混合型缓冲回填材料土水特征曲线测试与修正

 利用压力板法和水汽平衡法,对不同掺砂率和干密度的膨润土–砂混合物在脱湿过程中的土水特征曲线进行测试,探讨不同初始饱和条件、掺砂率与干密度对脱湿过程中混合物持水性能的影响。随着掺砂率和干密度增大,侧限饱和混合物试样持水性能下降;随着掺砂率增大,自由饱和混合物试样持水性能下降,而干密度对其持水性能基本没有影响。引入有效黏土密度和有效含水率的概念对混合物的实测土水特征曲线进行修正,发现饱和条件不同,混合物持水能力的控制因素也不同：侧限饱和条件下混合物持水性能主要受有效黏土密度控制,而自由饱和条件下混合物持水性能主要受掺砂率控制。根据试验数据拟合出侧限饱和与自由饱和后GMZ001膨润土–砂混合物修正土水特征曲线的经验公式,可用来估算高放废物处置工程中混合型缓冲回填材料的非饱和含水率。     

干密度对重塑花岗岩残积土土水特征曲线影响

采用压力膜仪法对深圳市福田区某基坑底部的花岗岩残积土重塑土样进行不同干密度下的土水特征曲线试验,研究不同干密度对花岗岩残积土土水特征曲线的影响,并采用Van Genuchten模型、FredlundXing模型及Gardenr模型进行土水特征曲线拟合,以给工程设计提供参考。试验结果表明:不同干密度对土水特征曲线有显著影响,且主要影响土水特征曲线的过渡段,干密度越大其持水性能越强、进气值越大; 3种模型均可较好模拟花岗岩残积土重塑样土水特征曲线。     

非饱和重塑弱膨胀土孔隙结构与土–水特征曲线试验研究

为研究压实程度对非饱和重塑弱膨胀土孔隙结构的影响,探讨膨胀土孔隙结构改变对土–水特征曲线的影响。以新疆哈密地区重塑膨胀土为研究对象,采用压汞试验测定压实程度对土样的微观孔隙结构影响,并利用热力学关系模型对膨胀土分形维数进行研究;采用滤纸法试验测量不同初始干密度下重塑土样的土–水特征曲线,提出可考虑压实作用影响的膨胀土土–水特征曲线双参数拟合模型,最后,基于土体孔隙特征,借助毛细管原理计算并结合滤纸法试验数据修正其土–水特征曲线。研究结果表明：随压实程度的增加,新疆哈密地区非饱和弱膨胀土孔隙呈初始三峰状并逐渐趋于双峰状分布,其在微观上表现为大孔隙的压缩引起的大孔隙向小孔隙的转化,孔隙内壁的粗糙程度及孔隙结构的复杂程度提高;土体基质吸力随含水率的增加而降低,相同体积含水率下,土体初始干密度越大,基质吸力越大,双参数模型可较好反映土体初始干密度对膨胀土土–水特征曲线发展规律的影响;基于毛细管原理的土–水特征曲线计算值同滤纸法实测曲线有相同函数模型发展规律,且随试样初始干密度的增加两者呈逐渐“远离”趋势发展;土–水特征曲线修正模型可统一描述非饱和弱膨胀土基质吸力随孔隙结构、含水率及密度状态的发...     

高庙子膨润土–砂混合料的三向膨胀力特性

 对不同干密度和含砂率高庙子膨润土–砂混合缓冲/回填材料的试样进行一系列三向膨胀力试验研究,结果表明,混合料三向膨胀力均随干密度呈指数关系递增,随含砂率呈指数关系递减。混合料存在各向异性,竖向膨胀力大于水平向膨胀力;含砂率和干密度均对试样的各向异性产生影响;当含砂率大于30%以后,含砂率的增加对其各向异性影响较小;混合料的各向异性随干密度的增大更加显著,并逐渐趋于稳定;对各向异性的机制进行分析,提出基于体积率的膨润土干密度这一指标,建立高庙子膨润土及其含砂混合料三向膨胀力的经验模型,并对模型进行验证和应用,研究成果可为高放废物深地质处置库中的缓冲/回填材料设计提供参考。     

