Soil Mechanics Principles Research Articles

The role of unsaturated soil mechanics in unconventional tailings deposition

Desiccation (water loss by drying or freeze-thaw sufficient to generate matric suction), can influence the performance of a tailings deposit both positively and negatively. The significance of desiccation is largest in tailings that have been dewatered prior to deposition, by thickening or filtration. Such tailings can be “stacked” or deposited with a significant slope, which usually implies that a substantial volume of tailings remain above water. Under such conditions the tailings, by accident or by design, may undergo desiccation before burial by fresh tailings. Desiccation can contribute substantially to strength, above and beyond the contribution arising from increase in density, through stress history effects. For some deposits, it is required practice that at least some tailings undergo desiccation to improve, particularly when those tailings for a structural part of a deposit. If, however, tailings remain exposed to the atmosphere in an unsaturated state for some period of time, this may have potential negative consequences through oxidation of sulphide minerals and the formation of acid drainage. This paper describes previous research on the strength gained through desiccation in tailings, and on modelling work that incorporates unsaturated soil phenomena into consolidation analysis. Both types of research are applied to a real field site, providing an example of how novel improvements to tailings management can arise out of application of principles of unsaturated soil mechanics.

Evaluation of soil damage degree under freeze–thaw cycles through electrical measurements

In permafrost regions, roads and soil slopes experience freeze–thaw cycles annually and the soil characteristics (such as strength and conductivity) change irreversibly. Several studies have been conducted on the evaluation of soil damage using soil mechanics principles and electrical measurements. Owing to the limitations of previous studies, such as low efficiency and the inapplicability of real-time testing for assessing damages, a real-time quantitative test method for assessing the damage degree of layered soil subjected to freeze–thaw cycles based on electrical measurements is proposed. An electrical measurement device was developed for testing the electrical parameters of multilayer soil. Combined with electrical measurements, freeze–thaw cycle tests were performed. The resistivity of each layer of soil under freeze–thaw cycles was determined. Additionally, microstructural parameters of the multilayered soil were determined through computed tomography to investigate the changes in the soil microstructure during freeze–thaw cycles. The porosity and pore distribution characteristics of the multilayered soil under freeze–thaw cycles were obtained using image processing methods, such as median filter and K-means clustering. As the number of freeze–thaw cycles increased, the soil porosity increased, and the pore structure became less complex. Based on the relationship between the soil resistivity and the microstructural parameters, the soil damage mechanism was described, and a damage factor was introduced in developing the model. Furthermore, the dynamic damage process of the multilayered soil under freeze–thaw cycles was evaluated using the damage model, and the settlement was used to verify the results. And the damage model can be applied to evaluate the damage state of soil in permafrost region

Mechanical behavior of a floating model pile in unsaturated expansive soil associated with water infiltration: Laboratory investigations and numerical simulations

Pile foundations are widely designed extending principles of saturated soil mechanics assuming drained conditions (i.e. effective stress) even in expansive soil deposits despite the fact the soil around the pile is in a state of unsaturated condition. The mechanical behavior of in-situ pile foundations are significantly influenced by suction changes associated with water infiltration. In this paper, a single model pile mechanical behavior variations associated with suction changes in an unsaturated expansive soil under a service load were investigated. Experimental results suggest pile settlement increases due to water infiltration. Such a phenomenon can be attributed to pile shaft friction reduction in the active zone associated with suction reduction due to water infiltration and volume expansion of the soil. Experimental results were also simulated and interpreted using a computation program, PEI (i.e. Pile behavior in Expansive soil upon Infiltration), which was proposed based on principles of unsaturated soil mechanics. A theoretical model was proposed for the estimation of threshold pile head load which distinguishes whether the pile would experience upward or downward movement upon water infiltration. The theoretical model and the numerical modeling approach presented in this are valuable for practicing engineers in the rational design of piles taking account of the mechanical behavior of expansive soils.

