This study presents a novel geotechnical engineering approach that utilizes naturally occurring processes to reduce soil permeability in-situ. This approach is inspired by a soil stratification process (Podzolization), where a low permeability layer is formed by metal-organic matter precipitates. In a field experiment, a direct aluminum-organic matter (Al-OM) floc injection was applied to create a continuous vertical flow barrier in a dike. Direct injection uses the shear-dependent size of Al-OM flocs. High-shear conditions (i.e., during injection) lead to the breakage of Al-OM flocs and thus allow their transportation in soils. When the injection stops and low-shear conditions prevail, the Al-OM flocs re-grow in size and block the pores, which ultimately reduces soil permeability. Two different Al-OM floc concentrations were applied in the field. Results show that a continuous flow barrier is only formed at lower concentrations; at higher concentrations a scattered permeability reduction was achieved. This demonstrates the viability of this approach in reducing soil permeability in-situ and shows that the spatial distribution of the flocs depends on input concentration.

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Stability of dikes is a national security issue for densely populated low-lying countries situated in delta areas, like the Netherlands. One of the dominant dike failuremechanisms in the Netherlands is piping, where high seepage flow rates transport sand particles and subsequently form a ’pipe’ under a dike structure. As such, one manner to reinforce dike lies in the modification of the seepage flow field. Though many of conventional approaches have demonstrated varied degree of success in creating flow barrier, which is a subsurface structure that can alter the seepage flow field, they are commonly costly in terms of energy and labour. Facing the ever-growing awareness of climate change as well as the large economic scale of the dike stability issue in the Netherlands, the development of alternative techniques is thus desired. The focus of this research project is to develop a cost-effective, robust and environmentally compatible technology for insitu permeability reduction of sub-surface systems. We took inspiration from nature, where a natural soil stratification process (namely Podzolization) shows the viability of organo-metallic complexes precipitation in reducing soil permeability in-situ. The aim of the research presented in this thesis is to quantitatively study the feasibility of using Podzolization-derived approaches to install flow barrier in dikes. Chapter 2 of this thesis presents two approaches for applying organo-metallic complexes to reduce soil permeability in-situ, which are derived from the detailed analysis of Podzolization and the flocculation process between metal salt with organic matter. The first approach bases on the in-situ mixing and reaction between two components (i.e., aluminium (Al) and organic matter (OM) solutions), while the second approach makes use of the direct injection of Al-OM flocs. To understand the feasibility of using these approaches to install flow barrier on site, a 3D process-oriented model was developed. An important aspect of this model development is to incorporate engineering conditions on site into the simulation of processes. A series of scenario analyses were therefore performed with the model in order to facilitate the design and evaluation of the full-scale experiments where the two delivery approaches were applied to install a flow barrier in two dikes. @en

Using naturally occurring processes to modify the engineering properties of the subsurface has received increasing attention from industry and research communities as they aid in the development of cost-effective, robust and sustainable engineering technologies. In line with this trend, we propose to use precipitates of aluminum (Al) and organic matter (OM) to reduce soil permeability in-situ. This process is inspired by podzolization: a soil stratification process where a layer with low permeability is developed at depth via the precipitation of metal-OM complexes. In this study, the concept of applying Al-OM precipitates for in-situ soil permeability reduction was for the first time applied in the field. The aim of the field experiment was to create a cylindrical flow barrier in a sand layer at depth. In order to design and engineer the field application, we performed a series of scenario analyses with a site-specific 3D reactive transport model. This led to an in-situ engineering approach where a flow barrier was created by separate injection of Al and OM using a combined injection/extraction strategy. During the field application, the local variation of soil conditions required significant modifications to the design. Further scenario analyses with the model were conducted to adapt the original design and to understand the consequences of these modifications. The results show that a cylindrical flow barrier was created after an injection period of 8 days. The precipitation of Al-OM is a highly localized process, where large amount of precipitates is formed in the close vicinity of the injection filter screens. Evaluation of pumping tests that were performed after the injection activities revealed that the permeability of the treated sand was reduced to 2% of its original value. This first full-scale field test demonstrates that applying Al-OM precipitates is a suitable bio-based engineering tool to reduce soil permeability in-situ.@en

Controlling subsurface flow is crucial to prevent: the spreading of contaminants, the failure of dikes or any undesirable water flow. Subsurface flow can be controlled by changing the soil permeability. In order to change soil permeability rigid structures are often installed (for example the installation of sheet pile walls into dike bodies), however, these solutions are costly and disrupt the environment. Alternatively, the SoSEAL project aims to provide an in-situ solution to reduce the permeability of highly permeable soil layers in an ecological friendly and cost-effective way. The SoSEAL project is inspired by podzolization, which is a soil formation process. It reduces soil permeability via the precipitation of metal-organic matter complexes in the pore space. During earlier practices a need has arisen for a cheap, environmentally friendly and naturally available aluminum source. Accordingly, the goal of this research is to investigate if gibbsite can be used as the aluminum source to form metal-organic matter precipitates.

