An Approach to the Interpretation of the Mechanical Behaviour of Intensely Fissured Clays

The paper discusses the intrinsic properties, the geological history, the natural structure and the mechanical behaviour of intensely fissured clays from Santa Croce di Magliano, a site located within the East-side border of the Apennine chain (southern Italy). The clays were originally deposited elsewhere and subsequently moved and largely deformed during the Apennine orogenesis, so that their structure appears severely disturbed and fissured. The applicability of a continuum-based approach to describe the mechanical behaviour of a Representative Element Volume (REV, hereafter) of the natural, fissured clay is experimentally investigated by analysing the clay specimen deformation under plane strain compression by means of False Relief Stereophotogrammetry. The results of such analysis show that, despite the intense network of pre-existing fissures, the patterns of deformation - including the eventual development of shear bands within the REV of the natural clay - are similar to those observed for unfissured clays. Triaxial tests were carried out on both natural and reconstituted clay samples and the results were compared with those recognised in the literature to be typical of unfissured clays. Based on such comparisons, the study provides a few essential elements that are useful to define a general framework for the mechanical behaviour of intensely fissured clays.     

STRENGTH AND CONSOLIDATION PROPERTIES OF BUSAN NEW PORT CLAYS

The sedimentary environment and the effects of sample disturbance on strength and consolidation properties of Busan New Port clays are examined through microfossils and radiocarbon age analyses and unconfined compression, K₀-consolidated undrained triaxial compression and extension tests and consolidation tests. In this study, only one or two samples, 74 mm in diameter and 100 mm in height, obtained from different depths, are used for the whole series of tests to provide small-sized specimens. The sedimentation rates of Busan New Port clays were (3.7-7.8) mm/year and higher than those for the coastal areas of the USA, Thailand and Japan. The in-situ undrained shear strength and consolidation parameters were estimated using Shogaki's method and compared with those of other test results and evaluated. Busan New Port clays are lightly overconsolidated clays. It can be seen that the consolidation settlements, which were greater than those estimated, were observed in Holocene Busan clay, are caused by the underestimation of the compression index and coefficient of consolidation values caused by sample disturbance.     

分散性粘土判别试验研究

应用碎块、针孔、双比重计、孔隙水可溶盐和交换性钠百分比等五种试验方法,结合物理化学性质和矿物成分分析,对西郊、三坪两水库大坝心墙土样进行了多种方案的分散性试验及长期渗流条件下的渗透变形试验和盐分运移试验。认为三坪土样中钠离子的分散作用小于西郊土样,钙镁离子的抑制分散作用大于西郊土样,并且三坪土样中的蒙脱石含量低于西郊土样,因此三坪土样属于非分散性土,西郊土样属于过渡型土。土样施工用水对土样的分散性鉴定结果影响较小,而冲蚀用水对土样的分散性鉴定结果产生大的影响。在长期的渗流条件下土样中钠离子随水流易迁移,河水中的钙离子含量大,进入土体后能抑制土样的分散性,促使土体向非散性的方向发展。结果表明土样中钠离子和钙镁离子含量多寡和相对含量对粘土分散性具有重要的作用,蒙脱石的存在是粘土分散性的重要因素,河水对粘土分散性能将产生显著影响。     

The mechanical behaviour of compacted Lambeth-group clays with and without fibre reinforcement

This study investigated the effect of fibre reinforcement on the large strain behaviour of compacted clay samples tested using large triaxial test equipment. A novel specimen preparation method was proposed where peds of clay are compacted to closely simulate the in-situ compaction. A large number of 100 × 200 mm triaxial tests and one-dimensional compression tests were performed using reinforced and unreinforced samples. The behaviour of unreinforced samples was observed to be similar to highly fissured clays; ped compaction generated a random fissure pattern due to the contact between peds. The addition of fibres to the compacted samples created fissures with higher mobility at lower friction than those in the unreinforced samples; hence, the state boundary surface of reinforced clay was below that of the unreinforced clay. With the addition of fibres, the failure mechanism changed from the formation of a shear plane to barrelling, demonstrating that the fibres transferred stresses further away from the shear plane, producing a more homogeneous stress distribution. The preparation method proposed here produced a fissure pattern in the clay that introduced transitional behaviour, which was drastically reduced with addition of the fibres, allowing better normalisation and the definition of a unique boundary surface.     

