In order to enable the use of Enzyme Induced Carbonate Precipitation (EICP) as a ground im-
provement technique in fine grained soils, the use of guanidinium hydrochloride (GndHCl) as a clay swelling inhibitor has been explored recently by Wennubst-Pedrini (2022). Guanidinium aggregates the clay by binding to the double layer of clay minerals. In that way, the particles become chemically and physically stable. However, it has been suggested that GndHCl as an additive has an effect on the calcium carbonate crystal sizes and polymorphs produced during EICP. However, the exact effect of guanidinium on calcium carbonate precipitation remains unclear. This study sets out to elucidate these effects. This study shows that the addition of GndHCl into the system strongly alters its starting pH from 8 to 4, leading to some samples not precipitating any calcium carbonate as the carbonate equilibrium was towards bicarbonate (HCO 3 ). However, the samples that did produce crystalline calcium carbonate, produced a comparable amount of about 0.65g per sample of the same polymorph (calcite), although crystal size was smaller. The morphology of the produced crystals suggested that guanidine altered the calcite production pathways. This study could not reproduce the promising results found by Wennubst-Pedrini (2022) and as such more research into the role of guanidinium in the EICP process is recommended. The promise that guanidine may hold in order to make EICP a market-ready technique in finer grained soils is worth further exploration.

Several Geological Disposal Facilities rely on clay-rich formations as the host geological formation or engineered barrier. During the lifetime of a GDF, gasses may be generated primarily due to the anaerobic corrosion of metal, triggering gas migration throughout the natural or engineered barrier. If significant gas pressures build up, preferential dilatant pathways may develop. Laboratory gas injection tests have shown that these pathways significantly affect gas migration by locally increasing the porosity and decreasing the air entry value. Material heterogeneity is one method that has been used to replicate these pathways in numerical modelling. The effect of modelling material heterogeneity on gas transport properties of clay-rich materials has been studied qualitatively, but no fully stochastic analyses have been performed thus far. As a result, uncertainties remain about the quantitative effects of heterogeneity on transport properties of clay. Furthermore, the role of the scale of fluctuation is unclear.
In this thesis, spatially correlated random fields of porosity were generated, and the Finite Element software LAGAMINE was used to model water permeability, gas diffusion and gas injection tests. Results indicate that the time necessary to reach a steady-state water or gas outflow is unaffected by heterogeneity. Monte Carlo analyses of permeability and gas diffusion tests showed that the variability of the effective permeability is higher than that of the apparent diffusion coefficient due to the nonlinear relation between the porosity and the permeability. Additionally, the variability of the effective permeability and apparent diffusion coefficient depends strongly on the ratio between the sample size and the scale of fluctuation. With larger samples, the effects of the random field are averaged, narrowing the distribution of results.
In gas injection tests, water saturation profiles suggest that the onset of desaturation occurs in zones of high porosity, reinforcing the hypothesis that preferential pathway development is related to material heterogeneity. A Monte Carlo analysis of a gas injection test shows that variability in the gas outflow and the degree of saturation not only depends on the scale of fluctuation but also the injection pressure due to the non-linear dependency of the air entry pressure on the porosity.
These results suggest that heterogeneity and the scale thereof are important aspects of gas migration in clay-rich materials. Specifically for laboratory testing of clay-rich materials, it appears that an understanding of the approximate scale of the heterogeneity is essential for estimating the minimum number of samples and the minimum size of those samples. Too small or too few samples will not allow for a reliable estimate of parameters of interest.
Some limitations apply to the findings of this research. First, the sensitivity of the results to factors such as the standard deviation of the distribution underlying the random field and anisotropy has not been addressed yet. Second, the dependency of the effects of the random field on the injection pressure in gas injection tests is not fully understood yet. Ideally, additional simulations are performed to separate these effects. Finally, hydromechanical coupling should be introduced to replicate experimental findings more accurately.

The integration of biological and (bio-)geochemical processes in soils is posed to become the next transformative practice in geotechnical engineering. Soil Sealing by Enhanced Aluminium and dissolved organic matter Leaching, also known as SoSEAL, is inspired by a natural soil formation process, called Podzolization. This process results in the formation of a nearly impermeable spodic B-horizon. A similar low-permeable layer is made by SoSEAL. The products of the chemical reaction between aluminium and organic matter generates organic precipitation, clogging the pore throats between sand grains. Past research on SoSEAL has primarily centered on its application as a low-permeable water retaining barrier used for dyke improvement. However, the broader impact of SoSEAL on the mechanical properties of soils beyond permeability remains largely unexplored.

