Most scientific literature on sulfide-rich soils address the environmental hazard caused by the oxidation of Potential Acid Sulfate Soil (PASS) in its unoxidized form to Acid Sulfate Soil (ASS) when exposed to oxygen. Sulfide-rich soils pose also significant geotechnical challenges due to high water content and organic content, fine particles, lack of consolidation and acid-generating potential. Excavation and disposal are often necessary for infrastructure projects in e.g. Sweden and Finland. This thesis investigates approaches managing excavated sulfide soils by enhancing their mechanical properties for geotechnical applications and optimizing water content in the soil to mitigate oxidation risks.  

The research comprises three interrelated components. The first focuses on field experiments evaluating the efficiency of in-situ mixing techniques in improving the uniformity and mechanical stability of sulfide soil-binder mixtures. Results indicate that multi-stage mixing substantially enhanced soil-binder homogeneity and strength of the stabilized soil. Mechanical stability of laboratory prepared samples consistently outperformed those mixed in the field. A predictive model was developed to explain the observed trends, highlighting the critical roles of binder dosage, porosity, and water content in achieving stabilization.  

The second component examines the stabilization of acid sulfate soils using Portland cement and Multicem binders. Portland cement was more effective in enhancing mechanical properties due to its rapid increase in alkalinity, particularly in highly acidic soils. Multicem, while effective under moderately acidic conditions, was less successful in environments with high acidity. The study emphasized the importance of understanding the interactions between binder type, soil composition, and environmental factors.   

The final part investigates the role of multi-layer soil cover systems in mine and sulfide soil landfill designs in controlling moisture dynamics. Sulfide soil landfills were analyzed using numerical modeling validated through field instrumentation. The findings demonstrated that cover material properties, thickness, and the inclusion of features like capillary breaks are significant in maintaining moisture and minimizing environmental risks by oxidation. These insights were also applies to mine covers.  

This thesis provides a framework for a more sustainable management of sulfiderich soils by integrating findings from field experiments, laboratory, and numerical modeling. This study aims to contribute to the development of practical strategies to reduce society’s reliance on landfills for managing excavated sulfide soils, promoting geotechnical applications while mitigating environmental impacts.

A laboratory investigation was conducted to identify principal variables-initial mixing water content, porosity, and binder content- impacting undrained shear strength (qu) of stabilized sulfur-rich silty soil. An equation for predicting qu of stabilized soil was established based on the experimental data. Initially, samples were prepared with soils sample with different initial water and binder contents. Multicem, a binder consisting of a mix of cement and cement kiln dust, was added to the samples. Three different percentages of Multicem were mixed at five different soil water contents to measure qu of stabilized mixtures to understand how water content and porosity levels in the samples affect the performance of the binder and their combined impact on the strength of the samples. The soil-binder mixtures were compacted and subsequently cured in laboratory-controlled environment. The prepared samples were tested in uniaxial compression test apparatus. The results evidenced that binder content and corresponding porosity affect the strength of specimens at an equal water content. The results showed that at equal initial mixing water content, the qu of a sample increased by increasing binder content. Furthermore, it was observed that increase of binder content has a reverse effect on porosity. It was appeared lowering the soil water content, initially increased the strength until an optimum water content. Further lowering water content increased the porosity and consequently decreased qu of samples. Moreover, a ratio of porosity/volumetric binder content was chosen to evaluate the impact of these two variables on strength of samples. This study showed that qu is an exponential function of porosity/binder volumetric content ratio which depends on initial mixing water content of mixtures. It was shown at water content lower than the optimum, results of stabilization are more effective than in soil at higher water contents. Therefore, reducing the water content and thereby porosity has more significant effect on improving qu than increasing the binder content.

Due to their silty texture and high organic content, sulfide soils are typically unsuitable for use as a foundation for construction projects. In most cases, excavation and replacement with other construction materials are needed to ensure the structural integrity of building or infrastructure. Landfills are commonly used for the disposal of excavated sulfide soil. Because of the chemical properties of these soils and the risk of acidification, these landfills need to be kept saturated to limit oxygen diffusion. However, the long-term behaviour of these landfills is not yet fully understood, highlighting the need for further research to investigate the fluctuation of the degree of saturation in the soil, under different conditions. This study examined the effect of seasons on the degree of saturation in sulfide soil landfills and its effect on oxygen transport. The objective of the current study was to validate a numerical model of a sulfide soil embankment, done using SEEP/W software, with data from the monitoring of a sulfide soil landfill, in Northern Sweden. A one-dimensional numerical model of the landfill was created using laboratory measurements of hydraulic conductivity and water retention capacity. This model was then validated by comparing it with data from installed sensors. The numerical model accurately predicted the degree of saturation changes over time in the landfilled soil. These findings allow engineers to optimize design and predict long-term performance under different environmental conditions. The study highlights the importance of numerical modelling in predicting long-term hydrological behaviour and offers valuable insights into sulfide soil landfill behaviour.

Sulfide soils are silty soils, often found in saturated conditions, under the groundwater level. Characteristics of these soils, including particle size distribution and consistency limits along with chemical composition and environmental properties, cause excavation to be necessary for construction purposes. The excavated sulfide soil usually is transported and deposited in landfills. These soils are either deposited in saturated conditions or chemical buffers are added to the soil to prevent acidification. Special conditions of these landfills complicate the disposal procedure and the landfill maintenance which makes those financially expensive. Reusing sulfide soil in construction is a solution to reduce the expenses related to the management of sulfide soils. Since the mechanical properties of these soils are not suitable for construction purposes, the first step is to improve soil characteristics to the level that fulfills the needs of construction applications. One solution to improve the mechanical properties of the soil is adding a binder to the soil.

