title: Characterisation and Mechanical Behaviour of Reconstituted and Stabilised South East Queensland Soft Soils
Creators: name: Liu, Yaxu
Publisher: name: Griffith University
Year: 2022
Subjects: subject: Soft soil stabilisation
subject: Mechanical and critical-state behaviour of soft soil
subject: Microstructural analysis
subject: Replacement of cement
subject: Reduction in carbon emission

description: Soft soils are deposited globally, especially in estuarine or coastal areas. In recent years, the land resource has lessened due to rapid urbanisation and population growth around the globe. It is crucial to develop land on poor ground conditions to solve the issue of land shortage due to urbanisation. South East Queensland is a particular region where soft soils are widely deposited. More construction is expected to be carried out on its soft soil deposits as the urbanisation continues. However, the existence of soft soils can cause construction complications because of the following reasons: having high compressibility and water content, accompanied by low shear strength and permeability. Therefore, the study of the mechanical behaviour of reconstituted and stabilised soft soils is significant in geotechnical engineering practice. There are limitations in previous research regarding the properties of soft soils. For example, the common particles in soft soils are clay, silt, and sand particles. The behaviour of clay and sand particles are unique and easy to identify. However, the behaviour of silt particles lies in between the behaviours of clay and sand. It is important that some previous studies found that the behaviour of silt is not in accordance with the critical-state framework adopted for clay and sand. It is suggested that the behaviour of silts is a transitional form between clay and sand. Some silts exhibit sand-like behaviour, while some exhibit clay-like behaviour. Consequently, it is important to understand silt’s physical, mineralogical, strength and microstructural behaviour, as it is presently recognised that gaps in understanding its fundamental behaviour exist. In addition, soft soils need to be stabilised by suitable ground improvement techniques before any structure can be safely constructed on it. It is widely known that in-situ soil mixing or stabilisation (e.g., mass mixing or deep soil mixing) has been proven to be an effective ground improvement technique in improving the engineering properties of soft soils. Cement is one of the commonly used cementitious materials which can be used to treat soft soil in the application of in-situ soil mixing. It can increase the soil mix strength and decrease the water content by triggering the hydration of cement and pozzolanic reactions. The use of cement to stabilise soft soils and the behaviour of cement-stabilised soils has been extensively investigated in many previous studies. However, the use of cement can cause environmental issues as the production of cement results in high emissions of carbon dioxide (CO2). Hence, it is essential also to consider other suitable types of stabilisation additives to reduce the amount of cement used in the stabilisation of soft soil. Fly ash and a commercially available additive DuraCrete, were investigated in this study as partial replacements of cement. The behaviour of specimens stabilised by cement, fly ash-blended cement, and DuraCreteblended cement under both unconfined compressive (UC) and consolidated isotropic undrained (CIU) conditions were investigated in this study. The experimental results proved that fly ash and DuraCrete can be used as partial replacements of cement to achieve more remarkable improvement results than just cement alone in stabilising soft soils. DuraCrete is more effective compared to fly ash because the addition of DuraCrete can reduce the amount of cement needed for the stabilisation while also improving the strength of stabilised specimens. This project seeks to investigate a) the mechanical behaviour of South East Queensland soft soil stabilised by cement with different cement content; b) the effect of the presence of silt particles on the mechanical behaviour of soft soils, such as evaluating the behaviour of silty soils within the critical-state framework; c) the effect of the presence of silt particles on the mechanical and microstructural of soft soils after stabilised by cement; and d) the use of fly ash and DuraCrete as partial replacements of cement in soft soil stabilisation. A series of laboratory tests consisting of consolidated isotropic undrained (CIU) triaxial tests, unconfined compressive strength (UCS) tests, and scanning electron microscope (SEM) tests were conducted in this study to achieve these objectives. South East Queensland soft soil was collected and stabilised by cement with varying initial soil water content and cement content. The mechanical and microstructural behaviour of natural and cement-stabilised South East Queensland soft soil was investigated. Some empirical equations were derived to estimate the strength of South East Queensland soft soil specimens with different cement content. The microstructure of cement-stabilised soil specimens was also analysed and interpreted. A series of triaxial compressive tests were conducted in this study on five types of soft soils with varying clay and silt contents, and therefore the effect of silt contents on the strength and critical state behaviours of soft soils were investigated. The empirical equations were proposed to evaluate the effect of silt content on the stress paths of reconstituted soft soils under consolidated isotropic undrained triaxial tests and the critical state parameters. Based on the observations from the CIU triaxial compression tests, it can be concluded that 1. For silty soils which have a plasticity index above 29%, even the soils are classified as silt by Atterberg limit testing results, but the soils show clay-like behaviour in the critical state framework, evidenced by the corresponding normally consolidated line (NCL) and critical state line (CSL) are parallel. 