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The choice of the characteristic value of a material property is fundamental in all civil engineering design and critical in geotechnical engineering. Based on our tradition, we strive to look for a cautious value, whose evaluation includes our engineering judgement. While the definition of a characteristic value is often given with the aid of statistics as a fractile value, its determination includes considerations of mechanics because “value” refers to the relevant quantity affecting the limit state. Soils are complex, often spatially heterogeneous and our knowledge of them is typically based on sparse incomplete data. This makes the definition and determination of a characteristic value for soils more multifaceted than for structural materials. This paper discusses the choice of the characteristic value for soils from various perspectives (statistical, mechanistic and practical) and summarises some recent findings to clarify the limitations of existing practice. Overly simplified statistical approaches may lead to unsafe or overly conservative designs. The authors conclude that a balance between practicality and incorporating salient features are needed. By increasing the value of data in design decisions, engineers will be motivated to collect, share, and utilise data as much as possible and bring our practice closer to the digital economy. publishedVersion

The continuous cone penetration test (CPT) measurements provide an advantageous liable rapid tool for stratification and soil behavior classification that can be employed in the sustainable design of the infrastructures. However, the CPT measurements are often interpreted by geotechnical experts because of the involved complexities and uncertainties. In this study, a novel stratification and soil type behavior (SBT) classification model is developed to identify the transition and thicker layers by integrating the geotechnical knowledge with the three submodels of (a) locally estimated scatterplot smoothing (LOESS), (b) a game theory model known as Nash–Harsanyi (N–H) bargaining, and (c) grey wolf optimizer (GWO). The LOESS and integrated N–H bargaining-GWO models are, respectively, used to approximate the outliers in CPT measurements and identify the SBT and layer changes. Attractively, in the proposed model, the engineer has the opportunity to judge on the precision of the stratification profile regarding their own preferences in a project. Solving simple algebraic equations, high speed, identifying thick and the interlayer transition layers, and small required training data are the other advantages of the developed model. Finally, the applicability of the proposed model has been assessed in an example. The compared estimated and two other models’ stratification profiles highlighted the potential of the proposed model to identify thin transition layers.

This study concerns the safety factor and the reliability calculation for structural codes. The Eurocodes are used as a reference. Safety factor calculation is a demanding task which necessitates using an appropriate root-solving algorithm with a sufficient numerical accuracy. This article introduces a simple algorithm to calculate the safety factors directly, as previously there has been no means to control the accuracy. Presently, the safety factors are defined indirectly through the reliability index. The basic safety factor calculation is presented here in six different equations with the same outcome but differences regarding the numerical calculation, which provides a method to check the accuracy and select a proper equation for the root solver. The safety factor calculation for the permanent and the variable load in the Eurocodes is based on the independent, i.e., random, load combination and single load pairs. The current approach of safety factor calculation applied in the Eurocodes is disclosed here. Simple analytical equations based on the convolution equation are presented. Those can be used instead of the computer programs applied currently. publishedVersion

The determination of creep properties still relies almost entirely on the incrementally loaded oedometer test introduced about 100 years ago. Although the simplicity of the test assures a robust evaluation of soil parameters, it also introduces some shortcomings like discontinuity of the evaluated parameters and the long duration of the test. In this study, the performance of the variable rate of strain (VRS) oedometer test for determination of creep properties of a sensitive soft clay is studied. The results from a comprehensive test series of VRS oedometer tests on a soft sensitive clay is presented. Three different setups for the strain rate variation were used, and each test was repeated thrice. The tests showed good consistency and yielded equal creep parameters compared to values from traditional incrementally loaded (IL) oedometer tests. The tests further verified that it is possible to describe the complex stress dependency of creep parameters with just one additional parameter to the primary deformation parameters. Compared to the IL tests, the VRS tests offers a faster determination and continuous creep properties for a wide range of stress.

