Study On The Swelling Characteristics Of Bentonite In Complex Environment

Bentonites have been often chosen as buffer material in nuclear waste repositories and underground landfill. It can fill up fracture because of high swelling potential, and its low permeability can prevent nuclear waste or contaminants contacting with the external environment. But these underground repositories landfill are often located in complex geological environment which will have a greater influence on the swelling characteristic of bentonite. In this paper, a comprehensive study on the swelling characteristic of bentonite in a complex environment has been conducted, in order to grasp the swelling characteristics of bentonite better to use its service to human society. The main contents of this paper are as follows:(1) Beginning with the presentation of swelling mechanism and structural characteristics of bentonite, this paper introduces double diffuse layer(DDL) model and fractal adsorption model. The applicability of two swelling model of bentonite are analyzed. A new prediction model of swelling pressure is proposed on this basis. The model uses DDL model to explain osmotic swelling phase and fractal adsorption model to explain crystal swelling phase. Besides, the swelling coefficient K in fractal adsorption model is calculated by DDL theory, and the factors affecting the value of K are discussed.(2) Details of the THMC coupling interactions in bentonite are presented. According to the different sizes of interactions, the coupling mechanism is simplified. Based on the THM coupling model established by preresearhers, introducing the new swelling model derived in this paper, the THM coupling mechanism is updated. In the re-established THMC multi-field coupling equations, different coupling constitutives are established between variables.(3) Based on the multi-field coupling model, the saturation processe of a bentonite clay barrier of nuclear waste repository is simulated. Its change of temperature, saturation, stress, displacement and other physical quantities are showen. The effects of various environmental factors on the swelling characteristics of bentonite are disussed.(4) Considering that the time scales of chemical reaction affecting swelling characteristics of bentonite are much longer than its saturation time scales. A transport chemical-reaction model is established in saturated bentonite to simulate the chemical field’s effect on the swelling characteristics of bentonite in long time scales.