Showing papers in "Computers and Geotechnics in 2014"

DEM simulation of the behaviour of geogrid stabilised ballast fouled with coal

Abstract: Geogrids are commonly used in railway construction for reinforcement and stabilisation. When railway ballast becomes fouled due to ballast breakage, infiltration of coal fines, dust and subgrade soil pumping, the reinforcement effect of geogrids decreases significantly. This paper presents results obtained from Discrete Element Method (DEM) to study the interface behaviour of coal-fouled ballast reinforced by geogrid subjected to direct shear testing. In this study, irregularly-shaped aggregates (ballast) were modelled by clumping together 10–20 spheres in appropriate sizes and positions. The geogrid was modelled by bonding a large number of small spheres together to form the desired grid geometry and apertures. Fouled ballast with 40% Void Contaminant Index (VCI) was modelled by injecting a predetermined number of miniature spheres into the voids of fresh ballast. A series of direct shear tests for fresh and fouled ballast reinforced by the geogrid subjected to normal shear stresses varying from 15 kPa to 75 kPa were then simulated in the DEM. The numerical results showed a good agreement the laboratory data, indicating that the DEM model is able to capture the behaviour of both fresh and coal-fouled ballast reinforced by the geogrid. The advantages of the proposed DEM model in terms of capturing the correct stress–displacement and volumetric behaviour of ballast, as well as the contact forces and strains developed in the geogrids are discussed.

Evaluation of coal longwall caving characteristics using an innovative UDEC Trigon approach

Abstract: The paper presents an innovative numerical approach to simulate progressive caving of strata above a longwall coal mining panel. A proposed Trigon logic is incorporated within UDEC to successfully capture the progressive caving of strata which is characterized by fracture generation and subsequent propagation. A new damage index, D, is proposed that can quantify regions of both compressive shear and tensile failure within the modelled longwall. Many features of progressive caving are reproduced in the model and found to fit reasonably well with field observations taken from a case study in the Ruhr coalfield. The modelling study reveals that compressive shear failure, rather than tensile failure, is the dominant failure mechanism in the caved strata above the mined-out area. The immediate roof beds act like beams and can collapse in beam bending when vertical stress is dominant or in beam shear fracture when horizontal stress is dominant. The proposed numerical approach can be used to guide the design of longwall panel layout and rock support mechanisms.

Assessment of soil-pile-structure interaction influencing seismic response of mid-rise buildings sitting on floating pile foundations

Abstract: The role of the seismic soil–pile–structure interaction (SSPSI) is usually considered beneficial to the structural system under seismic loading since it lengthens the lateral fundamental period and leads to higher damping of the system in comparison with the fixed-base assumption. Lessons learned from recent earthquakes show that fixed-base assumption could be misleading, and neglecting the influence of SSPSI could lead to unsafe design particularly for structures founded on soft soils. In this study, in order to better understand the SSPSI phenomena, a series of shaking table tests have been conducted for three different cases, namely: (i) fixed-base structure representing the situation excluding the soil–structure interaction; (ii) structure supported by shallow foundation on soft soil; and (iii) structure supported by floating (frictional) pile foundation in soft soil. A laminar soil container has been designed and constructed to simulate the free field soil response by minimising boundary effects during shaking table tests. In addition, a fully nonlinear three dimensional numerical model employing FLAC3D has been adopted to perform time-history analysis on the mentioned three cases. The numerical model adopts hysteretic damping algorithm representing the variation of the shear modulus and damping ratio of the soil with the cyclic shear strain capturing the energy absorbing characteristics of the soil. Results are presented in terms of the structural response parameters most significant for the damage such as foundation rocking, base shear, floor deformation, and inter-storey drifts. Comparison of the numerical predictions and the experimental data shows a good agreement confirming the reliability of the numerical model. Both experimental and numerical results indicate that soil–structure interaction amplifies the lateral deflections and inter-storey drifts of the structures supported by floating pile foundations in comparison to the fixed base structures. However, the floating pile foundations contribute to the reduction in the lateral displacements in comparison to the shallow foundation case, due to the reduced rocking components.

Geotechnical influence on existing subway tunnels induced by multiline tunneling in Shanghai soft soil

Abstract: Multiline tunneling construction in soft soil significantly impedes risk control and environmental protection. Current research has investigated on the effect of single-line shield excavation on surrounding environments and tunneling for parallel-crossing or perpendicular down-crossing underground structures. However, minimal attention has been given to soil disturbances induced by multiline tunneling and complex overlapped interaction mechanics for adjacent structures, such as existing above-crossing and down-crossing subway tunnels. Few studies focus on oblique crossing construction and setting rules for the operation parameters of shield machines. Based on the Shanghai Railway transportation project and in situ monitoring data, the deformation analyses of existing subway tunnels induced by an earth pressure balance (EPB) shield during the process of above-overlapped and down-overlapped crossing tunnels with oblique angles are presented. The deformation analyses employ the three-dimensional finite element (3D FE) numerical simulation method, and the simplified analytical method. The analysis results from the theoretical methods are consistent with the monitoring data. The setting rules of multiline propulsion main parameters, including the earth pressure for cutting open, and the synchronized grouting, are also established. This study may provide a theoretical basis for the development of properly overlapped crossing schemes and geotechnical protective measures during multiline tunneling construction in soft soil.