The determination of reliable geotechnical parameters from laboratory testing is highly dependent on sample quality. Over the past decades, undisturbed sampling of soft sensitive clays has been performed using various apparatuses and procedures. This paper outlines details of the design and performance of a new Laval type tube sampler employed for the investigation of five soft clay sites located in Finland. The investigation was conducted using the new tube sampler and two different piston samplers. The sample quality is evaluated based on the recompression volume during reconsolidation to the in situ effective stress in constant-rate-of-strain (CRS) oedometer tests. Test results show that tube samples are generally characterized by higher quality, especially in low plastic clays. In particular, the quality of piston samples is highly affected by the apparatus condition and sampling operations. Furthermore, the influence of storage time on tube samples is investigated. In order to guarantee a proper confinement, and thus reducing swelling, a pressurized system is applied to the tube samples obtained in two soft clay sites. Results demonstrate that the sample quality is not significantly affected by storage time as long as the soil is properly stored into the tube. Evaluation of sample quality from different sampling methods in Finnish soft sensitive clays

This paper presents the results of research aimed at studying the hydraulic and mechanical behavior of Finnish soft clays treated by quicklime. This research investigated the effect of water content and curing time on the characteristics of the compacted soil treated with 7% lime, with the aim of verifying the effectiveness of lime treatment and evaluating its possible re-use, thus avoiding landfill disposal. A laboratory-testing program was carried out both on treated and untreated soil, supported by microstructural investigation. Results have shown a general increase of the hydraulic conductivity due to the addition of lime, reduction of compressibility, and increase in the soil shear strength for a wide range of water contents (10%&ndash, 40%), proving the effectiveness of the lime treatment. Lime addition to the compacted clay at high water contents (90%&ndash, 130%) turned out to improve the mechanical characteristics to a lesser extent, while fractionated lime supply did not significantly improve the mechanical performance. The results of this research demonstrate that the re-use of sensitive clays, typically of high water contents, by lime addition require a drying process. Different drying procedures can be adopted (proposed in the paper) depending on the specific site conditions. However, the soil treatment is generally economically convenient in comparison to the disposal in waste landfills, which would represent the only alternative solution.

A well-established analytical model based on spherical cavity expansion and critical state soil mechanics theories is applied to piezocone soundings for profiling the yield stress and overconsolidation ratio of five soft sensitive test sites located in Finland. Yield stress is related to three piezocone parameters: net cone resistance, excess porewater pressure, and effective cone resistances. Input geoparameters include the effective stress friction angle, defined at both peak strength and at maximum obliquity, and the model directly provides the operational value of the undrained rigidity index. The piezocone-evaluated profiles compare favorably with results from laboratory constant-rate-of-strain consolidation tests for all the investigated sites. Based on the obtained experimental results, simplified correlations valid for Finnish soil conditions are derived. Their validity is assessed based on the bias factor, coefficient of variation, and coefficient of determination, showing a fairly good agreement between the predicted and the target values. acceptedVersion

The railway network on coastal areas of Finland is predominantly located in soft clay areas. The undrained shear strength of such clays is generally low, highly anisotropic, and rate dependent, and it exhibits post-peak strain softening under undrained conditions. A full-scale failure test was performed by Tampere University of Technology in Perniö, Western Finland, in 2009. A shallow railway embankment built on a soft clay deposit was equipped with a loading structure and loaded to failure in about 30 h. The embankment collapsed 2 h after the last loading step. In this study, data collected from the experiment are used, together with laboratory test results on high-quality samples, to conduct advanced finite element analysis of the Perniö failure test. The NGI-ADPSoft model is used for this purpose, which is capable of simulating the strain-softening behavior of the clay. Even though the observed rate effect is not taken into account in the analyses, the failure load can be predicted reasonably well. Good agreement is also observed for calculated displacements and failure mechanism with experimental observations.

