Effect of gravel content on creep behavior of clayey soil at residual state: implication for its role in slow-moving landslides

Soil creep is mostly manifested in slow-moving landslides. It is often the case for active slow-moving landslide with slip zone comprising clayey soil where creep would develop in the residual condition. If gravel is presented in clayey soil, this will have considerable impact on creep behavior of clayey soil. However, knowledge about creep behavior of the clayey soil containing gravel particles is scarce. This paper discusses creep behavior of natural clayey soil with gravel at residual state through a series of creep shear tests. Soil samples for this testing program were collected from the slip zones of two large slow-moving landslides in China. The collected soil samples consisted of clayey soil containing various amounts of gravel particles. The test results show that the soil specimens underwent two different creep patterns. An attenuating creep pattern was observed when the soil specimens were subjected to creep stress less than the residual strength, and a creep with increasing strain rate, or a nonattenuating creep pattern, with no evident secondary creep was noted when creep stress was intestinally increased to a level slightly greater than the residual strength. The creep patterns of clayey soil with gravel at the residual state observed in this study were noted to be very consistent with those of the gravel-free clayey soil reported in Bhat et al. (Int J Geomater 1(1):39–43, 2011, Nat Hazards 69(3):2161–2178, 2013) and Di Miao et al. (Eng Geol 162:53–66, 2013). This gives an indication that creep patterns of clayey soils with and without gravel at the residual state are essentially the same, or in other words, that the presence of gravel does not change the creep pattern of clayey soil. However, the test results in this study illustrate that the presence of gravel does have a notable effect on creep behavior of clayey soil. Specifically, the creep stress leading to creep failure of clayey soil and the minimum ratio of the creep stress to residual strength (RCSR) increased with gravel content, and the displacement until the tertiary creep was also larger in samples containing more gravel particles. It is postulated that creep behavior of clayey soil at the residual state in this study and its relationship with gravel content may be related to strength recovery and crushing of gravel-sized particles during creep.     

不同因素影响下层间错动带颗粒破碎和剪切强度特性试验研究

为了获得不同初始颗粒粒径分布和含水率对层间错动带颗粒破碎和剪切强度特性的影响,通过对比泥夹碎屑、泥夹粉砂、全泥型3种不同层间错动带类型与现场3种不同含水率（10%、7%和3%）试样在法向压力2～10 MPa作用下的反复直剪试验和剪切面颗粒粒径分析试验结果,可得出以下结论：①粗颗粒越多（d60越大）,采用相对颗粒破碎势Br量化的颗粒破碎程度越大;②较干颗粒（低含水率）由于磨损产生了更多的细小颗粒,而较湿颗粒（高含水率）由于破裂和摩擦产生了较大颗粒;③粗颗粒仅对峰值抗剪强度产生一定的影响,且粗颗粒越多,残余强度包线非线性越强;④黏聚力和内摩擦角随含水率线性减小,且低含水率试样残余强度包线非线性最强;⑤残余内摩擦角随颗粒破碎后的黏粒含量（<2 μm）线性减小。提出的残余内摩擦角初步预测公式可供实际工程参考。     

Residual strength of slip zone soils

Slip zones of ancient landslides are commonly composed of fine-grained soils with amount of coarse-grained particle. Residual strength of slip zone soil is an important parameter for evaluating reactivation potential and understanding progressive failure mechanism. In this study, the residual strength is examined by in situ direct shear tests, improved laboratory reversal shear box test, precut specimen triaxial shear test and ring shear test. Some residual shear behaviors are recognized. Field residual strength is the average operational resistance along the sliding surface not an ideal drained strength, which is less than peak and greater than residual strength measured in laboratory. Stress–displacement curves obtained from in situ shear and laboratory reversal direct shear demonstrate strain-hardening which have no significant peak, but the shear stress is decreased gradually with increasing displacement. Residual friction coefficient depends on the normal stress, and this dependence is relevant to the interaction of rolling and sliding of particles. Residual friction angle is closely related to coarse fraction and dry density, appearing a linear increase with increasing coarse fraction and a form of polynomial function with increasing dry density. The influence of shearing rate on residual strength can be negligible.     