Investigation of factors affecting the dispersibility of clays and estimation of dispersivity

Dispersive soils have caused failure of many slopes and earth fills due to external and internal erosion. This study aims to investigate various factors used for identification of dispersivity and to develop some new approaches for the prediction of dispersivity of clays. To achieve this purpose, physical and index properties, as well as degree of dispersivity of 29 clay samples taken from five different locations in and around the city of Ankara were determined. Various statistical prediction models were used for prediction of new dispersivity classes obtained by weighting ranking method. It was determined that dispersivity classes obtained from physical and chemical dispersivity tests performed on the same clay samples using distilled water were different from each other. In addition, crumb and pinhole tests were performed by using test waters with varying TDS values on five selected samples to find the impact of water chemistry on dispersivity. It is concluded from all dispersivity tests that total dissolved salts (TDS) values and sodium percentage (SP) remarkably affect the degree of dispersivity, and the use of these two parameters give more reliable results for the determination of dispersivity. By considering all these facts and to predict the most reliable dispersivity class, all dispersivity classes obtained from physical and chemical dispersivity tests were reevaluated by a weighted ranking system, and new dispersivity classes were assigned. In order to estimate these new dispersivity classes, various statistical models were established by using results of chemical analysis of pore water of clay samples. For this purpose, prediction models including soft computing methods such as decision tree and logistic regression are used and most reliable prediction models having the highest prediction performance are suggested.     

搅拌水泥土抗压强度特性试验

对深圳市某滨海软土区淤泥进行了不同水泥掺入比的搅拌水泥土抗压强度室内试验,试验结果可为类似滨海相淤泥搅拌水泥土抗压强度特性研究积累一些经验数据,也可为搅拌水泥土强度试验方法提供参考。     

Geo-mechanical behavior of clay soils stabilized at ambient temperature with fly-ash geopolymer-incorporated granulated slag

Geopolymer is a cementitious material that can replace ordinary Portland cement in several geotechnical engineering applications, such as soil stabilization, with the advantages of much lower harmful emissions and energy consumption. This paper presents a rigorous evaluation of the geo-mechanical behavior of different types of clay soils treated with geopolymer, including the influence of soil characteristics and mineralogy. Two natural clay soils in addition to a commercially available kaolin clay were used for this investigation. Laboratory experiments were performed including unconfined compressive strength (UCS) and consolidated undrained (CU) triaxial compression tests under different confining pressures. The UCS and triaxial tests indicated that the addition of geopolymer considerably increased the yield stress and initial stiffness of all examined clays. With the increase of geopolymer content, the stress–strain behavior of treated clays was found to develop progressively from ductile response into a post-peak brittle fashion. The CU tests also demonstrated that the addition of geopolymer changed the initial characteristics of remolded clays from quasi-over-consolidated to heavily over-consolidated, rendering high yield surface and more effective shear strength parameters (i.e., cohesion and friction angle). Moreover, although the overall qualitative stress–strain and stress path responses of the clays were similar, significant quantitative differences were observed, particularly in terms of the attainable yield strength, stiffness, and shear strength. These differences can be attributed mainly to the heterogeneity associated with the soil mineralogy and the corresponding differences in the interaction between the clay/non-clay minerals and geopolymer.     

Compressive Behavior During the Transition of Strain Rates

The application of a strain-rate-dependent model, for example, an isotache model, is very useful and highly effective for predicting the settlement due to consolidation, including secondary consolidation. In the isotache model, compression curves are not only determined by pressure, but also by strain rate. The validity of this model has been experimentally confirmed by many researchers using different types of oedometer tests, such as constant rate of strain (CRS) tests, incremental loading (IL) tests, etc. However, considerable scatter has been observed in the test results, which show the effects of the strain rate, and questions arise as to whether such scatter is caused by the heterogeneity of the soil samples or by the incompletion of the model. To avoid the heterogeneity of the tested samples, special CRS tests, in which the strain rate is not kept constant but is varied during the tests, were carried out on intact and reconstituted Osaka clay samples. The effects of the strain rate on the compressive behavior of these clays were carefully evaluated in terms of visco-plastic strain, assuming that the total strain consists of visco-plastic strain and elastic strain. It was confirmed that the stress and the visco-plastic strain relation of clay samples strongly depends on the visco-plastic strain rate. However, the effects of the strain rate, under a given constant visco-plastic strain rate, do not become constant when the visco-plastic strain rate becomes very small. The reason is assumed to be due to the development of structures under a constant small visco-plastic strain rate. The development of structures may restrict the applicability of the isotache model to the compressive behavior of clay.     

