Slaking characteristics of geomaterials in direct shear test

Numerous slope stability and other geotechnical problems worldwide have been reported due to the presence of mudstones. Similarly, in the world, infrastructure developments have been extended to hillsides and mountainous areas because of population growth, economics needs and other constraints. Soft sedimentary rock named mudstone is the most often encountered geomaterials during the major construction works that are undertaken in those areas. In the past decade, there has been a significant increase in knowledge with respect to the mechanics of geomaterials. Several fundamental issues have been solved, and important achievements have been made in certain areas. However, most of these achievements are either deal with rock or with soil because most of geotechnical engineers are generally used to viewing geomaterials either as soils whose behavior is highly susceptible to the fabric and water content of the intact material, or as a rock, with engineering behavior primarily controlled by fissures and joints. However, mudstones have two inherent properties: firstly, they are intermediate in behaviour between rock and soil, and secondly, they tend to transgress from rocklike to soil-like materials within relatively short time-frames. So, present experimental and theoretical methods used in geotechnical engineering practice are not inadequate for assessing the stability of mudstones. In general, the laboratory investigations on durability characteristics of mudstones are only made through the general slaking test. However, studies examining the effects of slaking on strength and deformation behaviour of mudstones are sparse. This research began by investigating the landslides to explore the probability of slaking induced landslides in soft sedimentary rock formations. Significant numbers of landslide events were found to be occurred after moderate rainfall, sometimes followed by drought. Similarly, intensive review of the studies that have been previously undertaken to study the slaking of mudstones was done to ascertain the current state of knowledge regarding this process. At the beginning of the experiment, the conventional slaking tests were performed to classify the tested material on the basis of slaking durability. In order to examine slaking effects on strength and deformation characteristics of crushed mudstone (Hattian Bala and Ishikawa), a series of direct shear tests were conducted on the crushed mudstone by simulating cyclic wetting and drying under different stress conditions by using a modified direct shear apparatus. In addition, a series of the monotonic loading tests on dry and saturated specimens were also performed to compare strength and deformation characteristics with those of the cyclic wetting and drying creep test. Similar tests were also conducted on conventional granular material such as Chiba gravel and Toyoura sand and Glass beads for the comparison of stress-strain behaviour after saturation and cyclic wetting and drying with crushed mudstones. Sieve analysis was performed after each experiment and degradation index was used to quantify the particle crushing due to slaking. The strength and deformation characteristics of crushed mudstones under dry, saturated and cyclic wetting and drying conditions were compared with standard granular soils such as Silica sand (Toyoura), crushed sand stone (Chiba gravel) and Glass beads as well. The saturated and the one with cyclic wetting and drying mudstone specimens exhibit largely different stress-displacement features from those for dry specimens while the difference in stress-strain-volume change behaviours between dry and saturated conditions is insignificant for both Toyoura sand and Chiba gravel. The effects of stress ratio, density of specimen, initial water content before wetting, slaking index and number of cyclic wetting and drying on slaking were investigated. It was concluded that the slaking induced shear displacement is the function of various factors such as stress ratio, density of specimen, initial water content before wetting, slaking index and number of cyclic wetting and drying. Drying induced slaking also observed when water content of the mudstone specimens became smaller than certain threshold value of water content.報告番号: ; 学位授与年月日: 2012-09-27 ; 学位の種別: 修士 ; 学位の種類: 修士（工学） ; 学位記番号: ; 研究科・専攻: 工学系研究科社会基盤学専