Despite progress, gaps persist in understanding SoSEAL’s influence on sand’s mechanical properties, particularly on the shear strength. The primary objective in this research was to investigate the impact of different concentrations of Al-OM (Aluminum and Organic Matter) flocs on sand’s mechanical characteristics through ex-situ mixing. This included the development of a testing procedure using a triaxial test apparatus, incorporating results from permeability tests and utilizing microscopy to analyze micro structural changes and the underlying mechanisms. The study aimed to deliver valuable insights into SoSEAL’s potential as a nature-inspired geo-engineering solution for soil improvement.

Undrained triaxial tests were conducted on ex-situ mixed sand with different concentrations of Al-OM flocs, namely 0, 0.1, 0.5 and 1%. These concentrations were defined as dry mass of flocs based on a metal/carbon ratio of 0.06. Through a carefully executed test procedure, involving Proctor’s test, permeability measurements and triaxial testing, the mechanical properties of the treated sand were investigated. Proctor’s tests were utilized to determine the maximum dry density and its corresponding moisture content of the (un)treated sand. These parameters were used for molding the sand samples for the triaxial test series. The untreated sand did not show a clear peak in its Proctor curve, which is typical for uniform graded fine/medium sands. The porosities, derived from the optimum dry density and corresponding water content, were found to be ≈ 0.4 (-) for all samples. Indicating the minor impact of the Al-OM flocs to the porosity of the sand samples.

The undrained consolidated triaxial test procedure, consisting of saturation, consolidation, and shearing, provided insights into the mechanical properties of SoSEAL. Although the consolidation phase did not reveal significant differences
in the presence of Al-OM flocs, elastic parameters, derived from the shearing phase, generally increased with higher Al-OM floc concentrations in sand. Young’s Modulus, E , increased by a magnitude between 2.11-2.62 times the untreated sand, while the shear Modulus, G, increased by a magnitude between 2.09-2.18. Nonetheless, exceptions such as test CU05-1 and CU10-2 were observed. Strength parameters, measured by maximum deviatoric stress at failure, exhibited an overall increase with higher floc concentrations. Finally, the results show that an alteration in floc concentration in sand did not have a significant impact on its permeability, contrary to previous measurements obtained in the absence of confinement.

The results from the Proctor’s test, permeability measurements and the triaxial tests highlighted differences and improved general knowledge of the impact of Al-OM flocs to the mechanical properties of sand. Variations in test results, seen
when comparing test CU05-1 to CU05-2 and test CU10-1 to CU10-2, underscored the complexity of factors such as compaction challenges and localized failures. From microscale examination using scanning electron miscroscopy (SEM), the increase in strength properties can be attributed to the cohesion between sand grains, evident in the formation of grain clusters. As the concentration of Al-OM flocs in sands increased, so did the quantity of grain clusters. The potential of Al-OM flocs in sand for dyke improvement is found in the observed increase in elastic and strength properties, providing enhanced resistance against erosion.

The frequent occurrence of catastrophic tailings dam failures underscores the urgent need to improve safety practices and minimise associated risks. This study introduces an advanced tool to evaluate the total Probability of Failure (PoF) of existing tailings dams systematically and effectively, supporting the rational prioritisation of mitigation efforts and minimise risk within the 'As Low As Reasonably Practicable' (ALARP) principle.

The tool utilises a semi-quantitative approach, combining observation frequency and expert judgment. A baseline PoF for each dam construction method and failure category is established using a database of 450 tailings dam incidents and accidents developed in this study. This baseline PoF is subsequently modified based on site-specific factors influencing failure prevalence. A total of 255 key contributing factors are identified based on a fault tree analysis, the Global Industry Standard on Tailings Management (GISTM), and experts in the field. The factors encompass site conditions, design elements, and the level of practise, which address all major credible failure modes and mechanisms. The factors are linked to the failure categories they influence and assigned relative weights through the analytical hierarchy processing method for each dam construction method and failure category. Subsequently, the weights are multiplied by modifiers to account for the effects of site-specific conditions (favourable: 0.2, neutral: 1, adverse: 5, and unknown: 2). Within the tool, users can choose fulfilment conditions for each factor from drop-down menus and the selected inputs are connected to the modifiers. The adjusted weights are multiplied by the baseline PoF of each failure category, given the dam construction method. The summation of these products yields the total PoF of the investigated dam.

