【作者】 郭青林；

【导师】 李最雄； 王旭东；

【作者基本信息】 兰州大学 ， 地质工程， 2009， 博士

【摘要】 著名的世界文化遗产,全国重点文物保护单位——敦煌莫高窟开凿在鸣沙山东麓的崖壁上,上下分为五层,南北长约1700m。最早开凿洞窟时间为前秦建元二年（公元366年）,后经十六国、北魏、西魏、北周、隋、唐、五代、宋、西夏、元等朝代的相继开凿,形成了一座内容丰富、规模宏伟的石窟群。至今仍保存有洞窟735个,壁画45000余m²,彩塑2415身,唐宋木结构窟檐5座,莲花柱石和铺地花砖数千块,是中国也是世界上现存内容最为丰富、规模最为宏大、保存最为完好的佛教艺术宝库,具有珍贵的历史、艺术和科学价值。受自然和人为因素的影响,加之制作材料较为脆弱等特点,敦煌莫高窟壁画产生了大量病害,其中如酥碱、空鼓、起甲、壁画脱落等多种病害均与可溶盐的富集和运移有关,而导致可溶盐运移的水汽来源一直是莫高窟保护中的研究热点和未解难题。本文从壁画结构和病害调查入手,开展了莫高窟区域地质与洞窟地层工程地质特征方面的调查与研究,研究了莫高窟地层含水、含盐及渗透特性,分析了莫高窟水环境特征和导致窟内壁画产生病害的水盐来源关系。主要内容包括可分为以下五个部分:（1）调查和分析了莫高窟壁画结构及制作材料,壁画盐害的种类、分布以及微观特征。莫高窟壁画由支撑体、地仗层、粉层和颜料层组成,这些层之间的接触面力学性质相对薄弱,适应环境能力差,是盐分容易富集而导致壁画产生病害的部位。壁画病害分为起甲、酥碱、空鼓、壁画大面积脱落、霉变、烟熏和变色等多种类型,除烟熏病害外,其他病害均与水汽和盐分的运移有关。导致莫高窟壁画产生病害的盐类主要为Na₂SO₄、NaCl等,从空间分布来看,莫高窟壁画盐害在下层洞窟最严重,上层其次,中层较轻微。（2）通过查阅资料、现场调查和室内试验,研究总结了莫高窟周边区域地质和洞窟开凿地层工程地质特征。莫高窟周边基岩绝大部分被第四系沉积物所覆盖,崖体及其下伏地层可划分为下更新统玉门组（Q₁）、中更新统酒泉组（Q₂）、上更新统戈壁组（Q₃）。按地层岩性和工程特性可将莫高窟崖体地层分为四个工程地质岩组,由上往下依次为A、B、C和D,洞窟主要分布在C组和D组中。受构造运动、长期风化卸荷及洞窟开挖的影响,在莫高窟崖体中分布有相互交错的构造裂隙、层面裂隙、纵张裂隙和卸荷裂隙,这些裂隙的相互作用与发展将直接影响洞窟围岩的稳定,同时这些裂隙也构成水或水汽运移的通道。（3）坑探和钻探成果、现场测定含水量、室内易溶盐测定、X射线CT以及高密度电阻率测定等大量野外和室内试验,较为全面地研究了莫高窟崖体表层、中部和深部地层的含水、含盐特征,以及影响水汽和盐分运移的地层渗透特性。通过研究得知,莫高窟崖体表部含水量大部分都在1%以上,最小为0.35%,最大为24.0%,含水量随崖体地层不同呈现不均匀性。钻探和温湿度监测结果显示莫高窟崖顶表面到深150m处地层中没有自由水,但有足以导致盐分潮解并在崖体内运移的水汽。易溶盐测定表明,莫高窟地层中的易溶盐主要为NaCl、Na₂SO₄,富集在地表至0.6m的范围内。敦煌莫高窟围岩不同地层具有不同的渗透性,通过现场渗透试验测得:Q₂-A层渗透系数6.50×10^-3cm/s;Q₂-C层的渗透系数为5.61×10^-5～4.50×10^-4cm/s;Q₃层的渗透系数1.11×10^-3cm/s;Q₄层的渗透系数1.08×10^-2cm/s。地层的水平渗透性能大于垂直渗透性能,由于冲洪积成因,即便处于同一地层岩组,渗透系数也有较大差别。利用高密度电阻率法可以反映莫高窟围岩的渗透特性和渗透速度,利用X射线CT计算出的孔隙比与实验中求得的孔隙比数值接近,在无法直接测定渗透系数的情况下,可利用X射线CT计算孔隙比推算出岩体局部的渗透系数,两种方法均适用于研究围岩的渗透性。（4）通过现场调查、长期监测以及室内测定,系统研究了莫高窟的水环境。研究表明,莫高窟多年平均降水量为36.45mm,且年内分配极不均匀。窟区唯一的地表水为大泉河,流量为0.0524m³/s,年径流量为165.64×10⁴m³/a,水质矿化度达2g/L,属微咸水。依据莫高窟北区洪痕标高,恢复历史洪水过水断面,自莫高窟建窟以来,大泉河最大洪峰流量约为2600m³/s～2800m³/s。大泉河冲洪积扇的中下部地带普遍分布有地下潜水,水位埋深变化较大,地下水的补给来源主要是党河流域地下水的径流补给,即由SW向NE方向渗流。（5）利用高密度电阻率、温湿度监测和易溶盐测定等手段深入调查和研究了导致莫高窟产生病害的水盐来源。研究表明,莫高窟底层洞窟,尤其是底层洞窟西壁盐害较为严重的原因主要是岩体内有较高的水汽含量,岩体内部与洞窟之间形成一个动态平衡系统。它们之间的湿度差导致岩体深处的水汽不断带动盐分向洞窟表面运移,并在相对湿度达到盐分潮解临界值的较浅部位聚集。由于盐分的存在使得西壁对洞窟环境的变化非常敏感,当洞窟内较为干燥时,盐分停留岩体较浅部位,当外界因素导致岩体浅部水汽含量超过盐分潮解临界值时,盐分便向外运移,导致壁画产生病害。可见岩体内部水汽向外运移是导致盐分在洞窟壁面表聚的主要因素,而外界环境变化导致窟内湿度的升高与降低的循环是洞窟壁画产生盐害的诱发因素。更多还原

【Abstract】 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.更多还原