基于CART对污染红黏土氧化物与强度特性的数据分析

作为一种区域性特殊土，红黏土裂隙发育，力学性质较差，而重金属污染将进一步加剧其不稳定性。为探究红黏土在重金属污染下的强度特征，通过XRF分析、直剪试验和抗压试验，分析了污染红黏土中Al₂O₃、TiO₂、K₂O等氧化物含量和力学性质与铬离子掺量的关系，并基于分类与回归树(CART)的方法，建模、拟合并预测了红黏土的力学性质随铬离子浓度的变化规律，分析了红黏土中氧化物对其强度的影响。试验结果表明，红黏土的抗剪强度、内摩擦角随铬离子浓度的升高呈下降趋势，而黏聚力随铬离子浓度的升高呈上升趋势，铬离子的引入使红黏土的无侧限抗压强度显著下降。同时CART建模表明土样中的氧化物可对红黏土的强度特性产生影响。     

含盐量对极细颗粒黏土强度影响的试验研究

 为深入研究滨海盐渍土等极细颗粒黏土的不良工程特性产生的物理化学机制,探讨在水的作用下由可溶盐离子引起土颗粒表面微电场的变化对极细颗粒黏土抗剪强度的影响,首先对一系列含盐量即不同孔隙液离子浓度下的人工土(膨润土、高岭土与膨润土的混合土、石英)进行常规直剪与液塑限试验,同时测试人工土的阳离子交换量与比表面积等微细观参数用于计算颗粒的表面电位,建立抗剪强度与表面电位之间的关系。测试结果表明,可溶盐离子通过扩散的双电层改变土颗粒表面电位,引起结合水膜厚度的变化致使土颗粒之间的变形阻力发生改变,在宏观上表现为土体强度的变化。结合科威特Bubiyan岛的海港盐渍土软基加固工程,研究可溶盐含量对当地滨海盐渍土的强度与液塑限的影响,证实孔隙液离子浓度对人工土的强度的影响机制与变化规律同样适用于天然极细颗粒黏土,为从微细观角度揭示盐渍土的不良工程特性提供理论解释与试验基础。     

重金属污染对土压实性及抗剪强度影响的试验研究

通过在实际重金属冶炼企业的尾矿库、废料堆填区采取重金属尾矿样品,与代表性净土进行掺和,配制了不同压实度、不同龄期、不同掺和比的重金属污染土样;在各自的最优含水量下,进行击实和直接剪切等物理力学试验,研究重金属污染对土的压实特性与抗剪强度的影响规律。试验表明:1)重金属污染对土体击实特性的影响,金属Mn的影响不大,Pb、Zn、Cd、Cr影响较大,后者表现为随掺和比增大,最优含水量减小,土的最大干密度增大;2)随着重金属污染土体压实度的增大,土的黏聚力C逐渐增大的,内摩擦角φ值逐渐降低,污染土的强度指标明显低于重塑净土的抗剪强度指标;3)重金属污染的作用效应是一个缓慢的过程,随着重金属污染物作用时间的延长,土的黏聚力c值下降,内摩擦角φ值上升;4)随着污染浓度的增加,土的黏聚力c值增大,则φ值呈降低趋势。     

重金属污染膨胀土膨胀变形试验研究

通过不同初始含水率、初始干密度和不同重金属离子(Pb2+、Zn2+、Cu2+)含量的无荷载膨胀变形试验,研究南阳膨胀土无荷载膨胀率与初始含水率、初始干密度以及重金属离子含量的关系。研究结果表明,膨胀土无荷载膨胀率随初始含水率的增加而减小,随初始干密度的增加而增大,回归分析发现,无荷载膨胀率与初始含水率和初始干密度成线性关系;重金属污染膨胀土中Pb2+、Zn2+、Cu2+含量存在界限值,当重金属离子含量小于界限值时重金属污染膨胀土无荷载膨胀率随重金属离子含量增加而增大,当重金属离子含量大于界限值时无荷载膨胀率随重金属离子含量增加而减小,但污染膨胀土无荷载膨胀率始终大于未污染膨胀土。     

