Unreinforced Masonry, a popular construction material with a rich historical legacy, has been resilient through centuries, despite its susceptibility to failure through various loading conditions such as seismic forces. Understanding the mechanical behaviour of masonry, particularly the cohesion and frictional mechanisms at interfaces between bricks and mortar, is crucial for assessing its structural reliability.
This thesis presents an experimental study characterising masonry frictional interfaces using a tribometer test, a novel approach in masonry research. The study aims to investigate the frictional behaviour of brick-mortar interfaces under varying precompression levels and test frequencies (3 Hz and 0.2 Hz). A novel sampling method was implemented, wherein the required specimens were extracted from a bonded masonry couplet specimen. The couplets were bond wrenched and then subjected to a mechanical extraction. Bond wrenching, through tension, separates the couplets, thereby isolating frictional response from the influence of cohesion by decoupling cohesion between the unit and joint. A tribometer was chosen for the investigation of frictional parameters, tribometers are typically employed with metal surfaces and rarely applied to masonry. Consequently, experiments were conducted using a reciprocating tribometer, featuring a novel specimen extraction procedure and a modified setup. The novel extraction involved mechanically altering a parent brick couplet to extract compatible specimens. Customised holders were procured and used to accommodate the prepared specimen. The obtained specimens were then, fastened into the appropriate sample holders. Tests were performed on the specimen with displacement, tangential force, normal force and time being recorded. Three key parameters, the mean Coefficient of Friction (CoF), the Energy Coefficient of Friction (ECoF), and tangential contact stiffness were analysed from the collected data. The above mentioned parameters were evaluated through two approaches, (i) Mean Curve approach and (ii) Cycle approach. A mean curve is the averaged curve of the measured hysteresis loop, which encompasses the overall behaviour exhibited by the hysteresis loop.
The experimental results revealed a dependency between the coefficient of friction and the applied pre-compression level, showing a non-Coulomb behaviour; this is in contradiction with assumptions of several models used for the analyses of masonry structures. Initially, experiments were conducted at a higher frequency (3 Hz) and followed by experiments at a lower frequency (0.2 Hz), and the results in both cases revealed a non-Coulomb friction behaviour characterised by a nonlinear connecting trend between kinetic and static friction region. The connecting trend between the kinetic and static region demonstrated a reduction in the degree of non-linearityan enhanced symmetry in the hysteresis profile as the precompression levels increased. The mean curve and the cycle approach yielded identical estimates for the coefficients of friction (CoF) and effective coefficients of friction (ECoF).
The evolution of Coefficient of friction (CoF), did not exhibit any clear systematic time dependency, while Energy Coefficient of friction (ECoF) showed a systematic decrease over time. The ECoF related the energy dissipated by friction across each cycle to a hypothetical Coulomb energy dissipation. Therefore the ECoF captures the energy dissipation due to friction during one cycle, while the CoF captured the static friction region. The energy dissipation across different precompression levels were also studied and it was observed that the energy dissipation increased for increasing precompression levels.
Tangential Contact stiffness refers to the tangential force required to effect a unit displacement in the tangential direction. The contact stiffness was calculated from the slope of the region of the hysteresis loop associated with displacement reversal. Contact stiffness was observed to increase with increasing precompression levels. This observation was also validated by the theoretical model which illustrates a proportionality between the applied normal load (precompression) and tangential contact stiffness.
Additionally, the study highlights the impact of frequency on the observed frictional behaviour, noting more prominent static and kinematic components at lower frequencies due to reduced inertial forces. A preliminary study using the LuGre model was used to plot the analytically derived frictional force-velocity relationship, to briefly study the impact of sliding velocity on frictional force. It was deduced that the velocity had an influence over the frictional force at the interface.
In conclusion, the findings provide valuable insights into the frictional characteristics of masonry interfaces under varying loading conditions. The results also shed light on the practical aspects of testing methods and offer considerations for further research in the field of masonry engineering.

The soil-grout interface friction was measured to be in between 0.0 [kPa] and 14 [kPa]. To measure this friction, a special research method was invented that enabled a tunnel to be recreated. With this setup, any formation could be used. The formation could be created with a hole in the middle which simulated a borehole. Next, by filling and pressurising this hole with a bentonite-water suspension, a stable borehole could be simulated. This borehole was then filled with drillgrout to recreate a tunnel filled with grout. This grout column could then be pulled out of the soil formation so that the friction between the drillgrout and soil could be measured. In this setup, no normal forces were present. Due to the absence of normal forces no friction angle could be deducted which made it impossible to estimate the effect of the friction at the depth where a pipe laid by HDD would generally be found. It was concluded that based on the data found, it is not recommended to use grout as a borehole filling around heat transportation pipes. Although the friction would be mainly determined by the filter cake strength, which is comparable to a weak clay, effects like irregular borehole diameters and large soil stresses at the depth of the pipe, would create a frictional force that is large enough to cause buckling or failure of the insulating PUR layer.