Assessing the Extent of Pavement Deterioration Caused by Subgrade Volumetric Movement Through Moisture Infiltration

Pavements across Free State in South Africa often experience seasonal moisture variations that trigger, volumetric movement of the subgrade soil. The interaction of moisture-suction relationship and associated cyclic loads also contribute to pavement failure across this region. The current study investigated the failure cause of existing pavement structures constructed on expansive subgrades in the Free State province of South Africa. Material samples were collected from three representative sites by core drilling. Series of consolidated undrained triaxial tests (CU), free swell index (FSI) tests, zero swelling tests (ZST), filter paper tests, and unsaturated California bearing ratio ( $${\mathrm{CBR}}_{\mathrm{u}}$$ ) tests were performed to investigate causes of pavement failure through the principles of unsaturated soil mechanics. The results revealed that the investigated pavements deteriorate by swelling of the subgrades. The findings also evaluated that traffic loads significantly affected pavement structure as resistance failure to moisture were triggered from the asphalt layer. The asphalt thickness layer failed to provide sufficient surcharge pressure to confine swelling stress from the subgrades. The result further revealed that the $${\mathrm{CBR}}_{\mathrm{u}}$$ and matric suction decreased by 52.33% on average, with an increase in swelling stress as moisture content increases. The study suggested that seasonal moisture variations associated with excessive traffic load could be the main cause of pavement failure, as its pavement was designed for low to medium volume road users. In comparison, swelling stress values from the dry side were 8.63% higher than that of the swelling stress values obtained from the wet-side. The result further revealed that swelling stress was the major cause of the pavements' failure. Thus, as swelling stress increased with an increase in moisture; this consequently led to a considerable decrease in shear resistance and bearing strength capacity of the subgrades.

Regional stability and adaptation measures slope failures due to rainfall in Singapore

As heavier rainfall intensities are expected in the future, it is important to employ preventive measures for the anticipated possibility of residual soil slope failures. This study primarily focuses on the principles of unsaturated soil mechanics to establish an appropriate framework for adaptation measures against extreme rainfall conditions. These adaptation measures consist of the development and evaluation of slope susceptibility maps, as well as the planting of vegetation for critical slopes based on regional stability analyses of residual soils within Bukit Timah Granite and Old Alluvium in Singapore. In this study, slope susceptibility maps were developed for three zones of residual soils within Bukit Timah Granite and Old Alluvium in Singapore. The pore-water pressure distributions and factors of safety within each zone were obtained from the Transient Rainfall Infiltration and Grid-based Regional Slope-stability model (TRIGRS) analysis and Scoops3D software, respectively. Results from the slope susceptibility maps were verified against two-dimensional (2D) numerical analyses. The 2D numerical models were verified through instrumentation data. The results from slope susceptibility maps agreed with the results from 2D numerical analyses. In addition, the inclusion of vegetation at the slope face was concluded to be an effective slope stabilisation measure from the simulated 2D numerical analyses.

Technological Soil Mechanics in Underground Construction

The article is devoted to the application of the technological soil mechanics principles in the design and construction of underground structures, also in problematic soils – loose water-saturated sands. The definition of technological soil mechanics is given. The relevance of the research topic is justified. A quantitative assessment of the stress strain behavior (SSB) of the soil mass containing the support systems of deep pits, depending on the technology and sequence of work during underground construction, was made. The possibility of the SSB of ground mass managing during underground construction in order to select the most economical options for design and technological solutions is proved. Recommendations for SSB management of underground parts of buildings and structures, as well as the surrounding soil mass, based on the technological soil mechanics application are formulated.

A process-based method for predicting lateral erosion rates

An accurate means of predicting erosion rates is essential to improve the predictive capability of breach models. During breach growth, erosion rates are often determined with empirical equations. The predictive capability of empirical equations is governed by the range for which they have been validated and the accuracy with which empirical coefficients can be established. Most empirical equations thereby do not account for the impact of material texture, moisture content, and compaction energy on the erosion rates. The method presented in this paper acknowledges the impact of these parameters by accounting for the process of dilation during erosion. The paper shows how, given high surface shear stresses, the erosion rate can be quantified by applying the principles of soil mechanics. Key is thereby to identify that stress balance situation for which the dilatency induced inflow gives a maximum averaged shear resistance. The effectiveness of the model in predicting erosion rates is indicated by means of three validation test cases. A sensitivity analysis of the method is also provided to show that the predictions lie within the range of inaccuracy of the input parameters.

Simple Approaches for the Design of Shallow and Deep Foundations for Unsaturated Soils II: Numerical Techniques

Numerical techniques are widely used in conventional engineering practice for obtaining the stress versus settlement or load versus displacement (P–δ) behavior, respectively, and use the information in the design of shallow and deep foundations, extending the principles of saturated soil mechanics. This Companion Paper II summarizes the details of numerical techniques that can be used in the design of shallow and deep foundations in unsaturated soils. The two key properties required for performing the numerical techniques are the shear strength and the modulus of elasticity of unsaturated soils. A user-friendly subroutine (USDFLD) has been developed for use in the commercial software, ABAQUS to predict the variation of these soil properties with respect to matric suction. The only additional information required for performing the numerical modeling in addition to the conventional soil properties is the soil–water characteristic curve. Good agreements were observed between the proposed numerical techniques and from theoretical approaches and experimental studies. The numerical techniques discussed in this paper are simple and can be used by the geotechnical engineers in the design of shallow and pile foundations in unsaturated soils.