The determination of a design soil property may include multiple sources of uncertainty. One of the sources originates from transformation model used to evaluate soil parameters when they are not measured directly. This study focuses on the transformation uncertainty related to three different transformation models used in evaluation of undrained shear strength from CPTu borings. The used correlation models are common models found in literature and calibrated at Tampere University. The CPTu data used in this study was taken from Knuuti and Länsivaara [2019. Variation of Measured CPTu Data. ISGSR], and it consisted of four different soft clay sites in Finland. The transformation uncertainty was calculated for each transformation model at each site. Moreover, every CPTu boring was analysed separately. The results showed that the transformation uncertainty was lowest for models based on the net cone resistance (COV = 0.033–0.084) and pore pressure (COV = 0.024–0.085). For the third model, the uncertainty was little higher as it included more uncertainty in the initial parameters. This suggests that the transformation models based on net cone resistanse (qNET) and pore pressure (u2) could be more suitable for practice. acceptedVersion

Understanding the behavior of soft sensitive clays is a key aspect of geotechnical design, especially in those regions where constructions are planned in marine clay areas. In Finland, one of the major geotechnical issues is the stability and deformation of the railway embankments located on soft soil deposits. The Tampere University of Technology (TUT) and the Finnish Transport Agency (FTA) have been carrying out a research project aiming to a better understanding of strength and deformation properties of Finnish soft clays. As a part of this project, a detailed site investigation was conducted at Perniö site in Western Finland. This paper presents the details of the geotechnical investigation, including field testing, sampling operations, and the laboratory testing program. In particular, an open drive tube sampler with 132 mm diameter, designed at TUT, was used to collect high quality samples. For comparison, samples were also taken using the conventional piston samplers and Sherbrooke Mini-block sampler. Field testing included piezocone, vane shear, and seismic piezocone. Laboratory testing consisted of index, consolidation, triaxial and simple shear tests. A comprehensive study of all these data is presented in a framework to describe Perniö soft clay. publishedVersion

The determination of the undrained shear strength of clays relies upon the goodness of the available in-situ and laboratory tests. Often, limited soil investigation data is available, or the collected data may suffer of low quality associated with poor test execution or sampling operations. The use of reliable correlations can then play an important role in geotechnical design. In that perspective, it is vital to choose the most appropriate correlation models that are suitable with the local soil conditions and that are possibly characterized by low uncertainty. The SHANSEP empirical model describes the undrained shear strength of clays in terms of normalized properties, where the soil strength is defined by means of the overconsolidation ratio (OCR) and two material coefficients that require empirical calibration. The SHANSEP model can be further combined with analytical solutions based on Critical State Soil Mechanics (CSSM) in order to define the undrained shear strength as a function of two fundamental properties, such as the preconsolidation stress (or the OCR) and the friction angle at critical state. The paper deals with the uncertainties associated with modelling the undrained shear strength of clays using a hybrid CSSM-SHANSEP model. The performance of the model is assessed by comparing the predicted undrained shear strength to an existing multivariate database of field vane data points from Finland. For each data point, the friction angle is estimated indirectly from the plasticity index using a correlation in the literature, while the OCR is taken directly from the database. Bias and uncertainties of the CSSM-SHANSEP model associated with the multivariate database are evaluated. Finally, a sensitivity study on the model parameters is presented.

The study focuses on the derivation of transformation models for undrained shear strength (su) of Finnish soft sensitive clays. Specific correlation equations for suof Finnish clays are presented in this work for the first time. Field and laboratory measurements from 24 test sites in Finland are exploited for this purpose and a multivariate database is constructed. The multivariate data consist of sufrom the field vane test, preconsolidation stress, vertical effective stress, liquid limit, plastic limit, natural water content, and sensitivity. The main objective is to evaluate the interdependence of su, consolidation stresses, and index parameters and provide a consistent framework for practical use. The new correlations are established through regression analyses. The constructed framework is further validated by another independent multivariate database of clays from Sweden and Norway as well as by empirical equations for Swedish and Norwegian clays. Existing correlations are evaluated for Finnish and Scandinavian clays. Finally, bias and uncertainties of the new correlations are presented.