High-level waste can be radiotoxic for thousands of years and should be carefully handled to prevent accidents. The research for geological disposal of radioactive waste focuses on the construction and durability of a geologic disposal facility in Dutch clays or salts. One of the challenges is assessing these host rocks’ capability to retard radionuclides and prevent them from entering the biosphere. This research focuses on disposal in clay. Because of clay’s low permeability, water movements are slow, and radionuclides transport is expected to occur predominantly by diffusion. The Peize and Waalre formations, situated on the interface between brackish and salt groundwater, serve as a natural analogue for targeted deeper, poorly indurated host rocks. The known disparity in chlorine levels between the aquifers adjacent to these clays qualify these formations for NaCl-tracer research. A cutting sampling study determined the conductivity of these clays’ porewater. Combining this with a one-dimensional modelling study, the saline history of this formation has been simulated. NaCl gradients were demonstrated at different drilling locations. The most manifested gradient is attempted to fit in the one-dimensional model. The model results suggest that this gradient originates from adjacent aquifer salinity, the clays’ physical properties and the difference in hydraulic head. The observed salinity discrepancy between aquifers and ion concentration gradient in the firstWaalre Clay confirm the assumption that the member can be a natural analogue. Uncertainty on the continuity of the total system prohibits concluding that diffusion-dominated transport in Dutch poorly indurated clays can be assumed. The best fitting scenario this research found fitting the empirical Waalre clay salinity curve is combined transport by diffusion and advection. The finding of diffusion-advection transport implies that the Waalre clay shows more complexity than initially expected.

Eternal aftercare for landfills is the standard in Dutch waste management policy after a law came into effect in 1996, meaning eternal waste management from potentially hazardous substances. A prescription of this policy is the application of watertight barriers on top of the wastebody. The policy also prescribes the renewal of the packing materials every 75 years. The wastebody is dried out, and the degradation has stopped so the wastebody does not change. The 'dry tomb' stays hazardous, thus eternal aftercare is implied.

The project 'introductie Duurzaam Stortbeheer (iDS) aims to create aftercare with an ending, or finite aftercare. It is looking for possibilities to stabilise the landfill mass, i.e. to eliminate the threat of pollutants by treating the wastebodies.

The goal of the CURE project is to develop fundamental insight into landfill processes in order to research the feasibility of wastebody stabilisation. To predict behaviour of landfills, and to monitor the processes, as much information as possible needs to be gathered through measurements. These consist of measurement of gas concentration and production rates together with the variation in leachate quality and volumes, as well as many more. This research, as part of the CURE project, presents the applicability of settlement as and addition to these measurements.

The main goal of waste body stabilisation is to actively reduce the amount of organic matter in a landfill. Uncontrolled landfills produce considerable methane emissions as well as high concentrations of nitrogen and heavy metals in the leachate, leading to groundwater hazards in the environment.

McDougall (2007) introduced the fundamental conceptual model upon which this research is based. This model is not publicly available, so one of the goals of this research is to provide a fundamental conceptual model. The relation between degradation and settlement has been studied and implemented in a 1D model which allows for hydrological systems, oxygen penetration, degradation and cell strain. The outcome is a relation between mechanics, biochemistry and hydrology. A few assumptions and simplifications were made to make the model versatile and adjustible, but also easy to read. For instance, the model uses oxygen as absolute limiting factor in degradation, neglecting all processes concerning anaerobic degradation. The model also assumes that at the beginning of the simulation, each cell has the same composition, because there is no conclusive data about the distribution of waste types. The model has been created in one dimension, thereby neglecting all multi-dimensional processes and limitations.

In the final scenario, the main limitation to degradation is the availability of dissolved oxygen. As irrigation provides infiltration of 5 mm water per day with 10 mg dissolved oxygen per liter, the daily reduction of oxygen demand is 50 mg. The total modelled oxygen demand of the landfill is over 9600 kg, which concludes that a different tactic needs to happen in order to stabilise the wastebody. To increase the degradation rate, more oxygen needs to be applied. An approximation of the effect of applying a partial vacuum above the water table in the landfill to attract air from the environment is modelled, giving the oxygen more transfer area into unsaturated water. An increase by a factor 10^{4} is applied to the effect of oxygen on organic matter to model this enhanced irrigation system. The results are modeled over 100 years, after which approximately 26.17% of the degradation has happened. This shows that aerobic degradation over a wastebody takes too long with conventional degradation, and the unsaturated voids might help the process.

The model shows credible results for an unspecified landfill with deterministic parameters. Settlement has been brought in relation to a simplified form of biodegradation. The need for further research, with a spatial fluctuation of these parameters and detailed multidimensional water flow is needed to predict landfill behaviour in more detail. \pagebreak

With changing climatic conditions, droughts are expected to increase in duration and intensity. Under drying conditions, desiccation cracks can form in clay soil, which is often used as dyke cover and in some cases also for the core of the dyke. These desiccation cracks have a potential impact on the stability of dykes. The main objective of this study is to gain a better insight into the potential influence of desiccation cracks on the macro-stability of Dutch river dykes. With the help of PLAXIS 2D software, a numerical model is used to study the hydraulic response and stability of a dyke with desiccation cracks. It is chosen to simulate the effect of cracks by adjusting soil parameters of the cracked layer. Steady-state scenarios as well as time-dependent scenarios, where high water events are simulated, are studied. The influence of precipitation is also taken into account. For the steady-state case, a maximum decrease of the factor of safety of 5.6% is found. In the time-dependent scenarios, the factor of safety decreases up to 10.8% if a cracked zone with a depth of 2 meters is present. The magnitude of this decrease is mainly depending on the hydraulic conditions and the crack parameters chosen to simulate the effect of cracks. Both inner and outers slope failure is observed. The largest difference between the factor of safety of the cracked and the uncracked dyke occurs when the dyke fails due to outer slope instability as the factor of safety decreased most in the case of a rapid drawdown. From these results it follows that cracks can have a negative impact on the macro-stability of a dyke, but the difference is not unambiguous. Precaution is advised if significant cracks develop at dykes, but smaller cracks are not likely to lead to failure immediately.

