Swell and microstructural characteristics of high-plasticity clay blended with cement

This study presents the effect of high plasticity on swell potential, swelling pressure and micro structural characteristics of kaolinite-bentonite mixed clays. Five different combining ratios of kaolinite bentonite mixture of 100:0, 90:10, 75:25, 50:50 and 25:75 in % by weight of dry kaolinite were used. All five synthesised soils were then mixed with 0%, 5% and 8% of cement by weight of dry soil, cured for 28days and subjected to Atterberg limit, one dimensional oedometer and scanning electron microscope test. The inclusions of 5% and 8% cement reduces the plasticity index of the treated soils as the percentage of bentonite increases. The effects of plasticity of treatment with 5% and 8% cement after 28 days curing period, was evaluated, and the results show that reduction in plasticity index resulted to decrease in swell potential and swelling pressure of the kaolinite-bentonite mixed clays. The results of micro-structural analysis of 5% cement treated soils show formation of flocculated fabric and cementation of soil particles, and filling with cementitious compounds of the voids of flocculated fabric in the soil. The reduction in swell can be attributed to the resulting compacted and dense mass of treated soils due to cementation of soil particles and cation exchange. The complex behaviour of swell of high plasticity kaolinite-bentonite mix has been explained using one dimensional oedometer test, by further experimental study and examination of the micro-structure of treated soils

Numerical analysis of evapotranspiration and its influence on embankments

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A decision support system for ground improvement method selection

Abstract unavailable please refer to PD

Hydro-mechanical analysis of expansive clays : constitutive and numerical modelling.

Bentonite-based materials are being currently considered in several countries as a backfill component in the multi-barrier concept for deep geological disposal of radioactive waste. The bentonite barrier fulfils several important functions: i) high swelling capacity to fill gaps and compress the excavation damaged zone and ii) very low hydraulic conductivity and important retention capacity which retards significantly radionuclides transport. Small-scale testing in geotechnical laboratories and in-situ experiments in underground research laboratories (URL) have demonstrated that initial state, water supply conditions and volume constrictions are the main aspects affecting the behaviour of bentonites. In this context, the main objective of the present study is the numerical simulation of the hydro-mechanical behaviour of expansive clays. For this purpose, a constitutive model has been developed to characterise the bentonite-based materials. The modelling of these materials is a quite challenging task. They exhibit a marked double-porosity system in which the swelling/shrinkage mechanism occurs at clay aggregate level and the collapsible behaviour comes from granular-like skeleton formed by the aggregates. In addition, several material configuration, with even more intricate fabric, have been proposed for the emplacement works of seals and plugs. The explicit consideration of two structural levels for the constitutive model seems to be suitable. Mechanical interaction and water mass exchanges between them can explain the short- and long-term behaviour. The model has been formulated using concepts of elasto-plasticity for strain hardening materials and generalized plasticity theory. The formulation has been implemented in the finite element code program CODE-BRIGHT and has been used to solve a variety of problems. The results provide relevant insights into the hydro-mechanical behaviour of double structure porous media, and they indicated the main aspects affecting the responses of expansive barriers. In particular, the relevance of the structural levels interaction has been demonstrated.Postprint (published version

Electrokinetic treatment of desiccated expansive clay

Structures built on shallow foundations in expansive clay, where vegetation and weather condition can significantly reduce the moisture content, must overcome the risk associated with excessive desiccation condition such as shrinkage and settlement. For many existing structures, removal of trees may not necessarily be an effective solution as it might cause swelling and heave on the long-term. This study explores a system that could be operated during prolong drought periods to rehydrate and neutralise the negative pore pressures of expansive clay avoiding the development of excessive desiccation. Electrokinetic (EK) is a proven method that has increasingly been adopted for dewatering and consolidation of fine-grained soils. Our hypothesis is to reverse the EK process by extracting water from saturated stratum below groundwater level and force it into the “early-stage” desiccated area, accelerating the rehydration and reducing suction. The theory was examined by a laboratory model using moderately expansive clay. After draining the soil for 23 days reaching moderate desiccation state, water was allowed into the lower part of the soil while applying the EK treatment using nine electrodes, placed in a radial pattern and inserted 110mm into the soil with approximately 30% of their lengths below the water level. As a result of the electro-osmotic process, the average moisture content increased in the model by more than two-fold within 8 hours. The suction measurement (taken in the middle of the desiccated area) showed an initial slow response followed by fast and consistent reduction rate, where suction dropped by 93% within 5 hours and ultimately down to 1kPa at the end of the treatment. The paper offers some insight into the factors controlling the proposed system and provides a good basis for future research to manage the risks associated with desiccation of expansive soils.University of Derby College of Engineering and Technology

THE DETERMINATION OF MOISTURE VARIATION PATTERNS IN CLAY SOILS UNDER A LIGHT STRUCTURE HOUSE

Published ThesisThe South African Government’s attempts to provide affordable, subsidized housing for the very poor have suffered from a large number of structural failures, many due to heaving foundations. These houses are particularly susceptible to damage by heaving clay because they are relatively light and clay heave can lift them very easily. Rational design requires knowledge of the pattern of heave which will occur under the foundation. The pattern of heave depends on the pattern of moisture movement. Currently available methods of rational design rely on assumptions about the shape of the mound which will develop due to moisture movement under the foundation. The shape assumed is largely guided by measurements made on mock foundations. Instrumentation has been installed under a Government Subsidy house in the Free State and moisture movement is being monitored. The actual pattern of moisture movement observed is substantially different to what is normally assumed and could point to more reliable estimates of the heave which needs to be designed for

