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.

Capillary barrier systems (CBSs) have been demonstrated to be effective in deviating infiltration in a landfill cover. However, their performance when combined with a cover soil optimised for methane oxidation had not been tested in the field yet. This study aimed to describe the water balance of a test field where such a configuration was built, located on the landfill in the Wieringermeer area, the Netherlands, over the period from 2009 to 2023.
During that time period breakthrough and deviated infiltration were measured. A one-dimensional finite difference model was built to model evapotranspiration and storage in the cover soil as well as the moisture retaining layer (capillary layer; CL). This model performed well in describing overall seasonal trends but generally overestimated evapotranspiration in spring and outflow in autumn, consequently underestimating the storage in these seasons.
The results show that the annual precipitation ranges between 770 and 990 mm. On average, 59% (494 mm) of this precipitation is evaporated, 33% (281 mm) is diverted by the CBS and 7% (63 mm) breaks through. Compared to other test fields and design standards, this breakthrough is high. A weakened functioning of the CBS by construction errors or the ingress of sand could be a reason for that. Another factor might be the coarse grain size distribution of cover soil, necessary for the efficient oxidation of methane. This type of soil has a relatively high hydraulic conductivity which does not control infiltration rates into the CBS as well as cover soils in other studies.
Furthermore, the results suggest that storage in the cover soil and CL is the main determinant for breakthrough to occur. Major breakthrough only occurred at a storage larger than the storage in the soil associated with field capacity, both on a seasonal and daily time scale. The available storage is mainly affected by the evapotranspiration which shows a seasonal cycle. Consequently, outflows for the CL and breakthrough occur mostly when evapotranspiration is low (autumn, winter). The relation of breakthrough with precipitation is less straightforward as a high rainfall does not necessarily lead to breakthrough on a seasonal or daily scale, depending on the available storage in the system. The distribution of precipitation can matter however. On a seasonal scale, precipitation in late summer can result in high storage in the beginning of autumn, potentially leading to breakthrough. Furthermore, a precipitation amount uniformly distributed over a day leads to less breakthrough than more concentrated precipitation.
Overall, with some adjustments regarding the design of the CBS and the cover soil, this landfill cover design is a promising alternative for the current standard.

In this work, a novel globally implicit framework for reactive multiphase flow and transport has been implemented using the simulator DARTS and the geochemical package Reaktoro. This framework applies the Operator-Based Linearization (OBL) approach together with a Gibbs Energy Minimization (GEM) scheme to model complex nonlinear reactive transport and flow. Using component- to element-based transport allows for numerically stable equations. This formulation is used to model the dry-out effect and subsequent precipitation of minerals in the near-wellbore region of subsurface aquifers when injecting CO$_2$. A kinetic rate in combination with Reaktoro equilibrium calculations is implemented to achieve more accurate precipitation results. The kinetic products are not considered inside of Reaktoro and also not as an element. The consistency at the phase transition boundary has been corrected by using a projection of a plane within the hypercube of zero charge balance. Highly sensitive reactions such as pH, require a high resolution to produce physical results. The parameterization grid can be implemented such that some species have a high resolution within a small variation of concentration. Reaktoro is highly accurate for the complex multiphase thermodynamic and chemical equilibrium calculation, but at phase transition boundaries, the equilibration of the state can be nonphysical. The results have been benchmarked against the existing geochemical simulator from Shell, showing that more physics needs to be implemented, but gives an overall accurate result.

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.