玄武岩残积土土-水特征曲线试验研究

利用非饱和土固结仪测得不同竖向应力及干密度下的玄武岩残积土土-水特征曲线,采用Van Genuchten模型对试验所得的土-水特征曲线进行拟合,比较和分析加压固结后,土体干密度、应力状态的改变对其土-水特征曲线的影响。结果表明:VG模型对所得数据的拟合度较高,适合玄武岩残积土土-水特征曲线的建模;竖向应力越大,土体水分越不容易排出,曲线越平缓,进气值、残余含水率与竖向应力呈正相关,当竖向应力超过400 k Pa时,竖向应力的增加对土-水特征曲线的影响减小;随着干密度的增大,残余含水率增大,土体的持水能力增强;竖向应力对土-水特征曲线的影响主要通过改变土体的干密度来实现,而土体应力状态的改变对土-水特征曲线的影响较小。     

石英砂掺量对膨润土–砂混合物泥浆样干缩开裂的控制机制

高放废物地质处置工程中,膨润土–砂混合物作为缓冲回填材料的干缩开裂特征对工程屏障的安全性有重要影响。以混合型缓冲回填材料为研究对象,分别制备掺砂率为0%～50%膨润土–砂的浆状试样,通过室内恒温干燥试验,研究混合物的干缩开裂特征。结果表明:当掺砂率小于30%时,混合物的收缩曲线与径向应变曲线均基本重合,石英砂悬浮在膨润土中,混合物的干缩开裂特性由膨润土决定;当掺砂率大于30%时,混合物中石英砂逐渐相互接触,增加了颗粒间的摩擦力且有大孔隙形成,进而能够抑制混合物的干缩开裂。石英砂颗粒的相互接触显著提高了混合物进气值,略微提高了缩限,限制了混合物的干燥收缩。大孔隙的形成导致毛细水作用力的降低;颗粒间摩擦力的增加,增强了混合物抵抗断裂的能力,进而抑制了混合物干燥裂隙的发展。最终确定了抑制膨润土–砂混合物干缩开裂的最低掺砂率为30%。     

高庙子膨润土在高温高压下的强度特性研究

高庙子膨润土是中国高放废物地质库的首选缓冲/回填材料,将长期在高温(100℃左右)高压(数十兆帕)条件下工作。为了探讨高庙子膨润土在高温高压条件下的强度特性,使用高温高压土工三轴仪,共做了81个三轴不排水剪切试验,系统研究了干密度、围压、温度和含水率对其强度特性的影响。研究结果表明:(1)围压和干密度对高庙子膨润土的破坏形态的影响很大;(2)高庙子膨润土的强度随含水率增大而减小;温度和干密度对其强度的影响比较复杂,干密度较低时,偏应力–轴向应变曲线的位置随温度升高而上移;但干密度较高时的情况则完全相反;(3)分别建立了高庙子膨润土的黏聚力和内摩擦角随干密度、含水率和温度变化的公式。本文的研究成果为分析缓冲材料的热–水–力耦合性状提供了科学依据。     

建筑垃圾细料生产流动化回填材料的性能

以灰砂比0.03、0.05和0.08,粉砂比0、0.05、0.1、0.15和0.2为设计参数,对建筑垃圾回填材料进行设计。通过试验对回填材料的流动性(流动度、泌水率)、无侧限抗压强度以及应力应变曲线、本构关系模型和弹性模量等进行研究。研究结果表明:回填材料的流动度受水固比影响较大,两者接近线性关系;流动度在200~250mm范围,泌水率在4%~8%之间;回填材料抗压强度与灰砂比和水固比之间存在很好的幂指数关系;回填材料应力应变曲线形状与普通混凝土的相似,在此基础上提出回填材料的本构关系模型;回填材料无侧限抗压强度与弹性模量之间存在很好的指数关系。     

考虑干密度和压实含水率影响的弱膨胀土土水特征曲线研究

通过采用常规压力板仪对压实含水率、制样干密度、土样结构等3种因素对非饱和弱膨胀土土水特征曲线的影响进行研究,通过采用Van Genuchten模型、Fredlund-Xing模型,通过最小二乘法对所测土水特征曲线进行拟合,着重分析了土样孔隙结构对土水特征曲线的影响。结果表明,常用的土水特征曲线模型Van Genuchten模型和Fredlund-Xing模型均可较好地对压实膨胀土样和原状样土水特征曲线进行拟合,但对于石灰改性膨胀土Fredlund-Xing模型的拟合效果较好。     