The results provide preliminary insight into factors significantly affecting the total PoF for the dam under investigation, aiding in evaluating whether the PoF reduction justifies costs. It facilitates preliminary, rational prioritisation of mitigation measures in accordance with the ALARP principle, contributing to ongoing efforts to improve tailings dam safety.

To validate the tool’s capabilities, two case studies with available data are analysed: the Aznalcóllar failure to examine the ability to identify high-risk factors and a recently improved dam to evaluate if the mitigation efforts are adequately reflected. The studies demonstrate the tool’s potential but also reveal uncertainties, inaccuracies, and limitations. These stem from discrepancies in the baseline PoF, weightings, modifiers, and unaccounted factors. Therefore, caution is warranted in the tool’s utilisation. Recommendations include various improvements and further verification and validation across a broader range of case studies. Value can be added by incorporating additional components and adapting the tool for new dams.

In dredging and land reclamation, there are growing economic and environmental pressures to make beneficial use of fine-grained/cohesive soils. Due to these pressures, there is renewed interest in researching ways to optimise the beneficial use of fine-grained soils in dredging and reclamation.
Stiff cohesive clayey soils soil break up into lumps rather than fluidising when being dredged from the sea floor. These lumps are hydraulically transported in the dredge pipe, and erode to a spherical shape due to the forces of scour and abrasion. The presence of clay lumps has a large impact on various parts of the dredging process, such as clogging of dredging equipment, head loss in dredge pipes, wear of equipment, bulking factor, and geotechnical properties (strength, consolidation) of a reclamation. Accurately estimating the presence and prevalence of clay lumps in early phases of a dredging project can significantly improve project outcomes.
The presence of clay lumps is for a large part dependent on the degree to which clay lumps erode during transport through dredge pipes. Currently, methods for predicting this consist of practical experience or design charts, and mostly result in a binary prediction rather than a quantified prediction. Alternatively, physical tests can lead to more quantified predictions, but are expensive and time-consuming. In order to improve predictions of clay lump erosion in the dredge pipe, an analytical approach may be taken. This could consist of a clay lump erosion model (CLEM) equation, representing remaining solid lump mass over time.
Inputs of a CLEM would consist of model parameters that can be correlated to soil parameters and other factors such as dredge pipe transport velocity. These correlations can be found by using an index test apparatus (clay lump erosion test) that simulates dredge pipe transport of clay lumps. Such a test can be used quantify the relationship between a soil parameter and clay lump erosion.

There is no consensus as to which equation is the most suitable as a CLEM, and no quantified correlations exist between soil parameters and clay lump erosion. In this thesis, a suitable CLEM equation is identified and the influence of coarse content (%>63 microns) on clay lump erosion is studied. 35 clay lump erosion tests were performed on various soils. Using this data, a suitable CLEM is selected consisting of an exponential decay curve, where a single model parameter c represents erosion resistance. For 17 of the clay lump tests, soil lump samples of varying coarse content were prepared by mixing clay and sand in various proportions. Fitting the selected CLEM to the results of these tests resulted in a clear relationship between coarse content and c. Lower coarse content corresponds to a higher c, indicating a higher erosion resistance.
These results consist of a starting point in a broader framework for prediction of erosion of clay lumps in the dredge pipe based on soil parameters and dredge pipe transport factors. In order to use a CLEM to make predictions of clay lump erosion in actual dredging projects, the relationship between clay lump erosion test results and clay lump erosion in actual dredge pipes must be studied.
Besides selecting a suitable CLEM equation and studying the relationship between coarse content and clay lump erosion, this research also resulted in a broad compilation of knowledge on clay lump erosion. Also, valuable experience was gained in conducting an experimental study on the relationship between a soil parameter and clay lump erosion in the dredge pipe.