Modeling 3-D desiccation soil crack networks using a mesh fragmentation technique

Abstract: The problem of desiccation cracks in soils has received increasing attention in the last few years, in both experimental investigations and modeling. Experimental research has been mainly focused on the behavior of slurries subjected to drying in plates of different shapes, sizes and thickness. The main objectives of these studies were to learn about the process of crack formation under controlled environmental conditions, and also to understand better the impact of different factors (e.g. soil type, boundary conditions, soil thickness) on the morphology of the crack network. As for the numerical modeling, different approaches have been proposed to describe the behavior of drying cracks in soils. One aspect that it is still difficult to simulate properly is the 3-D crack pattern typically observed in desiccated soils. In this work we present a numerical technique to model the behavior of drying soils. The proposed approach inserts high aspect ratio elements in-between standard elements of a finite element mesh. This mesh fragmentation technique can be easily adapted to standard finite element programs. We used this technique to analyze multiple case studies related to soil desiccation cracks developed under laboratory and field conditions. We focused our attention in some key factors that control the 3-D morphology of the drying cracks network in soils. We show that the proposed technique is able to simulate very satisfactorily the main patterns typically observed in cracked soils.

DEM analysis of “soil”-arching within geogrid-reinforced and unreinforced pile-supported embankments

Abstract: Geogrid-reinforced and pile-supported (GRPS) embankments have been widely used in road engineering due to their economy and effectiveness. The soil arching effect is a key factor in the load transfer mechanism of GRPS technique. In this study, a series of numerical simulations are conducted with the particle flow code PFC2D to study the evolution of soil arching with increasing surcharge. First, an Improved Multi-layer Compaction Method (IMCM) is proposed and applied to establish the Discrete Element Method (DEM) models with more reasonable initial stress states. Validation is then conducted by comparing the DEM results with experimental data. Second, detailed macro-behaviour (e.g., efficacy) and micro-behaviour (e.g., contact forces and fabric anisotropy) analyses are performed. Finally, parametric study is performed to identify the effects of two key factors, i.e., the stiffness of subsoil and the grain composition of embankment fill. Simulation results indicate that the strong force network (SN) serves as a load-carrying force chain, which is the main component of soil arching; while the weak force network (WN) acts as a support system. When a pile-supported embankment is high enough, the soil arching will experience three stages as the surcharge increases. In the first stage, the soil arching develops and its effect is strengthened gradually. In the second stage, the soil arching is fully mobilized and experiences a “forming-failure-reforming” process as the surcharge increases. While in the third stage, the embankment is unable to form a new stable soil arching and the efficacy decreases gradually. However, for the GRPS embankment, due to the reinforcement of geogrid, the third stage will not occur throughout the entire surcharge range of interest in this study. On the basis of the macro- and micro-results, the conclusion can be drawn that the presence of geogrid does not alter the failure mode of soil arching under the surcharge, though it is able to evidently improve the efficacy of load transfer and enhance the stability of soil arching.

A unified plasticity model for large post-liquefaction shear deformation of sand

Abstract: Based on previous experimental findings and theoretical developments, this paper presents the formulation and numerical algorithms of a novel constitutive model for sand with special considerations for cyclic behaviour and accumulation of large post-liquefaction shear deformation. Appropriate formulation for three volumetric strain components enables the model to accurately predict loading and load reversal behaviour of sand, fully capturing the features of cyclic mobility. Compliance with the volumetric compatibility condition, along with reversible and irreversible dilatancy, allows for physically based simulation of the generation and accumulation of shear strain at zero effective stress after initial liquefaction. A state parameter was incorporated for compatibility with critical state soil mechanics, enabling the unified simulation of sand at various densities and confining pressures with a same set of parameters. The determination methods for the 14 model parameters are outlined in the paper. The model was implemented into the open source finite-element framework OpenSees using a cutting-plane stress integration scheme with substepping. The potentials of the model and its numerical implementation were explored via simulations of classical drained and undrained triaxial experiments, undrained cyclic torsional experiments, and a dynamic centrifuge experiment on a single pile in liquefiable soil. The results showed the model’s great capabilities in simulating small to large deformation in the pre- to post-liquefaction regime of sand.