Ca~(2+)浓度对膨润土掺砂混合土渗透性能的影响

垃圾填埋场底部铺设含黏土或膨润土掺砂混合土的防渗系统,防止渗滤液流入地下污染周边环境。采用模型试验研究Ca~(2+)离子浓度和基础局部沉降对膨润土掺砂混合土渗透特性的影响。试验结果表明Ca~(2+)浓度对膨润土掺砂混合土的渗透性能有较大影响,渗透液中Ca~(2+)浓度超过一定值后混合土渗漏;Ca~(2+)离子浓度与发生渗透时间之间有很大关系,浓度越高发生渗透的时间越短;同样沉降量条件下,含Ca~(2+)离子溶液渗透的混合土均发生渗漏,而用蒸馏水作渗透液的没有出现渗漏;钠基膨润土遇Ca~(2+)离子溶液后,当Ca~(2+)离子浓度大于两倍Na+就会发生离子交换,钠基膨润土转化为钙基膨润土,混合土的防渗性能下降。因此开展Ca~(2+)离子浓度对膨润土掺砂混合土渗透特性的研究具有较好的理论和实际意义。     

锌污染黏土工程性质的试验研究

随着全球经济的持续发展,地基土重金属污染现象越来越严重,给工程建设带来严重的危害,而重金属污染土的工程性质研究较少。为进一步揭示锌污染黏土的工程性质,开展了人工制备锌污染土的系统室内试验。试验结果表明:随着锌离子浓度的增加,污染土的土粒相对密度增大,液、塑限减小,塑性指数增大。锌污染土的压缩性增大。压缩系数随锌离子浓度的增大而呈增大的趋势,压缩模量呈减小的趋势。污染土的抗剪强度随锌离子浓度的增大而减小。污染土的不排水不固结应力-应变关系曲线呈应变硬化型,呈塑性破坏特征。     

水泥固化重金属污染土的淋滤特性试验研究

固化稳定法是目前处理重金属污染土场地的常用方法之一。经过处理后的污染土,不仅在强度上有所提高,而且重金属污染离子亦能被有效固化稳定下来。目前,这方面的研究成果主要集中在固化污染土的工程性质变化方面,而对固化土中的重金属离子的滤出特性研究较少。通过系统的室内试验,以经水泥固化后的铅和锌污染土为研究对象,着重研究固化污染土中重金属离子的淋滤特性。试验结果表明:水泥固化重金属污染土后,随着固化剂掺量和养护龄期的增加,重金属的滤出率显著降低,并最终趋于稳定。在污染物掺量较低时,水泥对Pb2+的固化效果好于对Zn2+的固化效果;随着污染物掺量的增加,滤出液中Pb2+浓度的增幅要大于Zn2+浓度增幅。在污染物掺量较高时,水泥对Zn2+的固化效果好于对Pb2+的固化效果。     

水泥固化重金属污染土的淋滤特性试验研究

固化稳定法是目前处理重金属污染土场地的常用方法之一。经过处理后的污染土,不仅在强度上有所提高,而且重金属污染离子亦能被有效固化稳定下来。目前,这方面的研究成果主要集中在固化污染土的工程性质变化方面,而对固化土中的重金属离子的滤出特性研究较少。通过系统的室内试验,以经水泥固化后的铅和锌污染土为研究对象,着重研究固化污染土中重金属离子的淋滤特性。试验结果表明:水泥固化重金属污染土后,随着固化剂掺量和养护龄期的增加,重金属的滤出率显著降低,并最终趋于稳定。在污染物掺量较低时,水泥对Pb2+的固化效果好于对Zn2+的固化效果;随着污染物掺量的增加,滤出液中Pb2+浓度的增幅要大于Zn2+浓度增幅。在污染物掺量较高时,水泥对Zn2+的固化效果好于对Pb2+的固化效果。     

纳米膨润土加固黏土强度的试验研究

纳米膨润土是一种新型的土体加固材料,能有效提高土体的物理力学性质。掌握纳米膨润土加固土体的特性对于该技术的推广应用具有重要的意义。本文开展了10组无侧限抗压强度试验,研究了掺加纳米膨润土后黏土土样的力学性质,以及纳米膨润土掺量、土的含水率和干密度三个因素对土体无侧限抗压强度的影响。结果表明,纳米膨润土的掺加能有效提高黏土的抗压性能,且抗压强度随纳米膨润土掺量、土干密度的增加呈非线性增加;随土含水率的增加,抗压强度先减少后增加。黏土内掺入纳米膨润土后,由于土的孔隙会被填充,形成密集结构,纳米膨润土-黏土颗粒界面的有效接触面积增加,使黏土体的强度显著增加。同时,水分会在纳米膨润土-黏土界面起到润滑作用。本研究表明,这一土体加固技术能有效提高地基土的承载力和变形性能,因此具有巨大的应用潜力。     