The note presents the results of an experimental research study on the hydraulic behavior of Finnish soft clays treated by quicklime. The effect of a wide range of water contents and of curing time on hydraulic characteristics of the treated soil was studied. 7% of quicklime was identified as the optimum lime amount by means of initial consumption of lime tests and by evaluating the drying capability of different lime amounts. Soil-lime samples were prepared by wet mixing and Standard Proctor compaction. Hydraulic conductivity tests are carried out in flexible wall permeameters both on treated and untreated soil. A general increase in hydraulic conductivity due to the addition of lime is observed (2 order of magnitude at optimum conditions). For the treated specimens, a decrease of two orders of magnitude in permeability values was observed, increasing the water content of the soil. Reduction of permeability due to curing time is within 1 order of magnitude. Lime addition reduced water sensitivity and improves the draining capability of the soil.

It is well known that in undrained stability calculations, total stress and effective stress analyses do not give the same calculated factor of safety when FOS >1. This is due to the fact that shear strength is defined differently in these two approaches: In total stress analyses, the mobilised shear stress is compared to undrained shear strength, i.e. strength at failure. In undrained effective stress analyses, the shear strength is defined as corresponding to the mobilised effective stress state. This causes an overestimation of FOS in undrained φ′-c′ analyses. Modelling of excess pore pressure Δu has traditionally been source of most uncertainty in undrained effective stress analyses. Having the correct shear strength along the slip surface can be considered the most crucial detail in all stability analyses. It can be argued that in the context of Limit Equilibrium analyses where deformations are not considered, priority should be given to calculating the shear strength correctly, instead of attempting to obtain a “correct” mobilised Δu value. This paper gives a general introduction to the new HSU (Hybrid s u ) method. For the purposes of LEM analyses, Δu is calculated so that the resulting Mohr-Coulomb shear strength corresponds to the assumed failure state. This approach solves the inherent overestimation of FOS in undrained φ′-c′ analyses. To predict the effective stress at failure, a constitutive effective stress soil model is employed. Also presented is a concept of deriving undrained shear strength s u in LEM, based on an effective stress soil model. This makes it possible to conduct the LEM stability analysis in terms of total stresses, while deriving soil strength from effective strength parameters. The different approaches of calculating Δu and s u with the HSU method are compared using a theoretical stability calculation example. The relative merits of the different approaches are discussed.

The stress-strain response of sensitive clays tested in a laboratory setting can be significantly affected by disturbance effects caused by sampling, transport, storage and specimen preparation. Soil models for finite element analyses are commonly calibrated using the results from laboratory tests and, consequently, calibrated model parameters are likely to be affected by sample disturbance. For sensitive clays subjected to constant volume shearing, the stress-strain behavior is dependent on the direction of loading and, due to build-up of shear induced pore pressure, effective stresses will reduce with increasing strain in the post-peak regime. According to previous studies, peak strengths, strains at failure and post-peak behavior of sensitive clays are all significantly influenced by sample quality. Therefore, the relative quality of model predictions generated using a sensitive clay finite element model can also be expected to be notably affected by sample disturbance. In this study, the impact of sample disturbance on the determination of model input parameters for advanced finite element modelling of sensitive clays is addressed and critically discussed. Two advanced soil models are used for this purpose: the total stress based NGI-ADPSoft model, which is able to predict the anisotropic strain-softening behavior of saturated sensitive clays, and the effective stress based S-CLAY1S model, which is characterized by an anisotropic yield surface and is able to simulate soil destructuration. The practical implications of a thoughtful selection of the input parameters are evaluated through FE stability analyses of a sensitive clay slope.