Numerical analysis of seasonal heat storage in an energy pile foundation

Abstract: The use of concrete geostructures for energy extraction and storage in the ground is an environmentally friendly and easy way of cooling and heating buildings. With such energy geostructures, it is possible to transfer energy from the ground to buildings by means of fluid-filled pipes cast in concrete. By injecting thermal energy in summer and extracting it in winter, the ground in the area of a building’s piles can be used for seasonal energy storage, as long as the underground water flow in the storage remains low. This paper is a contribution to the improvement of the knowledge in the field of energy geostructures. The behaviour of a multi-pile seasonal storage system subjected to thermo-mechanical loading is examined numerically from both thermal and mechanical perspectives. The purpose of this paper is (i) to propose a thermo-hydro-mechanical 2D solution to the 3D problem, (ii) to explore the thermal behaviour of this type of storage and (iii) to evaluate its structural consequences. Coupled multi-physical finite element modelling is conducted. The efficiency of the storage is not dramatically affected by an increase in the annual mean temperature of the storage. It is shown that induced mechanical loads are less important when considering a wholly heated pile structure than when considering a single heated pile in a foundation. The evolution of stresses in the piles and in the soil during heating–cooling cycles also reveals possible critical phenomena.

Investigation of natural rock joint roughness

Abstract: The paper provides a comprehensive review on rock joint roughness measurement and quantification procedures Superiority of fractal based methods over JRC, statistical parameters and statistical functions in quantifying roughness is discussed in the paper Two of the best fractal based methodologies available in the literature, the modified 2-D divider and variogram methods, are used to quantify natural rock joint roughness in 3-D and 2-D, respectively The capability of these two methods in accurate quantification of natural rock joint roughness is shown in the paper by applying the procedures to four natural rock joints A good comparison has been obtained from the values obtained through the two methods Both these methodologies have two parameters to capture the stationary roughness The fractal dimension captures the spatial auto correlation of roughness; the other parameter captures the amplitude of roughness Anisotropic roughness has been studied by applying two other methodologies: (a) a triangular plate methodology and (b) a light source methodology to the same four natural rock joints A reasonably good comparison has been obtained through the results of these two methodologies All four roughness quantification methodologies can be applied to any size of sample covering from laboratory to field scales The results of the triangular plate and light source methodologies provided possible sliding direction values (under the gravitational loading) close to that reported in the literature for the rough discontinuity planes used in the study

Influence of heterogeneity on 3D slope reliability and failure consequence

Abstract: This paper investigates the influence of heterogeneity of undrained shear strength on the reliability of, and risk posed by, a long slope cut in clay, for different depths of foundation layer. The clay has been idealised as a linear elastic, perfectly plastic Von Mises material and its spatial variability has been modelled using random field theory, whereas slope performance has been computed using a parallel 3D finite element program. The results of Monte Carlo simulations confirm previous findings that three categories of failure mode are possible and that these are significantly influenced by the horizontal scale of fluctuation relative to the slope geometry. In particular, discrete 3D failures are likely for intermediate scales of fluctuation and, in this case, reliability is a function of slope length. The risk posed by potential slides has been quantified in terms of slide volumes and slide lengths, which have been estimated by considering the computed out-of-face displacements. The results show that, for a given horizontal scale of fluctuation relative to the slope geometry, there is a wide range of possible slide volumes and slide geometries. Indeed, the results highlight just how difficult it is to compute a 2D slope failure in a heterogeneous soil. However, they also indicate that, for low probabilities of failure, the volumes of potential slides can be small. This suggests that, for some problems, it may not be necessary to design to very small probabilities of failure, due to the reduced consequence of failure in this case. The techniques developed in this paper will be important in benchmarking simpler 2D and 3D solutions used in design, as there is a need to quantify slide geometries when benchmarking simpler methods based on predefined failure mechanisms.

Support vector machines applied to uniaxial compressive strength prediction of jet grouting columns

Abstract: Learning from data is a very attractive alternative to “manually” learning. Therefore, in the last decade the use of machine learning has spread rapidly throughout computer science and beyond. This approach, supported on advanced statistics analysis, is usually known as Data Mining (DM) and has been applied successfully in different knowledge domains. In the present study, we show that DM can make a great contribution in solving complex problems in civil engineering, namely in the field of geotechnical engineering. Particularly, the high learning capabilities of Support Vector Machines (SVMs) algorithm, characterized by it flexibility and non-linear capabilities, were applied in the prediction of the Uniaxial Compressive Strength (UCS) of Jet Grouting (JG) samples directly extracted from JG columns, usually known as soilcrete. JG technology is a soft-soil improvement method worldwide applied, extremely versatile and economically attractive when compared with other methods. However, even after many years of experience still lacks of accurate methods for JG columns design. Accordingly, in the present paper a novel approach (based on SVM algorithm) for UCS prediction of soilcrete mixtures is proposed supported on 472 results collected from different geotechnical works. Furthermore, a global sensitivity analysis is applied in order to explain and extract understandable knowledge from the proposed model. Such analysis allows one to identify the key variables in UCS prediction and to measure its effect. Finally, a tentative step toward a development of UCS prediction based on laboratory studies is presented and discussed.

