calculate the stresses caused by the excavation of the grotto and to estimate stresses that may be caused by future earthquakes. These calculations will aid in identifying zones of impending rupture. A further, more detailed but protracted calculation was made for some cracked areas of the ceiling. These calculations are more difficult to carry out than are similar ones currently used in rock mechanics for the analysis of storage dams and rock cavities.
Consideration has been given to the placement of monitors to signal an impending rockfall. These monitors are desirable because an accurate mechanical analysis of stability is beyond the scope of present geomechanical calculations. An indicator of insufficient stability is an
Increasingly abnormal wave transmission and emission behavior. Even though this relationship is qualitatively known in mining and earthquake engineering, a consistent mathematical predictor is not yet available. Therefore, at the present time, intuition based on experience is the best guide.
Lower Half of the Grotto
Toward the bottom of the grotto, nearly horizontal layers of clay are embedded in sandstone. Seepage of moisture from the loess cap of the sandstone formation migrates above the clay toward the cliff wall in the grotto. Capillary rise of moisture is nourished by this horizontal flow and strengthened by evaporation along the wall of the grotto. The lower half of the sandstone is wet, and water content decreases toward the ceiling. Dissolved salts, which migrate in the capillary water, crystallize at the surface to develop a white efflorescence. Part of the salt crystallizes below the stone surface; the resultant expansion has produced spalling of parts of the rock surface. In principle, this mechanism of weathering is well understood in geology and in the deterioration of monuments. Nevertheless, it is very difficult to analyze the process precisely and prevent further deterioration by technical means.
Such defects, caused by salt crystallization in the stone, can be seen in other areas of the grotto shown in Figure 4. Part of the foot of the eastern bodhisattva (area E) has been lost, and the entire statue could break down in the near future. The circuit behind the statues (area F) is expected to enlarge, causing the rock above to lose support. The base of the Great Buddha (area G) has become so soft that parts of it have already crumbled away. It is now inadequately supported by a buildup of sediments from the river and by rock material from the grotto.
Even without an analysis of mechanical stability, it is clear that this weathering will completely destroy the lower part of the grotto and the statues over the course of time. Some parts—as, for example, the right side of the Great Buddha's halo—are already approaching collapse and require immediate support. Disintegration of the rock face must be stopped or at least reduced.
One remedy to these problems, theoretically, would be to interrupt or reverse the flow of water and dissolved salts into the grotto. This step is not practicable, however, because surface layers would spall off after some time; drainage holes would divert only part of the seepage water; and vacuum or electroosmosis methods are not reliable for sandstone.
It will therefore be necessary to tolerate an ongoing influx of water and salts into the grotto. But further damage can be reduced with sacrificial plaster layers, sometimes used for the conservation of buildings. In this scheme, transported salts accumulate on and inside the layer and do, indeed, eventually destroy it—but the layer can be easily replaced later. Tests have been made in Karlsruhe, in cooperation with visiting Chinese scientists, to demonstrate the use of sacrificial plaster layers in combating salt transport.
World History
