INFLUENCE OF PRETREATMENT METHODS ON GRAIN-SIZE DISTRIBUTION PATTERN OF THE XIASHU LOESS

7 methods were adopted to pretreat the 30 Xiashu Loess samples from the Taishan Village at Nanjing to check the bias of different methods on grain-size distribution pattern. Our results indicate that:(1) Grain size frequency curves using Method A (without any pretreating) and method D (pretreated with hydrochloric acid) are all unimodal in distribution pattern, but the frequency curves using other five methods are bimodal. There is a small proportion of very fine grains in the fine grain fraction of the bimodal curve. The Xiashu Loess contains certain amount of organic matter, especially in the paleosol layers, which has significant influence on grain size distribution. It is a necessity to remove the organic matter before grain size analysis to be carried out. Owing to the hot and humid climate and strong leaching, carbonate content is relatively low in the Xiashu Loess, so it is not necessary to remove carbonate before grain size analysis. Moreover, hydrochloric acid may react with some minerals, and the excessive hydrochloric acid shall make the solution more acid, leading to flocculation of soil particles, that will make the grain size coarser, and deducing of the very fine component. In this regard, it is recommended that the method of G be used for pretreatment of the Xiashu Loess samples. At First, H₂O₂ is used for one night to remove the organic matter, and then add(NaPO₃)₆ as a dispersant to disintegrate the sample, and finally ultrasonic cleaning instrument is recommended before grain size analysis to be proceeded.