Foams are used in reservoir engineering for enhanced oil recovery, CO2 sequestration and environmental remediation of aquifers and soils. One of the main mechanisms for foam generation at steady state is Roof snap-off. In some cases, mechanistic models of Roof snap off, based on observations from 2D micromodels are used for reservoir simulation. The main problem with these experiments is in their 2D nature. Two-phase flow within a 2D medium requires that the fluids paths cross and compete for pore occupancy. This virtually ensures fluctuating pore occupancy and therefore puts into question the applicability of 2D mechanistic models for steady state foam generation in 3D media. Two-phase flow in a micromodel is analyzed with a lattice percolation model in order to determine under what conditions steady two-phase flow can be achieved. The gas network is established with bond percolation and liquid is allowed to flow across the sample with the help of liquid bridges. These liquid bridges enable the liquid to cross gas-occupied pore throats without snap-off. The calculated attribute for the gas and liquid networks is equivalent resistance, ΔP/Q. For this a new unit for hydraulic resistivity was used and is equal to the fluid viscosity divided by the pore radius to the third power, H = μ/R3. The equivalent resistance of the gas and liquid networks are found by applying rules from linear circuits of electrical resistances. Solutions for the equivalent resistivity of the gas network are calculated with the node elimination method and Kirchhoff’s solution for a random network of resistances. The liquid network’s conductivity is calculated as the sum of path resistances in parallel and is a theoretical maximum. The gas and liquid conductivity of nine pre-existing 16x16 networks from Holstvoogd(2020) are reevaluated and, in addition, twelve new samples of size 32x32 are evaluated. Functionally, the model’s behavior is as follows: gas conductivity is inversely proportional and liquid conductivity is proportional to the occupation threshold. It is found that the gas conductivity is a function of tortuosity and number of parallel flow loops. Conductivity decreases with increased tortuosity and increases with number of parallel flow paths. The ratio of liquid and gas conductivity for the twelve 32x32 models is calculated. When adjusted for gas viscosities of supercritical CO2 and Nitrogen gas it is found that it is in the order of 10-3 to 10-4. Therefore, it has been determined that it is practically impossible to achieve steady two-phase flow without fluctuating pore occupancy.

The erosion resistance of clay is an important aspect when assessing the integrity of a levee. This is because the clay layer of a dike must resist the forces that are imposed in the case of overflow or overtopping. Currently, the erosion resistance of clay in the Netherlands is being assessed based on the sand content, the liquid limit and the plasticity index. The quality of compaction is assessed by a density requirement. However, the effect of compaction on the erosion resistance of clay is not well understood and although its importance has been widely acknowledged by multiple literature sources, it has never been quantified. Therefore the goal of this thesis is to investigate what the are of compaction on the erosion resistance of clay and how large. To do this first of all a set-up has been designed and built, resembling the Erosion Function Apparatus as suggested by Briaud with some differences. Then 3 types of clay have been tested with differing initial erosion characteristics: one kind which fully complies with the current Dutch guidelines, 1 which does not quite suffice and 1 which does not suffice in any way. These 3 types of clay have been compacted at various energy levels (Modified Proctor, Proctor and 0.5*Proctor density). Also samples with differing water contents have been produced at each energy level and each clay. These samples were subsequently tested in the Erosion Function Apparatus at several flow-velocities in order to establish an erosion curve from which several erosion parameters were determined. \bigskip Having performed the erosion tests and determined the accompanying erosion parameters the results were interpreted in the Proctor curve. Plotting the results in the Proctor curve show that samples compacted at the optimum water content in the Proctor plane perform best, whereas they perform poorer the farther the water content is off from the optimum. Also it shows an increase in the compaction effort will generally result in a lower amount of erosion occurring. This result has been obtained by determining the critical shear stress, critical erosion velocity, the detachment coefficient and the velocity detachment coefficient and plotting these results in the Proctor curve. The first two parameters are measures of the amount of strength the soil has before it starts eroding at all, whereas the latter two are an increment meant to predict how much a soil might erode if the force imposed on the soil is above the erosion threshold. The results consistently showed the detachment coefficients were lowest with the samples compacted at an optimum water content and/or undergone a higher compaction effort (i.e. the soils erode less). On the other hand the critical shear stress/critical erosion velocity was highest at the optimum water content (i.e the soil erodes less), but did not necessarily increase with an increasing compaction effort. This is attributed to the loss of suction. Further, all tested soils showed their optimum erosion characteristics at or close to a degree of saturation of 85 \%. Several other methods were tried, but to no avail unfortunately. Also the rates of erosion were compared between the different types of clay. The clay which is in compliance with the Dutch codes showed the least amount of erosion, the one that did not quite suffice showed some more erosion and the clay that did not comply with the erosion guidelines at all showed the most erosion. However, it was also proved that the clay which did not quite suffice the requirements was (if compacted at optimum water content and at a sufficient energy level) only slightly more erodible than the clay which fully complied with the current Dutch regulations. Finally, the results of the tests carried out in this study were also compared to similar tests performed in the United States and the show consistent similar results. Overall, this study concludes that the use of (currently perceived) unsuitable clay as dike cover can be possible if compacted at or very close to the optimum water content and at a sufficient energy level. Also it can be concluded that compaction at the optimum water content can dramatically reduce the erodibility of a soil or even stop the erosion process. Increasing the compaction effort also reduces erodibility of a soil, although it must be kept in mind that the optimum water content also changes and thus if it is wise to do so. Finally this thesis leaves some suggestions for further research. These mainly concern the effects of cyclic wetting/drying cycles on the erodibility of the soil as well as trying to better understand the effects of the erosion process happening in the field versus the erosion process that is happening in the laboratory.