不同状态下黏性土剪切蠕变行为对比分析

低速滑坡的缓慢活动特征受其滑带岩土的蠕变行为控制。但是,新生型低速滑坡和复活型低速滑坡滑带岩土的蠕变行为显然分属于峰前状态和残余状态下的蠕变行为。两种状态下滑带岩土蠕变行为有何差异,国内外尚无明确认识。为此,本文以取自两个典型巨型低速滑坡——甘肃舟曲泄流坡滑坡和锁儿头滑坡滑带的含角砾黏性土为研究对象,分别采用直接剪切蠕变试验和直剪反复剪后的直剪蠕变试验方法,对比研究了峰前状态和残余状态下黏性土的剪切蠕变行为。研究发现,峰前状态和残余状态下,黏性土剪切蠕变均呈现阶段性,各阶段转换的临界剪应力均与正应力水平和角砾含量正相关。但是,峰前状态下黏性土蠕变具有明显的稳定蠕变阶段;残余状态下黏性土蠕变缺失这一阶段;前者加速蠕变临界剪应力大于后者、但小于峰值强度,后者加速蠕变临界剪应力略微大于残余强度。两种状态下,黏性土蠕变行为差异的根本原因在于土体内部的不同结构。     

巨型蠕滑滑坡滑带土特征强度特性试验研究

金坪子滑坡是一个位于金沙江右岸、上距乌东德水电站约900 m、总体积约6.25×108 m3的具有典型蠕滑特点的滑坡。为了进一步弄清控制该滑坡活动模式的内在机理,通过不同黏粒含量下滑带土的反复剪切试验,研究了滑坡滑带土的残余、峰值强度特性,以期为同类滑坡的防护和治理提供参考依据。研究结果显示,随着黏粒含量的增加,滑带土应变软化现象更加明显;而滑带土残余强度、峰值强度随着黏粒含量的增加,呈现非线性降低规律,但降低幅度随正压力的增大而增大;同时,残余内摩擦角、峰值内摩擦角与黏粒含量存在良好线性负相关关系,而残余黏聚力与峰值黏聚力随黏粒含量增加呈波动性增大,但在黏粒含量40%处出现一定降低幅度。关于黏聚力的波动性降低趋势,究其原因可能在于黏粒周围强结合水的分布对滑带土强度特性,尤其是黏聚力存在临界影响。本文数据和结论对金坪子滑坡的防护和治理以及不同粒径下蠕滑滑坡失稳演化进程有重要的借鉴意义。     

Correlation of mineralogy and index properties with residual strength of Iraklion marls

The marly formations of the city of Iraklion in Crete, Greece, are described and classified according to their mineralogy and physical properties. The residual angle of friction of these marls was determined using ring shear apparatus and the results were correlated with mineral composition and index properties, such as Atterberg limits and grain size distribution. Furthermore, for better understanding of the determined interrelationships between the physical characteristics and the residual strength of these marls, a number of marl-bentonite mixtures were also tested. Thus, the influence of the variation in clay mineral content on the residual strength was studied and correlations with clay-sized fraction and plasticity index were attempted.     

黄土完全软化强度与残余强度的对比试验研究

为了解决复活蠕滑型黄土滑坡强度参数的取值问题,开展完全软化强度与残余强度的对比试验研究。以山西地区典型黄土为研究对象,采用预压固结法制备饱和重塑试样,并进行反复直剪强度试验获取完全软化强度和残余强度参数。试验结果表明:黄土的完全软化强度以黏聚力为零、颗粒未发生定向排列为主要特征。完全软化强度与二次固结应力和黏粒含量有关。试样在二次固结应力小于300kPa时的应力-应变曲线呈应变软化型,完全软化强度大于残余强度,黏粒含量高的试样应变软化更显著; 试样在二次固结应力大于等于300kPa时的应力应变曲线呈理想塑性型,完全软化强度近似等于残余强度。完全软化内摩擦角与残余内摩擦角的差值和预压固结应力、二次固结应力及黏粒含量有关。完全软化内摩擦角与残余内摩擦角的差值随二次固结应力的增大而减小,最终趋于0。当预压固结应力小于300kPa时,内摩擦角差值及黏粒含量对内摩擦角差值的影响随着预压固结应力的减小呈乘幂性增大; 当预压固结应力大于等于300kPa时,完全软化强度近似等于残余强度,可用完全软化强度近似代替残余强度。研究结论为复活蠕滑型黄土滑坡稳定性分析时强度参数的取值提供了一定的参考。     

Microstructural study of a natural slip zone: quantification and deformation history

Microstructure of a natural slip zone was comprehensively examined using a combination of images captured systematically by optical microscopy (OPM) and backscattered electron microscopy (BEM) techniques. Microstructural features identified on these images were processed and evaluated using an advanced image analysis system, which proved that quantitative analyses could considerably enhance the understanding of shear behavior of slip zones. It was found that variations of porosity, abundance of platy clay particles and alignments of particles are significant indicators revealing nature of deformation processes. These indicators show that global mechanical behavior of the investigated slip zone can be conceptualized as that of normally consolidated clayey soils under drained conditions.

