Solifluction Research Articles

The dry closure of the Almagrera tailings dam: detailed modelling, monitoring results and environmental aspects

The Aznalcollar tailings dam failure moved the Spanish authorities to pay attention to tailings deposits. The Almagrera tailings dam holds one of the largest mining waste deposits in Andalucia. The dry closure of this dam has been detailed in this manuscript. Some serious difficulties had to be solved. Firstly, the dam had undergone up to five raisings before the closure operations started, and this process had not been properly documented. Secondly, the reservoir water was contaminated by the toxic tailings placed several metres below and, due to the high acidity of this water, the geotechnical characterization of the tailings deposit has been really challenging. Thirdly, the definition of the model itself has been a complex task due to the consideration of many phases and different hypotheses. In the finite element calculation, a constitutive model of perfect non-associated plasticity has been used for the dam and a soft soil creep model for the tailings. Next, it has been decided to decontaminate a closed mine by placing its abandoned material—Las Vinas fill—on top of the tailings deposit inside the reservoir. This operation generated important settlements on the tailings deposit. These settlements had to be accelerated by placing drainage wells to avoid the cracking of the final cap. The safety factor during the dry closure operations under dynamic loading was insufficient and a compacted rockfill reinforcement had to be laid on the downstream slope of the dam. Very few papers describe a successful dry closure of a tailings dam as is done here.

The settlement speed of the finite width foundation caused by the extrusion of weak soil layer from compressible base

The article presents the formulation and solution of quantifying the speed of the finite width foundation settlement due to viscoplastic or elastic-plastic extrusion of the weak soil layer from compressible base by analytical and numerical methods. During numerical simulations were used the finite element method with Mohr-Coulomb model (MC) and the model of Soft Soil Creep (SSC). According to the results of analytical solution were obtained the formulas to determine an initial ultimate load and a value and the foundation settlement speed.

Matric Suction Creep Characteristics of Reticulated Red Clay

Matric suction creep is the creep of unsaturated soil caused by loading constant matric suction. Matric suction creep characteristics of reticulated red clay with constant net confing pressure and step-loading matric suction was studied using stress-controlling unsaturated triaxial test apparatus. Test and analysis results show that: (1) with constant net confining pressure, axial and radial strain of the clay caused by creep due to matric suction increases obviously; (2) the increments of axial strain of the clay, caused by creep due to matric suction under constant net confining pressure, is not the same as that of radial strain, and the absolute value of radial strain is bigger than that of axial strain; (3) creep strain produced by the same amount of matric suction decreases as the increasing of the matric suction during step loading creep, which means that matric suction makes the clay hard; and (4) the creep strain-time curves of both axial and radial creep can be simulated very well by exponential decay function, and the parameters of the function are matric suction depend obviously.

Monitoring soil creep landsliding in an urban area using persistent scatterer interferometry (El Papiol, Catalonia, Spain)

Soil creep is characterised by slow displacement, with depths of a few meters and loosely defined limits. Buildings and infrastructure located on slopes affected by such landslides may suffer significant damages if their foundations are poorly dimensioned. The presence of soil creep in urban areas makes it necessary to develop landslide activity maps, derive hazard maps, and implement risk management plans. Even though both geological and geomorphological analyses can provide basic information, it is often necessary to use additional techniques to obtain information about ground displacements. This paper proposes a method to derive a soil creep activity map using a multi-approach analysis based on geological, geomorphological, and persistent scatterer interferometry (PSI) data. PSI is a satellite-based technique to estimate land displacement velocity. The work described in this paper was carried out in the town of El Papiol, in the metropolitan area of Barcelona (Spain). This is an urban area that has been heavily affected by soil creep over the past decades. The results achieved show that PSI data substantially improve the information provided by the geological and geomorphological analyses and make it possible to accurately define the landslide area and estimate its activity.

A combined method to predict the long-term settlements of roads on soft soil under cyclic traffic loadings

In order to investigate the mechanism of the long-term settlement of road on the soft soil, an in situ measurement of a highway in southeast China was conducted during both the construction and post-construction stage. In the theoretical analysis, the long-term settlement of the pavement on thick soft soils was divided into three main components: the consolidation settlement, the creep deformation and the traffic load-induced deformation. The equivalent time line model based on the viscous–elastic–plastic theory was adopted to simulate the road settlement caused by the consolidation and creep of the subsoil. A cyclic strain accumulation model, which was obtained from laboratory tests of soil elements, was adopted to consider the road settlement induced by cyclic traffic loadings. A pavement dynamic response model was used to calculate the dynamic stresses in the subsoil generated by moving traffic loads when the road was open to service. It was found that the equivalent time line model combined with the cyclic strain accumulation model could predict the road settlement accurately at both the construction and post-construction stage. Numerical results showed that the traffic speed had limited effect on the post-construction road settlements for the speed range considered. The post-construction settlement of the road, mainly composed of the traffic-induced settlement and soil creep deformation, could be reduced significantly by increasing the embankment surcharge during the construction stage. The creep component accounted for over 10% of the total post-construction settlement, while the percentage of the creep deformation in total post-construction settlement decreased rapidly as the embankment surcharge increased.