The current Dutch norms for the macro-stability of ood protection embankments and more specifically the determination of SHANSEP parameters has been subject of debate since its implementation in 2017. The main concerns arise from the apparent over-conservative nature of the guidelines and the apparent underestimation of shear strength of organic and silty clays. In this Master thesis, an assessment of the normalised normally consolidated undrained shear strength is performed for a dike stability reinforcement project between Gorinchem and Waardenburg in the South west of the Netherlands on a problematic clay layer called the `Gorinchem Clay' throughout this thesis. The main goal is the conrmation of the current norms or towards a more optimised and less conservative assessment of the strength parameters and in doing so, discusses one of the most recurrent questions in geotechnical engineering: do some of the measurements from laboratory tests reveal true material behaviour or do the limitations of such laboratory tests produce this particular behaviour? This analysis was performed on Triaxial Compression, Direct Simple Shear and Triaxial Extension tests from which the SHANSEP parameters are derived as input for the available shear strength along a slip surface according to the methodology developed by Ladd (1991) and following the Critical State Soil Mechanics. The structure of this thesis is as follows: rst, a literature study is performed on the strength parameters assessment in chapter 1, the principles of the Critical State Soil Mechanics and the diculties encountered in the determination of the strength parameters from laboratory tests in chapter 2. The laboratory results are then exploited in chapters 4 to 5 according to the current guidelines and more extended methods in which the limitations of the Classical Critical State Soil Mechanics are shown. The next chapters focus on a more fundamental understanding of the considered material consisting of modelling the material behaviour in chapter 6 using a simplied academic constitutive model featuring non-associative elasto-plasticity with mixed volumetric and deviatoric hardening allowing for hardening or softening. The outcomes of this thesis show Critical State conditions as traditionally understood could not be reached reliably for undrained conditions in Triaxial Compression, Triaxial Extension and Direct Simple Shear. The tests showed to be particularly unreliable beyond 10% axial strain in Triaxial Compression and Extension, and beyond 15% shear strain in Direct Simple Shear. The diculties encountered in determining the Critical State friction angle and undrained shear strength were shown to be minimised by performing drained and undrained Triaxial compression tests on slightly over-consolidated soil samples. Additionally, the limitations of the Classical Critical State theory were highlighted and a more advanced constitutive model including a non-associative ow rule and deviatoric hardening was successfully used to predict the behaviour in Triaxial Compression for undrained conditions. The predictions for drained conditions and particularly undrained Triaxial Extension were however limited.

Geothermal energy reservoirs can be used as sources of renewable and sustainable energy. The concept of geothermal technology relies on the harvesting of heat from the deep rock by injecting fluid that circulates through the porous, fractured reservoir rock between the injection and production boreholes, and thereby transports the heat to the ground surface. The presence of fractures can strongly increase the permeability of the rock and thus enhance the flow circulation in the system. Properly being able to model the fluid flow through the porous, fractured rock is therefore of importance in the optimisation of reservoir design. For that reason, this study proposes a new material model, which relates fracture initiation and propagation to a change in the permeability of the rock, assuming a smeared approach. The fracture-dependent permeability model is developed using the Diana FEA software, and can be used in combination with other fracture models and the mixture analysis for the simulation of hydro-mechanical behaviour of rock. The study investigates two different methods for the formulation of a fracture-dependent permeability, whereby both methods adhere to the cubic law, which defines the permeability as a quadratic function of its aperture. Evaluation of the model shows that it is capable of simulating extensional (mode I) fractures in the vicinity of geothermal boreholes. However, the analyses also indicate a mesh-dependency as well as a sensitivity to its input parameters. Furthermore, the fracture-dependent permeability model can only simulate hydro-mechanical behaviour in situations other than (enhanced) geothermal energy systems to a limited extent. Further model development is therefore recommended.

