Lime stabilisation is a ground improvement technique used to improve the engineering properties of cohesive fill materials. During earthworks operations, specialist plant is used to rotovate the clay fill material and intermix lime binder around clay clods. After completion of the lime treatment, the layer is compacted in the usual way. Immediately after mixing, the lime instigate a series of physico-chemical reactions within the clay soil. Where the chemical reactions are favourable and with time after compaction (curing) the material becomes progressively stronger and durable to environmental influences, e.g. inundation by surface or ground water. However, where sulphate is present within the soil, the reactions may change and the ingress of water into the layer can result in the expansive growth of deleterious minerals e.g. ettringite. While sulphate swell issues are relatively rare, when they do occur the degree of expansion can be very high. A high profile sulphate swell failure developed during the construction of the M40, Oxford, UK in 1989. Over the winter period after the lime stabilisation works, a 250mm deep lime treated layer heaved by up to 150mm - destroying the overlying road construction. Since the M40 failure, a substantial amount of effort has been undertaken to better understand the sulphate swell reactions and in this regard the state of scientific knowledge is relatively strong. A fundamental issue for field applications of lime stabilisation is that the vast majority of research has been undertaken on laboratory specimens prepared using methods which do not reflect site practice. Laboratory studies often use oven dried and finely crushed clay, whereas site operations will treat much larger clay clods to result in a more heterogeneous distribution of lime through the compacted soil body. With large clay clods, the chemical reactants must migrate through clods and this may cause the sequence of chemical reactions to change. A further challenge is that laboratory studies are typically undertaken with cure temperatures of 20°C, whereas a typical near surface temperature in the UK is <10°C. This is of particular relevance to sulphate swell failures which are reported to coincide with a reduction in ambient temperature over winter periods. Thus, the direct relevance of laboratory studies to site application was unclear. A series of laboratory experiments using a preparation method which reflects field applications of lime stabilisation was used to investigate the influence of large clay clods on the durability of lime stabilised clay soil. This method was applied to both low and high sulphate clay soils. A fundamental discovery from work on low sulphate clay is that the addition of lime binder to the surface of the clay clods causes a physico-chemical boundary to form. This boundary develops due to the rapid increase to the plastic limit of the clay preventing adjacent clods from joining together during compaction. This causes the engineering properties of each individual clod to develop independent to its neighbours and for each clay clod to be separated by an inter-clod pore space. The strength of each individual clay clod will increase with curing as the added lime dissociates into Ca2+ and OH- and migrates to form C-S-H deep within the clods. Where the material is compacted wet of the optimum water content, this condition improves ion migration and enables development of diffuse cementation deep within clods. The inter-clod porosity remains as a weakness throughout curing especially during specimen soaking, where the pore channels comprise a pathway, accelerating the ingress of soaking water. With low sulphate soil, the soaking water softens the treated material, however, with high TPS soil substantial sulphate swelling may develop. Thus, efforts to minimise this porosity during preparation is important and the use of quicklime with longer mellowing periods can cause the clay clods to develop high strength before compaction. The high strength clods resist compaction and the degree of inter-clod porosity in the compacted mass increases, worsening specimen durability to water ingress. The investigations into high sulphate clays included the development of a Novel Swell Test (NST) to assess volume change. A unique aspect of the NST was that the sulphate swell response of the lime treated material was investigated at site realistic temperatures of 8°C. It was identified that, when compared with standard laboratory test temperatures of 20°C the rate of sulphate swell is substantially higher at the low temperature. The mineralogical testing has permitted the hypothesis that, at 8°C the growth of crystalline ettringite becomes slower and the ettringite precursor, which has a high affinity to imbibe water, remains in this state for much longer. Thus, laboratory swell tests at 20°C may substantially underestimate the degree of swell that may develop in the field. As a pressing need, it is recommended that the industry adapt sulphate swell test methods to appraise the degree of swell at field realistic temperatures i.e. < 10°C. The work also identifies that the primary defence against sulphate swell is to condition the fill so that the risk of post compaction water ingress, via inter-clod porosity, is minimised. The use of GGBS and water addition during extended mellowing periods also reduces the degree of sulphate swell in natural clay soils. This work concludes that working methods for lime stabilisation of medium high plasticity soils of a potentially high sulphate content, should be adapted to encourage diffuse cementation and minimise the degree of (post compaction) inter-clod porosity. Practically this involves the use of hydrated lime and the addition of mixing water throughout extended mellowing periods. Fundamentally, the study recommends that where construction programmes allow, the long term durability of a fill material should be the priority over immediate strength.

