From soft soil modelling to engineering application

Soft soil engineering is still a challenge, in particular when we increasingly need to construct on densely populated urban areas next to existing structures, and need to deal with the effects of climate change. The paper discusses the systematic research by the author and her co-workers in the last 20 years that has resulted in the development and validation of advanced soil models, specifically geared for Scandinavian soft soil conditions. The culmination of this research is Creep-SCLAY1S, a rate dependent anisotropic model, which is backed by hierarchical development and systematic validation. Some recent examples of engineering applications where the model was used are are highlighted, in addition to the parameter determination and calibration

Volume Averaging Technique in numerical modelling of floating deep mixed columns in soft soils

The deformation behaviour of deep mixed columns in anisotropic soft soil is a threedimensional problem which has to be considered adequately in numerical modelling. However, in literature simplifications are often to be found due to computational costs of fully coupled 3D analyses by either modifying geometry of the problem or material parameters in 2D-plainstrain or -axisymmetric conditions. This paper uses an enhanced 2D volume averaging technique for numerical modelling of deep mixed columns. The method enables mapping the 3D problem in two dimensions, and yet modelling the two constituents (column and soft soil) appropriately within a homogenized material. The performance of the technique is demonstrated by considering unit cell simulations of floating columns in soft soil, in which the results from 2D finite element simulations utilizing volume averaging technique are compared against conventional and fully 3D coupled finite element analyses. It is shown that the quality of the match is very good but depending on the number of columns and stiffness ratio between columns and soft soil

BEST SOIL: Soft soil modelling and parameter determination

The report aims to give advice on parameter derivation for standard and advanced constitutive (soil) models, with focus on soft soil models. The soil models concerned include several strain-hardening models that are commonly used by geotechnical practitioners, installed in the Plaxis finite element (FE) suite, such as the Soft Soil model and the Hardening Soil model. These are referred to as the standard models. In addition, an advanced creep model developed at Chalmers, soon available for practicing engineers, is considered. Firstly, key features of the models are introduced, highlighting the main differences of the models. This is followed by recommendations for testing needed for reliable model parameter determination. It is highlighted that whilst for some of the models the determination of model parameters can be done easily based on typical Swedish site investigation and lab testing, for some models, this is not the case. Finally, advice on laboratory testing programme when intending to use geotechnical FE analyses is done

Permanent Sheet Pile Wall in Soft Sensitive Clay

Reports on the serviceability stage and long-term performance of embedded retaining structures are scarce, even more so in areas with background settlements. This paper presents a case study of an excavation with a permanent sheet pile wall in soft sensitive clay in Uppsala, Sweden. The monitoring data span the short-term and four years of the serviceability stage. The monitoring data are compared with finite element analyses using the rate-dependent Creep-SClay1S model. A historic groundwater drawdown is accounted for in the modeling and challenges with respect to the proper initialization of the in situ stress state, preconsolidation pressure, fabric, and bonding are discussed. The results show that even though the background settlements are somewhat underpredicted, the horizontal displacements as well as anchor forces were captured rather accurately. This valuable case study of the short- and long-term behavior of a permanent sheet pile wall in soft sensitive clay confirms the performance of using this cost-effective solution for a permanent construction. Furthermore, the paper highlights the challenges of an accurate description and modeling of the initial in situ state

Modelling of undrained shearing of soft natural clays

stress-strain response of soft natural clays is characterised by anisotropy, destructuration and rate-dependency. An accurate constitutive description of these materials should take into consideration all of the characteristics above. In this paper, two constitutive models for soft soils, namely the SCLAY1S and Creep-SCLAY1S models are used to simulate the undrained response of two soft natural clays, Gothenburg clay from Sweden and Otaniemi clay from Finland. The SCLAY1S model accounts for the effect of inherent and induced anisotropy and destructuration, while the Creep-SCLAY1S accounts also for the creep and rate effects. The model simulations are compared against triaxial compression and extension tests on anisotropically consolidated samples. The results demonstrate the need to incorporate all features represented in the Creep-SCLAY1S model when modelling structured natural clays

Numerical analyses of stone column installation in Bothkennar clay

The paper presents the results of numerical simulations studying the installation effects of stone columns in a natural soft clay. Stone column installation is modelled as an undrained expansion of a cylindrical cavity, using the finite element code PLAXIS that allows for large displacements. The properties of the soft clay correspond to Bothkennar clay, a soft Carse clay from Scotland (UK). The complexity of this material is simulated via two advanced recently developed constitutive formulations able to account for the soil structure, namely S-CLAY1 and S-CLAY1S. Modified Cam Clay model is also used for comparison purposes. The paper shows the new stress field and state parameters after column installation and the subsequent consolidation process. This sets the basis for including installation effects in studying the settlement reduction caused by stone columns

Influence of stone column installation on settlement reduction

The paper presents numerical simulations investigating the settlement reduction caused by stone columns in a natural soft clay. The focus is on the influence of the soft soil alteration caused by column installation. A uniform mesh of end-bearing columns under a distributed load was considered. Therefore, the columns were modelled using the ‘‘unit cell’’ concept, i.e. only one column and the corresponding surrounding soil in axial symmetry. The properties of the soft clay correspond to Bothkennar clay, which is modelled using S-CLAY1 and S-CLAY1S, which are Cam clay type models that account for anisotropy and destructuration. The Modified Cam clay model is also used for comparison. Column installation was modelled independently to avoid mesh distortions, and soft soil alteration was directly considered in the initial input values. The results show that the changes in the stress field, such as the increase of radial stresses and mean stresses and the loss of overconsolidation, are beneficial for high loads and closely spaced columns but, on the contrary, may be negative for low loads, widely spaced columns and overconsolidated soils. Moreover, whilst the rotation of the soil fabric reduces the settlement, in contrast the soil destructuration during column installation reduces the improvement

Viscoplastic cyclic degradation model for soft natural soils

Cyclic loading affects the long-term response of geostructures build on natural soils. Soft soils are particularly susceptible to the development of large deformations, induced by the repetitive nature of loading. A new viscoplastic cyclic accumulation model is presented, which is an hierarchical extension of the Creep-SClay1S model, to model the long-term permanent deformation resulting from undrained cyclic loading of natural soft clays. The cyclic cumulative strains are incorporated by means of an additional viscoplastic multiplier. This cyclic viscoplastic multiplier adds four additional model parameters that are derived from undrained cyclic triaxial tests. The model is calibrated using experimental data from undrained cyclic triaxial tests performed on high quality block samples of natural Ons\uf8y clay, at different average shear stresses, shear stress amplitudes and loading periods. The accuracy of the proposed model is demonstrated by comparing the element level simulations with the experimental data. The applicability of the proposed model is further illustrated with a boundary value problem, where an embankment submitted to cyclic loading is simulated. The use of the new model enables the simulation of the response of cyclic loaded foundations on soft soils, where the serviceability limit state over a long period of time is governing the design