A literature study was conducted to investigate the interplay between gibbsite and dissolved organic matter (DOM). Four processes are primarily studied in this research, they are: the release of free Al via dissolution of gibbsite; the complexation between free Al and DOM; the protonation and subsequently precipitation of DOM and the adsorption of H⁺/DOM onto the gibbsite surface. Thereafter the following hypotheses were formulated:

•The extent of gibbsite dissolution is larger at low pH. However, due to its low solubility and slow dissolution kinetics, a low concentration of free Al is expected.
•The organic matter (OM) source used in this study is a humic acid, therefore it is expected to become completely insoluble when the pH is below two.
•The contradicting pH dependency of gibbsite dissolution and OM solubility leads a narrow transitional pH range, that is favourable for the occurrence of complexation between free Al and DOM.

These hypotheses are tested by performing experiments in the laboratory, where the pH varies from 2 to 8 and the aluminum/carbon (mol/mol) ratio is 0; 0.05; 0.1; 0.3 and 1. A synthesized crystalline gibbsite powder and a potassium humate are used.
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The results are subdivided into three pH ranges in which similar behaviour was found. In the low pH range (pH = 2 - 2.5), the protonation and subsequently precipitation of DOM is the dominant process in short term. However, when considering the long term behaviour, gibbsite dissolution is the dominant process as the DOM has precipitated. Due to the difference in time scale, there is very little complexation between free Al and DOM. In the transitional pH range (pH = 3 - 3.5), the competition for H⁺ between DOM and gibbsite is profound. This increases the solubility of DOM and slows the dissolution of gibbsite down. As a consequence, this favours the gradual complexation between free Al and DOM. Nevertheless, due to the ambiguity of free Al measurement method used in this research, it is difficult to quantify the complexation. In neutral to high pH range (pH = 4 - 8) DOM undergoes protonation over time but stays soluble. The release of free Al via the dissolution of gibbsite is negligible in this pH range. Therefore complexation between free Al and DOM should not be expected.

The results presented in this research indicate that gibbsite may not be suitable as the aluminum source to form precipitates with organic matter in engineering solutions that require fast soil permeability reduction. However, the slow release of free Al from gibbsite in combination with the gradual formation of precipitates with organic matter might be interesting for robust problems that require a self-healing ability. For efficiency concern, another OM source that is rich in fulvic acids (higher solubility at low pH than humic acid) and a natural gibbsite source (higher solubility than synthesized gibbsite) should be investigated.

Using naturally occurring processes to modify the engineering properties of the subsurface has gained increasing attention from industrial and research communities as they aid in the development of cost-effective, robust and sustainable engineering technologies. In line with this trend, we propose to use the interaction between aluminum (Al) and organic matter (OM) to reduce soil permeability in situ. This is inspired by podzolization: a soil stratification process where the mobilization of Al, iron and OM in the topsoil is followed by their precipitation at greater depth. In this study this newly developed engineering technique has been applied for the first time in the field. The aim of the field test was to create a cylindrical flow barrier (5 m i.d.) in a sand layer, located at a depth between 7 to 13 m below ground surface (bgs). A 3D reactive transport model was developed via the coupling between Darcy’s law and solutes transport, meanwhile Al-OM precipitation and its impact on permeability are included. The model was used to design and analyze the results from the field test. At the site Al and OM solutions were injected separately through 20 injection wells distributed in two circles with a radius of 2.5 m for Al injection and 3 m for OM injection. An extraction well was placed in the center of these two circles to control in situ mixing and precipitation of Al and OM and precipitation of these two components in a specific zone. Results demonstrated that after a period of 8 days, we successfully created a cylindrical flow barrier where precipitates formed in close vicinity of the injection filter screens. The permeability of the treated sand was reduced to 2.3 % of its original value. Pumping tests conducted 6 months after the treatment showed no change in the achieved permeability reduction, indicating the stability of the Al-OM precipitates during this period. Further investigation is however necessary to evaluate the long-term stability of the flow barrier. This field study demonstrates the viability of using Al and OM complexation and precipitation as an in situ engineering tool to reduce soil permeability. By separate injection of the two components and a combined injection/extraction strategy, we were able to induce in situ mixing of Al and OM and control the geometry of the formed flow barrier. @en

