Hydraulic behaviour of bentonite based mixtures in engineered barriers: The Backfill and Plug Test at the Äspö HRL (Sweden)
Hydraulic behaviour of bentonite based mixtures in engineered barriers: The Backfill and Plug Test at the Äspö HRL (Sweden)
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发表时间：
2003-07
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通讯作者：
C. Mena
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作者：
C. Mena
1996年，瑞典放射性废物管理局（SKB）管理的Aspo硬岩实验室（瑞典）开始了回填和堵塞测试项目。回填和堵塞测试项目是SKB研究的重要组成部分，目的是以安全的方式将核废料储存在深层地质处置库中。ENRESA（西班牙放射性废物管理局）在该项目中与瑞典公司SKB和Clay Technology合作，对用于回填廊道的回填材料的水力机械行为进行表征。试验研究了钠基膨润土MX-80与花岗岩碎石（最大粒径为20 mm）按重量比为30%混合后的充填体。实验活动考虑了盐水对其流体力学行为的影响（高达16 g/L）。由于膨润土的含量，与天然粘性土相比，其活性相当大。因此，孔隙流体化学性质的变化影响该混合物的流体力学行为。由于目前的应用的混合物，作为候选人的密封画廊在未来的核废料储存库，兴趣集中在孔隙流体中的盐浓度的变化相关的渗透率的变化。进行了不同含盐量渗透试件的固结试验和不同含盐量渗透试件的吸水试验。还用蒸馏水和盐水进行了压实试验。研究了钠基膨润土与砂在相同质量比下的不同混合物的渗透吸力。用石英砂代替花岗岩，用晶体管干湿球温度计测定。混合物的实验研究包括一个新的迷你测压计，允许进行恒定和可变的头在饱和粘性土测试的设计和校准。还开发了一些数字工具，用于分析在实验室和现场进行的脉冲试验（2003年3月下旬在AHRL全尺寸实验室的ZEDEX画廊进行的试验）。不同的方法（解析，半解析和数值）被用来分析在实验室和现场进行的脉冲测试。最后，论文重点是使用一种新的有限元模拟器，它解决了热-水-机械-化学（THMC）的问题，在一个完全耦合的方式（吉马良斯，2002年），模拟饱和过程的障碍，核废料由这种混合物。主要研究了孔隙流体中盐分浓度变化对混合物水力特性的影响。考虑了Na+和Ca ~（2+）离子交换反应的简单地球化学模型。从实验信息中获得的渗透率随孔隙流体盐浓度变化的经验规律，在有限元程序中实现。研究了混合物分子扩散的影响，指出了在求解溶质在多孔介质中运移问题时，该参数的重要性。总吸力计算为基质吸力和渗透吸力之和。通过获得盐浓度并使用Van 't霍夫方程计算渗透吸力。这样，渗透吸力就不像工程实践中通常认为的那样被假定为常数。本论文的贡献，材料的特性，这是很难调查，它提供了准确的信息回填渗透性，设计核废料处置库时的关键参数之一。
In 1996 the Backfill and Plug Test Project started at the Aspo Hard Rock Laboratory (Sweden) managed by SKB (the Swedish Radioactive Waste Agency). The Backfill and Plug Test Project makes up an important part of SKB's research in order to store nuclear waste in a deep geological repository in a safe manner. ENRESA (the Spanish Radioactive Waste Agency) collaborates in this project with the Swedish companies SKB and Clay Technology in characterising the hydro-mechanical behaviour of the backfill material used to backfill a gallery. The backfill, obtained by mixing 30% of sodium bentonite MX-80 and 70% of crushed granite rock by weight, which maximum grain size was 20 mm, has been experimentally investigated. The experimental campaign took into account the salt water effects (up to 16 g/L) on its hydro-mechanical behaviour. Because of the bentonite content, its activity is quite large if compared with natural clayey soils. Therefore, changes in the pore fluid chemistry influence the hydro-mechanical behaviour of this mixture. Due to the current application of the mixture, as candidate for sealing galleries in a future repository for nuclear waste, interest was focused on the variation of permeability related to variation of salt concentration in pore fluid. Oedometer tests on specimens permeated with different salt water contents and water uptake tests permeated with different salt water contents were performed. Compaction tests with distilled and salt water were also performed. Osmotic suction was investigated on a different mixture of sodium bentonite and sand keeping the same weight ratio. Crushed granite was substituted by sand in order to use transistor psychrometers were used in this determination. The experimental study of the mixture included the design and calibration of a new mini-piezometer which allows performing constant and variable head tests in saturated clayey soils. Some numerical tools were also developed to analyse pulse tests performed in laboratory and in situ (tests performed in late March of 2003 in the ZEDEX gallery in the AHRL full-scale laboratory). Different methods (analytical, semi-analytical and numerical) were used to analyse the pulse tests performed in laboratory and in situ. Finally, the thesis focused on using a new finite element simulator, which solves thermo-hydro-mechanical-chemical (THMC) problems in a fully coupled way (Guimaraes, 2002), to simulate the saturation process of a barrier for nuclear waste made up with this mixture. Interest was mainly focused on simulating the influence on the mixture hydraulic behaviour when salt concentration in pore fluid changed. A simple geochemical model, which took into account the ion exchange reaction between of Na+ and Ca2+, was considered. An empirical law of permeability variation with pore fluid salt concentration, obtained from the experimental information, was implemented in the finite element code. Effects of mixture molecular diffusion were also investigated, pointing out the importance of this parameter when transport of solutes in porous media is solved. Total suction was computed as the sum of matric suction and osmotic suction. Osmotic suction was calculated by obtaining salt concentrations and by using the Van't Hoff equation. In this way, osmotic suction was not assumed constant as it is usually considered in engineering practice. This thesis has contributed to the characterisation of a material, which is difficult to investigate, and it has provided with accurate information of the backfill permeability, one of the key parameters when designing a nuclear waste repository.