Abstract:

The relics carrier of Yungang Grottoes is a huge sandstone lens deposited by river in Jurassic period and the sandstones are weathering rapidly and seriously in present conditions. Based on the field investigation, there are close connections between the damages with the deposition surrounding and structure of every subdivide layer. Furthermore, a standard has been presented for evaluating the weathering degree depended on tests of rock samples to different deteriorated degrees. It has been shown from the tests of the sandstones that the erosion of the cements and alteration of feldspar are the main chemical weathering reasons and the mechanical weathering, crystallization of the secondary salts and cycle of freeze and thaws should be paid attention to. Conclusions have been drawn from the weathering simulation tests that an inside to outside damage can be induced by the secondary soluble salts which come from the erosion of the acid on the grottoes rock surface directly and that the resistance for freeze-thaw cycle of the sandstone is good because of very low porosity, permeability and strong cementation. The suggestions has been presented for the relics protection that water controls such as the decrease of the surface flow on the cliff, the seepage from fracture inside caves and capillary water from the clay base of cliff are major ways to alleviate the weathering.

Key words: Yungang Grottoes, sandstone weathering, soluble salt, freezing-thaw