Simple Approaches for the Design of Shallow and Deep Foundations for Unsaturated Soils I: Theoretical and Experimental Studies

Bearing capacity is one of the key properties required in the design of shallow and deep foundations. In conventional engineering practice, simple approaches are widely used for determining the bearing carrying capacity of foundations based on the shear strength parameters of saturated soils. However, foundations are typically placed in part or fully in the soil zone above the natural ground water table, which is in a state of unsaturated condition. The shear strength of unsaturated soils is significantly influenced by the matric suction. The bearing capacity of foundations cannot be reliably determined by extending conventional soil mechanics principles for soils that are in a state of unsaturated condition. This Companion Paper I, introduces how the shear strength can be used as a tool in the interpretation and prediction of the bearing capacity of foundations in unsaturated soils. In addition, both theoretical and experimental studies related to the bearing capacity of unsaturated soils are succinctly summarized. Numerical techniques that can be used for predicting the stress versus settlement behavior used for the design of shallow and deep foundations are summarized in Companion Paper II. The succinctly summarized information in the companion papers are valuable for geotechnical engineers for understanding and implementing the mechanics of unsaturated soils in the design of shallow and deep foundations.

Performance estimation of a shallow foundation on an unsaturated expansive soil slope subjected to rainfall infiltration

In the last two decades, there has been a significant increase in infrastructure development on slopes of hilly regions of the world, due to population growth. There are many infrastructures on unsaturated expansive slopes, especially in semi-arid and arid regions. Rainfall infiltration is one of the major factors that contributes to the slope and infrastructure foundations failures on hilly slopes with unsaturated expansive soils. In the current study, a rational approach is proposed considering the combined influence of the foundation-slope behavior based on the principles of unsaturated soil mechanics. This is achieved by a novel numerical modelling approach using the commercial software Geo-studio to investigate the performance of strip foundation located on the top of the unsaturated expansive soil slope subjected to various rainfall infiltration conditions. Hydro-mechanical coupling analysis is conducted to evaluate the rainfall water infiltration influence combined with slope stability analysis using limit equilibrium method. Comparisons are made between both the foundation bearing capacity, slope stability before and after rainfall water infiltration. Different failure mechanisms of the foundation and slope system are presented with and without foundation loading for various rainfall scenarios. Results summarized in this paper are helpful for the geotechnical engineers for understanding the performance of shallow foundations on unsaturated expansive soil slopes considering the influence of rainfall infiltration conditions.

A numerical technique for modeling the behavior of single piles in unsaturated soils

Pile foundations are widely used in both saturated and unsaturated soils. In certain scenarios, these foundations are subjected to combined vertical and lateral loads. Conventionally, saturated soil mechanics principles are routinely used for the design of pile foundations in unsaturated soils. Such approaches contribute to unreliable estimates of the behavior of piles due to ignoring the influence of matric suction. In this paper, a comprehensive numerical technique is proposed for simulating the behavior of single piles subjected to combined vertical and lateral loads in unsaturated soils by taking account of the nonlinear behavior of shear strength and the elastic modulus of unsaturated soils. This is achieved through a subroutine that was developed for use in the ABAQUS software. The proposed numerical method provided reliable prediction of the vertical load-displacement behavior of a published model pile tested in saturated and unsaturated sands. In addition, 3D finite element analysis was extended to simulate the influence of variations in ground water table (GWT) on the vertical bearing capacity and the influence of vertical loads on lateral response of piles. The proposed numerical technique is a promising tool for implementing the state-of-the-art understanding of the mechanics of unsaturated soils into conventional engineering practice.