Thixotropic mechanism of clay: A microstructural investigation

Clays with high sensitivity exhibit varied degrees of thixotropic characteristics. Previous research has been focused on dilute suspensions, but only a few studies have investigated normal clay with water contents lower than the liquid limit. Given the limited studies performed, the thixotropic behavior of clays and the underlying mechanism remain ambiguous, particularly in the microscopic scale. In this study, samples of Zhanjiang clay from southern China were used to elucidate the thixotropic process. The samples were agitated and then subjected to unconfined compressive strength and pocket penetrometer tests, scanning electron microscopy, and mercury intrusion porosimetry to evaluate their regain strength and microstructure. After 500 d of thixotropy, the strength of the Zhanjiang clay was 2.58 times higher than that after the disturbance and significantly higher than that of the other clays reported in the literature. These findings provide evidence that thixotropy is a process in which the dispersed structure is transformed into a flocculent structure through the homogenization of the pore size distribution caused by particle movement. This phenomenon may be due to the changes in the interaction of attractive and repulsive forces between particles. Given the experimental findings, a thixotropic mechanism for clay is proposed. This study will be beneficial for predicting the strength of disturbed soils.     

天津滨海地区两种典型软黏土蠕变特性试验研究

通过三轴蠕变试验,研究了天津滨海地区两种典型淤泥质黏土和粉质黏土的非线性蠕变特性。蠕变试验采用分级加载的方式进行,利用“陈氏法”对实测数据进行处理后得到两种软黏土在不同应力状态下的蠕变试验曲线。在此基础上,提取相关数据得到两种软黏土的应力–应变等时曲线,根据该曲线的变化规律,采用 Merchant 模型对两种软黏土的蠕变曲线进行拟合,确定模型参数,建立了适用于天津滨海地区两种典型软黏土的非线性蠕变模型。最后,对两种软黏土蠕变模型参数的变化规律进行了对比分析得出： Merchant 模型中虎克弹簧的弹性模量 E _H 随主应力差增大呈负指数规律变化, Kelvin 体的弹性模量 E _K 随主应力差增大呈线性规律变化的经验公式。     

Fabric of ioldian clays and its influence on their strength

Petrographic investigations of clay fabric in undisturbed ioldian clays have shown that in a macroscopically uniform clay stratum with constant mineralogical and granulometric composition, there are five types of fabric : non-oriented, oriented, oölitic, blocky and reticular. Shearing and penetration tests on samples of each fabric type have revealed the effect of fabric on the strength characteristics of the rocks. All other conditions being equal, the strength values of rocks are the smallest in samples with non-oriented fabric, substantially higher in samples with oriented fabric, and the highest in samples with oölitic and blocky fabric. The fabric of clayey rocks has once again been shown to exert great influence on their physico-mechanical properties.     

SIMULATION OF SHEAR AND ONE-DIMENSIONAL COMPRESSION BEHAVIOR OF NATURALLY DEPOSITED CLAYS BY SUPER/SUBLOADING YIELD SURFACE CAM-CLAY MODEL

Naturally deposited clays exhibit complicated mechanical behavior that differs from that of remolded clays. For example, clay in a normally consolidated state commonly exhibits softening in undrained shear tests or “rewinding” in a heavily overconsolidated state. The Super/subloading Yield Surface Cam-clay model (Asaoka et al., 1998, 2000, 2002) was proposed in an attempt to clarify the complicated mechanical behavior in naturally deposited clays. In this constitutive model, the concepts of “structure,” overconsolidation, anisotropy, and their evolution laws, are introduced into the modified Cam-clay model. In the present study, undrained triaxial compression tests and oedometer tests were carried out on two types of naturally deposited undisturbed clay, Pleistocene clay and Holocene clay, and the behavior was then simulated using the Super/subloading Yield Surface Cam-clay model. The findings of the present study are as follows:

• 1)For the two types of undisturbed clay, the Super/subloading Yield Surface Cam-clay model can simulate undrained triaxial compression behavior ranging from the normally consolidated state to the overconsolidated state, corresponding to various isotropic pressures using a single set of material constants.
• 2)In addition, the model can simulate one-dimensional compression behavior using the same material constants as those used for the simulation of the undrained triaxial compression behavior.
• 3)Through the simulation, the mechanical behavior of Pleistocene clay and Holocene clay, which have different loading histories and have undergone different aging effects, can be described by the different evolution parameters, as well as the elasto-plastic parameters.

