Strength property of mudstone deteriorated by drying process followed by wetting

Summary A series of uniaxial compression tests were conducted to investigate effects of water content and drying process followed by wetting on the strength of the mudstone collected from the middle area of Niigata, Japan. The mudstone had been formed geologically during the Neogene Period. The intact mudstone was excavated by a backhoe from the rock formation, and collected into the laboratory. The mudstone collected is classified into siltstone based on a grain size analysis of soil, and its compressive strength measured at the completely dried condition is almost same as Portland cement. Following remarks are concluded from the laboratory tests: 1) The compressive strength of the mudstone collected is strongly affected by its water content. By comparing the strength of the test specimens intact, dried in air, dried in an oven box and immersed in water, it is found that the compressive strength of the mudstone increases with Its water content decreasing. 2) The compressive strength of the mudstone is deteriorated by repeat of drying process followed by wetting. The drying process followed by wetting was repeated four times, with the result of the largest decrease in strength of the mudstone during the first attack of wetting after drying. 3) Degree of the deterioration in the compressive strength of the mudstone does not depend on the difference of water content between drying and wetting processes. Some change in the water content caused by the drying process followed by the wetting is thought to be an important trigger which deteriorates the strength of the mudstone. The degree as well as the process of deterioration in strength of the mudstone may be affected by the type and the content of clay mineral such as montmorillonite, illite and kaolinaite included in it. The laboratory investigation based on a clay mineral analysis of mudstone should be conducted for better understanding of its mechanical properties.