Soft Computing Based Prediction of Unconfined Compressive Strength of Fly Ash Stabilised Organic Clay

The current study uses machine learning techniques such as Random Forest Regression (RFR), Artificial Neural Networks (ANN), Support Vector Machines Ploy kernel (SVMP), Support Vector Machines Radial Basis Function Kernel (SVMRBK), and M5P model tree (M5P) to estimate unconfined compressive strength of organic clay stabilized with fly ash. The unconfined compressive strength of stabilized clay was computed by considering the different input variables namely i) the ratio of Cao to Sio2, ii) organic content (OC), iii) fly ash (FAper) content, iv) the unconfined compressive strength of organic clay without fly ash (UCS0) and v) the pH of soil-fly ash (pHmix). By comparing the performance measure parameters, each model performance is evaluated. The result of present study can conclude the random forest regression (RFR) model predicts the unconfined compressive strength of the organic clay stabilized with fly ash with least error followed by Support Vector Machines Radial Basis Function Kernel (SVMRBK), Support Vector Machines Ploy kernel (SVMP), Artificial Neural Networks (ANN) and M5P model tree (M5P). When compared to the semi-empirical model available in the literature, all of the model predictions given in this study perform well. Finally, the RFR and SVMRBK sensitivity analyses revealed that the CaO/SiO2 ratio was the most relevant parameter in the prediction of unconfined compressive strength

An overview of soil–water characteristic curves of stabilised soils and their influential factors

Since unsaturated soil conditions are normally experienced above the groundwater table, most treated or stabilised soils for roadworks, earth dams’ embankments, landfill sites, hydraulic barriers etc. could be regarded as existing in this region. The soil–water characteristic (or retention) curve (SWCC) is a useful conceptual tool by which an evaluation of unsaturated soil's property functions and corresponding macro-scale behaviour (strength, volume change, hydraulic conductivity, fluid flow, diffusivity, etc.) can be carried out. Hence, an examination of some of the various factors that could affect the hydraulic or water retention property of the stabilised soil is very vital both for laboratory studies and field practice. However, a thorough assessment of the water retention behaviour of stabilised soils can be understandably limited sometimes. This could be partly due to some of the perculiar conditions associated with soil preparation methods, soil type, soil-stabiliser mix proportion used, curing conditions, method of compaction, durability assessment modalities and other logistical issues surrounding either laboratory instrumentation or in-situ application. This article presents a critical and comprehensive review of these factors on the stabilised soil’s water retention behaviour and also provides a systematic understanding of the mechanisms of stabilisation occurring at the micro- and macro-mechanical levels. Recommendations are also made to stimulate further discussions on the synthesis of SWCC of stabilised soils vis-à-vis factors influencing them with possible interpreted engineering behaviours such as shear strength and soil consolidation

A Soil Suction-Oedometer Method and Design Soil Suction Profile Recommendations for Estimation of Volume Change of Expansive Soils

abstract: The experience base of practitioners with expansive soils is largely devoid of directly measured soil suction. This historical lack of soil suction measurement represents an impediment to adoption of modern unsaturated soil engineering to problems of expansive soils. Most notably, soil suction-based analyses are paramount to proper design of foundations in expansive soils. Naturally, the best method to obtain design suction profiles is to perform an appropriate geotechnical investigation that involves soil moisture change-appropriate drilling depths, sampling intervals, and requisite laboratory testing, including suction measurement. However, as practitioners are slow to embrace changes in methodology, specifically regarding the adoption of even relatively simple suction measurement techniques, it has become imperative to develop a method by which the routine geotechnical procedures currently employed can be used to arrive at acceptable approximations of soil suction profiles. Herein, a substitute, or surrogate, for soil suction is presented, such that the surrogate agrees with observed field soil suction patterns and provides estimates of soil suction that are acceptable for use in practice. Field investigations with extensive laboratory testing, including direct suction measurement, are used in development of the soil suction surrogate. This surrogate, a function of water content and routinely measured soil index properties, is then used in estimation of field expansive soil suction values. The suction surrogate, together with existing geotechnical engineering reports, is used to augment the limited existing database of field soil suction profiles. This augmented soil suction profile database is used in development of recommendations for design suction envelopes and design suction profiles. Using the suction surrogate, it is possible to proceed from the beginning to the end of the Suction-Oedometer soil heave/shrinkage analysis without directly measuring soil suction. The magnitude of suction surrogate-based heave estimates is essentially the same as heave estimates obtained using direct soil suction measurements. The soil suction surrogate-based approach, which uses a complete-stress-state approach, considering both net normal stress and soil suction, is an intermediate step towards the adoption of unsaturated soil engineering in expansive soils analyses, wherein direct soil suction measurements are routinely made.Dissertation/ThesisDoctoral Dissertation Civil, Environmental and Sustainable Engineering 201