高液限花岗岩残积土的物理特性和剪切特性

高液限花岗岩残积土广泛分布于华南地区,针对其中的含粗粒细粒土进行了物理特性和剪切强度特征的试验研究,包括土的组成和微观结构、土的物理状态、土的峰值强度和残余强度等。基于试验结果分析了细粒含水率及对土体物理状态的影响,归纳了土中粗粒含量与天然含水率之间、土的天然状态指标与抗剪强度参数之间的统计规律,探讨了应用反复剪切试验方法确定此类土残余强度的注意事项及大剪切位移时的剪应力特征。由于此类残积土具有高液限土和粗粒土的综合特征,因而需基于粗粒含量的影响来认识有关基本特性。     

Shear strength estimation of plastic clays with statistical and neural approaches

Because shear strength parameters highly influence the bearing capacity of soils, several researchers have carried out large number of experimental and theoretical studies aimed at understanding soil strength behaviors. Within this context, the determination of correlations between soil index properties and shear strength parameters for specific soil types is possible. The aim of this study is to observe the performance of statistical and artificial neural network (ANN)-based methods on establishing correlations between index properties and shear strength parameters of normally consolidated plastic clays. To collect modeling data, consolidated-undrained triaxial tests were performed on normally consolidated plastic clays obtained from the same region. Additionally, detailed statistical analyses were conducted on the test data. Results indicate that the ANN-based model is superior in determining the relationships between index properties and shear strength parameters. However, in order to get appropriate outcomes, specific care must be dedicated when applying ANN-based correlation models.     

不同黏性土的残余强度及其抗剪强度指标特性研究

利用环剪仪系统地研究了17种不同塑性指数的饱和黏性土在不同固结状态下的残余强度的变化规律,分析了当前法向应力、超固结比、塑性指数和多级剪切方式对残余强度的影响,重点给出了大变形条件下的抗剪强度指标随塑性指数的变化规律及实用公式。试验结果表明:残余强度随当前法向应力的增大而增大;超固结比对残余强度没有显著的影响;随着塑性指数的增大,残余强度逐渐减小;残余强度在较小的当前法向应力水平下具有明显的非线性;在大变形条件下,残余内摩擦角随塑性指数的增大而减小;最后给出了残余内摩擦角与塑性指数的函数关系式。     

滑带土环剪剪切面的微观观测与分析

 滑带土的残余强度一直是边坡工程研究的重点。由于剪切面微观结构在剪切前后的变化与宏观的土体强度存在必然的联系,所以针对一古滑坡滑动带的含砂黏土进行环剪试验,采用电镜扫描技术分析剪切前后微观结构的变化,探讨微观结构与土体宏观力学特征的关系。研究表明,含砂黏土的强度特征不同于一般黏性土,没有明显应变软化特性,粗粒含量对峰后强度有比较明显的影响。剪切过程会导致颗粒出现明显的定向性排列,微观结构发生改变,出现颗粒破碎、孔隙率提高,颗粒形状变狭长的现象,这与滑带土的宏观强度变化有密切的关系。另外,通过周长–面积法对剪切面颗粒形态的分形研究,发现剪切面土颗粒微观形态具有明显的分形特征,分形维数与滑带土的残余强度指标呈线性负相关关系。     

抗剪强度概率特性的相关与非相关回归统计法

正确统计土体抗剪强度指标的均值和标准差是确定抗剪强度参数合理取值的关键手段。由于实验组数较少，相对均值而言，标准差或变异系数的统计显得十分困难。特别是传统的线性回归方法并不能获得真正的抗剪强度指标标准差。基于概率论和泰勒展开方法，提出了能计算抗剪强度指标标准差和相关系数的回归统计方法，同时也给出了简单有效的不考虑强度指标互相关性的回归统计方法，并利用大围压下土体抗剪强度变异性主要来源于f的特性，将大围压下的数据进行综合统计，解决了土工实验中棘手的样本数量问题。应用不同的方法对小浪底心墙料320个三轴实验成果进行了统计分析，发现不同方法都能较好地预测强度参数的不确定性。然而较大的负相关系数的存在使得矩法得到的标准差要高于非相关回归统计的结果，因此在实际应用中，利用矩法的成果时不应忽略相关系数，而在诸如分项系数设计或其它默认相关系数为0的条件下，应使用基于非相关假定的回归统计成果，否则将高估抗剪强度的不确定性。 方法     