Discontinuities have an important influence on the bulk behaviour of geomaterials. Discontinuities can be physical openings like fractures and joints, interfaces between different material types, or zones with different material behaviour like mortar or reinforcement material. Bulk material behaviour is often determined by the behaviour of the discontinuities by providing preferential pathways for flow or by acting as failure planes.
FEM simulations with continuum elements are typically not suitable for modelling discontinuities with no width, due to the inability to model discontinuous fields. One solution is to represent discontinuities using zero-thickness interface elements. Inserting zero-thickness interface elements allows for discretely modelling discontinuous behaviour between neighbouring continuum elements. The continuum elements are then used to represent the bulk behaviour and the zero-thickness interface elements discretely model the discontinuity.
A new constitutive law for zero-thickness interface elements that expands the cohesive zone model with friction is formulated in this thesis. The tangential stress response of the interface elements consists of a cohesive and frictional contribution. The cohesive component of the constitutive law is based on the Crisfield’s cohesive elasto-damage model, which is characterised by a linear elastic response to increased relative displacement, followed by degradation of the cohesive strength and stiffness. The elasto-plastic frictional component is based on the Dahl friction model and implemented in parallel to the cohesive component.
The implemented new constitutive law is verified in a large variety of loading conditions, including tangential loading, tangential loading with a reversion of loading direction and combinations of mixed-mode loading. The constitutive laws are validated based on shear box experiments performed on London clay where interface elements are used to model a predefined shearing plane.
The cohesive and frictional constitutive law can represent confinement-dependent tangential stress and residual stress at large relative displacements. These features, observed in the shear box experiments, could not be reproduced by the purely cohesive constitutive law. This new constitutive law can be used to better model discontinuities in geomaterials represented by zero-thickness interface elements.

The geo-technical quest to couple technical, environmental, and economic innovation, has increased recent attention towards bio-inspired soil strengthening techniques. This thesis presents a proof-of-concept for a coupled clay inhibition and bio-cementation treatment tested in sand-bentonite, referred to as Bio2Cementation. Fine particles are first aggregated using a nitrogen based compound. By binding the electrical double layer of clay minerals, the aggregates become chemically and physically stable. Thereafter, bio-cementation treatments hydrolyze urea to precipitate calcium carbonate crystals within the pore space. The crystals bind the mineral particles, increasing the strength and stiffness of the soil.
State of the art considerations regarding enzyme induced calcite precipitation and guanidinium hydrochloride research are used as the theoretical foundation for the treatment’s design. A diverse sand-bentonite matrix is tested, comprised of 10% and 30% bentonite, to evaluate the applicability limits of the technique. The implementation is tested in flow-cells, whereby soils are injected with Bio2Cementation treatments. Experiments show the dominant role of guanidine for stabilizing clay particles – the matrix aggregates, hydraulic conductivity improves by two orders of magnitude, surface charge interactions are minimized, and swelling is halted irreversibly. Hydraulic conductivity calcultations, unconfined compressive strength tests, image analysis of Micro-CT scans, and scanning electron microscope imaging evidence to notably improved bio-cementation following guanidine injections. Consistently, the optimized enzyme induced calcite precipitation was found to crystallize vigorously in 10% bentonite samples, but less successfully in optimally compacted 30% bentonite soils. The concept of Bio2Cementation is proven to work within certain limitations.
Future research should explore the role of different clay minerals such as kaolinite, in view of better defining the treatment’s engineering applicability in-situ. Additional strength testing, soil-structure interaction analyses and environmental impact studies are also recommended

This study aimed to evaluate the field scale potential of Microbially Induced Desaturation (MID) to improve the compactibility of silty sands. Conventional soil compaction techniques for saturated silty sands often require either high compaction effort or even fail to reach aspired relative densities. This thesis is a follow-up study based on the promising small scale results of Stals 2020. The proposed solution consists of a two-stage method in which, firstly, a soil sample was desaturated by means of Microbially Induced Desaturation (MID) through denitrification and, secondly, dynamically compacted. The objective of this, and previous, research was to assess whether biogenic gas formation could be used as a pretreatment to increase the effectivity of soil compaction techniques. Tests with a height of 1m and 2.5m were developed, opposed to the previous 15cm small scale tests. Series of tests were conducted on a silt-sand mixture of 16% fines. Each series was compacted with both a different compaction method and energy input. The investigated scale effects included: the desaturation stage, bubble growth, bubble distribution, gas migration and final degree of compaction. All treated soils were able to desaturate to, and even below the optimum water content. In this research the in-situ shear strength was linked to swelling and bubble production. Based on a qualitative inspection, the gas distribution in the sample was found to be homogeneous over depth. In order to estimate swelling and venting of the sample during the desaturation stage, a model was produced based on a saturation dependent gas conductivity. It was suggested that a continuous gas phase was responsible for the highest amounts of compactive strain. With a non-intrusive compaction method, more compactive energy led to faster, but not more, release of gas. All 2.5m tests ended with a residual amount of gas. A 1m test series, consisting of three tests resulted in higher degrees of compaction for the treated tests opposed to the untreated tests. The final relative density of the treated tests was at least two times higher than the final relative density of the untreated test. A 2.5m test, which included surcharge, had a final RD 2.8 times higher opposed to a similar untreated test without surcharge. Taking into account the uncertainties of the results, it was concluded that a field trial is feasible. A hypothetical field case was set up of 500 m3. The costs of the proposed method were a factor 2.2 times cheaper opposed to the current soil improvement for silty
sands, namely a stone column vibroreplacement technique.