Natural soils in work-sites are sometimes detrimental to the construction of engineering projects. Problematic soils such as soft and expansive soils are a real source of concern to the long-term stability of structures if care is not taken. Expansive soils could generate immense distress due to their volume change in response to a slight change in their water content. On the other hand, soft soils are characterised by their low shear strength and poor workability. In earthwork, replacing these soils is sometimes economically and sustainably unjustifiable in particular if they can be stabilised to improve their behaviour. Several techniques have evolved to enable construction on problematic soils such as reinforcement using fibre and planar layers and piled reinforced embankments. Chemical treatment using, e.g. lime and/or cement is an alternative method to seize the volume change of swelling clays. The use of lime as a binding agent is becoming a popular method due to its abundant availability and cost-effectiveness. When mixed with swelling clays, lime enhances the mechanical properties, workability and reduces sensitivity to absorption and release of water. There is a consensus in the literature about the primary mechanisms, namely cation exchange, flocculation and pozzolanic reaction, which cause the changes in the soil characteristics after adding lime in the presence of water. The dispute is about whether these mechanisms occur in a sequential or synchronous manner. More precisely, the controversy concerns the formation of cementitious compounds in the pozzolanic reaction, whether it starts directly or after the cation exchange and flocculation are completed. The current study aims to monitor the signs of the formation of such compounds using a geotechnical approach. In this context, the effect of delayed compaction, lime content, mineralogy composition, curing time and environmental temperature on the properties of lime-treated clays were investigated. The compaction, swelling and permeability, and unconfind compression strength tests were chosen to evaluate such effect. In general, the results of the geotechnical approach have been characterised by their scattering. The sources of this dispersion are numerous and include sampling methods, pulverisation degree, mixing times and delay of compaction process, a pre-test temperature and humidity, differences in dry unit weight values, and testing methods. Therefore, in the current study, several precautions have been set to reduce the scattering in the results of such tests so that they can be used efficiently to monitor the evolution in the properties that are directly related to the formation and development of cementitious compounds. Four clays with different mineralogy compositions, covering a wide range of liquid limits, were chosen. The mechanical and hydraulic behaviour of such clays that had been treated by various concentrations of lime up to 25% at two ambient temperatures of 20 and 40oC were monitored for various curing times. The results indicated that the timing of the onset of changes in mechanical and hydraulic properties that are related to the formation of cementitious compounds depends on the mineralogy composition of treated clay and ambient temperature. Moreover, at a given temperature, the continuity of such changes in the characteristics of a given lime-treated clay depends on the lime availability.

Yes / This study aims to provide a thorough evaluation for the changes in the microstructure and evolution of strength of highly reactive clays that were treated with 7 % lime over a period of curing time as a function of the mixing moisture content. Three series of testing were carried out on specimens with 100 %, 85 % and 75 % of bentonite content and prepared with different moisture content of 10, 20, 30 and 40 % above the corresponding optimum moisture content. Specimens of 100 % bentonite were treated with 7 % of lime, compacted to achieve a predetermined dry unit weight and cured at temperatures of 20 OC and 40 OC for up to 28 days whereas the specimens with 85 % and 75 % of bentonite content were prepared by the addition of sand and were cured at 20 oC for up to 7 days. Unconfined Compressive Strength tests and Scanning Electron Microscopy were conducted to observe the strength and the microstructural changes resulting from increasing mixing moisture content. California Bearing Ratio and Resilient Modulus were correspondingly determined based on correlations with the Unconfined Compressive Strength. The failure pattern was also studied to better understand the ultimate behaviour of lime stabilised clays. The results revealed that the strength of treated bentonite increased with the increase in the moisture content up to 30 % above the corresponding optimum moisture content and with increasing the curing time and temperature. Nevertheless, substituting bentonite with sand on the specimen resulted in a significant reduction on the attained strength. Furthermore, the results of California Bearing Ratio and Resilient Modulus showed that values for both parameters are significantly enhanced with lime treatment. The microstructural analysis provided visual evidence to the improved strength in which the pozzolanic reaction was found to be significantly affected by the amount of moisture in the mixture. The results suggested that compacting lime treated expansive clays with moisture content moderately higher than the optimum moisture content would result in a significant enhancement to the attained strength over the period of curing.

Construction on problematic soils is challenging owing to the potential of volume changes due to variation of moisture content. Lime stabilisation can be used to treat problematic soils. The main drawbacks of lime addition to the clayey soils are the need for lengthy curing periods and relatively large quantities of lime for significant improvement and also loss in ductility. Using eco-friendly agricultural and industrial wastes, that can partially be substituted by the material responsible for greenhouse gases such as lime, can overcome these drawbacks and decrease global warming. In the current study, variables controlling the unconfined compressive strength of lime treated clay with a focus on assessing the effects of moisture content were investigated. Furthermore, the effects of adding agricultural waste (Dried Athel Leaves (DAL)) and industrial waste (Silica Fume (SF)) on hydromechanical properties of lime treated clay were assessed. The performance of the treated mixtures was examined based on results attained from unconfined compressive strength, swelling pressure and permeability. Specimens were treated with deferent percentages of lime and cured at different periods and temperatures to observe the strength behaviour. In oedometer tests, the specimens were prepared and tested immediately after compaction. The failure patterns were also studied to better understand the ultimate behaviour of lime stabilised clays. The appearance and presence of cementitious products were identified by using the scanning electron microscope and energy dispersive X-ray spectrometer techniques to elucidate their strength development. The findings indicated that the effect of moisture content is controlled by the clay content and unit weight. The addition of 7% lime to clay caused a remarkable increase in the unconfined compressive strength by 363%. The incorporation of 2% DAL and 5% SF within lime treated clay further increased the strength by 6% and 33% respectively after curing of 28 days at 20 °c in comparison with those attained by lime treatment only. The improvement of the strength of the lime­ treated clay augmented with both wastes continued in long term. Temperature and lime content have positive effects on the improvement of strength, however, increasing lime content to 11% negatively affected the strength of lime treated specimens with 2% DAL. The formation of cementitious products was observed in SEM images and detected quantitatively through EDS analysis. The results of the recorded oedometric tests for lime-DAL and lime-SF mixtures revealed that incorporation of the 2% DAL and 5% SF reduced the clay swelling pressures by 25% and 10% compared to that attained by lime treatment only resulting in total reductions of 93.6% and 68% from that recorded on untreated clay. In addition, the impermeable clay transformed into permeable material by adding DAL and SF. Of the two types of wastes considered in this research, DAL demonstrated more superior improving capability. A further study was conducted to develop ANN model based on collated laboratory data for the prediction of the UCS values of lime treated soils. The promising outcomes of this research suggest that the drawbacks of lime stabilisation can be overcome by the addition of agricultural and industrial wastes. Consequently, the findings attained could be considered in future practice standards with regards to the requirement of lime stabilisation. / Ministry of Higher Education and Scientific Research in Libya