Several waterway protection structures in the Netherlands dealt with rapid degrading armourstone after construction. This suggests that the current selection procedure of armourstones is not flawless to all degradation mechanisms, especially in dynamic environments. In this work armourstone is meant to include all rock that is used in river, coastal and offshore constructions, including rip rap and rock fill. Armourstones are in the Netherlands selected by their CE marking, which lists the description of the armourstone product and information on regulated characteristics. These characteristics are tested according to the European standard EN 13381-1&2:2002, which specifies the properties of aggregate acquired by processing natural materials for use as armourstone. This research is executed to provide a contribution in the quality control and assessment of armourstone and rock fill to ensure proper handling and installation in coastal and waterway protection structures. The research started with a durability investigation according to the standard EN 133831&2:2002 on sampled sandstones and limestones that will be used in a submarine trench backfill, to check the regulated armourstone characteristics. Next, a petrographic analysis under the microscope was performed on methylene blue (MB) stained thin rock sections to investigate the presence of deleterious constituents and structures. This analysis was followed by extensive index testing to investigate the applicability of simple testing tools in a durability investigation. These tools include the Brazilian tensile strength (BTS) test, an indirect tensile strength test, the Equotip test, a surface hardness tester that records the surface rebound of an impact body, and the MB adsorption and staining test. MB is a dye that colours constituents with an excess in negative electric charges. The tests according to EN 13383-1&2:2002 suggested that the sampled armourstone bulk satisfies the required durability parameters and no evidence was found that indicates rapid degradation during the engineering lifetime. Nevertheless, variability in the armourstone pieces was spotted and some individual rock pieces approached or exceeded minimum durability requirements. The BTS test revealed considerable variation in tensile strength, a key parameter to assess degradation, between armourstone blocks and within a single block when a similar orientation of the bedding was maintained. Moreover, the orientation of the bedding in the sandstone had large influence on the tensile strength, where the values obtained perpendicular on the bedding were twice as high than parallel to the bedding. Single impact method (SIM) Equotip measurements on rough, untreated aggregate surfaces were consistent. The rebounds correlated to visual features like grain size, cracks, surface roughness and degree of weathering within handheld specimens. Equotip measurements on the rock cores and sawn surfaces obtained higher mean rebound values and smaller standard deviations. The mean and standard deviation are considered most suitable to be used in a durability investigation. The repeated impact method (RIM) by the Equotip was not successful to indicate the degree of weathering of single hand specimens. The mean Equotip values on the aggregate correlated well to the water absorption when divided into proper density and size classes. The mean rebounds on the cores distinguished well between weathered and intact cores when saturated, and correlate well to the unconfined compressive strength (UCS) values. Furthermore, the Equotip mean rebound value related to the BTS value when performed on isotropic rock disks. The MB adsorption test and MB staining of the thin sections indicated the presence of localised spots and laminae rich in clay or organic matter. The staining of sawn aggregate surfaces agreed with some of the deleterious structures in the thin sections, yet was not consistent throughout the tested rock pieces and varied between the dark coloured limestone and light coloured sandstone. The Equotip test, BTS test and MB adsorption test are quick, easy and cheap methods to obtain more understanding in variability and rock behaviour which are not necessarily captured by the standard laboratory tests according to EN 13383-1&2:2002. The simple index tests should always be accompanied by the standard laboratory tests to provide a proper reference and understanding. Detailed mechanical durability tests, like the slaking test or wet-dry cyclic tests, should be performed when the durability assessment indicates a high amount of deleterious minerals and structures, to identify the amount of degradation caused by swelling behaviour of these minerals.

Mechanical dredging of a stiff clay results in clay lumps. Those clay lumps can be reused for the construction of land reclamation projects, and hereby economic and environmental value is generated. Placement of the clay lumps
in water result in a matrix of clay lumps and an interlump void space. The collapse of the interlump void space will cause large settlements and needs to be overcome before the site can be used for construction. Preloading is
an effective method to close the interlump voids. Stiff clays soften over time due to unloading and swelling. As a result, the strength and stiffness of the clay lumps decrease over time. The presence of discontinuities accelerates
the softening process. It was proposed by Leung et al. (2001) that the interlump void space closes under a reduced
preload of 25 kPa. The interlump void space closes under a reduced preload because the lumps soften over time.
The question rises if closure occurs within a normal construction timespan, i.e., 1 - 2 years, under this reduced
preload. In this study, a combined experimental and numerical approach is applied to determine the influence of soil
characteristics, softening and the presence of discontinuities on interlump void closure. The influence of softening due to chemical and hydro-mechanical swelling is tested by experimental swell-load tests on stiff overconsolidated
Boom clay samples. Additionally, the presence of discontinuities is studied by CT images, and a miniature clay fill test is performed to study the softening time and the rearrangement effect. Furthermore, a numerical study is performed in which the influence of specific soil characteristics on void closure was researched by a sensitivity analysis. By the experimental tests, it is shown that a pore water chemistry change alters the degree of swelling and its compressibility. Furthermore, fissures were identified in the sample material by CT images. The smallest microfissures could not be identified due to the resolution of the images. Consequently, it is impossible to estimate the effect of fissures on the hydraulic conductivity. Therefore, literature data was used to estimate the hydraulic conductivity and this was used as the input in the calculations. In the numerical study, it was shown that MPM could model the softening behaviour over time. The sensitivity analysis showed that the MPM model responds consistently to parameter sensitivity analyses. This leads to the conclusion that MPM can be used as a investigation tool to increase
the understanding of the influence of parameter variability on the final interlump void closure problem. The resulting strains of the numerical model comply with the theoretically calculated strains. It was expected
that the numerical strains were smaller than the experimentally determined strains, due to the presence of interlump voids in the numerical model. But, the numerical strains are larger than the experimental strains. In the sensitivity
analysis, it was found that the final strains and closure time are highly dependant on the soil characteristics. It is likely that the input of the MPM model differs compared to the true sample material and thus different strains result.
The time until the interlump void space closes was determined by a simplified geometry in MPM under a preload of 25 kPa. For an unfissured Boom clay, interlump closure takes place after approximately 16 years while for a
fissured Boom clay it only takes 2 months. Thus, closure of the highly simplified geometry takes place within a normal construction timespan for a fissured Boom clay. It must be kept in mind that the results are highly dependant
on the geometry, lump size and soil characteristics. Therefore, these results can not be generalized into an estimate of the interlump void closure time for any stiff lumpy clay fill. In conclusion, the feasibility of MPM was explored and it turned out to be a promising method to model the interlump void closure problem. Further studies are required to check if the model gives plausible results for more advanced constitutive soil models and geometries. If the results are positive, MPM can be used as a investigation tool to increase the understanding of the interlump void closure time for more refined geometries.