Soilphysics: An R package for simulation of soil compaction induced by agricultural field traffic

Agricultural traffic is the main mechanism responsible for the increase in compacted fields. Strategies to minimize the impact of field traffic on the soil includes knowledge of stress-strain relationships, which involve the use of soil compaction models; however, one of the challenges for dissemination of these models among researchers and academics is its availability as computational tools, easily and freely accessible. In this work, we present a computational tool for simulation of compaction induced by agricultural field traffic based on soil physical and soil mechanics principles, which includes the calculation of contact stress, stress propagation in the soil profile, simulation of bulk density variation in response to the applied stress and assessment of the risk of compaction through readily available soil properties and machinery parameters. Furthermore, the soil inputs of the model is facilitated by a set of pedo-transfer functions for estimation of the required soil properties. The model was computationally implemented as a set of R functions (R software), as well as on an interactive web page, named PredComp, both available in the R package soilphysics (available from https://arsilva87.github.io/soilphysics/ or http://cran.r-project.org/web/packages/soilphysics/index.html).

CFD-Based Framework for Analysis of Soil–Pipeline Interaction in Reconsolidating Liquefied Sand

Submarine buried pipelines interact with shallow soil layers that are often loose and prone to fluidization/liquefaction. Such occurrence is a possible consequence of pore pressure build-up induced by hydrodynamic loading, earthquakes, and/or structural vibrations. When liquefaction is triggered in sand, the soil tends to behave as a viscous solid–fluid mixture of negligible shear strength, possibly unable to constrain pipeline movements. Therefore, pipelines may experience excessive displacement, for instance, in the form of vertical flotation or sinking. To date, there are no well-established methods to predict pipe displacement in the event of liquefaction. To fill such a gap, this work proposes a computational fluid dynamics (CFD) framework enriched with soil mechanics principles. It is shown that the interaction between pipe and liquefied sand can be successfully analyzed via one-phase Bingham fluid modeling of the soil. Postliquefaction enhancement of rheological properties, viscosity, and yield stress can also be accounted for by linking soil–pipe CFD simulations to a separate analysis of the pore pressure dissipation. The proposed approach is thoroughly validated against the results of small-scale pipe flotation and pipe dragging tests from the literature.

Arching in granular soils: limit state equilibrium

One of the most critical assumptions in the application of arching theory is an accurate estimate of the coefficient of lateral stress, K, used to relate vertical and horizontal stresses on the assumed failure plane. Historically, this has been treated as an empirical constant, and previous mathematical estimates based on consideration of principal stress rotations in the soil have not been able to match experimental values. This paper presents an analytical solution based on limit equilibrium to calculate K based on the rotation of principal stresses at the failure plane and the formation of a continuous arc of major principal stresses across the trapdoor, as observed from centrifuge tests specifically conducted to study arching in granular soils. The predicted K values from the analytical method showed a good match to historical measurements of K, and were successfully used to predict the vertical stresses from the centrifuge tests conducted in this study. The proposed method gives a new analytical basis from soil mechanics principles and an understanding of the rotation of principal stresses to calculate this factor.

Predicting of land surface uplift caused by the flooding of underground coal mines – A case study

The liquidation of underground mines as in flooding of mining excavations and goafs often leads to movements of the rock mass and land surface as a result of pressure changes in the flooded zones. The changes resulting from the rising water table caused by the changes in the stress and strain state as well as the physical and mechanical properties of rock layers can lead to damage to surface construction objects and environmental changes, such as chemical pollution of the surface water. This is why the ability to predict the movements of rock masses generated as a result of mine closure by flooding serves a key function in relation to the protection of the land surface and buildings present thereon. Currently, the world literature presenting methods of forecasting ground surface uplift during mine flooding is very modest and is limited to three analytical methods: Pöttgens', Fenk's and Sroka's method. However, the numerical methods used in this subject area are limited only to modeling the phenomenon using the back analysis method and are not used for forecasting. The author's methodology based on the finite element method presented in the article allows on, after calibrating the 3D numerical model using surface subsidence measurements, as well as geological and mining data, to simulate the flooding process by changing the density of the medium according to classical soil and rock mechanics principles and then obtain prognostic values for ground surface uplift. This methodology has been used in the example of a German underground coal mine, which has been flooded in 1999. The authors have built a 3D numerical model and then performed simulations. The results of calculations were compared with the results of measurements obtaining compliance at the level of 5%.