Argillaceous (clay-rich) host formations for radioactive waste geological repositories are extensively researched for their capability to decrease the hydraulic conductivity of fractures over time (i.e. self-sealing). This self-sealing behavior is beneficial in the context of radioactive repositories as the excavation of these spaces unavoidably induce fractures in a region called the excavation damaged zone (EDZ), allowing for radioactive particles to leak into the surrounding environment and groundwater. Despite knowing the mechanisms for self-sealing (e.g. swelling of clay minerals), more research is needed into the complexities of self-sealing to fully understand and effectively utilize it widely. To facilitate research of swelling in induced fractures, a quantified method of fracture characterization using morphological descriptors is developed to provide insight into the morphological state of the fracture surface and its influence on the self-sealing capacities of argillaceous rocks. This was carried out by identifying morphological descriptors of a rough surface and their relation to roughness, as it pertains to macroscopic and microscopic roughness. A dynamic script was developed capable of: analyzing and quantifying morphological descriptors of roughness in 2 and 3-dimensions, calculating the surface area of a rough surface, and calculating hydraulic conductivities (kx and ky) of a discrete fracture. This method was tested using two samples of Opalinus Clay (OPA) from the Mont Terri underground research laboratory (URL) in Switzerland. Furthermore, derived data of heterogeneous morphology, along with surface area and hydraulic conductivity, is used to discuss the limitations and constraints of the developed method and compare 2-dimensional versus 3-dimensional morphological analysis to determine what combination of analysis fully captures the breadth of morphological information. It was determined that the morphological descriptors of amplitude, amplitude density, and the bearing area curve (BAC) are suitable descriptors to define the microscopic and macroscopic roughness of a rough surface, where both forms of roughness are defined in the context of influence from local maximums (asperities) and local minimums (valleys) of the surface. Comparison of the 2 and 3-dimensional analyses of these morphological descriptors shows that a combination of the 2D BAC, 3D amplitude, and 2D amplitude densities both fully capture the breadth of morphological information and provide an easier means of interpretation. Using Delaunay triangulation on the surfaces of both OPA samples, the calculated upper and lower bounds of surface area for sample 1 is: 2267.37mm2 and 942.09mm2 and is: 2377.64mm2 and 959.73mm2 for sample 2. Compared to the planar surface area of each OPA sample analyzed, this corresponds to an 847 - 2039% increase for OPA sample 1 and a 998 - 2399% increase in surface area for OPA sample 2. The calculated hydraulic conductivities kx and ky for sample 1 is: 1.78×10−7 m/s and 7.75×10−7 m/s and is: 8.01×10−9 m/s and 2.1×10−7 m/s for sample 2. This is compared to the hydraulic conductivity of the Opalinus clay material, which is 2×10−13 m/s. Future research into discrete fracture characterization using morphological descriptors should focus on increasing the computational efficiency of the existing code, improving the surface area calculations, and transforming the matrices of data into one points of data for easier comparison of discrete fractures across multiple samples. In doing so, it is hoped that this small contribution will aid in understanding morphology influence on swelling development in induced fractures in argillaceous geomaterial.

In the Netherlands the design condition for a dike is surmised to occur during winter where it is assumed that the dike is fully saturated. Preliminary results by Van Duinen (2020) indicate that suction remains present, albeit ever decreasing, during the winter. This means that it would be possible for a dike to dry out during the summer period such that it becomes unsaturated, and that any effects may not have fully disappeared during a normative event.