Irreversible damage induced by stress in brittle rock is accompanied by the formation of micro-cracks. The strain energy released during the fracturing process is released in the form of acoustic emission. This thesis applied the non-intrusive method of acoustic emission monitoring to assess the deformation process of brittle rock during cyclical loading and loading to failure in a standard uniaxial compressive strength test set-up. It is confirmed that the stress-strain curve is clearly separated into five phases and that the cumulative hits recorded throughout the failure process correspond with these five phases. Furthermore it is found that there is an obvious rise in trend in the amplitude of the acoustic events as the rock nears failure, but that individual events of high amplitude should not be considered indicative for the damage in the rock. Additionally it is found that high amplitudes characteristic for near failure stress are recorded at 25\% of the failure stress if the rock has been under high stresses, indicating a change in fracture mode. The Kaiser Effect, the phenomenon defined as the absence of detectable acoustic emission events until the load imposed on the material exceeds the previous applied level, was confirmed during uniaxial cyclical loading. Through loading samples from a highly stressed pillar in the Nepheline Syenite Stjernoya Mine in Norway, it is found that the onset of acoustic emission may be indicative of the stress in the pillar. These findings are useful to further develop acoustic emission as a monitoring method in a range of applied earth science applications.

The heterogeneity of layered systems leads to variation in rock or soil mechanical properties, influencing the resistance to failure. A formation breaks by fracture propagation when the effective stress surpasses the formation strength. The propagation of a fracture through a mechanical interface depends on whether the formation strength of the second formation is overcome. This critical effective stress level could be trespassed by high natural or industrial induced pore pressures. Understanding fracture propagation in multi-layered systems with variable pore pressure regimes has important implications and applications to many industries such as quarrying and hydraulic stimulation. The slope stability of Westerwald Clay Quarries is influenced by inter-bedding of thin sand layers. Surface and slope fractures within the clay formation originated from a high observed hydrostatic head within the sand layers and a reduced confining stress from mining activities. The slopes of the quarry are key in determining the volume of economically mineable clay, these slopes are in turn controlled by the size/extent of fractures and whether they extend through multiple formations. This study examines the effect of fracture continuation from sand layers into the stiff Westerwald Clay Formation. The soil parameters (cohesion and friction angle) of the different lithologies within a Westerwald Clay Quarry are determined for slope stability analysis by shearbox testing. Soil classification has been done in terms of plasticity, grain size and mineralogy by Atterberg Limits, Sieving and XRD & XRF respectively. The results show that fracture initiation within the Westerwald quarries is a combination of mining activities lowering the confining stress and a constant natural hydraulic head. The hydraulic head within the small sand formation lowers slope stability by causing fracture initiation and water infiltration into the clay formation. Slope stabilisation occurs by artificial water pumping or natural water dissipation lowering the hydraulic head. Slope stability is decreased by embankments of low permeability backfill and increased by high permeability backfill. Inter-bedded systems are common target locations for an unconventional reservoir systems and can be found both within source rocks as well as in conventional geological traps. Improvement in recovery from these tight systems often depends on the extent and continuity of fractures through heterogeneous interfaces. This study examines the propagation and continuity of the fractures in an artificial heterogeneous layered system. The fractures will be initiated by hydraulic fracturing in a dried layered system via water injection in a triaxial cell. Fracture propagation is analysed through Micro-CT scans. The mechanical properties such as acoustic wave velocities, unconfined & confined compressive strength and tensile strength are all determined for the analysed layered systems. The results show that hydraulic fractures initiated within the weakest layer are arrested at the interface between a mechanically weak and strong formation, whereas fractures initiated within mechanically stronger layers prograde through the interface. Hydraulic fractures are initiated when local pressure difference at the interface exceeds the formation’s critical tensile stress, the formations critical tensile strength is dependent on the confining pressure.