不同干密度下玄武岩残积土土水特征曲线分析

采用美国SOILMOISTRUE综合压力板仪法,测试了原状与重塑玄武岩残积土样在不同密度下的土水特征曲线。试验结果表明:不同干密度下玄武岩残积土的进气值不同,干密度大的土样进气值较小,即相同含水率条件下土样的基质吸力随干密度的增大而增大;当基质吸力超过850kPa后,体积含水率随基质吸力的变化很小,重塑土与原状土土水特征曲线基本平行。进一步地采用Van Genuchten模型(简称VG模型)、Fredlund 3参数模型和Fredlund 4参数模型对试验所测的曲线进行了拟合,结果表明:VG模型能较好地拟合本次试验结果,同时给出了拟合参数,拟合的相关系数均大于0.99;对比Fredlund 3参数模型和Fredlund 4参数模型的拟合数据,可以看出假定残余含水率θr等于0是合理的,三种模型对玄武岩残积土土水特征曲线均具有良好的适应性,但VG模型拟合效果最为良好。     

混合型缓冲回填材料抗剪强度特性研究

高放废物深地质处置中,缓冲回填材料的抗剪性能对处置库的安全设计、施工及运营有重要的影响。本文从工程应用的角度出发,测试不同掺砂率时膨润土-砂混合物标准击实样在最优含水率条件下的抗剪强度指标。研究结果表明:随着掺砂率的增大,膨润土-砂混合物的抗剪强度指标略有下降;纯膨润土的抗剪强度指标随着干密度的增大而增大,随着含水率的增大而减小。通过测试和分析不同干密度和含水率时纯膨润土的抗剪强度指标,引入有效粘土密度和有效含水率的概念,对最优含水率条件下不同标准击实样的抗剪强度指标进行分析。     

石英砂掺量对混合型缓冲回填材料抗剪强度的控制机制

高放废物深部地质处置工程中,向膨润土中加入一定比例的石英砂可以优化缓冲回填材料的热传导性和可施工性。从"混合土"的概念出发,设计干密度和含水率相同、掺砂率不同的压实膨润土-砂混合物试样,通过剪切试验揭示掺砂率对抗剪强度的控制机制。试验材料选用内蒙古高庙子膨润土(GMZ001膨润土),石英砂按照质量0%～50%的比率添加。剪切试验结果表明:随着掺砂率的增大,压实混合物由应变软化型向应变硬化型过渡,黏聚力及内摩擦角逐渐减小,即抗剪强度降低。基于混合土孔隙结构假说,通过类比分析,对黏土-砂混合物抗剪强度的界限掺砂率进行估计,提供孔隙结构的扫描电镜证据。引入有效黏土密度和有效含水率的概念,描述石英砂颗粒之间的黏土基质的物理状态,阐释石英砂掺量对混合型缓冲回填材料抗剪强度的控制机制。     

Effects of Dry Density and Grain Size Distribution on Soil-Water Characteristic Curves of Sandy Soils

The soil-water characteristic curve (SWCC) of soil plays the key roll in unsaturated soil mechanics which is a relatively new field of study having wide applications particularly in Geotechnical and Geo-environmental Engineering. To encourage the geotechnical engineers to apply unsaturated soil mechanics theories in routine practice, numerical methods, based on the SWCC and saturated soil properties, have been developed to predict unsaturated permeability function and unsaturated shear strength properties which are expensive and time consuming to measure in laboratories. Further, several methods have been proposed to predict the SWCC in order to avoid difficulties in measuring the SWCC in laboratories. It is time consuming and it may require special techniques or apparatus to measure the SWCC in laboratories. However, it is important to have laboratory measured data of SWCCs to enhance and verify the proposed numerical methods. Hence, employing a Tempe pressure cell apparatus, the present study aims to investigate the effects of dry density and grain-size distribution on the SWCCs of sandy soils. Drying and wetting SWCCs were obtained for four sandy soils with different dry densities. The test data were best-fitted using the Fredlund and Xing (1994) equation and found that the fitting parameter, a, increases linearly with increasing the air-entry value of the SWCC and the fitting parameter, m, decreases with increasing the residual suction of the SWCC. The results revealed that soils with a low density have lower air-entry value and residual suction than soils with a high dry density. Further, the maximum slope of drying SWCC and hysteresis of drying and wetting SWCCs decrease with increasing density of soil. The air-entry value, residual suction, and hysteresis (the difference between the drying and wetting SWCCs) tends to decrease when the effective D₁₀ of the soil increases. A soil with uniform grain-size distribution (the steeper slope in grain-size distribution) has a less hysteresis and a greater slope of drying SWCC than those of a non-uniform soil.     