The geometric patterns of the microstructure of the slip zone are similar to the S–C fabrics seen in tectonic shear zones. It is suggested that combined progressive bulk simple shear and pure shear modes enable to realistically reconstruct the kinematic history of the slip zone, through which particle movements and microstructural evolution were accomplished via various types of particulate flows. The results of this study show that clay mineralogy plays a more important role in the development of the slip zone than abundance of clay-size particles, while both clay mineralogy and relative proportions of each particle size fraction control the response of particles to shear deformation. Among the fractions present in the slip zone, fine silts are the strongest indicator of global shear stress characterized by their highest degree of alignment, whereas clay particles are the weakest. Highest degree of shape preferred orientation is also found within fine silt domains.     

Insight into the effects of clay mineralogy on the cyclic behavior of silt–clay mixtures

Results of a systematic testing program showed that the cyclic behavior of silt–clay mixtures is greatly influenced by the dominant clay minerals in the mixture. In particular, it was demonstrated that given the same amount of clay/clay mineral and/or same value of plasticity index, the montmorillonitic soils have the highest cyclic strength, followed by the illitic soils, and then by the kaolinitic soils. Moreover, the rate of increase in cyclic strength with increasing % clay mineral and PI is again the highest in the montmorillonitic soil, lowest in the kaolinitic soil and intermediate in the illitic soil. Therefore, without considering clay mineralogy, the % clay fraction, % clay mineral and plasticity index are unreliable indicators of the liquefaction susceptibility of fine-grained soils. The differing adhesive bond strength each clay mineral develops with the silt particles is deemed to largely explain the observed differences in the response of the three different soil mixtures to cyclic loading.     

Landslides and residual strength in marl profiles in Israel

Many areas in northern Israel show evidence of old landslides in marly profiles, and new slides are often activated in these regions. Such a slide occurred in the town of Rechasim, near Haifa, during an exceptionally wet winter in 1992. The present paper analyzes the causes for this slide, and suggests that it was due to a combination of downslope flow in the overlying, severely cracked chalk–limestone layer, and mobilization of residual strength at the marl–chalk interface. The residual strength of Israeli marly soils has been studied, and it is shown that while many of these consist predominantly of clay-sized (< 2 μm) particles, and may, therefore, expect to have a very low residual friction angle, the amount of clay sized carbonate in the soil is of major influence on the residual friction angle. When the clay-sized material includes less than about 11% carbonate, the clay particles dominate the shearing mechanism and result in a residual friction angle of the order of 12°; this was the case at Rechasim. However, when more than about 30% of the clay-sized material is carbonate, the carbonates control the shearing mechanism, and residual friction angles can be as high as 30°. Results of a preliminary study of the change in residual strength with time are also presented.     

Correlation between incompetent beds and slope deformation at Badong town in the Three Gorges reservoir, China

Badong town is a new immigration area in the Three Gorges reservoir, China, which is built on many giant deep-seated landslides. In this region, the slope deformation is very severe and it is strongly correlated with the incompetent beds, which are distributed widely in the rock mass. In this paper, two giant deep-seated translational rock landslides used as study cases are Huangtupo landslide and Zhaoshuling landslide. Firstly, the composition materials, structures and deformation characteristics of the two landslides are analyzed. Then, the position, structure, mineral composition and the formation mechanism of the incompetent beds are studied in detail. Finally, based on the comparison of the position, mineral and structure between incompetent beds and sliding zones of the landslides, the correlations between incompetent beds and giant landslide are discussed. The results indicate that 13 large incompetent beds exist in the middle Triassic Badong Formation strata, which can be divided into three types as weak interlayers, crushed beds and groundwater corrosion zones. The dominant minerals in the incompetent beds are illite, chlorite, quartz and calcite. The contents of the clay minerals in different positions are quite distinct, and the maximum difference of clay content exceeds 70 %. In addition, it is found that the contents of minerals in incompetent beds are similar to the slide zones of the landslides. The initial deformation of slopes normally developed along the incompetent beds, which induced subsequent shear displacement easily. Under the effect of gravity, the deep slip zones were generated mainly along the incompetent beds and the failure mode evolved from creep to integral slide gradually. The results can provide an important reference for the mechanism analysis and prevention of landslides in Badong town.     