     

Stabilization of soft clay using short fibers and poly vinyl alcohol

In this study, the effect of the combined addition of fibers and a nontraditional polymer on the mechanical behavior of a clay was investigated. Poly vinyl alcohol, PVA, used as a solution with concentrations of 0.1%, 0.3%, 0.5%, 1.0% and 1.5% and 1,2,3,4 Butane Tetra Carboxylic Acid, BTCA was added as a crosslinking agent at concentration rates of 0.1%, 0.3% and 0.5%, respectively. Short polypropylene fibers were added to the clay at proportionate quantities of 0.25% and 0.50% of the dry weight of the soil. Clay samples were prepared for unconfined compressive strength (UCS) tests at two different initial void ratio values, denoting relatively stiff and markedly soft states. UCS tests were conducted on both 1-day and 14-day cured samples. The results confirmed significant UCS improvements with combined fiber reinforcement and PVA-BTCA stabilization when samples were cured for 14 days. It was also observed that fiber reinforcement outperformed PVA-BTCA stabilization for clays with the lower initial void ratio. PVA-BTCA stabilization was however found to be superior to fiber reinforcement in clays with a relatively higher initial void ratio. The effect of fiber reinforcement and PVA-BTCA stabilization on the stability of soils subjected to excessive wetting was also evaluated using soaking tests. Stabilization with PVA and BTCA was found to enhance the stability of soaked samples significantly. The results of soaking tests proved that BTCA made PVA-stabilized samples more durable when exposed to soaking.     

Experimental investigation of the influence on static and cyclic deformation of structural soft clay of stress level

This paper presents the experimental results performed to study the static and cyclic deformation behavior of undisturbed and remolded soft clays sampling from Xiaoshan. The consolidation tests indicated that the vertical strains of undisturbed soft clay could be divided into three stages with load increasing, however, the remolded clays were almost independent of stress level. The two cut-off points of these three stages are the preconsolidation stress and the structural yielding stress of the original clay, which could be determined by shear wave velocity measurement. The strains developed during cyclic tests of undisturbed and remolded soft clay, both having one turning point under different amplitude of cyclic stress. The strain developed slowly and stayed at a low level at the early stage, but developed quickly in a different way when the turning points were achieved and finally became great. The turning strains changed with different cyclic stress amplitudes, but they almost fell on a linear line whether undisturbed or remolded. Furthermore, the turning points of the remolded clay all fell on the same line of different confining stresses, including which of the undisturbed soft clay under confining pressure was larger than structural yielding stress. It was also found that the deformation characteristic of undisturbed and remolded Xiaoshan clay tend to be consistent when the structure of undisturbed soft clay is damaged. __________ Translated from Chinese Journal of Rock Mechanics and Engineering, 2006, 25(5): 937–944 [译自: 岩石力学与工程学报]     

Effect of compaction method on the undrained strength of fiber-reinforced clay

The use of discrete fibers in reinforcing soils is of interest to the geotechnical engineering community. Two limitations exist in experimental studies involving fiber-reinforced clays. First, fiber-reinforced clay specimens are generally prepared in the lab using conventional “impact” compaction, whereas the compaction of clay systems in the field typically involves “kneading” action. Second, the majority of tests reported in the literature use synthetic fibers to the exclusion of other types. This paper addresses these limitations through an experimental triaxial testing program that: (1) supplements the scarce data available in the literature on the undrained load response of clays reinforced with “natural” fibers and that are compacted by “kneading”, and (2) assesses the capacity of the experimental procedures that involve “impact” compaction to produce responses that are relevant to actual field conditions. Results from 73 unconsolidated undrained triaxial tests indicate that the percent improvement in the undrained strength of the fiber-reinforced clay is highly dependent on the compaction method, with specimens that are prepared using impact compaction yielding improvements up to three times larger than identical specimens prepared by kneading. This discrepancy in the behavior can be traced back to differences in the fiber orientation distributions between specimens that were compacted by impact and kneading.     