Features of optimization of model parameters for solving geotechnical problems

At present, numerical methods of calculations, which are implemented in a large number of software complexes, are widely used in geotechnical practice and the definition of input parameters of the ground is very important and necessary to reflect the real work of the foundation of geotechnical structures [1-4]. There are often cases when the results obtained during laboratory tests of soils are not accepted by software complexes, errors are given, recommendations are proposed for changing the parameters in the direction of increasing or decreasing. In connection with these problems, the question arose about the need to optimize soil parameters obtained as a result of laboratory tests to compare and correct these parameters, based on the degree of approximation of model tests with laboratory tests [5]. Optimization of soil parameters can be carried out in the subroutine Soil test, incorporated in the PLAXIS geotechnical software [6]. Using the Soil test, the triaxial and compression tests are simulated based on the input parameters of the soil and the initial test data. The purpose of this study was to describe the methodology for optimizing the parameters of the Hardening Soil model and the Soft Soil Creep model using the PLAXIS 3D software geotechnical complex, as well as a comparative analysis of the results of laboratory soil tests with modeling results in software complex.

Embankment prediction and back analysis by means of 2D and 3D finite element analyses

The paper summarises key results of a class A prediction obtained from 2D and 3D finite element analyses performed for the Embankment Prediction Symposium 2016. In addition class C predictions are included and the changes in input parameters for the finite element model are discussed. Although different constitutive models have been employed in the class A prediction where short term displacements, i.e. after embankment construction, have been considered in addition to long term consolidation and creep settlements only long term displacements are dealt with in this paper. Thus, only results obtained with the Soft Soil Creep model implemented in the finite element code Plaxis are presented. Input parameters have been chosen based on laboratory and in situ experiments and verified by back calculation of undrained triaxial tests and CRS tests. 3D FEA have been carried out confirming that a 2D representation is sufficient for this problem.

Class A and Class C numerical predictions of the deformation of an embankment on soft ground

The performance of a trial embankment on soft ground treated with wick drains has been predicted, in terms of total surface settlement, lateral movement, and dissipation of excess pore pressure. The settlement performance was predicted on the basis of hand calculations, carried out in a spreadsheet, using the classical radial consolidation equations of Barron and Hansbo. This was followed up with numerical analysis using 2-D finite element code in PLAXIS 2D2016, employing their soft soil creep model. Analyses were carried out on a single cell, for comparison with the hand calculations, using both axisymmetric and plane strain models. An analysis was also carried out in plane strain for a section through the whole embankment. This was used to predict the lateral movements and also the dissipation of excess pore pressures at selected points, for comparison with measured data from instruments.After the results of monitoring the trial embankment were published, the numerical model has been modified and shows excellent agreement with the measured data. A description of the modelling is provided, as well as details showing the limited way in which the model has been modified.

Numerical prediction and back-calculation of time-dependent behaviour of Ballina test embankment

The paper describes the Class A prediction and Class C back-calculations of the Ballina test embankment using the finite element program Plaxis and the Soft Soil Creep Model (SSCM). The prediction underestimated the measured settlement 3years after construction by about 20%. This was mainly due to too high stiffness in the transition zone beneath the clay and that SSCM underestimated the shear deformation of the clay. Furthermore, the horizontal permeability of the clay was overestimated. In the back-calculation, it was possible to obtain a excellent match with the measured settlements by reasonable modifications of the input parameters.

Predicted and measured behaviour of a test embankment on Ballina clay

A test embankment was established on the soft Ballina clay and practising engineers and academics were invited to make predictions of the behaviour of the embankment. Comprehensive geotechnical investigations were performed and the testing data were provided to the predictors. This paper presents Class A and Class C predictions made by the authors for the embankment. In Class A prediction, the soft clay was modelled as a single homogeneous layer. The Class A prediction showed good agreement with measurements for settlement, but not for pore pressure and lateral ground movement. Class C prediction divided the soft clay into two layers based on the information of ground investigation, and achieved better predictions for settlement, pore pressure and lateral movement. Finite element analysis using Plaxis 2D was carried out for the prediction, and Soft Soil Creep model was used for the soft clay. Consolidation parameters were assessed by statistical analysis of the oedometer test data. Sensitivity studies were performed to investigate the influence of parameters on the analysis results. Results of the predictions and sensitivity studies are discussed in the paper.

