Swelling of unsaturated GMZ07 bentonite at different temperatures
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Bentonite
Void ratio
Saturation (graph theory)
This paper investigates the swelling behaviour of compacted MX80 bentonite under hydration with the presence of initial voids that simulate the technological voids in real nuclear waste repositories. With water introduced into samples from the initial voids, the swollen bentonite first filled up the initial voids, and then swelling pressure was generated. At different hydration times, the microstructure was investigated at different heights (initial void, top, middle and bottom) using mercury intrusion porosimetry, together with the determination of dry density, water content and suction. It was observed that the microstructure was characterised by the presence of a large- and a small-pore families at bottom and middle layers, but by the creation of a new medium-pore family with dominant pore mode around 0·04–2 μm at the top and initial void layers after the void space was filled by swollen bentonite. From bottom to void, the significant increase in medium pores was accompanied by an increase in inaccessible pores and a decrease in large and small pores. Further examination showed that the sample could be divided into compression and swelling zones based on the variation of void ratio with swelling pressure: at the compression zone, the soil was compressed over time, characterised by a significant reduction of medium and large pores; whereas at the swelling zone, the soil was still undergoing swelling, represented by the increase of inaccessible pores and medium pores. In addition, the changes in microstructure along the sample were found to be well correlated with changes in dry density and water content: the water content was decreasing and the dry density was increasing from the initial void to the bottom positions; over time the dry density of the upper part was increasing due to compression, while the dry density of the lower part was decreasing due to swelling. This provided evidence for the main mechanism of compacted bentonite swelling with the presence of technological voids: the swollen bentonite fills the initial voids first and then undergoes compression by the swelling of the bentonite behind.
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The density of bentonite and of bentonite-sand mixt ure is the prime criterion in the evaluation of the swelling pressure and deformation of buffer material which must be ta ken into consideration in the design of any type of waste disposal facilities. A series of laboratory swelling pressur e and deformation tests using variable dry density of the specimens has been carried out to investigate the characteristics of buffer material for radioactive waste disposal. Initial dry density and loading pressure on the specimens has a noticea ble influence on maximum swelling rate. Temperature is also an important factor in the control of the swelling rat e of compacted bentonite. The void ratio increased in high initial dry density material and decreased for low level initia l dry density when compared with the initial state at the end of swelling due to static load. The swelling pressure fluctuated with elapsed time in respect to tempera ture. The maximum swelling pressure is dependent on the initi al dry density and the content of bentonite in bent onite-sand mixture.
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A multi-ring and a newly developed swelling pressure apparatus were adopted for measuring swelling pressure of three low dry density bentonites (sodium type: MX-80 and Kunigel-V1; calcium type: Kunibond). Subsequently, slices from the multi-ring were observed using X-ray diffraction to obtain basal spacings. Distance between particles was calculated from a basal spacing database. Relations between microstructural changes, including basal spacing and distance between particles, and swelling pressure are discussed. Results show that calcium-type bentonites have greater swelling pressure than sodium-type bentonites during saturation. When bentonites have the same cation type, lower montmorillonite content bentonite obtains smaller swelling pressures than the high montmorillonite content ones. Basal spacing increases drastically first and then maintains a stable value during saturation. All three bentonites gradually reach the 3 w state along with wetting (MX-80 and Kunigel-V1: 1–3 w; Kunibond: 2–3 w). The distance between particles increases with the saturation time. During saturation, swelling pressure generally rises with increasing basal spacing and increasing distance between particles. The mechanism for swelling pressure development is discussed.
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Bentonite
Void (composites)
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A series of experiments has been conducted at the G eosphere Research Institute of Saitama University, Japan to evaluate the effect of void ratio on swelling and p ermeability of bentonite. Void ratio is the prime c riterion for swelling and permeability of bentonite and bentonite-sand mi xtures when it is used as a buffer material for was te disposal facilities. At the end of swelling void ratio incre ased 7 to 8 times when compared with the initial st ate of the bentonite and bentonite-sand mixture. Permeability is also in creased as a result of an increase in void ratio.
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Bentonite
Consolidation
Expansive clay
Swell
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Abstract Swelling deformation tests of Kunigel bentonite and its sand mixtures were performed in distilled water and NaCl solution. The salinity of NaCl solution has a significant impact on the swelling properties of bentonite, but not on its surface structure. The surface structure was characterized using the fractal dimension D s . Based on the fractal dimension, a unique curve of the e m – p e relationship ( e m is the void ratio of montmorillonite and p e is the effective stress) at full saturation was introduced to express the swelling deformation of bentonite–sand mixtures. In mixtures with a large bentonite content, the swelling deformation always followed the e m – p e relationship. In mixtures with a small bentonite content, when the effective stress reached a threshold, the void ratio of montmorillonite e m deviated from the unique e m – p e curve due to the appearance of a sand skeleton. The threshold of vertical pressure for mixtures in different solutions and the maximum swelling strains were estimated using the e m – p e relationship. The good agreement between estimates and experimental data suggest that the e m – p e relationship might be an alternative method for predicting the swelling deformation of bentonite–sand mixtures in salt solution.
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Distilled water
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Investigation on swelling characteristics of buffer/backfill materials during hydration is an important issue in the design of artificial barriers in high-level radioactive waste (HLW) disposal repositories. In this work, for clarifying the characteristic of void ratio-suction relationship for compacted bentonite on hydration path, suction-controlled swelling deformation tests under constant vertical stresses 0.001~40 MPa were carried out on compacted bentonite specimens. Four different types of void ratio-suction curves indicated that swelling-collapse behavior under hydration depends on suction and over-consolidation ratio (OCR), based on which the swelling index was defined. Then, equations were proposed for describing the swelling-collapse characteristic of void ratio-suction curves. Simulation results of suction-controlled swelling deformation tests show that the different types of the hydration deformation curves could be well described by the proposed equations. Obviously, the proposed equations could be used for description and prediction of swelling characteristics of compacted bentonite during hydration, which is also of great importance for the safety assessment of the HLW repositories.
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Bentonite
Void ratio
Void (composites)
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