Relating equal smectite content and basal spacing to the residual friction angle of soils

The residual shear strength of soils is increasingly becoming an active research area, as landslides occur almost daily all over the world. Since Skempton published his landmark paper in 1964, several researchers proposed correlations between soil index properties and residual friction angle of soils for preliminary estimates. However, recent data gathered from Japanese colluvium soils indicate that previously proposed charts correlating soil index properties with residual friction angle yield very poor estimates. In this study, first previously published Japanese data are reevaluated to establish a theoretical foundation for such correlations. Then, it is shown that equivalent smectite content (ESC) largely controls the residual friction angle of soils. In the second stage of the paper using the data of several researchers, it is shown that there is piecewise linear correlation between the equivalent basal spacing (EBS) and the residual friction angle. This study lays down a foundation for theoretical correlation between EBS and residual friction angle of soils.     

The residual shear strength of some Hellenic clayey soils

Summary A programme of residual shear strength testing has been undertaken on a number of Hellenic soil types: Marls, Clays and Flysch. The residual strength was determined using the Bromhead ring shear aparatus. For the applied normal stress range the residual strength envelope was straight and the resultant residual friction angle values are correlated with the index properties, such as Atterberg limits and grain-size distribution. The influence of mineralogy on the residual friction angle is also considered.     

高压力下层间错动带残余强度特性和颗粒破碎试验研究

针对某水电站层间错动带的原状样与重塑样,模拟现场地应力条件,在试验法向应力高达10 MPa条件下开展了反复直剪试验。试验结果表明：原状样峰值强度高于重塑样,但强度下降较快,二者残余强度趋于一致;此外,试样发生了大规模的颗粒破碎,采用颗粒相对破碎势Br量化重塑样剪切面附近区域与非剪切面附近区域颗粒破碎程度,对比发现,前者颗粒破碎程度高于后者,且破碎机制不同;剪切面颗粒破碎是高压力下残余强度包线非线性的根本原因：剪切强度的下降（残余强度与峰值强度的比值）与Br和S2（试验前后小于粒径< 2 µm 颗粒含量的比值）呈线性关系,颗粒破碎导致的能量释放和不断产生的黏粒（< 2 µm）降低了剪切强度。     

汶川地震区崩滑堆积体强度现场直剪试验研究

以汶川震区漩口一带地震诱发的松散堆积体为研究对象,开展碎石土原状样和重塑样的现场直剪对比试验,探讨不同法向应力、不同粒度组成和不同含水率等条件下碎石土的剪切强度特性。研究结果表明,地质成因和岩土体结构相似、粒度组成不同且级配不良的碎石土的剪切强度特性具有相似性;原状样剪切强度明显高于相同干密度和含水率的重塑样;级配良好的碎石土应变硬化程度略高于级配不良的碎石土,当粒径大于5 mm的粗颗粒含量大于42.9%时,随粗颗粒含量增加,碎石土的内摩擦角增加,而粘聚力则先减小后增大;抗剪强度指标与含水率呈线性负相关关系,随着含水率增高,碎石土抗剪强度降低,其中粘聚力较内摩擦角下降更明显。综合前人研究和本次试验结果,建议汶川震区类似结构组分碎石土天然状态下的剪切强度指标c值取15±3 k Pa,φ值取30°±2°。     

基于环剪试验的湟水河流域大型泥岩滑坡滑带剪切特征初探

大型泥岩滑坡是青海湟水河流域最主要的地质灾害类型之一。为了揭示此类滑坡运动过程中的滑带土动态剪切特征,本文以湟水河流域两处典型泥岩滑坡滑带土为例,首先通过基本物理性质测试,判定该泥岩滑带土具有中膨胀性;然后利用环剪试验方法,得到了不同含水率的滑带土在不同剪切速率的环剪作用下的剪切特征曲线。结果显示:剪切过程表现出明显的应变软化特性,试样从峰值应力到残余应力有快速明显的应力降（降幅多介于30%~40%之间）;试样的剪切速率和含水率与剪切强度呈反相关关系,残余应力对上述因素的响应小于峰值应力;剪切速率（含水率）变化条件相同时,含水率（剪切速率）越大,剪切应力和剪切强度的变化越大。上述结论为湟水河流域大型泥岩滑坡的成因机理分析和稳定性研究提供了参考。     