Prediction of embankment performance using numerical analyses – Practitioner’s approach

A 3m high embankment with prefabricated vertical drains was constructed over Ballina clay. It has been thoroughly instrumented for monitoring over three years after construction. Based on the data available at the site, the authors undertook Class A predictions of embankment performance using two approaches: a simple 1D consolidation analysis and a sophisticated large strain finite element analysis (FEA) using Soft Soil Creep (SSC) model. Class C predictions were then conducted using the SSC model in FEA, with and without large strain. It is demonstrated that the SSC model can give satisfactory results when large strain FEA was used.

Creep behavior and the microstructural evolution of loess-like soil from Xi'an area, China

Creep behavior of loess-like soil is closely related to microstructural changes. The microstructure of loess-like soil collected from the Xi'an area, China, was investigated using triaxial creep tests. Qualitative microstructural observations were made by examining vertical and transverse sections from the undisturbed and tested samples with a polarizing microscope. Quantitative data were acquired by image analysis of SEM images of those sections. The image analysis documented changes in particles and pore structure under different axial stress and confining pressures. The results show that: (1) Undisturbed samples have plank-macrospore cline-cementation structure with the particles distributed randomly. The number and area of four kinds of pores decrease in the decay creep stage. The orientation of particle long-axis gradually aligns (weakly) in the steady creep stage. The samples have a lightly cemented mosaic microporous structure and plastic minerals like illite significantly aligned in a preferred orientation, and rigid quartz and feldspar grains become slightly arranged along the illite preferred direction in accelerated creep stage. The orientation of particles and pores interconnections eventually result in the creep failure of sample. (2) The prefer orientations of pores are most significant in accelerated creep stage, and the pores have insignificant preferred orientation in creep failure stage. (3) Mesopores cover the largest area, and they are the most important factors leading to loess-like soil creep. Undisturbed samples contain the largest number of pores which cover the largest area among all of the samples. As creep progresses, both number and area decrease gradually before accelerated creep stage, and finally the pore area has its largest in creep failure stage. (4) The pores in these samples are relatively oblate in shape with rough outlines in undisturbed samples, and the greater the deviatory stress is, the slender the pores with smoother outlines will be.

Exploration of the creep properties of undisturbed shear zone soil of the Huangtupo landslide

The time-dependent soil creep behavior affects the strength and deformation of the shear zone in landslides. Limited studies have been carried out on the undisturbed soil samples from the deep shear zone due to the difficulties in obtaining them. This problem is investigated using a case study of the well-known Huangtupo landslide, which is the largest landslide in the Three Gorges Reservoir area of China. The creep properties of the undisturbed shear zone were studied using triaxial creep testing. Two creep stages (the primary stage and the attenuation secondary creep stage) were observed, which were characterized by a creep rate that approached zero, instantaneous deformation and the development with a constant speed. The long-term shear strength of the shear zone was calculated based on the test results. The creep stages and long-term shear strength are significantly different from those of direct creep tests of reconstituted shear zones due to the influence of the confining pressure and coarse particles. The Computerized Tomography (CT) scanning technique was also applied, and the results showed clear rotation of some coarse particles in the middle parts of the samples, which was reflected by their shapes. The changes in particle position were more significant at the bottom of the sample. Based on these results, an empirical constitutive model was proposed to describe the relations among stress, strain and time, and this model fits the experimental data well. The inhomogeneity of the undisturbed soil samples leads to different creep properties from those in ideal theoretical models; therefore, new stochastic constitutive models should be explored and developed for further studies.

The influence of climate and microclimate (aspect) on soil creep efficiency: Cinder cone morphology and evolution along the eastern Mediterranean Golan Heights

AbstractAlthough hillslope evolution has been subject to much investigation for more than a century, the effect of climate on the morphology of soil‐mantled hillslopes remains poorly constrained. In this study, we perform numerical simulations of volcanic cinder cones in the Golan Heights (eastern Mediterranean) to estimate soil transport efficiency across a significant north–south gradient in mean annual precipitation (1100 to 500 mm). We use the initial cinder cone morphology (constrained by stratigraphy), the modern hillslope form (surveyed with sub‐meter accuracy) and the eruption age (based on 40Ar–39Ar chronology) to predict the best‐fit value of the soil transport coefficient (‘diffusivity’) based on a nonlinear transport model. Our results indicate that the best‐fit diffusivity (K) varies from 1 to 6 m2 ka−1 among the five cinder cones in our field area. Diffusivity (K) values vary systematically with precipitation and hillslope aspect; specifically, K is higher on south‐facing (drier) hillslopes and decreases with mean annual precipitation. We interpret this climate dependency to reflect vegetation‐driven variations in apparent soil cohesion, which increases with root network density, and attenuation of rain splash and overland flow erosion, which increases with vegetative ground cover. To assess how vegetative root mass and ground cover vary with precipitation and aspect, we quantified the spatial distribution of NDVI (normalized difference vegetation index) from ASTER satellite images and observed spatial variations that correlate with our calibrated values of K. Analysis of previously studied cinder cones in the USA can be used to extend our framework to arid domains. This endeavor suggests a humped relationship between K and precipitation with maximum diffusivity at mean annual precipitation of 400–600 mm. Copyright © 2017 John Wiley & Sons, Ltd.

