The Study On Shrinkage,Cracking And Moisture Transfer During Desiccation Of Mixed Buffer Backfill Materials

At present,deep landfill is widely used in the treatment of nuclear waste in nuclear dominated countries.This method puts forward strict requirements for the performance of buffer backfill material between nuclear waste storage container and foundation pit.The main choice used in the industry is that the bentonite is selected as the main material and supplemented by various additives to improve its engineering properties.The most suitable mixed buffer backfilling materials for engineering practice in China is bentonite-sand mixture.Its optimum mass ratio and many engineering properties need to be studied.Based on the Archimedes principle,this paper innovatively measured the effect of different sand content and different soluble salts on the soil shrinkage characteristic curve of bentonite-sand mixture at constant temperature and humidity by using balloon method.The MCG-B model was used to validate the reliability of the data measured balloon method.At the same time,an orthogonal laboratory drying test was designed according to the thickness and sand content of slurry sample at constant temperature,in order to study the moisture transfer and the cracking behavior of bentonite-sand mixture.GMZ01 sodium bentonite and quartz sand from Yongdeng County,Gansu Province were used as test materials.The study of soil shrinkage characteristic curve shows that the shrinkage of bentonite-sand mixture is significantly inhibited when the sand content reaches 30% or higher due to the skeleton effect of quartz sand particles.At the 30% sand content,the shrinkage of mixture is inhibited with the increase of soluble salt concentration and cationic valence in pore water.The mixing of quartz sand could increase the evaporation rate,while the soluble salt reduces the evaporation rate when the sample in saturated state,because of the additional osmotic suction.The fitting results show that the balloon method is accurate and reliable,but it is not suitable to determine the soil shrinkage characteristic curve of soil samples containing crystalline soluble salts.The study of cracking and water transfer shows that increasing sand content and increasing sample thickness could weaken the development of main fissure on the surface of samples.The increase of sand content will induce more small surface cracks,and with the increasing of sample thickness the minimum surface cracks is found when the sand content is 20%~30%.The influence of sample thickness and sand content on water retention capacity of soil samples is opposite,and the thickness of sample plays a dominant role in this competition effect.Through introducing drying coefficient k,it can be found that the evaporation rate is affected by both the air entry value and the actual soil-air contact area.According to the change of actual soil-air contact area,the water evaporation process can be divided into four stages: initial stage,air entry value stage,stable air channel stage and air filling stage.