云冈石窟覆盖层特征及其对石窟渗水的影响

Characteristics of the Overlying Sediments in Yungang Grottoes and Its Impact on Surface Water Infiltration

云冈石窟砂岩山体存在一定厚度的第四系覆盖层,能够起到阻滞降水入渗、减缓石窟渗水危害的作用。覆盖层下部砂砾层透水性强,对阻滞石窟渗水作用不大,而上部的粉土层属于低渗透介质,其厚度是决定石窟存在渗水风险与否的关键。人工入渗试验结果表明,当粉土层厚度小于0.7m时,容易导致地表积水快速下渗到砂岩顶部,而其厚度大于1.1m可以有效阻滞水分下渗;地表积水可以在具有薄层粉土的地方快速下渗,然后通过侧向渗流影响石窟渗水;植物根管的存在会显著加快水分下渗的速率,因此,云冈石窟山顶应尽可能少植树以降低石窟渗水风险。

The quaternary sediments overlying on the sandstones in the Yungang Grottoes with a certain of thickness are able to slow down surface water infiltration and reduce the damage of the grottoes from water intrusion. The gravel sand layer in the lower part of the sediments is not important because of its high permeability, whereas the upper silty soil layer plays an essential role for stopping infiltration due to its low permeability. The thickness of the silty soil is the key factor. When a surface pond of water exists, based on artificial infiltration test, it is indicated that the water would quickly move to the top of sandstone if the thickness of the silty soil is less than 0.7m, however it could be effectively blocked if the thickness of the silty soil is greater than 1.1m. Surface water could also quickly infiltrate into a thin silty soil and then have influence on the grottoes through lateral seepage. The root canals of plants would significantly speed up the infiltration rate. Accordingly, in order to reduce the risk of water intrusion, planting trees on the top of Yungang Grottoes should not be encouraged.