膨润土-砂混合材料三向膨胀力试验研究

利用改进的三向胀缩仪研究了掺砂率、干密度对高庙子膨润土-砂混合型缓冲/回填材料膨胀特性的影响。试验结果表明:混合物的三向膨胀力与掺砂率和干密度均呈指数关系,随干密度增加而增加,随掺砂率增加而减小;混合物的竖向膨胀力始终大于水平向膨胀力,在试验研究的范围内,水平向膨胀力与竖向膨胀力的比值随干密度变化较小,其值稳定在0.8左右;水平向膨胀力与竖向膨胀力的比值随掺砂率增加大致呈线性降低;混合物的三向膨胀力发展呈两个阶段,初始阶段膨胀力发展迅速可达到最大膨胀力80%左右,第二阶段膨胀力发展缓慢。最后从晶体扩张理论和双电层理论分析了混合物的膨胀特性机理。     

Swelling characteristics of Gaomiaozi bentonite and its prediction

Gaomiaozi （GMZ） bentonite has been chosen as a possible matrix material of buffers/backfills in the deep geological disposal to isolate the high-level radioactive waste （HLRW） in China. In the Gaomiaozi deposit area, calcium bentonite in the near surface zone and sodium bentonite in the deeper zone are observed. The swelling characteristics of GMZ sodium and calcium bentonites and their mixtures with sand wetted with distilled water were studied in the present work. The test results show that the relationship be- tween the void ratio and swelling pressure of compacted GMZ bentonite-sand mixtures at full saturation is independent of the initial conditions such as the initial dry density and water content, hut dependent on the ratio of bentonite to sand. An empirical method was accordingly proposed allowing the prediction of the swelling deformation and swelling pressure with different initial densities and bentonite-sand ratios when in saturated conditions. Finally, the swelling capacities of GMZ Na- and Ca-bentonites and Kunigel Na-bentonite are compared.     

Evaluation of hydro-mechano-chemical behaviour of bentonite-sand mixtures

Bentonite-sand mixtures are widely used in engineering barrier of deep geological disposal of high-level radioactive nuclear waste and anti-seepage barrier of civil geotechnical engineering. Under the action of groundwater solution infiltration and external stress, the hydro-mechanical (HM) behaviour of bentonite-sand mixtures, i.e. the swelling characteristics and permeability, will change. Once the anti-seepage and filtration effect is weakened or lost, the pollutants will spread to the biosphere. Therefore, it is necessary to study the swelling characteristics and permeability of bentonite-sand mixtures under coupled mechano-chemical (MC) effect and to establish corresponding prediction model. For this reason, swelling tests under salt solution with different concentrations are conducted on pure bentonite and its mixtures with 30%, 70% and 90% sand contents, the compression tests are carried out on saturated samples, and the saturated permeability coefficient k of the sample under each load is calculated by Terzaghi's one-dimensional consolidation theory. The concepts of true effective stress p_e, montmorillonite void ratio e_m and critical sand content α_s are introduced to determine the e_m-p_e relationship and finally the k-e_m relationship of bentonite-sand mixtures. It is found that when the sand content α ≤ α_s, the e_m-p_e relationship of the mixture is linear and independent of the salt solution concentration, and when α > α_s, the e_m-p_e relationship of bentonite-sand mixture is bi-linear with the true effective deviatoric stress p_esα as the intersection. In addition, the e_m-k relationship also shows the linear trend when α ≤ α_s, and the slope of the line increases with the increase of the salt solution concentration. When α > α_s, the k-e_m relationship will deviate from the linear relationship. Moreover, the larger the sand content is, the farther the deviation is. On the basis of summing the regularity, a model for predicting the HM behaviour of bentonite-sand mixture under the coupled MC effect is proposed. By comparing the swelling and permeability test results with model prediction results of different types of bentonite and its sand mixtures, the predictive model is verified. The study on the HM behaviour of bentonite-sand mixtures under salt solution infiltration and the model establishment can provide experimental and theoretical basis for the design and construction of anti-seepage engineering by bentonite-sand mixtures.