Identification of the Microfabric of Kaolin Clay and its Impact on the Shear Strength Behavior

A new method for identifying the microfabric (geometric arrangement of platelets) of cohesive soil using X-ray diffraction technique without the need for a pole figure device is introduced in this study. This method is based on identifying the occurrence of basal and prism peaks in a given X-ray diffraction pattern and their relative intensities observed in the pattern. Five different specimen preparation techniques are used to prepare the Kaolin clay specimens with different particle orientations to identify their microfabrics using X-ray diffraction analysis, and the results are assessed in conjunction with high magnification SEM (Scanning Electron Microscope) images. The proposed method does not require impregnation of clay with alternate pore fluids/epoxy, and is able to identify the undisturbed microfabric of clay sample at its natural water content. This method is further used for evaluating the variation in microfabric of slurry consolidated specimens of Kaolin clay due to triaxial shearing under compression and extension loading conditions. Based on the lubricated end triaxial tests on the clay specimens with two distinct microfabrics (flocculated and dispersed) and the X-ray diffraction analysis of these specimens before and after shearing, this study shows that the extension shearing may cause a significant re-orientation of particles within the specimen unlike the effect of compression shearing. The impact of micro fabric on shear strength behavior of Kaolin clay is studied for the variation of confining pressure, stress history, and loading/boundary conditions by perfonning a series of triaxial experiments using lubricated ends. The importance of using appropriate drainage conditions for the lubricated ends in triaxial device is discussed based on the experimental observations during drained and undrained triaxial tests performed at three different drainage conditions. Good drainage conditions of lubricated end platens offered significantly higher values of shear strength, effective friction angle, volumetric deformation, and axial strain at failure conditions in comparison to the corresponding values obtained for the poor and intermediate drainage conditions. Using improved drainage conditions, a series of triaxial compression and extension tests are performed on normally consolidated (NC) and heavily overconsolidated (HOC, overconsolidation ratio =10) specimens of Kaolin clay with two extreme micro fabrics ( dispersed and flocculated) under drained and undrained conditions. Three different values of confining pressure (207, 276, and 345 kPa) are used to evaluate the effect of confining pressure on shear behavior of dispersed and flocculated specimens of Kaolin clay. A comparison between the drained and undrained shear response is presented for the variation in the loading conditions (compression and extension) and the stress history (NC and HOC) of soil. This study also includes a discussion on the occurrence of strain localization within the clay specimens tested using lubricated ends, which is believed to be a successful method of reducing non-uniformity in stress-strain and deformation behavior of soil based on the importance of lubricated ends over frictional ends. The impact of microfabric, confining pressure, loading conditions, stress history, and drainage conditions on the pattern of strain localization within the solid cylindrical specimen of Kaolin clay is discussed based on the digital image analysis of its deformation profile

Strain localization patterns of cohesive soil with controlled microfabric under cyclic loading using DIA technique

This paper focuses on the shear behavior and strain localization patterns of cohesive soil under cyclic and drained/undrained (axial) loading conditions for soil's two extreme microfabrics using lubricated end boundary cyclic triaxial system facilitated with DIA setup. The Digital Image Analysis (DIA) is an advanced technique for studying the strain localization patterns of soil, which has been developed for the cyclic triaxial system and used throughout this experimental research. The impact of soil's microfabric (dispersed/flocculated), specimen's boundary conditions (Lubricated-end/Frictional-end), equipment's testing and loading conditions (drained/undrained, axial/cyclic) on strain localization patterns of Kaolin clay has been studied using obtained experimental data. This research reveals that the initiation and formation of intense straining zones occurs at much earlier stage of shearing in undrained tests than the drained tests. In cyclic triaxial tests, the strain localization patterns at low strain levels shows the linear shear banding in compression & extension modes of a cycle and undefined zones of intense straining at the end of the cycle. At higher strain levels of cyclic loading, the maximum local strain experienced by the soil in a test has been observed to be significantly higher in dispersed microfabric as compared to the flocculated.by Ajanta Sacha

Effect of Intermediate Microfabric on Shear Strength and Strain Localization Response of Kaolin Clay Under Compression and Extension Loading

The present paper evaluates the role of microfabric in strain localization patterns observed in soil specimens during its shear deformation in compression and extension triaxial testing. A series of compression and extension lubricated end triaxial tests are performed on Kaolin clay with extreme as well as intermediate microfabrics, which are obtained using slurry consolidation technique by varying calagon content from 0 to 3 %. Intermediate microfabric is the geometric arrangement of particles within the soil mass, which lies in between the particle orientation during two extreme microfabrics; flocculated and dispersed. Flocculated has random orientation of particles with face-to-edge particle contacts and dispersed has parallel orientation of particles with face-to-face particle contacts. When the specimen is subjected to large stress levels in triaxial testing, the particle orientation and geometric arrangement get affected due to the force acting on the clay platelets. In this experimental research, the variation in clay’s stress–strain and pore pressure response and initiation, propagation and formation of shear bands at different levels of compression and extension shearing is evaluated using digital image analysis setup associated with triaxial system.by Ajanta Sacha