基于直剪试验的金坛盐岩力学特性研究

 采用RMT–150C试验机对江苏金坛拟建储气库埋深段盐岩试样进行一系列直剪试验研究。将盐岩作为多晶聚合体考虑,深入分析试验过程中的剪切应力、剪切变形、剪胀以及破坏特性。试验结果表明：盐岩剪切破坏是一种延性破坏,盐岩剪切峰值对应的剪切位移一般为4～6 mm,个别试样可达7～8 mm,且整个剪切应力–剪切位移曲线都较平缓,剪切峰值不明显,这主要是由于局部位错交替导致的;得到盐岩抗剪强度参数c,? 值分别为3.16 MPa,44.7°;发现残余强度基本上与法向应力呈正比例关系,且残余应力较大,约为峰值应力的50%～80%,表明盐岩摩擦承载能力较强;盐岩剪胀终止发生在峰值应力之后、残余应力之前,且在较大法向应力作用下剪胀起始应力与剪胀终止应力接近;盐岩的剪切破坏位置不是一个面,而是一个破碎带,破碎带上下一定范围内有不同程度损伤;表面局部有明显擦痕,类似于摩擦学的“犁沟效应”,有利于提高其抗剪能力。试验研究成果对深入理解金坛盐岩破坏机制及地下盐岩储气库稳定性研究具有一定的参考价值。     

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。     

内蒙古好鲁库地区硅藻土物理力学性质研究

硅藻土结构性显著,是非常罕见的特殊性岩土,通过室内含水率、侧限压缩、直接剪切等试验,对比分析了内蒙古某高速公路不同埋置深度硅藻土的基本物理性质,研究了压缩特性,探讨了硅藻土抗剪强度参数与埋置深度的相关性。结果表明,硅藻土质地轻、孔隙比大、有机质含量高,虽然在天然状态下硅藻土多为半固态或固态,但具有极高的含水率;e-p曲线可根据孔隙比随竖向荷载减小速率的变化分为2个阶段,对应E_s-p曲线压缩模量基本平稳和显著提升两阶段,好鲁库地区硅藻土E_s-p曲线上难以找到明显的结构屈服应力;在不高于1 600 kPa的荷载条件下压缩变形稳定后,好鲁库地区硅藻土还具有大孔隙比和较小的压缩模量,可能产生可观的后期变形;抗剪强度参数与深度线性相关性不显著,确定抗剪强度参数标准值可以不考虑参数与深度的相关性。     

Shear strength of an unsaturated weakly expansive soil

To study the weakly expansive clay obtained from a slope along Wuhan—Shiyan expressway in Hubei Province,soil-water property tests and some unsaturated triaxial tests with suction control were conducted,and the soil-water retention curve（SWRC） and unsaturated shear strength of this soil were obtained.Results show that the air-entry suction and the residual degree of saturation of the tested soil are 106 kPa and 8%,respectively.The boundary effect zone and the transition zone can be identified on the desorption curve,but the residual zone is not so obvious.The unsaturated shear strength increases as suction increases within the range of controlled suction in the test,and friction angle,b,in the triaxial shear test is 17.6°.Based on the results,constitutive models for predicting the unsaturated shear strength using the SWRC were evaluated,and comparisons between prediction and measurement were made.It is concluded that for engineering purpose,the constitutive model should be carefully selected based on soil properties when predicting the unsaturated shear strength using the SWRC.     