黄淮地区深部黏土工程性质试验研究

总结了黄淮地区深部黏土的物理力学性质,包括密度、含水量、颗粒大小、矿物成分、膨胀性、压缩性、抗剪强度等。研究表明,黄淮地区深部黏土主要为高液限饱和膨胀土,其物理力学性质指标随埋深或液性指数变化一般表现出总体上增大或减小,而局部呈无规则振荡变化的规律。对深部黏土重塑土的常规三轴试验表明,其内摩擦角较一般浅层黏土低,而且在高围压下强度包线出现向下的转折。     

The Influence of the Dielectric Constant and Electrolyte Concentration of the Pore Fluids on the Undrained Shear Strength of Smectite

It is common knowledge that the engineering properties of clays are greatly influenced by the type of pore fluids. However, the impact of the pore fluids on the geotechnical properties of the samples is even more dependent on the type of mineral and interlayer ions. Completely different behaviours could be observed with identical pore fluids but different clay minerals and vice versa. The liquid limit, plastic limit and undrained shear strength were determined for two types of smectites with different interlayer cations, namely Ca and Na-smectite. The pore fluids were varied by using different dielectric constants (ε) and electrolyte concentrations (n₀). The results show that the two kinds of soils respond in a similar way but with a different magnitude depending on the ion occupation.     

水泥固化稳定重金属污染土的工程性质试验研究

随着社会的快速发展,因工业生产以及人类活动引起的地基土重金属污染现象越来越严重。地基土受到重金属污染后,其工程性质会发生改变。固化稳定法是处理重金属污染土地基的常用方法之一。以人工制备的铅或锌重金属污染土为研究对象,通过系统的室内试验,着重研究水泥固化稳定重金属污染土的工程性质。试验结果表明,土体受到污染后,其强度降低;掺入水泥固化稳定的重金属污染土的强度随水泥掺入量以及养护龄期的增加显著增大。通过试验还发现,较低浓度重金属离子的存在可以促进水泥固化土抗剪强度的提高。     

南海北部陆坡软黏土全流动强度试验研究

鉴于海底软黏土强度测试困难和精度不足的现状,研发了一种适用于低强度、高含水率土体强度测试的新型全流动贯入仪,并进行了有效性校验。在此基础上,针对南海北部陆坡区典型软黏土,开展了多组原状试样的全流动强度试验,分析了试验中初始阻力系数N与重塑阻力系数N_(rem)的取值范围,给出了软黏土扰动前后不排水剪切强度沿深度的分布特征及变化趋势,并结合微观孔隙面积比和宏观构造灵敏度,探究了研究区土体的强结构特征。最后,基于重塑不排水剪切强度与含水率/液限间的关联性分析,提出了适用于研究区土体的不排水剪切强度归一化模型,为南海北部陆坡区海底能源开发、海洋工程基础设计与地质灾害预测提供参考。     

用圆锥试验确定粘性土性质指标的新方法

本文用圆锥试验和常规试验对原状土和重塑土的物理力学性质进行了大量的对比试验,根据这些试验成果提出了确定粘性土性质指标的新法。这个方法十分简捷,一个试验人员在一小时内可完成一个土样的液限、塑限、天然稠度、渡性指数、灵敏度等试验,并得出原状土和重塑土的无侧限抗压强度和不排水抗剪强度。如果采用常规试验,将要许多天才能完成。此外,本文还分析了圆锥入土深度和抗剪强度之间的关系,并给出了液塑限和不同稠度的抗剪强度。     

Properties of very soft clays: A study of thixotropic hardening and behavior under low consolidation pressure

In a study on the properties of very soft clays, bender element testing was used to evaluate thixotropic hardening behavior; that is, to measure the stiffness with resting time under constant volume and water content. A laboratory vane test, which measures the undrained shear strength of the materials, was also carried out for comparison purposes. To investigate the mechanism of the thixotropic phenomenon, a consolidation test with very low pressure was also performed in a cell equipped with bender elements. The most important findings from this study are as follows: (1) regardless of soil types, the effect of thixotropy was significant around the liquid limit state and less remarkable at the lower and higher ranges; (2) the shear modulus at the liquid limit after 24 h resting is around 200 kPa; (3) the correlation between the shear modulus and the undrained shear strength of very soft clays is similar to that of cement-treated soil proposed by Seng and Tanaka (2011); (4) the increment of the shear modulus developed in the thixotropy process appears to be noticeably higher than that in the secondary consolidation process. It is believed that these findings are very useful to establish a new theory for the consolidation of ground filled by very soft clays or dredged soils with extremely high water content as well as to understand the effects of ageing on the consolidation properties of natural soils.     