In Finland, undrained shear strength is commonly measured using the field vane shear test (FV). Currently, the most commonly used field vane testers are the Nilcon vane and the electrical vane with shear rotation and measuring systems located above the ground level. Vane testing is normally carried out using vanes equipped with slip coupling, while the use of casing for protecting the vane is not very common. Recent studies from Finland have shown that the undrained shear strength of clays can be significantly underestimated when casing is not used. Experimental observations suggest that the slip coupling might not always be sufficient to remove all of the rod friction effects that occur during testing. Tampere University of Technology has recently purchased an innovative field vane apparatus with a vane tester unit, where torque and rotations are measured right above the vane. In this way, the effect of rod friction is minimized and the measured stressrotation behavior is less biased. In this study, issues related to practical applications, testing devices and interpretationmethods are discussed. Then, a critical comparison between test results in soft clays from both the traditional and new field vane testers is performed. acceptedVersion

Energy involved in disintegrating of sensitive clays from an intact to a fully remoulded state is one of the key aspects in assessing the post failure movements of sensitive clay landslides. This energy is referred to as remoulding energy. In this paper, the energy approach is conceptualised using an analytical approach. A comprehensive review of the empirical, laboratory and field techniques to estimate remoulding energy are presented and discussed in detail.

This paper investigates the feasibility of Least square support vector machine (LSSVM) model to cope the problem of implicit performance function during first order second moment (FOSM) method based slope reliability analysis. LSSVM is firmly based on the theory of statistical learning. In LSSVM, Vapnik’s e -insensitive loss function has been replaced by a cost function which corresponds to a form of ridge regression. Here, LSSVM has been used as a regression technique to approximate implicit performance functions. A slope example has been presented for illustrating the applicability of LSSVM based FOSM method. The developed LSSVM based FOSM has been compared with the artificial neural network (ANN) and least square method. The result shows that the approximation of LSSVM can be used in the FOSM method for slope reliability analysis.

Limit equilibrium methods, satisfying both force and moment equilibrium can be formulated using assumptions on the internal variables or the external variables. Even though most stability methods are based on force and moment equilibrium, as well as the Mohr–Coulomb yield criterion, there are great differences between the results of the different formulations due to variations in the assumptions. The authors believe that the use of the interslice force function f(x), the thrust line or the base normal forces should provide an equivalent concept at the ultimate/failure state. In the present study, the authors have used the well-known bearing capacity solutions to determine f(x), the thrust line and the base normal forces for a “horizontal slope”. The equivalence between the different formulations under the ultimate condition is demonstrated. It is shown that it is not important which forces are used in the stability formulation, external boundary forces or internal forces, if only that the ultimate state is considered. It is also demonstrated in the present paper that the maximum extremum from the limit equilibrium analysis is equivalent to the slip line solution using a classical bearing capacity problem.

This paper examines the potential of relevance vector machine (RVM) for slope reliability by using first order second moment method (FOSM). The FOSM demands the values and partial derivatives of the performance function with respect to the design random variables. Such calculations could be time-consuming or cumbersome when the performance functions are implicit. The analysis of slope by limit equilibrium method gives implicit performance functions. Here, RVM has been used to predict implicit performance functions. RVM rely on the Bayesian concept and utilize an inductive modelling procedure that allows incorporation of prior knowledge in the estimation process. In this paper, an example is given regarding how the proposed RVM-based FOSM analysis can be carried out. This study shows that the proposed RVM-based FOSM is viable alternative for slope reliability analysis.

An improved harmony search algorithm is proposed which is found to be more efficient than the original harmony search algorithm for slope stability analysis. The effectiveness of the proposed algorithm is examined by considering several published cases. The improved harmony search method is applied to slope stability problems with five types of procedure for generating trial slip surfaces. It is demonstrated that the improved harmony search algorithm is efficient and effective for the minimization of factors of safety for various difficult problems, and the method of generating the trial failure surfaces can be important in the minimization process.