冻融循环对冻土-混凝土界面冻结强度影响的试验研究

为研究冻融循环作用对冻土-混凝土界面冻结强度的影响,对不同冻融循环次数、法向应力、试验温度及土体初始含水率条件下的冻结界面进行了系列直剪试验,研究经历冻融循环后界面峰值剪切强度、残余剪切强度及强度参数的变化规律。试验结果表明:冻融循环对界面剪切应力与水平位移曲线形态影响很小,经历20次循环后曲线仍是应变软化型。冻融循环对峰值剪切应力的影响强于对残余剪切应力的影响,表明其对界面胶结冰含量产生影响。当土体初始含水率较低且温度较高时,冻融循环使界面峰值剪切强度增加,但变化量较小。然而在含水率较高(20.8%)及试验温度较低时(-5℃),峰值剪切强度随着冻融循环增加而降低。因此在土体含水率较高且冻结温度较低时,对于发生小变形的冻结界面需要重视冻融循环对峰值剪切应力的影响。不同初始含水率、试验温度下冻融循环对残余剪切强度的影响较小且变化规律不明显。在试验温度为-1℃,-3℃,-5℃时,峰值黏聚力随冻融循环增加分别表现为增加、波动和下降,推测是由于界面胶结冰含量不同而引起。峰值摩擦角和残余摩擦角随冻融循环次数增加略有变化。     

岩石力学特性分析与高速滑坡启动速度预测

斜坡高速滑动之前，在静力极限平衡状态时形成与抗剪强度相当的抗滑力。一旦锁固段突然被剪断，由于峰残强降效应，原有下滑力的一半得到释放，赋存其中的弹性应变能则转换为滑坡剧动的动能。因此，滑坡体在启程的瞬间产生相当高的初速度，并使其沿着滑动面高速下滑。以云南昭通头寨沟大型高速远程岩质滑坡为例，通过常规三轴压缩试验，研究了滑坡区主要工程地质岩组(玄武岩)岩芯的变形和强度特性。试验结果表明，玄武岩的峰值强度与残余强度之间存在显著的差异，峰残强降率为50％左右。并在试验研究的基础上，对头寨沟滑坡启程初速度进行了预测。     

Residual shear strength of fatigue cracked slender web panels

A series of quasi-static and fatigue tests on short span, slender plate girders subjected to predominantly shear loading is described. The fatigue test results are presented in terms of the normalised load range versus the number of load cycles to failure, and compared with other available test data. A simple procedure for predicting the residual shear strength of a fatigue cracked web panel, based on an assumed equilibrium stress field, is developed and shows reasonable correlation with the test results.     

重金属污染土物理性质强度定量关系试验研究

通过室内试验研究土体孔隙液中重金属离子影响土体物理力学性状的机理,采用不同浓度的重金属Cu2+、Zn2+、Pb2+掺入高岭土、伊利石类黏土和钠基膨润土,研究土体界限含液率、不排水抗剪强度随重金属离子浓度的变化规律。试验结果表明:以高岭石、伊利石为主要黏土矿物的低活性土的液限、塑性指数随离子浓度的增加而增大,钠基膨润土则表现出相反的变化规律;低活性黏土的不排水抗剪强度随离子浓度增加而增大,高活性黏土强度则随之减小;基于已有的无污染土体物理力学性状定量联系,分析重金属污染土的不排水抗剪强度与液性指数的定量关系,发现重金属离子引起的不排水抗剪强度的变化可以归结于重金属离子引起的液塑限变化,表明重金属Cu2+、Zn2+、Pb2+污染过程基本没有产生化学反应,其物理力学定量关系与已有的无污染土经验关系式一致。     

Moisture content effect on the ring shear characteristics of slip zone loess at high shearing rates