循环荷载作用下软黏土不排水累积变形特性

在原状样与重塑样循环三轴试验的基础上,研究了软黏土在无静偏应力和有静偏应力循环荷载作用下的不排水瞬态累积变形特性,提出了考虑循环应力、循环振次、超固结比及静偏应力影响因素的土体应变本构模型。结果表明:软黏土变形规律受固结应力水平和土结构破坏影响甚大,当固结压力大于土体结构屈服应力时,其变形规律将趋近于重塑样;在循环荷载作用下,土样变形存在转折点,标志着土结构的破坏,本构模型宜按试样破坏前、后分段描述。     

软粘土抗剪强度参数试验研究

从软粘土的抗剪强度参数试验出发 ,一方面通过三轴UU剪切试验 ,得出了试样饱和度对软粘土抗剪强度参数的影响关系 ;另一方面通过大量的固结快剪试验 ,得出软粘土在不同先期固结压力Pc、不同固结度U下的强度变化规律。可为在软粘土上的工程设计和稳定性分析提供依据     

由于应力释放引起的深水土样扰动性研究(英文)

从深水海底取得的土样要经受很大的应力释放。模拟孔隙水中溶解有不同含量气体的深水黏土取样过程 ,对Lierstranda黏土进行了一系列三轴和高压固结试验。试验结果表明 ,自重应力下的孔隙比变化与孔隙水中的气体饱和度 η 成线性增长关系。其它测得的土性参数 ,如不排水强度、峰值应力下的应变、剪胀系数和先期固结压力等均随着 η 有规律地变化 ,显示土样的扰动性也随着 η 的增加而增加。据此 ,本文提出了对可能含有气体的深水土样的最佳贮藏与处置方案。     

Proposal of a Simple Method for Assessing the Susceptibility of Naturally Deposited Clay Grounds to Large Long-Term Settlement due to Embankment Loading

A simple method that utilizes the results of laboratory tests has been proposed for determining the susceptibility of soft clay grounds to large residual consolidation settlement due to embankment loading. It was found that there is a possibility of large long-term settlement if the sensitivity and compression index ratios of the clay material that constitutes the ground are equal to or more than 8.0 and 1.5, respectively. The compression index ratio is defined in this paper as the ratio (Cc/Ccr) of the steepest gradient of the compression curve of an undisturbed sample to that of the remolded sample. Through the SYS Cam-clay model, an elasto-plastic constitutive model that describes the actions of the soil skeleton structure, it was found that clays with large sensitivity and compression index ratios are characterized by initially highly structured soils and that decay/upgradation of the structure can easily occur due to plastic deformation. In addition, by following Schmertmann's graphic method for in-situ compression curve (1953), this paper proposes a method of deducing the in-situ initial conditions from the results of laboratory consolidation tests on undisturbed samples. These investigations revealed not only that large delayed settlement is facilitated in clays, which have higher degrees of structure and faster rates of structural decay, but also that the Δe method and other simple methods of predicting settlement may underestimate the amount of settlement.     

Comparison of Mechanical Behavior of Two Overconsolidated Clays: Yamashita and Louiseville Clays

A site investigation program was carried out at the Yamashita site, Yokohama, Japan, by means of laboratory as well as field tests. It was found from the investigation that the clayey layer is overconsolidated with an OCR of about 2.0. It is evident from geological consideration that this overconsolidation may not have been caused by stress change, but was probably due to structure effects such as cementation or delayed consolidation. The physical and mechanical properties of the Yamashita clay is compared with those of Louiseville clay, a Champlain Sea Clay widely distributed in Quebec, eastern Canada, and a well known cemented clay. Louiseville clay was retrieved by Japanese sampler and transported to the Port and Harbour Research Institute (PHRI) for laboratory investigation. Comparison of the two clays shows that strength properties such as the stress-strain curve or stress path for both clays are very similar to each other. However, the pattern of the e-log p curve measured by Constant Rate of Strain oedometer, particularly beyond the yield consolidation pressure, is remarkably different between the two clays.