The main focus of the research was to improve the mechanical properties of soil. The research activities were divided into two parts. The first part was conducted in a laboratory environment to develop mixtures, while the second focused on transferring the results to field conditions. The laboratory tests included mixing soil and binder i.e., cement was added to the soil at different percentages to evaluate the soil improvement. An unconfined compressive strength (UCS) test was conducted on the stabilized sample to evaluate the efficiency of the stabilization. The resultsof UCS for the stabilized samples were compared. Since the soil contains a high amount of water, the traditional sample preparation was not suitable. Therefore, an alternative method was developed and evaluated. Moreover, the effect of curing time on the strength and consistency limit of stabilized samples was evaluated. At last, the effect of different variables, including porosity, binder content and initial water content, on the UCS of soil was investigated to identify potential correlation between UCS and different soil variables.

The results of the tests showed that adding a binder, regardless of the type of sulfide soil, positively affects the UCS of prepared samples and increasing the curing time increased the UCS of the samples. At higher cement content, the effect of curing time was more significant. Also, it was shown that at higher water content, the effect of binder is lower in comparison with the same soil at lower water content. By lowering the water content, the strength of stabilized soil reaches a maximum and drying further the soil, below the optimum water content, led to strength reduction. A correlation between UCS of sample and porosity/binder ratio was employed to predict the strength behavior of stabilized soil based on variables such as porosity, initial water content and binder dosage.

In order to evaluate if laboratory results can be applied to geotechnical applications, the second part of this research included a field mixing experiment for a large-scale mixture of soil and cement. The effect of the mixing procedure with common equipment on the homogeneity of industrial-size mixture was investigated. A sampling strategy for collecting representative samples of mixture was selected and assessed. the number of mixing steps and the effect of binder dosage on the uniformity of samples were studied. Results of UCS of samples prepared from field and laboratory mixture were compared and evaluated. A field evaluation was conducted to determine the quality of the mixture and how many mixing steps are required to reduce variability between samples. Two different percentages of binder were added to the 5 Tons of soil. The UCS test samples were prepared from the soil-cement mixture in the same way as they were prepared in the laboratory and cured for a specific time. The UCS test was conducted on cured samples. The test results were compared to evaluate the mixture homogeneity in the field.

The results showed that homogeneous mixtures can be obtained in the field with the available equipment. Assessing the sampling strategy showed that increasing the sampling sections from 5 to 12 and preparing single UCS sample from the collected soil provides representative samples from the soil mixture pile. Additionally, it was shown that by increasing mixing steps from 2 to 3, it was possible to eliminate samples with notable lower strength than average UCS. A greater number of mixing steps improves homogeneity while reducing the average UCS. It was found that mixing soil and binder in the laboratory improves strength better than mixing them in the field. When applying laboratory results to field design, this point must be taken into account.

In this study, a mixing procedure of sulfur-rich soil and cement-based binder to enhance the soil’s unconfined compressive strength (UCS) was tested in field conditions for geotechnical applications. The focus was to evaluate uniformity of industrial size soil-binder mixture, blended by existing method. This paper outlined sampling strategy and the number of samples needed for a valid uniformity evaluation. Moreover, this study emphasized the difference between field mixing and laboratory mixture preparation by comparing UCS of stabilized soil samples in the field and UCS of corresponding samples mixed and prepared in the laboratory environment. In the field, soil and cement were blended in two to four stages with 5% and 7% cement—the percentages being based on the soil’s dry weight under field conditions. Samples were taken from the field mixtures after each stage. Since the number of samples needed to be representative of mixture characteristics for large-scale mixing is not standardized, this field experiment included two phases. The first phase was dedicated to finding a sampling strategy for a large soil pile along with measuring UCS of collected samples. In the second phase, sample collection was conducted based on the results of sampling strategy from the first phase. In the laboratory, samples with percentages of binder similar to the amount of binder in the field were also prepared. Both field and laboratory samples were prepared using the tapping method in the laboratory for UCS test. Samples were cured under similar conditions for 28 days. The results showed that the uniformity of mixture improved after each additional mixing stage. In addition, while spots with low UCS were observed in the second mixing step, these spots were eliminated in the third mixing step, and results of the UCS tests were comparatively uniform. Moreover, comparison of the samples revealed that the UCS of the laboratory mixture is higher than that of the field mixture. This showed that even though the UCS is a good standard for comparing the strength of different soils stabilized with different percentages or types of binders in the field mixing, the actual strength of the stabilized mixtures under field circumstances is lower than that in laboratory prepared mixtures.

Sulfide-rich soils contain iron monosulfides and disulfides, which can oxidize under aerobic conditions when the soil is drained, facilitating oxygen diffusion and leading to the acidification of surrounding watercourses. This type of soil is often excavated due to its unsuitable geotechnical properties. As a result, soils with acidification potential need to be safely transported and deposited in landfills. Moisture content plays a crucial role in controlling the oxidation process, as maintaining optimal moisture levels limits oxygen diffusion, thereby preventing oxidation and acid generation. This study uses numerical modeling using SEEP/W to predict moisture fluctuations in sulfide soil landfills under various conditions. It further investigates the effects of soil properties, weather conditions, and landfill cover thickness on landfill performance, focusing on their role in maintaining moisture levels that inhibit sulfide oxidation. Results indicate that soil hydrological properties have a significant impact on saturation levels in the landfill, independent of other parameters. Adding a capillary break positively affects the moisture levels period studied, and a thicker cover enhances moisture retention. Keeping the water table close to the surface helps the system recover water lost due to evaporation and weather changes. These findings showed the current cover design practices, mandating uniform 1-meter cover thickness for all sulfide soil landfills is over-design in some cases and provide helpful insights for developing effective management plans to mitigate environmental risks and optimize landfill construction.