The residual strength of slip zone loess is strongly affected by the moisture content. To explore such an effect, slip zone loess obtained from Baqiao landslide was used in ring shear tests under moisture content ranging from 6% to 29%. Laboratory tests were utilized for the determination of the stress–displacement relationship and residual strength parameters for the effect of moisture content in loess. The experimental results showed that the residual strength generally decreases with increasing moisture content. In addition, for a given shearing rate, the residual strength at any moisture content is approximately proportional to the normal stress. The stress fluctuation of shear stress was found to be greater with high shearing rates. Furthermore, the effect of moisture content on the residual strength parameters (cohesion and friction angle) was presented: the residual cohesion of slip zone loess was observed to increase with moisture content to a certain limit that approaches the plastic limit, above which it decreases. Also, the friction angle of slip zone loess was found to decrease with increasing moisture content. Additionally, due to the increase of the moisture content, the scratch color on the shear surface changed from the original color yellow to brown and the thickness of the shearing band of samples reduced. Finally, the mechanism of the influence of moisture content on the macroscopic morphology of the shear surface was also analyzed and illustrated. The study may provide a basis for understanding the shear behavior of slip zone loess under different moisture contents, as well as guidelines for loess landslide stability prevention and prediction.

     

Evaluation of shear strength of clayey soils by using their liquidity index

 It is well known that overconsolidated clays have higher shear strength than unconsolidated clays. The liquidity index makes possible an evaluation of the consolidation degree of clays. However, there is no empirical information about this relation. In this study, clayey soil samples have been collected from various locations and tested. The tests include the determination of liquidity index and shear strength. Obtained parameters were correlated and regression equations were established among liquidity index and undrained shear strength, presenting high coefficients of correlation (R=–0.93). So, an equation [c_u=e^{(0.026–1.21 IL)}] that makes possible a rough evaluation of the shear strength of clayey soils by using their liquidity index value is an improvement. Received: 10 December 1999 · Accepted: 4 April 2000     

Ring shear characteristics of discontinuous plane

Residual shear strength is an essential parameter in evaluating the long-term stability of reactivated landslides in geotechnical engineering. According to previous studies, earthquake-induced landslides may occur on discontinuous planes, such as bedding planes, between weathered and unweathered mudstones having different cementation properties resulting from diagenesis. However, the shear behaviour at the contact surfaces between the cemented and the non-cemented soil layers has not yet been sufficiently investigated. The objective of this study is to elucidate the residual strength characteristics of artificial bedding planes that model the actual behaviour of slip surfaces occurring between two layers with different degrees of cementation. The experimental tests were conducted with a ring shear test apparatus. Additionally, in order to simulate the realistic mechanical behaviour of naturally cemented clay, artificial cementation bonds were created by adding a cementing agent at different ratios to clay slurry. A series of ring shear tests was performed under various conditions on one-layer non-cemented and cemented kaolin samples, respectively, as well as on two-layer specimens composed of one layer each of cemented kaolin and non-cemented kaolin. The test results showed that the residual friction angle of the two-layer combinations of non-cemented and cemented kaolin was approximately 33.6% lower than that of pure kaolin. In contrast, the residual friction angle of cemented kaolin may be as much as 6.2° greater than that of non-cemented kaolin. At cement ratios of up to 2%, the stress ratio of cemented kaolin increased as the shear displacement rate increased. As the cement content was increased beyond 2%, the degree of increase was not significant. These results suggest that the residual strength of cemented kaolin at cement contents greater than 2% is independent of the shear rate.     

循环荷载作用下脆性材料剪切性能试验研究

在自制的MTS振动台试验设备上对混凝土、岩石类脆性材料（砂浆材料）进行静力和动力剪切试验,试验结果表明,（1）动力荷载情况下位移-剪力包络线形状同静力荷载情况下曲线破坏形状相似,均可划分为相同的4个阶段;（2）在相同垂直压力作用下试件的动力抗剪强度与静力抗剪强度相比有明显的增大趋势,强度较低的试件强度增加较大;（3）在相同频率情况下,随着垂直压力的增大,峰值剪切强度、残余抗剪强度随之增大;（4）静力荷载情况下其破坏处水平位移要大于动力荷载情况下破坏处的水平位移,其他条件相同时破坏处水平位移值随峰值抗剪强度的增大而增大;（5）剪切过程中有明显的剪胀现象出现。最后根据试件的破坏形状,初步分析混凝土、岩石类脆性材料的动静态剪切特性的机制。     

