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Comparing calculation methods of deep excavations in soft clay - Comparison of analytical and numerical methods with field measurements for sheet pile wall excavations in soft clay
(2026) Johansson , Elin; Larsson, Erica
Using computational tools has become standard practice in all engineering fields. In geotechnical engineering, there are a large number of calculation programs available to capture the complex conditions associated with underground construction. These advanced models can achieve efficient and optimised designs in a short amount of time. But can the complicated conditions of soil construction be too complicated to capture in advance models? This thesis will try to answer when the implementation of numerical tools is the most appropriate option. The thesis has studied three cases of sheet pile wall excavations, one of which is based on a real case with measured data from a single-strut excavation, and the other two are based on empirical values for typical Gothenburg clay. In the last two cases, one is a single-strut excavation, and the other has two levels of struts. The cases were assessed using four different calculation methods. Two of the four methods use a numerical approach, including the Mohr-Coulomb and NGI-ADP material models. Numerical results include a parametric study of certain stiffness parameters, such as the bending stiffness of the sheet pile wall, the soil stiffness, and the axial stiffness of the supporting strut. The results include active and passive earth pressures, horizontal displacements and waler reaction forces. Results show that, in general, it is very difficult to create a model that accounts for all parts of a construction process, and that an advanced model does not always yield more realistic results than a simpler model. However, complicated models are more flexible to work with and easier to implement for finding optimised designs. The parametric study showed that earth pressures are not significantly affected by changes in stiffness inputs, whereas deflections and waler forces are more sensitive. Further, the soil stiffness input has a greater influence than the stiffness inputs of the structural elements. The effect of the strut stiffness in particular could be considered negligible, and the bending stiffness of the sheet pile wall only showed a minor influence on the results.
Detailed investigation of lime-cement columns in quick clay during construction
(2026) Backlund, Fanny; Sellgren, Alva
Numerous landslides associated with deep mixing installation in quick clay has been reported. Deep mixing is a ground improvement method commonly used to stabilise roads and railway embankments. However, during the deep mixing installation process, the method temporarily reduces the stability of the area before the soil improvement is achieved. This temporary reduction in stability is mainly caused by excess pore pressure in the permeable soil layers and by remoulding of the clay. Due to the high sensitivity of quick clay, remoulding can result in a significant loss of shear strength. The excess pore pressure is generated by the air pressure used to inject the binder during installation. This thesis aims to investigate a safe framework to be able to perform deep mixing with a reduced risk of failure during construction. Numerical analyses and liter ature studies were conducted to investigate the effect of deep mixing during the construction phase and see how the trigger factors influence the stability of the slope, and identify which trigger factor that is the most critical. The results indicate the importance of accurately characterising the permeability of the permeable soil layer, usually laying underneath the clay. The permeability is correlated to the dissipation rate of excess pore pressure, which could be considered when planning the installation procedure and the construction sequence. The analysis also indicated that the distance to the injection point within the friction layer had a limited influence on the pore pressure recovery time. It should be noted that the PLAXIS 2D analysis was performed with simplified geometry of the friction layer. Furthermore, the results demonstrate a linear relationship between the coverage ratio of the lime-cement columns and the factor of safety. The coverage ratio therefore represents a design parameter regarding both the temporary reduction in stability caused by remoulding during construction and the long-term increase in stability achieved after completion.
Comparison of Ground Improvement Techniques - Comparison of Different Techniques Based on Technical, Environmental and Economic Aspects
(2026) Carrin , Albin; Öhgren, Gabriel
This master’s thesis compares three ground improvement techniques: deep mixing, stone columns, and lightweight fill. The methods are designed to achieve comparable settlement levels and are evaluated for two case studies with clay soils in western Sweden, representing different soil depths and conditions. In addition to settlement performance, the impact on slope stability is analysed. Furthermore, the environmental impact and cost of the techniques are assessed and compared. The results indicate that deep mixing is generally the most effective technique in terms of set tlement reduction and slope stability improvement relative to cost. However, it also exhibits by far the highest environmental impact among the studied methods. Lightweight fill, on the other hand, generally results in the lowest environmental impact and represents a competitive alternative, particularly in deep soil conditions where column-based methods would otherwise require floating solutions. Stone columns are most suitable for sites with thinner clay layers and favourable ground conditions, and they have the additional advantage of significantly reducing the consolidation time. Overall, the findings show that the optimal ground improvement technique is highly dependent on site specific conditions, as well as the relative importance of technical performance, cost, and environmental impact.