The maintenance and improvement of river embankments plays a crucial role in granting the safety and ensuring the quality of life of the inhabitants of the Netherlands. While the threat posed by storm surges has been greatly reduced since the implementation of the Deltaworks, riverine areas in the west of the country are characterised by the highest risk of flooding.
The use of relief wells is one of the viable solutions to improve the safety of river embankments against instability mechanisms such as uplifting. Throughout the Schoonhovenseveer-Langerak project (SLA), the implementation of a system of relief wells constituted one of the key measures to increase the safety of the embankment structure during high-water events of the river Lek. In the current study, the influence of relief wells was simulated employing a 3D coupled model built with the Finite Element software PLAXIS 3D. Using soil properties from former site investigations, a calibration was performed based on in situ pore water pressure measurements, in order to devise adequate boundary conditions. The safety assessment of the structure was then performed by applying a time-dependent high-water wave and employing strength reduction to yield a Factor of Safety. An intermediate 2D model was employed to examine the evolution in time of the Factor of Safety, the influence of mesh refinement on the safety results, and the shape and the location of the failure mechanism. By means of the 3D model, different design choices of the system of relief wells were tested and their influence on the stability of the structure and on the water discharge could be determined. A comparison between the results obtained in the 2D and 3D models showed that a 3D stability analysis does not necessarily yield a less conservative result than the 2D equivalent.
Comparing the values of the Factor of Safety obtained with time-dependent and stationary boundary conditions showed that the results obtained in the latter case are considerably more conservative, suggesting the potential benefits of a time-dependent fully-coupled analysis.

The utilization of natural processes for in situ permeability reduction has seen a growing interest in recent years since controlling infiltration or seepage of water is one of the most challenging tasks in water management and civil-engineering. We hereby propose a novel geoengineering tool for in situ permeability reduction, namely Soil Sealing by Enhanced Aluminum and organic matter Leaching (SoSEAL). SoSEAL makes use of the interaction between organic matter (OM) and aluminum (Al). Complexation and subsequent precipitation of OM by Al results in the formation of soil layers with reduced permeability; a process which is well known from podzols. This study demonstrates the suitability of the SoSEAL technique for permeability reduction in laboratory experiments. All experiments have been performed using humic acid (HUMIN P775, Humintech, Germany) as an OM source and aluminum chloride as the metal component. Batch experiments were conducted to study the interaction between OM and Al at various metal to organic carbon (M/C) ratios and pH levels. Results show that the precipitation of Al-OM flocs depends on the Al concentration and therefore the M/C ratio, which is well-known from OM removal in drinking water treatment. Precipitation of the hereby used humic acid starts to occur at a molar M/C ratio of 0.01 and almost all OM is removed at M/C ratios larger than 0.04. The size of the Al-OM flocs ranges between 20 and 1000 m, which enables them to cover micro- and mesopores in porous media and therefore reduce the permeability. In order to quantify the permeability reduction that can be achieved by Al-OM flocs, saturated column experiments were performed using sand with three different grain size distributions and applying various injection strategies to induce in situ mixing of the two separately injected components (i.e. Al and OM). We were able to reduce the hydraulic conductivity in the sand column to a range between 10 and 40% of its initial value. Results show that the reduction in permeability depends on several factors including the sand type, the injection technique, mixing and reaction of the two components in situ, and the orientation of the precipitation band. We conclude that the precipitation of Al-OM flocs induced by in situ mixing of Al and DOM can significantly reduce the permeability of different sand types. These results are the proof of principle of the SoSEAL concept. @en

The utilization of natural processes for engineering purposes has been widely discussed in recent years since they might enable the development of cost-effective, robust and environmentally compatible engineering technologies. Biomineralization is one of the many possible biogeochemical processes that is currently investigated in detail. We propose the use of another natural process, namely podzolisation, as a novel geoengineering tool for in situ permeability reduction. Podzolisation is a soil formation process where the mobilization and subsequent leaching of aluminium, iron and organic matter (OM) in the topsoil is followed by their precipitation at greater depth. The accumulation of Al/Fe-OM precipitates results in the formation of an almost impermeable soil layer [1]. In situ permeability reduction is interesting for several engineering questions, e.g., prevention of piping, leaking water bodies, and contaminant spreading. Preliminary experiments and modelling results revealed that Al-OM precipitates can reduce the hydraulic conductivity in sand by up to 4 orders of magnitude. In order to apply the podzolisation process for engineering purposes, it is, however, necessary to control the reaction kinetics and ensure Al-OM precipitation over the entire desired treatment zone within a porous media. Therefore, a 2D experimental setup (80x160x5 cm) equipped with numerous pressure and electrical resistance tomography (ERT) sensors is used to tests different kind of injection strategies and their effect on the permeability reduction within a porous media. A reactive transport model coupling a MATLABbased toolbox and ORCHESTRA (equilibrium reaction processor) is used (1) to design the injection strategies and (2) to simulate the solute transport, the geochemical reactions and their effect on the permeability within the experimental setup. The experimental results will be used to validate the model and to implement the most promising injection strategy in the field. @en