"Landfills form the final stage in waste processing and are sources of multiple environmentally harmful emissions. Due to the biodegradation of organic material, methane, a strong greenhouse gas, is released into the air and soluble pollutants, like ammonium or heavy metals, are flushed out of the landfill waste. In modern landfills, the waste body is isolated from the environment by an impermeable liner and both the gas and the leachate fluxes are collected and treated. However, sealing the landfill reduces the emissions on the short term but gives no sustainable solution as eternal active aftercare is needed and the emission potential is not reduced. The goal of the CURE project is to develop methods to enable the sustainable management of Dutch landfills. One of these methods is piloted at the Braambergen landfill, which is the in-situ aeration of the waste body with the goal of speeding up the biological degradation of organic matter to reduce the emission potential relatively quickly below the values in the Dutch regulations, such that the eternal active aftercare is not required anymore. The issue that the landfill operator, Afvalzorg, is facing is that in compartment 11Z of the landfill the aeration efficiency is poor due to ponding leachate. The objective of this thesis is to quantify the relation between the water level in the landfill and the efficiency of aeration, and to research the spatial and temporal variability of the water levels, leachate composition and hydraulic conductivity of the waste body to investigate the nature of the water and to find a suitable solution to mitigate the reduced aeration efficiency. To reach this goal, multiple types of field and lab measurements have been performed and the results of a water balance model have been used for a numerical evaluation. In the field, water tables have been measured as well the magnitude and composition of landfill gas extracted by the aeration wells. Pumping tests have been performed on the wells to assess the hydrological behaviour of the leachate surrounding the wells and to determine the horizontal hydraulic conductivity of the waste. Leachate samples from all the wells have been collected and analysed in the lab on pH, redox potential, electrical conductivity, DOC and ammonium concentration. Also were CTD-divers installed in some of the wells to monitor the response to rainfall and the medium long term behaviour of the water level and the electrical conductivity. The general conclusion of this thesis is that the studied compartment 11Z is homogenous in its heterogeneity. With all the different measurement types performed it was in some way the objective to see whether the behaviour or the properties of the landfill and the leachate can be grouped spatially, but this was not the case. Therefore can be concluded that the only scale for which a measurement can be representative for the entire compartment is on the scale of the compartment itself. This is in line with the expectations of large heterogeneity in landfills and the unique situation of aeration system on the Braambergen made it possible to proof this. The efficiency of the aeration system is particularly low in compartment 11Z because the conditions are unfavourable for both water and gas flow. The conditions in compartments 12 and especially 11N are more favourable because the waste, especially at the bottom of the compartments, is more permeable. Even if the amount of water in 11Z is reduced and the gas flow increases, the total amount of extracted LFG will not reach the same level as for the other compartments.

Anaerobic digestion (AD) is a traditional technique to treat sewage sludge and produce biogas. Thermal hydrolysis process (THP) is a feasible pretreatment for AD by improving the biodegrdability of organic compounds. Partial nitritation/anammox (PN/A) process could be applied in side-stream for treating the reject water from AD since the reject water has a low ratio of COD/N. The prime microorganisms involved in PN/A process are anammox biomass, denitrifiers, ammonium oxidizing organisms (AOO) and nitrite oxidizing organisms (NOO). However, the compounds generated in THP may cause negative effect on microbial activities. In this research, characteristics of the reject water was clarified, and manometric method and respirometry method were used to determine the microbial activities by measuring the conversion rates of the substrates. A higher concentration of soluble COD, total ammonium as nitrogen (TAN) and refractory compounds were in the wastewater treatment plants (WWTPs) with THP installation (i.e. Tilburg, Hengelo, Apeldoorn, and Amersfoort) than the ones in the WWTPs without THP pretreatment (i.e. Sluisjesdijk and Olburgen). Sluisjesdijk has one step PN/A process, while Olburgen has two steps PN/A process. Maximum specific anammox activity(SAA) among the studied WWTPs was found in Sluisjesdijk as 1.32±0.02 gN/gVS/day. THP installation could suppress the SAA value. The presence of refractory compounds may lead to the decrease of SAA. The denitrification activity was enhanced by the high concentration of COD generated in THP. The ratio of AOO activity to anammox activity were similar to the stoichiometry value of 1.32 in Sluisjesdijk Sharon reactor, Tilburg, Hengelo and Amersfoort. Molecular methods may recommended for further study in the area of this research.

Conceptual hydrological model parameters represent characteristics of a catchment. They are an integration of spatial heterogeneous parts of the system. Finding adequate parameter values based on field observations, which represents the heterogeneity of a catchment on the spatial resolution and scale of the model, is considered challenging since the scale of the observations do not meet the scale of the parameters.

The objective of this thesis is to analyze the extent to which it is possible to make an estimation of parameter distributions based on field observations (the precipitation, evaporation and discharge) and a given hydrological model structure. The goal is to avoid the use of uninformed prior parameter distributions during calibration by using available field data to generate informed prior distributions.