2. For silty soils, which have a plasticity index between 19% and 29%, the soils show a transitional behaviour between the clay-like and sand-like behaviour, as the corresponding normally consolidated line (NCL) and critical state line (CSL) are becoming non-parallel. 3. For silty soils, which have a plasticity index lower than 19%, it shows typical sand-like behaviour. These types of soft soils were then stabilised by cement with varying cement content. A further series of unconfined compression tests were conducted for each group of cement-stabilised soil specimens. As the silt content might exhibit a different influence on the strength of cement-stabilised samples, a varying dosage of cement content was considered in this study. The experimental results indicate that silt content plays a different role in soil stabilisation under different cement contents. The effect of cement content and silt content on the microstructure development of stabilised soils were also analysed by utilising the Scanning Electron Microscope (SEM) images. With the increase of cement dosage, the number of cementitious products, such as reticulated CSH and needle-shaped ettringite, was notably increased, resulting in a denser structure. This can be attributed to the hydration of cement and the pozzolanic reactions. As for the effect of silt content, since particle size plays a significant role in microstructure development, both cement and silt contents can dramatically affect the pore size distribution. When the cement content is lower than 10%, clay platelets can fill the pore spaces and the cementitious products can enhance the inter-cluster bond strength by aggregating clay and silt platelets together to form larger and denser aggregates responsible for the strength improvement. When the cement content is between 10% and 20%, the stabilised soil strengths increase with the increase of silt content and then decrease when silt contents are higher than 50%. This is because the strength gained from cementitious product enhancement was partially countered by the increment of pore size caused by the excessive cement and silt contents. When the cement content is higher than 20%, the strength shows a negative correlation with silt content, which can be attributed to the incomplete reaction of cement due to the reduction of clay content. Regarding the partial replacement of cement by adopting fly ash and DuraCrete, the UCS and CIU testing results show that both fly ash and DuraCrete are very effective as partial replacements of cement to reduce the cement content and CO2 emission. Fly ash can the provide the highest reduction in the cement replacement content, and it can also provide the highest reduction in CO2 emission. However, at the same mixture content (e.g., 25%), the UCS of the specimens stabilised by fly ash-blended cement is lower than that stabilised by cement only. Thus, more material is needed when using fly ash to partially replace cement to maintain the same UCS. Even though, the CO2 footprint can still be reduced because the CO2 emission rate of fly ash is much lesser than that of pure cement. Therefore, fly ash is effective as a partial replacement of cement to reduce the use of cement and CO2 emission. Compared to fly ash, DuraCrete is more effective as a partial replacement of cement in some circumstances. For example, the total mixture content is reduced to achieve a target strength of 500 kPa when using DuraCrete-blended cement instead of pure cement only. The reduction in total mixture content is an essential advantage by using DuraCrete compared to using fly ash. Comparing the proportional quantities of fly ash and DuraCrete required, the quantity of fly ash required is between 6.1 times and 9.7 times the proportional quantity of DuraCrete required. Even though the use of DuraCrete can reduce the amount of cement used and reduce the total mixture content, it cannot provide as much reduction in cement as fly ash does. This is because there is a ‘saturation point’ with the DuraCrete replacement ratio. If this saturation point is exceeded, DuraCrete will not be as effective anymore, being mainly a magnesiumbased additive. Therefore, when the maximum reduction in cement is the only factor under consideration, fly ash is more suitable than DuraCrete, as it facilitates a greater reduction in cement. However, suppose both reduction in cement and the total mixture content are considered. In that case, DuraCrete might be more appropriate, as it not only reduces the use of cement but also reduces the total mixture content required. Most importantly, unlike cement and fly ash, the production of DuraCrete is not carbon-intensive. The production of DuraCrete does not produce carbon emission as it does not require a furnace, nor is it a by-product of a carbon emitting process. These critical outcomes can help engineers reliably customise the soil stabilisation design to achieve optimal strength, environmental friendliness, and cost-saving. As such, engineers can have more design options to meet the strength requirement while having the opportunity to minimise the negative impact on the environment by reducing the use of cement. They can also achieve a balance between the reduction in cement and the budget, hence the important contribution of this study.