This problem is two-fold, as (i) the strength of an unsaturated soil is higher compared to a saturated soil, and (ii) it is unknown how the strength associated with the initially unsaturated zone can be modelled in a macro-stability calculation as it varies temporally. This report investigates the strength of the initially unsaturated zone in clay river dikes in the Eastern part of the Netherlands and it explores the opportunities which the better understanding of the strength may present for the factor of safety in a macro-stability calculation

The feasibility and applicability of the SSCC as formulated by Lu and Likos (2006) for conventional geotechnical laboratory tests is investigated. A literature review bundles international knowledge on unsaturated soil behavior which is relevant in the context of dike design. Silty clay samples from the dike trajectory of Ravenstein-Lith were investigated using the SSCC. It can be successfully applied to K0-CAU triaxial test when determining strength parameters using strain-compatibility. Compared to its conventional variant, the saturation stage must be skipped. Samples are air-dried under laboratory conditions to a prescribed volumetric water content which can then be linked to an amount of suction using the SWCC and a comparison can be made with the closed form of the SSCC (Lu et al., 2010).

The SSCC cannot be applied to DSS tests successfully. In DSS tests both slip and a diagonal failure plane was observed. This research was able to pinpoint at which shear strain slip would occur, and hence when tests are valid. DS tests were used to confirm the validity of DSS tests using CCSM.

A case study performed on a representative cross-section found a positive effect on the factor of safety due the additional strength related to the initially unsaturated zone. The FoS increased by at least 1.7% in the most conservative case and at least 5.0% in a scenario supported by field measurements. The current limited number of field measurements restricts a more precise determination. It is advised to perform more of these such that the initially unsaturated zone in a clay river dike can be modelled effectively and reliably with a high degree of certainty.

To conclude, with the application of the SSCC concept, the additional strength produced by considering the initially unsaturated zone in clay river dikes in the Netherlands leads to an opportunity where the dike can be designed more efficiently by reducing conservatism. Thus, taking the initially unsaturated zone in a dike into account can be considered as a valuable contribution to the toolbox of Dutch geotechnical engineers.

The current plan of bottom to top bottom sublevel stoping for the Kearney gold vein at the Cavanacaw mine, Omagh, Northern Ireland may not be the most effective in terms of stability. The Kearney ore body is a narrow vein gold deposit, which has been previously exploited through an open pit and is currently being developed as an underground operation using sublevel stoping (modified Avoca mining method). Stability within the mine is one of the key factors to be considered when it comes to hard rock mining. It should be considered equally as important from a safety and economic point of view. The extraction sequence plays an important role when considering the stability of a designed mine. This thesis aims to establish if the current planned sequence of extraction of bottom to top sublevel stoping is the most effective in terms of overall rock stability, or whether an alternative plan would be better? In the context of this thesis, the modified Avoca mining method is a form of sublevel stoping where material is extracted (stoped) between two drives (blind tunnels) and then backfilled.
The project addressed, a conceptual study, field testing and laboratory testing in order to yield the information required to build several numerical models. The numerical modelling was carried out on several different stoping orders which met the constraints set out by Galantas, using the Hoek-Brown model within Plaxis2D. The analysis was conducted on the total displacements, phase displacements, predicted failure points and safety factors. The analysis of the different models showed that an alter- native stoping method of middle to top bottom to middle sublevel stoping peformed better in terms of stability. This improvement in stability was shown by an increase in the minimum safety factor from 3.20 to 3.50, over the current plan. There is further evidence in the reduction of the total number of predicted failure point by 25%.

Study of self-sealing and self-healing in clay is a fascinating topic which can be applied in many fields namely in construction sector or as the underground nuclear waste repository. However, lack of references still hinders the advancement of such field. Meanwhile, more advance researches are available for self-sealing and self-healing in concrete. Therefore, this project is held to find whether any valuable lessons from extensive researches of self-sealing and self-healing in concrete which can be applied to the development of the study in clay. This research is a comparative literature study of self-sealing and self healing mechanism between clay and concrete based on four aspects i.e. hydraulic, thermal, chemical and mechanical which based on the published papers from scientific journals and books. In terms of hydraulic aspect, hydraulic gradient plays an important role for sealing and healing mechanism in both clay and concrete. For thermal aspect, hydraulic conductivity is not affected by the temperature increased in clay while sealing and healing rate are improved for the similar condition in concrete. In chemical aspect, high pH environment increases sealing rate of concrete while high salinity decreases the capacity of sealing and healing of clay. For mechanical aspect, both concrete and clay experience a decreasing hydraulic conductivity under increasing confining stress. All of the results showed that both clay and concrete experience self-sealing and self healing mechanism with different influences by each aforementioned aspect.