In this research, six different expert-knowledge inverse modelling methods are developed to find four parameter distributions. Each method uses sub-periods in the data and is coupled to the parameter of the model component representing that specific type of sub-period.

To test the methods, the study was conducted in a synthetic environment, which made it easier to validate the parameter distributions obtained with the methods. In this synthetic experiment, discharge data was produced by a model driven by real rainfall data and potential evaporation data. All forms of uncertainty were excluded in this test.

The effect of data uncertainty in the methods was investigated separately by conducting a sensitivity analysis. The same synthetic data was used; however the synthetic data was corrupted to simulate data uncertainty.

Last, an application of the methods upon real measured data was completed. The performance of the methods to find parameter distributions can no longer be assessed since in the real world the “correct” parameter values are not known. However, a comparison of the informed prior parameter distributions of the methods with uninformed prior parameter distributions could be made with a Monte-Carlo sampling strategy calibration.

In the synthetic experiment, all parameter distributions of the investigated model were correctly determined using the expert-knowledge inverse modelling methods. The sensitivity analysis revealed that the method to determine the maximum percolation rate parameter (Pmax) distribution was sensitive to data uncertainty. The determined Pmax parameter distributions did not include the original parameter of the corrupted synthetic data. However, this issue does not lead to other parameter distributions that do not include the original parameters.

In real-world application, insight is gained into the performance of the developed methods to find parameter distributions. An uncertainty interval was constructed with the Generalized Likelihood Uncertainty Estimation (GLUE) method. The total area of the constructed uncertainty interval using the calibration results of the informed prior parameter distributions was less than half than the uncertainty interval constructed using the uninformed prior parameter distributions. The posterior parameter distributions of the informed parameter distributions was two to five times smaller than for the uninformed parameter distribution. The model performance of both calibrations did not deviate significantly, indicating a sufficient model structure for the catchment and an adequate performance of the methods to find parameters.

A considerable contribution to the greenhouse effect originates from landfills. This contribution originates from the degeneration of waste in the landfill. The gas which is created during the degeneration consists of 40 to 60% methane. One mole of methane has a global warming potential which is 28 times higher than that of carbon dioxide. Therefore, emissions of methane should be reduced. One method which can be used to reduce this emission is the oxidization of methane to carbon dioxide. This can be done by the construction
of a cover soil with methanotrophic bacteria in the soil. These bacteria oxidize methane as their source of carbon and energy. This oxidation process discriminates against the heavier isotopes present in the gas, this shifts the isotope signature. A method to determine the oxidation efficiency of the methane is based on the evaluation of this change in isotope signature over the cover soil. Next to the fractionation of isotopes due to oxidation, fractionation also occurs due to diffusion. However, fractionation due to diffusion is generally
assumed to be negligible. Mostly, this assumption is valid, since the dominant part of transport is advective; which does not discriminate against an isotope.

In this thesis the effect of compaction and saturation on two sands are evaluated. Using this evaluation, the criteria during which diffusion becomes an important transport phenomena are determined. This is determined by a series of experiments, where the concentration of methane is monitored while the gas diffuses through a soil sample. In this thesis the effect of the soil matrix on the fractionation of methane is also evaluated. This is evaluated by performing isotope spectrometry on samples of gas, taken at different times during the experiments. These two aspects are combined to evaluate if the oxidation efficiency can still be determined using fractionation of stable methane isotopes when diffusion plays a considerable role in gas transport.

During the experiments the decrease of methane in the chamber was measured. Spread over two soils, for a total of 18 variations in compaction or saturation the decrease of methane over time was measured. The results of the experiments show that for diffusion no distinction between variation in compaction and saturation needs to be made. Compaction and saturation can be described together for both soils using the air-filled porosity. The relation between the air-filled porosity and the effective diffusion coefficient is linear for both soils. Due to the large sand fraction the share of coarse pores is high for the whole range of compaction, meaning that no significant increase of tortuosity is visible in the relation between air-filled porosity and the effective diffusion coefficient. When the effective diffusion coefficients are compared to effective permeability values of these soils, it showed that the diffusion is more important for drier and more compacted soils. Thus, diffusive transport becomes important for gas transport over a cover soil when the pressures are lower, the soil is drier, and the soil is more compacted.