As the demand for green energy is growing, geothermal energy will play an important part in the future energy mix. Geothermal energy is widely used for both direct-use and electricity generation. A viable geothermal reservoir must have a sufficiently high temperature, a fluid pathway through the rock (permeability) and a fluid that can collect and transport the heat to the surface. Deeper reservoirs often have high temperatures and a low permeability, whereas shallow reservoirs often have a high permeability but low temperatures. When the permeability is too low, the reservoir can be stimulated. This is conventionally done by fracking the reservoir rock; that is, creating fractures to enhance the flow. This is a very expensive and potentially hazardous operation. Another technique is Radial Jet Drilling (RJD), which uses a high pressure focused fluid jet in order to drill small diameter horizontal holes (laterals) from the vertical borehole. Laterals can be drilled in multiple directions of the borehole and can be as long as 100 metres, potentially increasing the production 3-8 times (Blöcher et al. 2016). However, the destruction mechanism of rocks during jetting is not yet fully understood. Experiments and simulations were conducted in order to investigate the destruction mechanism of a rock during jet impingement. First, experiments and simulations were carried out which had the aim to find a relation between stagnation pressure (exerted pressure on the rock by a water jet) and jetted cavity depth. The experiments provided information about the maximum jettable depth (with a static nozzle), while the simulations provided a continuous pressure depth relationship. Combining the simulation and experimental results leads to the conclusion that pressure fluctuations are key for the destruction process, which is consistent with the theorem of hydraulic fracturing. Furthermore, a Finite Difference Method (FDM) solver was developed in order to investigate the pore pressure inside a porous rock during jet impingement. The solver was used to investigate (1) the difference in pressure between the first layer of pores (adjacent to impinged area) and the fluid pressure of the jet and (2) the influence of an increased pore pressure in the whole sample (i.e., back pressure). It became clear that the pressure fluctuations in time between the pores and jet are significant and sufficient enough to induce hydraulic fracturing. Increasing the back pressure resulted in higher pressure fluctuations, but only when the back pressure is higher than the average jetting pressure. There are still some doubts in the literature in whether or not cavitation erosion is the governing destruction mechanism. Cavitation is the formation of vapour bubbles in a fluid due to fluid pressure drop below the vapour pressure. Once the bubbles travel to a higher pressure regime, they implode, creating either a shock wave or a mini jet, resulting in erosion of material. Hahn et al. (2019) and (Kumagaiet et al. (2011) evaluated the location of cavitation erosion on a flat surface and concluded that cavitation erosion is not the governing erosion mechanism as there was no damage at the impinged area. But those studies did not prove that cavitation erosion does not occur inside a hole. Experiments and simulations were therefore performed to investigate cavitation erosion inside a cavity. It is found that cavitation erosion does occur inside a cavity, even at cavity depths of >25mm. However, experiments on one of the rocks showed that the maximum jetting depth was 25mm. From these results, it was concluded that although cavitation erosion occurs inside a cavity, it cannot be the governing destruction mechanism. Based on the results in this study, it is concluded that hydraulic fracturing is the governing erosion mechanism, because of the proved dependency of destruction on pressure fluctuations and the disprove of surface erosion due to shear forces (in literature, Buset et al. (2001)) and cavitation erosion being the governing mechanism. Another, in literature, proposed mechanism is pore-elastic tensile failure, which is partly interconnected with the theorem of hydraulic fracturing and not further investigated in this study. Based on this conclusion, a 'jetting correlation' was developed which is able to predict whether a rock is jettable or not. Combined with the pressure-depth relationship, it can also predict the maximum jetting depth. The correlation matched with the experimental results, which is another indication that hydraulic fracturing is the governing destruction mechanism.

Cyclic loading tends to affect the strength and stiffness parameters of soils, degrade its structure and results in accumulation of the excess pore water pressure. Such behaviour often leads to premature failure of the soils. Over the past few decades, geotechnical research community has developed numerous constitutive models to predict the behaviour of soils with variable degrees of success. The constitutive models based on the concept of bounding surface plasticity have gained much attention owing to the simplicity in describing the development of stiffness. This thesis analyzes the performance of bounding surface SANICALY model in reproducing the stress path and stress-strain behaviour of kaolin clay under undrained cyclic loading conditions .
A driver was developed in MATLAB for the chosen constitutive model to simulate undrained triaxial loading conditions. The performance of the driver was verified against the data published from literature. Further, sensitivity analysis was carried out on chosen model parameters. This was followed by validating the model with the experimental data on kaolin clay. Particularly, model performance was examined with varying initial conditions such as change of over-consolidation ratio, change of initial anisotropy, variation of initial pressure and strain controlled loading. The obtained results from sensitivity analysis have shown to increase the strength and stiffness response of the model with increase in model parameters such as the rate of evolution of the anisotropy, bound for evolution of the anisotropy and change of initial stress-induced anisotropy. In the context of calibrating the model parameters against the experimental data, it was initially noticed that the experimental stress path in monotonic loading was not being reproduced by the model with various combinations of the initial parameters. Owing to such performance the model was subsequently assessed qualitatively. When the model is subjected to different initial loading conditions, certain aspects of the experimental behaviour were qualitatively captured by the model. These include faster rate of accumulation of pore water pressure with increase in the amplitude of cyclic loading, reduction in the rate of development of strains with increase in OCR values, increase in the hysteretic damping with increase in the amplitude of strains. However, with change of OCR there were differences in the development of stress path. Also contrasting results were observed with regard to the development of the stress-strain response with change of amplitude of cyclic loading and initial pressure. Analyzing the model formulations revealed that the chosen model did not take into consideration the fabric anisotropy and hence it explains the deviation of the stress path from the experimental stress path in monotonic conditions. In the chosen model, during the process of cyclic loading, stagnation in the evolution of stress path is observed whenever the plastic volumetric strains stop evolving. It is recommended to incorporate plastic deviatoric strains in the evolution of the bounding surface in order to stimulate the further development of stress path even when the plastic volumetric strains stop evolving. It is also suggested to validate the model against different clays since this thesis focused only on Kaolin clay.