储层砂岩破坏特征与脆性指数相关性影响的试验及数值研究

 储层砂岩破坏特征及其脆性评价对油气田开采工程具有重要意义,针对胜利油田某油井隔层和油层砂岩岩芯,开展压缩条件下的物理试验及数值模拟。研究结果表明：储层岩芯在三轴压缩条件下的破坏模式以剪切破坏为主,同种围压的隔层岩芯脆性高于油层岩芯,脆性指数B较好地描述了岩芯脆性的变化趋势;分析了储层岩芯主要力学参数与脆性指数B的相关性,即在围压压缩条件下,岩芯峰值强度、扩容点、弹性模量、泊松比、残余强度及残余强度系数随脆性指数B的增加而降低,且用线性关系拟合各参数与脆性指数B的相关性较好;油层砂岩黏土矿物含量大于隔层砂岩是造成其脆性较高的内因,单轴压缩条件下的岩芯断面CT扫描图从细观上反映了      2种岩芯脆性破坏特征的不同;在单轴压缩条件下,数值岩芯脆性随弹性模量、压拉比、内摩擦角的增加而增加,随泊松比、残余强度系数的增加而降低,与峰值强度之间的关系不大;在声发射模式上,储层砂岩以前震–主震–后震型和群震型为主,根据物理试验和数值计算结果,总结了脆性指数B与岩芯破裂声发射模式之间的内在关系,即在相同加载速率条件下,低脆性区、中脆性区和高脆性区岩芯声发射模式分别为群震型、前震–主震–后震型及主震型,通过对灰岩试样和不同种类煤体声发射模式的对比,验证了机制讨论的正确性。     

土工格栅–土体界面特性大型直剪试验研究

土工格栅与土体的界面特性直接影响了加筋土工程的安全和稳定性,土工格栅两侧为不同材料的界面特性研究还较少。采用双向土工格栅为加筋材料,对其两侧分别为不同含水率粉质黏土及不同粒径石英砂的界面特性开展一系列的大型室内直剪试验,分析法向应力、粉质黏土含水率、剪切速率、石英砂粒径及粉质黏土压实系数等因素对土工格栅–土体界面抗剪强度的影响。结果表明:土工格栅–土体界面抗剪强度与法向应力呈线性相关,符合莫尔–库仑理论;粉质黏土含水率的变化对土工格栅–土体界面抗剪强度有较大的影响,在最优含水率时其界面抗剪强度指标最高;剪切速率的大小和石英砂的粒径变化对土工格栅–土体界面的抗剪强度有一定的影响,其影响范围分别在±10%和±7%内;粉质黏土压实度的增加能有效增加界面抗剪强度,压实系数越高,其提高幅度越大。这些影响应在工程应用中适当考虑。     

Evaluation of liquefaction strength of Japanese natural sandy soil using triaxial and simple shear tests

The aim of this note is to reexamine the stress and strain conditions of cyclic triaxial and cyclic simple shear tests for evaluating the liquefaction strength of natural sandy soil. The conventional method of determining the liquefaction strength relies on the shear stress components. However, the use of stress invariants eliminates the need to specify the stress components, thereby making it a more rational approach for determining the liquefaction strength. A comparison of the results using the conventional stress ratio and the stress invariant ratio is done in this study. A strong correlation is seen between the liquefaction strengths obtained from the triaxial and the simple shear tests when using the stress invariant ratio. Despite this strong correlation, large scattering still remains due to the effects of the soil fabric of the natural sandy soil. A torsional hollow cylindrical device is used here to perform simple shear tests on undisturbed natural sandy soil.