Some relations among fall cone penetration,liquidity index and undrained shear strength of clays considering the sensitivity ratio

This note describes some relations among fall cone penetration, d, liquidity index, LI, and undrained shear strength s_u, of clays. Fall cone tests are used to assess the liquidity index considering also the sensitivity ratio of undisturbed and remoulded soils, based on the cone penetration. Considering the British and the Swedish cones, it is possible to observe some differences between d values. Because the cone penetration amount is dependent on the undrained shear strength and that undrained strength of clays can be related to LI, a relation correlating LI based on the fall cone method and the cone penetration amount d for remoulded soils was obtained. It was observed that in remoulded soils, the LI−d relationship for each cone is unique irrespective of the soil type, testing equipment and operators. The correlation between liquidity index and undrained shear strength is provided considering new data and literature values taking into consideration the sensitivity ratio as well. From the LI−s_u−S_t relationship, it is possible to obtain a general overview about the strength characteristics of soils. Sensitivity ratio can also be extrapolated by liquidity index values considering a coefficient ‘a’ which might be related to the sedimentary environment (e.g. salt content), mineralogical composition and structure of clays, which however must be confirmed by further research. Finally, the predicted sensitivity ratio S_t^* is evaluated against the measured one within a range of (0.5-2)S_t.

     

不同重金属离子对C S H凝胶影响及其固化稳定性

通过溶液法人工合成不同钙硅比C-S-H凝胶,借助XRD,SEM,XPS,TEM,ICP等测试方法,分析研究了CSH凝胶对不同种类、不同含量重金属离子(Cr3+,Cr6+,pb2+,Zn2+,Cd2+等)的俘获能力及其稳定性.结果表明:钙硅比不同,CSH凝胶对重金属离子固化稳定性及其固化机制均存在一定差异,高钙硅比CSH凝胶对重金属离子固化稳定性较强;重金属离子种类及价态不同,其在C-S-H凝胶中的结合方式及固化能力也不同,其中CSH凝胶对Cu2+,Cd2+,Zn2+等离子的固化能力较强,对Cr6+的固化能力较弱.水化液相中,CSH凝胶对Cr3+,pb2+,Zn2+等多种重金属离子的固化能力低于其对单一重金属离子的固化能力;水化液相中Cr6+的存在,会对CSH凝胶对pb2+的固化及其稳定性产生巨大影响,而Cr3+的存在对其影响则较小.     

Effect of spatial variability of block-type cement-treated ground on the bearing capacity of foundation under inclined load

Deep mixing methods are widely used for stabilizing soft clayey soils and improving their bearing capacity. However, spatial variability in the shear strength of the cement-treated ground introduces uncertainties in estimating the bearing capacity for design. This paper evaluates the reliability of, block-type, cement-treated foundation under inclined load conditions using random field numerical limit analyses. The undrained shear strength is modelled as a random field which is characterized by a log-normal distribution and a spatial correlation length. Monte Carlo simulations are then used to interpret the stochastic bearing capacity factor and failure mechanisms for inclined concentric loading conditions at selected ratios of the shear strength ratio of cement-treated ground to original clay, the coefficient of variation in undrained shear strength and correlation length of the cement-treated zone. Variability of the undrained shear strength can reduce the expected bearing capacity of the cement-treated ground by 50–70% compared to homogeneously mixed clay.     

Static liquefaction-triggering analysis considering soil dilatancy

The failure of a sloping ground due to static liquefaction occurs when the shear stress applied by a monotonic triggering load exceeds the undrained yield (peak) shear strength of the saturated liquefiable cohesionless soil. Current practices for determining the in-situ undrained yield strength for grounds subjected to static shear stress rely on either a suite of costly laboratory tests on undisturbed field samples or empirical correlations based on in-situ penetration tests, which fail to account for the effect of soil dilatancy in decreasing the degree of strain-softening and the brittleness of cohesionless soils with an increasing penetration resistance. In this study, the effect of soil dilatancy on the static liquefaction failure of cohesionless soils is characterized by an empirical relationship between the soil brittleness index and the undrained yield strength from a database of 813 laboratory shear tests collected from the past literature. The application of this relationship for estimating the static liquefaction-triggering strength of cohesionless soils under sloping ground conditions is validated by comparing several cases of liquefaction flow failures. Finally, a procedure is briefly demonstrated for evaluating the triggering of static liquefaction in a dyke to the north of Wachusett Dam and Duncan Dam which incorporates the dilatancy behavior of cohesionless soils in a semi-empirical procedure based on in-situ penetration tests.