2D and 3D Numerical Modelling of Sheet Pile Walls in Soft Clay - Comparison of NGI-ADP and Creep-SCLAY1S
(2026) TOLL WESTER , EMIL; SJÖLUND, OLIVER
This thesis examines the behavior of sheet pile walls in soft clay in 2D and 3D finite element analyses using PLAXIS, with a focus on comparing the constitutive models NGI-ADPandCreep-SCLAY1S.ThestudyisbasedonthegroundconstructionofHotel Draken at Järntorget in Gothenburg, where sheet pile walls of type PU12 and AZ18 were used to retain the soil during a staged excavation. The northern outer sheet pile wall was selected as the primary element of analysis, since inclinometer measurements were available for this wall. Both 2D and 3D models were created to simulate the construction sequence, including installation of sheet pile walls, staged excavation, casting of concrete, and installation of waling beams and corner struts. To address the impact of surcharge loading from ma chines or other equipment, two load scenarios were investigated. A sensitivity analysis was also performed for the sheet pile walls on the anisotropic plate input parameters in 3D. The results from the analyses show that the 2D plane-strain assumption overestimates the horizontal displacement compared to 3D. However, the choice of constitutive model also proved equally, or even more influential than the choice of dimensionality even for a non-symmetrical complex geometry. The more advanced Creep-SCLAY1S model produced results closer to the measured inclinometer data compared to the NGI-ADP model. The inclusion of surcharge loads from construction machinery was found to significantly affect the predicted displacement profile both in 2D and 3D, particularly in the upper portion of the wall. The sensitivity analysis of the anisotropic plate param eters regarding the area moment of inertia I2 and I12 in the 3D model showed to have negligible influence on computed wall displacements.
Displacements of buried pipelines due to sheet pile extraction in soft clay - A study on pipeline displacement using numerical modelling
(2026) GODENIUS , TEODOR; PETTERSSON, EDVIN
Large parts of the Gothenburg region consist of soft and sensitive soils, making the ground prone to settlements when ground conditions are altered. Such changes commonly occur during installation and replacement of water and wastewater pipelines. These projects are often considered as a low-budget project, which often results in insufficient geotechnical investigations and measurements, and have in many cases led to disputes and expensive settlements following soil deformations. Pipeline installation is often carried out in trenches supported by retaining walls, often sheet pile walls, and while settlements caused by the unloading and reloading during excavation are relatively well understood, available studies on the effects of the sheet pile extraction remain scarce. The aim of this study is to investigate how sheet pile extraction influences soil deformations, how the void created from the sheet pile is filled and further enlighten that sheet pile extraction can cause significant pipeline settlements in soft clay. The work was carried out by using numerical modelling in PLAXIS 2D together with comparison with a previously conducted full-scale field test in the Gothenburg region. The numerical model in PLAXIS 2D was calibrated against observed horizontal and vertical displacements of the full-scale field test for the initial phases leading up to the sheet pile extraction, in order to capture the soil behaviour. Thereafter, different modelling approaches for simulating sheet pile extraction in PLAXIS were evaluated based on their ability to reproduce the observed horizontal displacements. The selected method, where low stiffness- and strength- parameters are applied to the soil surrounding the sheet pile, was then used to simulate the vertical settlements induced by the sheet pile extraction. The results show that the numerical model was able to reproduce the general soil behaviour for all phases during pipeline construction. Furthermore, the results indicate that the majority of settlements originate from reloading the excavation with heavier material, but that the extraction of the sheet pile contributes to considerable settlements that should be taken into consideration. Lastly, this study contributes to an improved understanding of the additional settlements induced by sheet pile extraction in soft soils, although further and more extensive research is still considered necessary.