A Novel Experimental Technique to Investigate Soil–Pipeline Interaction under Axial Loading in Saturated and Unsaturated Sands

Abstract Comprehensive experimental and numerical models are required for reliable investigation of unsaturated soil-structure interaction problems. In this study, a reliable testing technique and a numerical modeling methodology were developed to study the performance of a buried pipeline system in sand subjected to relative soil movement in a direction parallel to its axis under both saturated and unsaturated conditions. A novel testing system was developed using a specially designed test box of 1.5-m length, 1.2-m width, and 1.1-m height. The system utilizes the hanging column technique to achieve different soil matric suction profiles. The experimental results on the tested sand suggest that the measured axial force exerted on a pipe in unsaturated sand is significantly higher compared with the saturated condition. The measured behavior of the prototype pipe was numerically modeled using the commercial software SIGMA/W extending the principles of saturated and unsaturated soil mechanics. The predicted axial force, following the suggested finite element analysis (FEA) methodology, is in good agreement with the measured axial force on the tested prototype pipe. Both the experimental technique and numerical method proposed in this article are useful for the practicing engineers in the rational design of pipelines in sandy soils, taking account of the influence of saturated and unsaturated conditions in sandy soils.

Sensing and monitoring for assessment of rainfall-induced slope failures in residual soil

Real-time monitoring can be used to assess the effect of climatic conditions on slope stability. However, this can be costly. There is therefore need for a methodology to select critical slopes for stability monitoring using comprehensive field instrumentation throughout the year. In this study, the Transient rainfall infiltration and grid-based regional slope stability (TRIGRS) model was used to generate a slope susceptibility map (SSM) of Singapore. This model was selected as it can incorporate unsaturated soil properties in spatial analyses of water infiltration and factor of safety (FoS) calculations. Available studies on establishing SSMs by incorporating unsaturated soil mechanics principles are limited and most focus on mountainous areas. This paper presents relatively new research on the development of a SSM for a densely populated area like Singapore. The study area was limited to residual soil from Bukit Timah Granite in Singapore. TRIGRS helped to determine zones that are highly susceptible to failure; the instrumentation can be used to monitor high-risk areas during dry and rainy periods. A selected slope was instrumented with tensiometers, moisture sensors, piezometers and a rain gauge, all connected to a data logger for real-time monitoring. The changes in pore-water pressure and soil moisture with time during wet and dry periods were used for slope stability analyses. FoS variations over time can be used to assess slope stability, especially during periods of heavy rainfall.

Physico-chemical nature of clayey soils strength

The principles of soil mechanics based on Mohr–Coulomb strength theory and Terzaghi effective stresses theory give us the idea about the nature of clay strength. However, these theories consider the clay as a solid body and do not take into account their internal structure. In the present paper, the problems in determining the strength of clayey soils are considered from the standpoint of the physicochemical theory of effective stresses. This theory is based on the ideas of the internal structure of clays. The main idea implies that the structure of clays depends on the contacts formed between mineral particles, where the external stresses transmitted to the soil are concentrating. Water in clayey soils not only fills the pore space but also interacts with the mineral particles forming hydrate films around them. The prevailing type of structural contacts in clays determines the properties of latter. Strength is an intrinsic property of soil determined by its composition and internal structure, independent of the conditions of load application to it and characterized by the actual effective strength value. The value of actual effective strength in clays is determined by the total strength of individual contacts. This is the maximal stress transferred to the soil contacts, the structure being ruined upon exceeding this value. The actual effective strength in soils with the same predominant type of contacts depends on the number of contacts. The numerical strength characteristics of contact types are determined for different clay soil varieties to characterize the actual effective strength. The parameters of relationship between the actual effective strength in clays and their strength characteristics are obtained from the soil testing by standard methods.

State of practice for use of the soil-water characteristic curve (SWCC) in geotechnical engineering

Routine geotechnical engineering practice has witnessed a significant increase in the usage of unsaturated soil mechanics principles. Laboratory measurement of the soil-water characteristic curve (SWCC) for a soil has been labelled as a primary reason for the improved understanding of unsaturated soil behaviour. Laboratory measurement of the “shrinkage curve” has yielded further insight into the estimation of unsaturated soil property functions (USPFs). The USPFs provide the necessary information for the simultaneous numerical modeling of the saturated and unsaturated portions of the soil profile. This paper presents a state-of-practice summary of the engineering protocols that have emerged amidst the numerous research studies reported over the past couple of decades. It also introduces issues related to hysteresis associated with the SWCC and suggests a pathway forward.

Role of unsaturated soil mechanics in geotechnical engineering

The understanding of unsaturated soil mechanics principles is of interest to a wide spectrum of geotechnical problems associated with soils above water table and compacted soils. This paper describes the stress state variables and constitutive equations based on the unsaturated soil mechanics principles. In addition, the basic concepts for characterization of unsaturated soils and measurements of matric suction (or negative pore-water pressures) are also explained. The application of unsaturated soil mechanics theories is illustrated through the use of capillary barrier system for minimizing rain infiltration into residual soil slopes.