Surfactant-Alternating-Gas (SAG) is a popular enhanced oil recovery method that utilizes foam to decrease gas mobility and subsequently improve reservoir sweep efficiency. SAG is known to have many benefits, such as mitigating corrosion effects and increasing gas injectivity. Despite this, liquid injectivity is typically very poor during the SAG process. Currently, there is not a consistent model for liquid injectivity during SAG. However, Gong et al. recently conducted core-flood experiments to gain clarity on how gas and liquid injectivity are affected during SAG in near-wellbore conditions. Gong et al. determined that gas and liquid injectivity are represented by the propagation of multiple banks. For gas injectivity, there are two banks: the collapsed-foam bank and the foam bank. For liquid injectivity, there are three banks: the collapsed-foam bank, the gas dissolution bank and the liquid-fingering bank.
Conventional foam simulators, such as CMG’s STARS, use the Peaceman equation to obtain an estimate for well pressure as well as liquid and gas injectivity. Gong et al. came to the conclusion that conventional foam simulators do not take the propagation of the collapsed-foam bank into account. Thus, Gong et al.’s results indicate that the Peaceman equation greatly underestimates gas and liquid injectivity during SAG in a 100 by 100 meter grid block. Subsequently, this paper aims to determine how refining the 100 by 100 meter grid block will alter well pressure and liquid and gas injectivity, especially in the near-wellbore region.
We used CMG’s STARS, a conventional foam simulator, to test the impact of grid refinement on injectivity and well block pressure. Our grid set-ups include a base grid of 5x5 equal blocks and two refined grid cases, in which the center grid blocks are partitioned into 9 and 25 equal block pieces. Our results indicate that as we refine the grid, the dimensionless pressure drops occur quicker and the magnitude of pressure decreases. When compared to the base grid, the pressure drops of the refined grids were discovered to be approximately 9 and 25 times faster for the 3x3 center grid case and the 5x5 center grid case respectively. Additionally, we observed additional dimensionless pressure peaks in the refined cases, which can be explained by a drop in relative mobility when the foam reaches a new grid block.
Although our foam parameters were based on Gong et al.’s foam scan, our foam parameters are not exactly the same as the parameters listed in Gong et al.’s paper because we did not include foam shear-thinning properties. In order to build on this paper’s research, future research should look into comparing our STAR’s results to the fractional-flow theory. Additionally, future models and research should look into reducing the simulation’s time step in order to increase the number of iterations calculated by the simulator, especially around the region in which the pressure drop occurs. Lastly, it would also be important to look into finding the best method to represent the well block pressure for a refined grid case.

