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The mechanical properties of very soft clays, silts and organic soils can be improved with deep mixing, a soil improvement technique in which stabilising agents, such as lime and/or cement are mixed into the soil in-situ by using auger-type mixing tools. Deep mixed columns are nowadays extensively used to reduce settlements and improve the stability of embankments and foundations constructed on soft soils. The problem analysed involving a regular arrangement of columns under an engineering stmcture, such as embankments or strip footings, is a fiilly three dimensional problem. As 3D analyses are computationally very expensive, an enhanced 2D technique using the so-called volume averaging technique has been developed as part of this research. The basic idea is to describe the column-improved ground as a homogenized composite material and map the the 3D problem into 2D.

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Increase in environmental awareness over the past decades has resulted in increasing attention to industrial pollution and waste management control. The use of waste is becoming increasingly important in construction. Such materials including Wastepaper Sludge Ash (WSA), can be used to modify certain engineering properties of soils for specific uses to conserve non-renewable natural resources. hi lime-stabilization of sulfate-bearing clay soils, there has been increasing concern over the damage caused by the expansion which is produced when sulfate-bearing soils are encountered. The main objective of this research was to investigate the potential of utilizing WSA, an industrial by-product, as a soil stabilizer with or without blending it with quicklime (CaO), Portland Cement (PC) or with Ground Granulated Blastfurnace Slag (GGBS). The engineering behaviour (plasticity characteristics, compaction, unconfined compressive strength (UCS), linear expansion and California Bearing Ratio (CBR)) of the sulfate-bearing Lower Oxford Clay (LOG) soil and of a non sulfate-bearing soil-industrial Kaolinite (control)-were investigated. Compacted cylinders of LOG and Kaolinite stabilized with quicklime ((CaO) at typical 2wt.%, 4wt.% and 6wt.%) and with various stabilizers incorporating WSA (WSA-Lime, WSA-PC and WSA-GGBS, at 10wt.%, 15wt.% and 20wt.%) were made under controlled laboratory conditions. The cylinders were made under either mellowed (compacted 3 days after mixing) or unmellowed (compacted immediately after mixing) conditions and then moist cured for 7, 28, 90, 180 and 365 days prior to UCS tests. CBR tests were also carried out, but only on selected stabilized LOG samples, at the lowest and highest stabilizer contents. Linear expansion of stabilized cylinders during moist curing and during subsequent soaking was monitored for at least 100 days. The results obtained showed that the blended stabilizers incorporating WSA reduced the plasticity index (PI), reduced the maximum dry density (MDD) and increased the optimum moisture content (OMC) of both LOG and Kaolinite. The UCS values of stabilized systems incorporating WSA for both LOG and Kaolinite were higher than those systems stabilized with the traditional CaO. When WSA was blended with lime, PC or GGBS, the results indicated that in the LOG stabilized system, the strength development of unmellowed samples was generally better than for the mellowed samples. This is in contrast with the Kaolinite stabilized system where it is the mellowed samples that recorded higher strength than the unmellowed samples. The CBR values of the unmellowed samples were also higher than those for the mellowed samples. The linear expansion of unmellowed stabilized LOG system was significantly reduced. This is again quite the opposite in the Kaolinite system, where mellowed samples showed reduced expansion relative to the unmellowed ones. Sulfate and thermogravimetric analysis results suggest that the presence of sulfate in a soil plays a major role in the mellowing process. In conclusion, the findings in this research suggest that whether or not to mellow depends primarily on the stabilizer used. Other variables include the target material and site conditions, besides possibly other factors. There are technological, economic as well as environmental advantages of utilizing WSA and similar industrial by-products, in the stabilization of sulfate bearing and other clay soils, as an alternative to the traditional stabilizers of lime and/or Portland Cement.