Shear testing data of soil: a function of boundary friction in triaxial setup

by Ajanta Sacha

Dynamic characteristics of natural kutch sandy soils

Kutch region has experienced widespread liquefaction and related damage during a series of past earthquakes; however, cyclic behavior of sandy soils of the region is yet to be explored. The current study is focused on the dynamic characteristics of natural soil deposits of the high seismicity Kutch region. In the previous studies, variations in fines content (FC) and the nature of fines (plastic, non-plastic) has been controlled and systematic and based on either non-plastic or plastic fines using mostly standard or river sands. However, the present study is different in the approach as the dynamic characteristics of natural sandy deposits with simultaneous presence, and random variations of both the plastic and non-plastic fines are explored. Results from cyclic triaxial tests indicated sandy soils of Kutch region are severe to moderately prone to liquefaction with a strong dependence on plasticity rather than FC. Pore pressure ratio (ru), mean effective pressure (p?) and cyclic stress ratio (CSR) for silty-sands signified effective stress failure, which was found to be coinciding with initial liquefaction. Clayey-sands exhibited strength degradation instead of liquefaction, and the plasticity index (PI) was found to control the magnitude of degradation. Large degradation in secant shear modulus and reduction in damping ratio with the number of cycles was observed in silty-sands whereas clayey-sands exhibited relatively lesser degradation in secant shear modulus and damping ratio.by Majid Hussain and Ajanta Sacha

Experimental study on static and cyclic liquefaction on cohesionless chang dam soil

by Majid Hussain and Ajanta Sacha

Matric suction, swelling and collapsible characteristics of unsaturated expansive soils

Unsaturated expansive soils are recognized as one of the most problematic soils owing to its swelling-shrinkage characteristics. The presence of such soils in highway/railway embankments, slopes and earthen dam sites manifests various critical issues during and after the construction of structures. The current research aims on the evaluation of the suction, swelling and collapsible characteristics of four expansive soils possessing different expansiveness and mineralogical composition. A series of constant volume swell pressure, double-oedometer and incontact filter paper tests were performed on four different expansive soils to acquire swelling pressure, collapse potential and matric suction at different degree of saturation. Swelling pressure, collapse potential and matric suction of all expansive soils degraded substantially with increment in the degree of saturation. The results revealed significant impact of magnitude of matric suction on volumetric deformation (swell and collapse) behavior of expansive soils. Swelling and collapse potential were observed to be higher for the expansive soils with larger matric suction. Higher matric suction governed the water intake within interlayer spacing of the Montmorillonite mineral present in expansive soil, which contributed to higher crystalline swelling response. Larger collapse potential indicated development of larger localized deformations within the soil mass owing to higher matric suction.by Saloni Pandya and Ajanta Sacha

Effect of microstructure on stress�strain and pore-pressure response of sabarmati sand under the influence of mica

Micaceous soil is believed to be detrimental for civil engineering constructions due to the effect of high compressibility, low compacted density and low shear strength. Individual mica particle has numerous intact mica flakes foliated over each other making it flexible upon loading and rebound upon unloading due to its low hardness and resilient nature. Hence, micaceous soils with mica content more than 10% are considered undesirable for highway pavements, embankments and railway track constructions. When platy mica particles are sufficiently numerous to interact with spherical sand particles, bridging and ordering phenomena are augmented within the soil mass creating unique sand-mica particle orientation (MS microstructure) unlike sand-sand particle orientation (PS microstructure). The current experimental research was conducted to evaluate the variation in stress�strain, pore pressure and effective stress path response of Sabarmati sand under the influence of mica (sand with 30% mica and pure sand) with MS and PS microstructure respectively. Effect of particle crushing on stress�strain and pore pressure response was also studied on Sabarmati sand with MS and PS microstructure. Distinctive macroscopic response was observed in Sabarmati sand with MS microstructure under the influence of mica as well as mica particle crushing.by P. Seethalakshmi and Ajanta Sacha

Dynamic behaviour of Kutch soils under cyclic triaxial and cyclic simple shear testing conditions

The dynamic behaviour of soils from Kutch (seismically active region of Gujarat, India) was explored under cyclic triaxial (CTX) and cyclic simple shear (CSS) testing conditions. Under both the CTX and CSS conditions, the soil specimens exhibited cyclic strength that decreased rapidly at lower plasticity index and lower fines content. The reduction in cyclic strength was evaluated to be higher and faster for the clayey-sand specimens as compared to clayey specimens. Specimens with low plasticity index and lower fines content showed higher pore pressure response in both CTX and CSS tests. Pore pressure ratio under CSS conditions was observed to be higher as compared to CTX conditions. Degradation in shear modulus was found to be higher and quicker for clayey-sand specimens as compared to clayey specimens. Degradation index increased with plasticity index and fines content at a given number of loading cycles under both CSS and CTX conditions.by Majid Hussain and Ajanta Sacha

Effect of Inter-granular Void Ratio on Volume Compressibility and Undrained Shear Response of Base-sand and Natural Silty-sand of Kutch

by Majid Hussain and Ajanta Sacha