The fractionation factors due to diffusion, which have been determined for a selection of experiments, showed no trend with any other measured parameters. The precision of the measured values from which the fractionation factors are determined is high. Thus, the confidence in the fractionation factors is high.
The lack of a trend present means that in sand the soil matrix has no effect on the fractionation factor. This means that the fractionation factor due to diffusion in soil is the same as in free air. For the calculation of the oxidation efficiency with a relevant diffusive flux the ratio of diffusive to advective flux needs to be
determined but the fractionation factor due to diffusion is constant. Before the oxidation efficiency can be calculated, first the load needs to be corrected for any loss through hot-spots. Next, the advective flux and diffusive flux are combined, this is done by adding the fluxes together. The fluxes can be added because there are no interdependent effects, each flux only increases the total flux. Thus, the oxidation efficiency can be calculated using the changes in isotope signatures, even when a significant diffusive flux is present.

The construction of the new airport of Mexico City (NAICM) is challenging, as the subsoil at the site is very weak. The airport, which is under construction at the moment, is located on top of thick layers of Mexico Clay soils. These clayey layers consist of a mixture of both clay particles and pyroclastic volcanic materials. These unique soils are characterised by a high water content and compressibility. Upon loading, the soft soils will settle as a result of the consolidation process. This consolidation is defined as the dissipation of excess pore water as a result of an increase in stress. The rate of consolidation is governed by the hydraulic conductivity of the soil, which is in general very low in soft soils, and the drainage path. In order to speed up this time consuming process, vertical permeable elements can be installed in the soil. Upon installation of these elements, a reduction in drainage path is realised. This reduction in drainage path allows faster dissipation of excess pore water pressure, hence reducing the consolidation time. Due to the conditions at the site, the design of the runways is challenging. The differential- and residual settlements need to be within strict boundaries in order to keep the runways operational. Prior to the design of the runways, trial embankments have been constructed in order to measure the performance of different soil improvement techniques at this specific site. Based upon this field test results, prefabricated vertical drains (PVDs) were selected as the most efficient soil improvement technique. Besides the PVD trial, the sandpile trial also showed promising results. The PVDs and sandpiles both decrease the drainage path length inside the soft soil layers. The main difference is the relative high stiffness of the sandpiles, the stiffness of the PVDs is negligible. By installing stiff elements in the soft soil, the loads applied on the top of the soils are transferred to deeper and stiffer soils. Hence, the total settlements of the top soft soil layers are reduced. The sandpiles could therefore be feasible alternative to PVDs, applied at locations in which the total settlement needs to be limited.
The investigation into the behaviour of sandpiles in Mexico Clay soils is performed in multiple stages. First, analytical models are used for gaining knowledge about the expected sandpile behaviour. In the second stage the FEM software Plaxis 2D is used for the numerical modelling of the sandpiles, using an axisymmetric model setup. The model is constructed in multiple steps, with increasing complexity. The first models simulate the material behaviour with the Mohr-Coulomb model, in the advanced models the Soft Soil Creep and Hardening Soil models are used. The final numerical model is verified by the measurement data obtained from the field trials.
After fitting the numerical model to the field data, the verified model is used in the sandpile sensitivity analysis. In this analysis the sensitivity of the material properties and geometry of the sandpile on the performance are researched. The residual settlements or performance, which is defined as the difference in settlement after construction and over a period of 8 years, is affected by the length, radius and centre to centre (ctc) distance of the sandpile. Adjusting the pile stiffness has minimal effect on the performance of the sandpile. An optimum in performance is found by varying both the pile radius and ctc distance. A combination of a small pile diameter with a small ctc distance results in the best performance. When comparing the performance of both sandpile and PVD, the PVD is found to be more effective in terms of performance. The additional stiffness of the sandpile is not reducing the total settlements, on the contrary, the self-weight of the piles increases the total settlements by providing an additional load to the soft soil layers located underneath the pile tip. In conclusion the sandpiles have no additional benefit over the use of PVDs, therefore the application of sandpiles at the NAICM site is not a feasible alternative to the use of PVDs.