“Failure to converge” during slope stability analysis for complicated slopes with external surcharge or soil nails using the “rigorous” methods are experienced by many engineers. The two main reasons for failure to converge are: (1) the fulfillment of the assumptions for a specific method of analysis; (2) the limitation of the numerical method in evaluating the factor of safety. It will be demonstrated that there are problems where convergence for the “rigorous” methods is not achieved by the conventional iteration analysis, but physically acceptable results may exist for some of these “non-converged” problems, if the failure to converge is due to the second reason as given above. With reference to some of the designs by the engineers in Hong Kong, the authors will also demonstrated that the minimum factors of safety obtained by the current design practice may be over-estimated due to the convergence problem in the stability analysis.

In this paper, the factors of safety and the locations of critical failure surfaces obtained by the limit equilibrium method and strength reduction method are compared for various slopes. For simple homogenous soil slopes, it is found that the results from these two methods are generally in good agreement except when ϕ′ is zero. It is also found that the strength reduction method (SRM) is usually not sensitive to the dilation angle, soil moduli or the solution domain size and the choice of these parameters is not critical in the analysis. While the SRM may perform well in many cases, two major limitations of this method are found in this study: it is sensitive to the nonlinear solution algorithm for the case of a soft band with frictional material and it is incapable of determining other failure surfaces which may be only slightly less critical than the SRM solution but still require treatment for good engineering practice.

A full-scale embankment failure experiment was conducted in 2009 in Perniö, Finland. A small, extensively instrumented railway embankment on a soft clay foundation was brought to failure by loading over a period of 30 h. Instrumentation consisted of over 300 different measurement points, including 37 piezometers and nine automatically monitored inclinometer tubes. The relatively rapid loading simulated a heavy train coming to a standstill on the embankment. The primary purpose of the experiment was to gather field data of a failure caused by a rapidly applied load, with an emphasis on the pore pressure response in the clay foundation layer. The test was also used to assess the suitability of various instruments for real-time stability monitoring. The embankment failure was an asymmetric bearing capacity mechanism that is hypothesised to have been triggered by an undrained creep rupture.During the last 2 h of the experiment, pore pressure and displacements increased at an accelerating rate while the external load was kept constant. The time-dependency of the pore pressure and displacement responses was a key factor in the experiment. With regards to monitoring of similar in-service train embankments, proper placement of instruments according to predicted failure mechanisms was found to be important. publishedVersion

This article discusses undrained effective stress stability analyses on normally consolidated soft clay. A key issue in such analyses is that the yield induced pore pressure is properly accounted for. Yielding of natural clays is affected amongst other factors on the shape and size of an anisotropic yield surface and the deformation properties of the clay, including creep. These then have a large effect also on the undrained capacity of the soil. The implementation of yield induced pore pressure into stability analyses conducted by both limit equilibrium method (LEM) and finite element method (FEM) is presented. Calculation examples are given for a full scale embankment failure experiment using various methods to include the yield induced pore pressure. It is shown that it is possible to account for such effects even in simple LEM analysis to obtain a realistic failure load, while in advanced FEM analysis it is possible to also account for time effects.

The use of partial safety factors has, with the adaptation of the Eurocodes, become the dominant safety concept, also for slope stability in most European countries. The article discusses the partial safety factor approach based on considerations of uncertainties involved and consequences of failure. Partial safety factors are calculated based on reliability theory, following first the present ideas of the Eurocodes, and second an alternative approach aimed to improve the shortcomings in the code. In the alternative approach, all uncertainties are placed on the material safety factor, which are then calculated for different target reliability index values. Two simple examples are presented, where the differences of the two approaches are outlined.

The paper presents an elastoplastic model developed for the overconsolidated region of clays. In the model, the stiffness of the soil is related to its previous history introducing memory points. The stiffness is thus depending on the size and the direction of the new stress/strain increment compared to previous stress/strain states. The plastic strains are constructed from three components. The first plasticity mode is for pure volumetric behavior, the second one for pure shear behavior, and the third one is a Coulombian plasticity term which includes coupling effects. The performance of the model is discussed in relation to laboratory tests. The model has been found to comprise several essential characteristics of low strain level soil behavior, like stress history and direction dependency of stiffness and strain accumulation in cyclic loading within a relatively simple framework.