WATER INDUCED DETERIORATION CHARACTERISTICS IN SANDSTONE OF TERTIARY FORMATION AT QINHUAI EAST RIVER IN NANJING CITY

The Qinhuai East River, in Nanjing city, is designed to pass through the section of Tongjiashan hill, where the Huangmaqing group of Tertiary (T₂h) will be excavated for the new river. The excavated stratum is mainly composed of sandstone, cemented by mud and iron. The maximum height of the future slope will arrive at 70 m. Results of the field investigation and drilling holes show that the rock mass near the ground surface has intense cracks and low integrity, but the rock mass at the deep location has good integrity and few cracks. The cracks are closed with the favorable cementation strength. The wet and dry cycle tests are carried out for the rock cores. When the samples are tested for the five times of wet and dry cycles, a majority of the samples are cracked and disintegrated. But when the samples are immersed under the natural conditions, after twenty cycles, the samples are continuative and not cracked. This phenomenon can be explained that the small cracks in the rocks, where the water is not easy to enter under natural condition but enter easily in the vacuum, are very developed. When the water enters the small cracks, the cements are softened and the small cracks open, water swelling. The deterioration of the rocks due to water are caused by the double mechanisms. The mineral composition analysis is taken by the XRD apparatus. The results indicate that the more clay minerals in the rocks are the important causation due to softening of the rock, which result in the weak expansion. After the cycles of dry and wet tests, the wave velocity, unconfined compression strength and elastic modulus decrease obviously. The physical and mechanical characteristics are sensitive to the water. With the cycle numbers increase, under the compressive stresses condition, the pore compaction and elastic deformation process of the rock gradually disappear. After excavation of the high slope for the Qinhuai East River at the section of Tongjiashan hill, the effect of water on the stability should be considered adequately.