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Several well established techniques of soil stabilisation and soil reinforcement are available to improve properties of geotechnical materials. However, the addition of fibre into soils has its unique potential as a reinforcing agent. This is because a friction between fibre and soil particles increases the bonding between the particles of soils and this can improve the plasticity, stress-strain behaviour and failure characteristics of both cemented and uncemented soils. It also reduces the brittleness of the cemented sand. Numerous experiments on fibre-reinforced granular materials have been carried out by several researchers. However, the behaviour of fibre-reinforced cemented granular soils has not been fully understood yet. Furthermore, most experimental studies of fibre reinforced cemented materials have been carried out at relatively low confining pressures. As a result, more experiments are still needed to understand complicated behaviour of soil-cement-fibre composite materials. The main objective of this thesis is to analyse the behaviour of fibre reinforced cemented sand under wide range of confining pressures. For this GDS high pressure triaxial cell apparatus and Bishop and Wesley conventional triaxial cell apparatus have been used to carry out the tests at wide range of confining pressures from 50kPa to 20MPa. Drained and undrained tests have been carried out on polypropylene fibre reinforced sand with and without the addition of cement. Samples with varying fibre and cement content were prepared by the method of undercompaction and were cured for 28 days prior to testing. The experimental results indicate that there is a significant effect from the addition of fibre and/or cement contents and confining pressures on the mechanical behaviour of Portaway sand. Particularly, these effects were noted in drained and undrained triaxial tests, particularly peak strength, strength parameters, shear banding, particle crushing, yielding, and stress-dilatancy relationships. The addition of fibres increases the peak, yield, and ultimate strengths. Increase in confining pressure also increases the strength but the individual effect of the addition of fibres was more pronounced at low confining pressures. Progressive suppression in the dilation by the gradual increase in confining pressures as well as an increase in dilation with the addition of fibres during triaxial compression was also worth noticeable. Although, no noticeable affect was observed in isotropic compression due to the addition of fibre in both cemented and uncemented sand. An extensive series of tests were carried out but due to time constraint only one type and length of fibre was used. Therefore, more research needs to be carried out at different fibre lengths and types in order to see that whether these change the behaviour observed in this research.

Soil stabilisation is a useful civil engineering technique that enables the insitu material to be used as part of an engineered structure. Stabilised layers are used in road foundation; working platforms and for slope stabilisation and sea defences. Chemical stabilisation involves the use of a hydraulic binder (and sometimes additional pozzolans). Commonly, quicklime (CaO) or slaked-lime (Ca(OH)2) is used. On mixing into the ground, this reacts with the aluminosilicates of the clay fraction, reducing its overall water content and plasticity. Further additions increase the insitu pH. Above pH 10.4, the aluminosilicates become soluble in the pore solution. They are then able to form a range of insoluble mineral hydrates which constitute a cementitious matrix. This results in both an increase in mechanical strength and a decrease in dimensional stability. If the insitu material contains sulfur bearing mineralogies, these can react with the hydraulic binder and the aluminosilicates to form expansive minerals. If this occurs after the initial setting and hardening of the stabilised layer has occurred, it can lead to severe dimensional instability and mechanical weakening. This is termed sulfate heave and the principal agent of this heave is a hydrous calcium sulfoaluminate hydrate, ettringite (AFt). The fundamental processes of ettringite formation and associated expansion are little understood in stabilised soils. This research used a range of artificial sulfate bearing, lime stabilised blended soil samples subject to two immersion tests used for material suitability assessment in the UK. The physicochemical response (in terms of dimensional heave and mechanical weakening) was assessed as a function of soil composition and the environmental conditions imposed by the two immersion tests. The fundamental microstructure and phase composition was characterised using a range of analytical techniques (XRD, SEM-EDX, dTGA). The relationship between the observed macro-physical properties and underlying chemical environment and microstructure was explored. Key findings include that the mechanism of ettringite formation and expansion was found to be governed by the fundamental structure of the bulk clay. This explained the greater swell response of the kaolin based soils compared to those of the montmorillonite. The SEM-EDX analysis identified a primitive, Ca-rich, AFt phase termed ‘ball ettringite’, in stabilised soils. This has only relatively recently been reported in studies of cement mortars. Also, small amounts of sulfate in the bulk soil actually increase soil strength. It was suggested that the preferential formation of monosulfate (AFm) plays an important role in this mechanism. The introduction of water to the pore solution is key to the formation of ettringite. This was evidenced by X-Ray CT of the damage caused to soil specimens on immersion, as well as low angle XRD studies of the principal AFt peak. Based on the limited testing undertaken one of the immersion tests (European accelerated volumetic swell test, EN13286-49), appears to be more onerous than the other (UK CBR linear swell test, BS1924-2).