Salt caverns formed by solution mining may cause soil subsidence, because the surrounding adapts to fill the void (cavity) created in the strata. The cavern volume is hence not only a function of salt extraction (estimated from mass balance) but also a function of salt creep. The cavern size can be measured by sonar, but a less costly way would be to correlate cavern volume to cavern compressibility. The size of the cavern can be obtained by a compressibility test. The compressibility test consists of injecting brine or fresh water into the cavern, causing a pressure build-up. The pressure increase depends on the cavern volume and compressibility. Bérest et al. (2006) [1] derived an equation that expresses the pressure response to injection of a volume V of fresh water. The equation includes creep that can be derived from a separate geometric creep model that relates the strain to the stress history using the Norton-Hoff law for describing an Arrhenius type of response model, and thermal expansion effects influenced by the geothermal temperature. It also considers Darcy flow and leakage through the permeable salt cavity. This thesis investigates the sensitivity related to the various mechanisms in the Bérest and Van Sambeek (2005) model. It also extends the model by introducing chemical dissolution of salt effects and taking into account the impact of the sump (insoluble precipitates at the bottom of the cavern). We state the model equations and solve them for an example of interest. We can also compare the calculated pressure response to the results of a pressure test. In this pressure test a given volume of fresh water is injected above the sump and produced at the top of the cavity. The produced salt concentration is also one of the parameters measured in the test. There are at least two widely used methods to obtain or predict cavern size of active salt solution mining caverns, namely sonar surveys and leaching simulators. However, these data can be flawed due to uncertainties of the methods and the accuracy of measuring devices like flow meters. The compressibility test can then be used as an additional survey method. The test execution is simple and inexpensive, making it possible to be performed more frequently than a sonar survey. The cavern volume is obtained from a compressibility test data by using a solution that includes not only the injection volume, but also volumes introduced by other phenomena. These other phenomena are caused by thermal, hydraulic, mechanical, and chemical influences. Previous work has attempted to introduced solutions incorporating these influences for idle caverns. The proposed solution incorporates these effects in an active leaching salt cavern. It also introduces the impact of sump (insoluble on cavern bottom) to be included in the model. The proposed solution is then tested against field data. The work here differs from previous work that it presents a comprehensive description of the various methods used in the literature and practice. Where current models predict cavity volumes with a mean absolute percentage error of 33% (Thiel’s method for BAS3O) to 127% (Bérest et al. for BAS4) for the studied caverns (BAS3O and BAS4), the model here, which includes the presence of insoluble particles, and a well-established creep model is able to predict cavity volumes to an accuracy of 33% (proposed model for BAS3O) to 12% (proposed model for BAS4).

Simulation of multiphase flow in natural subsurface formations include selection of time-step size, i.e., the discrete snapshots (steps) over which the time-dependent nonlinear process is investigated. The simulation results would naturally depend on the size of the discrete time step, according to the order of accuracy of the implemented numerical scheme. For accurate analyses the required time steps might be needed to be very small, which then leads to very costly simulations, which are often times impractical in real-world simulation applications. Taking coarse-scale time-step sizes can reduce the computational costs, however, it results in lower simulation accuracy. Note that a common simulation practice is to take constant time-step everywhere in the computational domain. To resolve this challenge, this MSc thesis report presents a novel local time-stepping (LTS) method for multiphase flow in porous media. In this method, different time-steps can be applied in one computational domain; allowing for employing small time steps in sub-domains with higher sensitivity to the time step size and bigger time steps elsewhere. The LTS method is developed for all types of sequentially-coupled and fully-coupled simulation strategies. For all of these developments, the interfacial flux continuity is the key requirement to connect the sub-domains of different time zones. The fluxes chosen for the inner stages of the coarse time-step are consistent with the chosen simulation strategy. More precisely, in the case of the implicit-pressure-explicit-saturation (IMPES) method these fluxes are lagged in time (explicit), while for the sequential implicit and fully implicit (FIM) methods they are calculated at the current (implicit) time-step. For implicit transport method, the refined zone partition matrices are developed to algebraically form the refined zone Jacobian matrix and refined zone residual vector. The numerical results show that only a fraction of the domain can be simulated with small time steps; while the rest can be simulated at a bigger one. Local time-stepping method provides accurate simulation results compared with the fine-scale time-step simulation results for all three simulation strategies. Specially, the local time-stepping for FIM simulations, which is the mostly common commercial-grade simulation approach, can preserve the accuracy of the results and also the computational efficiency. For the studied cases, the computational efficiency gain of LTS for sequential implicit method is not as significant as that of the FIM. To exploit full adaptivity in simulation, as presented in Appendix B, one can directly apply the developed method of this thesis to adaptive implicit method (AIM); where sequential and fully implicit coupling approaches are applied in different sub-domains of the reservoir. Overall, LTS casts a promising approach to optimize accuracy-efficiency tradeoff when it comes to time step selection.

Feasibility study of Dutch concept of deep geological repository for radioactive waste is at present widely conducted in the Netherlands. In this project, transport properties of selenate ion in clay materials were investigated. Apparent diffusion coefficients of selenate ion were obtained as results. To study possible alternations of Boom Clay by alkaline concrete pore water, elemental composition and pore structure of raw Boom Clay samples were characterized.