黏粒含量对磁县段膨胀土抗剪强度影响的试验研究

利用电动应变控制式直剪仪及直剪/残余剪切试验仪对南水北调磁县段不同黏粒含量的原状膨胀土进行快剪、饱和快剪、饱和固结快剪和反复直剪试验,研究黏粒含量对其抗剪强度的影响。研究表明:饱和后试样的抗剪强度明显降低,固结后强度提高,且饱和作用对黏粒含量较大的中膨胀土强度的削弱作用更为显著,固结作用对黏粒含量较小的弱膨胀土强度的治愈作用更显著; 随黏粒含量的增大,黏聚力逐渐减小,内摩擦角则先减后增,其临界值在32%左右; 峰值强度后的抗剪强度降低幅度随黏粒含量的增加而增大; 土体的峰值强度f随黏粒含量则先减后增,变化趋势比较平缓; 残余强度r随黏粒含量增加逐渐减小,成指数关系; 残余强度内摩擦角r与黏粒含量成对数关系,黏聚力cr则比较离散。     

宽级配砾质土作为GCls防渗垫保护层的抗剪强度试验研究

水化后的土工织物黏土垫层(GCls)是良好的复合防渗材料,但同时表现出较低的抗剪强度特性。为改善水化后的GCls抗剪强度低的缺陷,拟采用取自天然并经人工调配的宽级配砾质土料代替黏土作为GCls防渗垫的保护层,共同构成复合防渗系统。文中利用TGH直剪摩擦拉伸仪对筛选出的宽级配砾质土及其与不同水化条件下的GCL接触面进行了试验研究,得到宽级配砾质土样及其与质量含水率分别为50%和完全水化的GCL接触面的抗剪强度试验数据并进行了整理分析。结果显示,宽级配砾质土样的抗剪强度大于其与不同水化条件下的GCL接触面的抗剪强度,且宽级配砾质土与GCL接触面的抗剪强度随GCL含水率的增加而减小。因此,可以根据外荷载作用下土中应力会发生扩散的原理,利用抗剪强度高和压缩性低的砾质土来承担部分甚至全部荷载,以弥补水化后的GCls抗剪强度低的缺陷。     

Comparative analysis of contributing parameters for rainfall-triggered landslides in the Lesser Himalaya of Nepal

In the Himalaya, people live in widely spread settlements and suffer more from landslides than from any other type of natural disaster. The intense summer monsoons are the main factor in triggering landslides. However, the relations between landslides and slope hydrology have not been a focal topic in Himalayan landslide research. This paper deals with the contributing parameters for the rainfall-triggered landslides which occurred during an extreme monsoon rainfall event on 23 July 2002, in the south-western hills of Kathmandu valley, in the Lesser Himalaya, Nepal. Parameters such as bedrock geology, geomorphology, geotechnical properties of soil, and clay mineralogy are described in this paper. Landslide modeling was performed in SEEP/W and SLOPE/W to understand the relationship of pore water pressure variations in soil layers and to determine the spatial variation of landslide occurrence. Soil characteristics, low angle of internal friction of fines in soil, medium range of soil permeability, presence of clay minerals in soil, bedrock hydrogeology, and human intervention were found to be the main contributing parameters for slope failures in the region.     

矿物成分、特征地球化学组分对水在滑带形成中作用的指示意义:以三峡库区大型滑坡为例

弄清滑坡滑带抗剪强度降低的根本原因是滑坡形成机理研究和滑坡活动趋势预测的主要内容之一。水是滑带形成过程中导致其抗剪强度衰减的最活跃因素之一。三峡库区两个大型滑坡实例分析显示,滑带及其周围岩土矿物成分、地球化学成分的变化特征指示滑带形成与水-岩(土)物理、化学作用的方式及作用程度,从而证实矿物学、地球化学研究方法和理论是揭示滑带抗剪强度降低内在机理的有效途径之一。三峡库区黄土坡滑坡临江I#崩滑体和泄滩滑坡滑带土及其周围岩土矿物学、主量化学元素的含量变化特征指示:前者滑带形成过程中,滑带部位地下水因大气降水补给、地下水的氧化作用活跃,导致滑带土抗剪强度减低,其主要原因是其中泥灰岩碎屑的水解泥化作用、方解石溶解作用和伊利石向伊-蒙混层矿物的转化作用;后者滑带形成过程中,滑带部位地下水与外界水力联系较差、地下水的还原作用强烈,滑带部位长石化学风化、次生粘土矿物增多,可知由伊利石转化的伊-蒙混层矿物增多是导致滑带土抗剪强度衰减的主要原因。