Numerical modelling of the effect of curing time on the creep behaviour of a chemically stabilised soft soil

This work studies the effect of the curing time on the creep behaviour of a stabilised soft soil, using volumetric and deviatoric creep laws associated with constitutive models. Results of unconfined compressive strength tests for several curing times are used to define the time evolution of the mechanical and creep properties. The models/creep laws are validated by oedometer and triaxial creep tests, for 28 and 90days of curing. The long-term behaviour of an embankment built on a soft soil reinforced with deep mixing columns shows that the effect of curing time decreases the settlement and increases the improvement factor.

Preliminary Investigations of Creep Strain of Neogene Clay from Warsaw in Drained Triaxial Tests Assisted by Computed Microtomography

Abstract The study concerns soil creep deformation in multistage triaxial stress tests under drained conditions. High resolution X-ray computed microtomography (XμCT) was involved in structure recognition before and after triaxial tests. Undisturbed Neogene clay samples, which are widespread in central Poland, were used in this study. XμCT was used to identify representative sample series and informed the detection and rejection of unreliable ones. Maximum deviatoric stress for in situ stress confining condition was equal 95.1 kPa. This result helped in the design of further multistage investigations. The study identified the rheological strain course, which can be broken down into three characterizations: decreasing creep strain rate, transitional constant creep velocity, and accelerating creep deformation. The study found that due to multistage creep loading, the samples were strengthened. Furthermore, there is a visibly “brittle” character of failure, which may be the consequence of the microstructure transformation as a function of time as well as collapse of voids. Due to the glacial tectonic history of the analyzed samples, the reactivation of microcracks might also serve as an explanation. The number of the various sizes of shear planes after failure is confirmed by XμCT overexposure.

On the dynamic smoothing of mountains

AbstractAfter their formation, mountainous landscapes gradually evolve toward smoother geometries controlled by the interplay of erosion and sedimentation. The statistical mechanical properties of this process and the link between topography and geology have remained largely unexplored. We analyze the slope statistics of different mountains worldwide, showing that landscape age is fingerprinted in their distribution tails. Data reveal a universal relaxation process, through an algebraic decay progressively replaced by an exponential one, with exponents described by a global monotonic function. We then investigate the dominant components of this dynamic smoothing using a landscape evolution model, showing that the time evolution of slope statistics results from a delicate balance between diffusive soil creep, noise, and advective river incision, with the relaxation phase mainly dominated by diffusion. Results may suggest ways to formulate reduced order topographic evolution models for geomorphological and climatological applications, and to explore similarities in surface evolution in different contexts.

Contemporary overview of soil creep phenomenon

AbstractSoil creep deformation refers to phenomena which take place in many areas and research in this field of science is rich and constantly developing. The article presents an analysis of the literature on soil creep phenomena. In light of the complexity of the issues involved and the wide variety of perspectives taken, this attempt at systematization seeks to provide a reliable review of current theories and practical approaches concerning creep deformation. The paper deals with subjects such as definition of creep, creep genesis, basic description of soil creep dynamics deformation, estimation of creep capabilities, various fields of creep occurrence, and an introduction to creep modeling. Furthermore, based on this analysis, a new direction for research is proposed.

A frozen soil creep model with strength attenuation

A frozen soil creep model with shear strength attenuation was proposed based on a soft soil creep model. K 0 compression and triaxial shear tests were conducted to obtain the model parameters. With triaxial creep tests, the performance of soft soil creep model was verified at first. It was shown that different creep stages can only be described separately, which was due to a constant failure line used in the model. After incorporating strength attenuation into the original model, relative position between stress point and failure line is time dependent, and the progressive development of creep strain from primary to tertiary stage can be captured reasonably.

Debris flows, soil creep and heavy rains in the Wadi Queilbeh, northern Jordan

Debris flows, soil creep and heavy rains in the Wadi Queilbeh, northern Jordan