Origin Of Water And Salts Responsible For Wall Paintings Disease At Dunhuang Mogao Grottoes

The Dunhuang Mogao Grottoes, the well-known world heritage site and the national key cultural relic's conservation unit, is located on the cliff at the east side of Mingsha Mountain. It has five layers from top to bottom and almost 1700 meters in length from north to south. The earliest time of excavated cave is former Qin Dynasty (366 years B.C.) and the rest caves excavated successively from dynasties of 16 states, the Northern Wei, the Western Wei, the Northern Zhou, Sui, Tang, Five Dynasties, Song, Western Xia and Yuan dynasties which engendered these abundant and magnificent grottoes. There are still 735 caves survived at present and they contain 45000 square meters of wall paintings, 2415 painted sculptures, 5 eaves of Tang and Song dynasties wooden structure and thousands of lotus pillars and pieces of paving tiles. Mogao Grottoes with its world's largest scale and best preserved condition is the famous treasure house of Buddhist art and it has precious historical, artistic and scientific value.With the influence of natural and man-made factors, combined with inherent susceptibility of fragile wall painting materials, the wall paintings of Mogao Grottoes produced a large number of diseases, such as efflorescence, flaking and large wall paintings detachment. These diseases can be connected with the accumulation and migration of soluble salt caused by moisture transform, so study on the source of moisture at Mogao Grottoes is a hot-topic and difficult subject. This paper began with the survey of wall painting's structure and disease and made investigation and research on regional geology environment and geological engineering characteristics of the strata in the Mogao Grottoes. For the first time a more comprehensive study on the strata's moisture, salt, permeability and analysis of relationship between water environment characteristics and source of moisture in the caves which lead to the wall painting's disease. Main contents can be divided as the following five sections:1. Investigation and analysis of wall painting's structure, materials,types, distribution and micro-features. The wall paintings of the Mogao Grottoes consist of the support layer, plaster layer, render layer and pigment layer. The interfaces between these layers have weak mechanical properties and thus poor adaptability to the environmental change. So these areas are easier for salt enrichment which leads to the wall painting disease. Wall painting disease can be divided into efflorescence, flaking, detachment, sootiness, mildew, discoloration, etc. In addition to sootiness, the rest diseases are related to the migration of moisture and salt. The types of salts that lead to wall painting disease of Mogao Grottoes are mainly Na₂SO₄ and NaCl. The wall painting's salt hazard of Mogao Grottoes is most serious at the bottom, upper second and slight in the middle for the spatial distribution.2. The around regional geological environment and geological engineering characteristics of the strata in which caves are studied by field investigation and laboratory test. Most of strata around Mogao Grottoes have been covered by the Quaternary sediment. The cave's cliff and its underlying strata can be divided into Pleistocene Yumen group (Q₁), middle Pleistocene Jiuquan group (Q₂), the Pleistocene Gobi group (Q₃). According to strata lithology and engineering character, the stratigraphy of cave's cliff can be divided into four engineering geological group, namely A, B, C and D from top to down. The caves mainly distribute in the C and D group. There are cross-cutting fissures, level fissures, longitudinal cracks and unloading fissures distributed in cliff of Mogao Grottoes by the influence of tectonic movement, long-term weathering and excavation of caves.The interaction and development among these fractures will directly affect the cave's surrounding rock's stability and meanwhile these fractures are also channels for migration of water and moisture.3. Various methods have been used in this study, such as geological pit, boring, moisture monitoring, soluble salt analysis, X-ray CT, high-density electrical resistivity measurement, as well as a large number of field and laboratory tests, comprehensive study on moisture, salinity characteristics and strata permeability which dominates the migration of water and moisture through the surface of cliff, middle and deep part of strata.Study shown that the water content of the surface is more than 1% with a maximum data 24.0% and minimum data 0.35%. The water content showed heterogeneity with different strata of cliff. The data from drilling and temperature-humidity monitoring indicate that there is no free water from cliff's surface to 150m deep strata, but there are enough moisture that can cause salt to dissolve and migrate. Test showed that the types of soluble salts concentrated from the range of earth surface to 0.6 meter high at Mogao Grottoes are mainly NaCl and Na₂SO₄. The different rock strata of Dunhuang Mogao Grottoes have different permeability. Measurement from on-site permeability test: the permeability coefficient of Q₂-C layer is 5.61×l0^-5-4.50×10^-4 cm/s, Q₂-A layer is 6.50×10^-3cm/s, Q₃ layer is 1.11×10^-3cm/s, Q₄ layer is 1.08×10^-2cm/s.The strata lateral permeability of strata is larger than the vertical permeability. Even in the same stratigraphic rock group, the permeability coefficient is also quite different because of alluvial and flood origin. The use of high-density resistivity method can reflect penetration characteristics and penetration velocity of surrounding rock, the void ratio values obtained from calculation through X-ray CT are close to the experimental values. In case that the direct measurement of permeability coefficient is inconvenient, it can be calculated from the void ratio values acquired from X-ray CT calculation.4. Systematic study on the water environment of Mogao Grottoes through field investigation, long-term monitoring and indoor measurement. Studies have shown that the annual precipitation in Mogao Grottoes is 36.45mm and the distribution is very uneven with a year. The Daquan river which is the only surface water of grottoes area has a flux of 0.0524m³/s, annual runoff 165.64×104m³/a, water salinity 2g/L and it belongs to brackish water. Form the flood marks elevation on the north side of Mogao Grottoes, the restoration of historic flood cross section can be obtained. The maximum peak flow is calculated about 2600m³/s～2800m³/s after the time the Mogao Grottoes has been built. The groundwater is widely located in the middle and lower zone of Daquan river alluvial fan and the water table changes greatly with depth, recharged by the Dang River from the southwest to northeast.5. Investigations and studies have been down on the source of moisture and salt which lead to wall painting's diseases by means of high-density resistivity, temperature & humidity monitoring and measurement of soluble salts. Studies have shown that the salt hazard is mainly occured at the bottom layer of Mogao Grottoes, particularly in the West wall at the bottom of the caves due to the higher moisture content and dynamic equilibrium system between the rock and the air in the cave. The humidity difference lead to the moisture accumulation and even the salt to migration from inside of the rock to cave's surface when the relative humidity reaches the threshold at which the salt can be crystallization and aggregation. The presence of salt makes the western wall of the cave very sensitive to changes of the environment. The salt stays at the shallower parts of rock when the cave was relatively dry, and dissolved and migrated to the surface, leading to the wall painting's disease, when the moisture content induced by external factors exceeds the critical value.Therefore, it can be concluded that the salt accumulation on the wall surface of the caves are firstly responsible to the moisture migration within cave rocks, and secondly to the changes in air humidity of the cave induced by external environmental variation, such as weather change and guest resperation etc.