Recent developments in bio-based soil improvement techniques include the use of the bacterially mediated process of denitrification to alter engineering properties of soils. Pham (2017) used the denitrification process to stimulate the production of gas and precipitation of calcium carbonate as a soil improvement method. The ability to desaturate a soil to its optimum saturation by means of gas production inspired the use of this method to improve the engineering properties of soils more efficiently by means of a two-stage process of desaturation and compaction. Andrag (2017) started a research to investigate the potential of this concept and developed a test setup for the assessment of its efficiency. The prospects of this biogenic soil pretreatment method are further investigated in the current study. The objective of the research is to assess how the compactability of an initially saturated soil is influenced by the in-situ formation of gas bubbles and if this gas can be used to reduce the energy requirements for compaction of a soil to a given target density. A review on existing literature is initially carried out to grasp the latest developments on this topic. Information from literature is used to determine the typical characteristics of soils on which compaction activities are often found to be ineffective. Based on this information, a representative silty sand with a high gas storage capacity is created for further experiments. It is found that gradation is one of the main properties controlling the water content-dry density relationship of soils. The water content-dry density relationship of soils composed from industrially produced material is found to be often significantly different from natural soils, which can be explained by differences in gradation. A testing strategy is developed and a setup is established for assessment of the concept of biogenic soil pretreatment for reducing energy requirements for compaction. Denitrifying bacteria are cultivated with a chemostat setup which can supply bacterial inoculum for subsequent experiments. The effect of the biogenic pretreatment is evaluated by comparing the energy requirements for compression of treated and untreated soil samples from initial conditions to a given target density with a constant strain-rate. Soil samples are successfully desaturated to targeted saturations by means of an engineered treatment regime. Concentrations of the substrates are determined based on the stoichiometry of the biochemical reaction, Henry’s law, Boyle’s law and the ideal gas law. Gas is produced at a steady rate inside the soil samples with a very limited amount of gas escaping, confirming the high storage capacity of the assembled sample material. The determined optimum saturation of the silty sand (80%), is therefore achieved without exceeding the gas percolation threshold of the soil. The gas that escapes the sample during the gas production stage and the loading stage, is captured with a gas trap that is included in the setup. The changes in water content and dry density of the soil during the experiment can be accurately predicted by means of a volume balance. The major part of the initially available nitrate in the pore fluid is consumed at the end of the experiments, with a limited amount of accumulated nitrite. Moreover, the denitrification process is successfully buffered by including calcium within the engineered substrate solution.
It is initially presumed that energy requirements for static compaction of a silty sand in the Rowe cell can be reduced by bringing the soil closer to optimum conditions in terms of saturation, before starting the loading stage. Based on an analysis of the work input per volume, it is found that the energy requirements for compaction increase slightly as a result of the biogenic pretreatment. In terms of compactability, no clear benefit is obtained from the biogenic pretreatment according to the energy-based assessment method that is used in this research. The slightly higher energy requirements for compaction of treated samples can possibly be explained by the formation of biofilm or entrapment of gas bubbles in the drainage lines of the setup. Cementation effects as a result of calcium carbonate precipitation are not expected to play a role, since calculations show that the relative amount of calcium carbonate in the soil is very limited at the end of the experiments. It is recommended to further investigate the potential of the denitrification-based pretreatment for the compaction of soils by exploring new methods of assessment. It is suggested to focus on implementation of a dynamic loading component to the test method and to investigate the options for execution of a field experiment. Representation of conditions that apply in practice is thereby essential. The potential of biogenic pretreatment for improvement of the engineering properties of silts and clays can also be investigated as it might be a sustainable alternative for the relatively scarce and costly coarse-grained fill material that is normally used nowadays.

The term peat is commonly used to describe organic soils, composed of fragmented plant remains and fibers. Despite the extensive research on the behaviour of peat during the last 50 years, the vast majority of the attention was devoted to the volumetric behaviour. Only few attempts are reported to model the deviatoric behavior of peat, which is fundamental in many geotechnical applications as dykes founded on peat layers. This Thesis represents an attempt to understand the current knowledge on the deviatoric behaviour of peat with the aid of an hierarchical simple elasto-plastic model.

The constitutive model was conceptually developed aiming for the use of a limited number of parameters with an easy way of determination. This choice aligns with the needs of engineering practice, which demands the use of models with few and comprehensive parameters. The constitutive ingredients of the model have been derived from direct experimental evidences. The model requires 10 parameters, 5 out of them are the classical parameters of the well-known Modified Cam-clay model.\\
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The high compressibility of the peat along with its high friction angle intensify the end restraints occurring due to the platens of triaxial apparatus. In order to account for the effects of the boundaries, the constitutive model has been implemented in the Finite Element Programme LAGAMINE and the tests have been simulated as Boundary Value Problems. The model is in qualitative agreement with the experimental data and, in particular, with quantitative agreement until 20\% of strain for the case of consolidated drained triaxial tests. To be able to reproduce properly the case of consolidated undrained triaxial tests which impose an isochoric response, one parameter associated with the flow rule of the model should be changed. This change of a parameter is related to the fact that the fibers in the peat specimen are not considered explicitly in the constitutive equations, although they can alter the deformational response of peat under some kinematic constraints and influence the mechanical response in a predominant way at high strain levels.

Overall, the constitutive model developed in this Thesis gives good quantitative results until 20\% of axial strain. Further research on the influence of fibers on the response of peat under different strain paths and on the consequent matrix-fiber interaction should be done in order to have a model with predictive capabilities until high strain levels.