DEM–SPH simulation of rock blasting

Abstract: A numerical model is proposed for the simulation of rock blasting. A bonded particle system is utilized to mimic the behavior of rock. The particles interact at the contact points through normal and shear springs to simulate rock elasticity. To withstand the deviatoric stresses, the particles are glued to each other. If the applied force exceeds the contact strength, local failure occurs and microcracks are developed in the synthetic rock. For simulation of gas flow, the smooth particle hydrodynamic method is implemented. The interaction of gas particles with the rock grains is assumed to follow a perfect plastic collision model in which the initial momentum of the colliding particles is preserved. A detailed examination of the interaction of gas with blast hole is investigated. It is shown that the proposed hybrid model is capable of simulating the induced shock waves in the gas together with wave propagation in the rock material. The model successfully mimics crack propagation in rock. In particular, the crushed zone around the borehole, radial cracks, and surface spalling are all captured successfully. The results of numerical analysis suggest that gas–rock interaction can, in fact, generate a few successive compressive waves in the rock specimen, causing further extension of radial cracks with time as the weaker secondary and tertiary waves interact with the crack tips.

Optimization of site exploration program for improved prediction of tunneling-induced ground settlement in clays

Abstract: Excessive settlement caused by tunneling during construction often damages adjacent infrastructures and utilities. Such excessive settlement can also present a challenge in the maintenance of subways during their operation. Thus, it is important to be able to accurately predict tunneling-induced settlement. The uncertainties in geotechnical parameters, however, can lead to either an overestimation or an underestimation of the tunneling-induced settlement. Such uncertainties can arise from many sources such as spatial variability, measurement error, and model error; in this paper, the focus is on the geotechnical parameters characterization from site exploration. The goal here is to determine an optimal level of site exploration effort so that effective predictions of the tunneling-induced settlement in clays can be achieved. To this end, a Monte Carlo simulation-based numerical model of site exploration is first established to generate artificial test data. Then, a series of parametric analyses are performed to investigate the relationship between the level of site exploration effort and the accuracy of the tunneling-induced ground settlement prediction. Through the assumed different levels of site exploration effort, statistics of soil parameters are estimated using the maximum likelihood method and the tunneling-induced ground settlement is then analyzed using the probabilistic method, and finally the effect of site exploration effort is assessed. The knowledge generated from this series of analyses is then used to develop the proposed framework for selecting an optimal site exploration program for improved prediction of the tunneling-induced ground settlement in clays. Examples are presented to illustrate the proposed framework and demonstrate its effectiveness and significance.

A load transfer approach for studying the cyclic behavior of thermo-active piles

Abstract: Unsatisfactory understanding of thermally induced axial stress and mobilized shaft friction in the thermo-active piles has led to a cautious and conservative design of such piles. Despite the fact that the number of construction works using this type of piles has been rapidly increasing since the last 20 years and none of them witnessed any structural damage, the question that still remains is how to overcome the cyclic thermal effects in such piles to optimize the design method. This paper presents a soil–pile interaction design method of an axially loaded thermo-active pile based on a load transfer approach by introducing a proposed t–z cyclic function. The proposed t–z function comprises a cyclic hardening/softening mechanism which is able to count the degradation of the soil–pile capacity during two-way cyclic thermal loading in the thermo-active pile. The proposed t–z function is then compared to a constitutive law of soil–pile interface behavior under cyclic loading, the Modjoin law. Afterwards, numerical analyses of a thermo-active pile located in cohesionless soil are conducted using the two cyclic laws in order to comprehend the response of such pile under combined mechanical and cyclic thermal loads. The behaviors of the pile resulting from the two laws show a good agreement in rendering the cyclic degradation effects. At last, the results permit us to estimate the change in axial stress and shaft friction induced by temperature variations that should be taken into account in the geotechnical design of the thermo-active pile.

On the heat transfer coefficient between rock fracture walls and flowing fluid

Abstract: Two analytical solutions are derived to model the heated flow-through experiments for granite fractures in the literature. The first model, which assumes an identical/continuous temperature between the bulk fluid and fracture surfaces, represents an upper bound solution of water temperature in rock fractures. The second model including the empirical parameter of heat transfer coefficient is used to calculate the average heat transfer coefficient based on the available experimental data. The obtained heat transfer coefficients are smaller than that from the thermal boundary layer theory for flat plates, but larger than the previous estimates. A power function is fitted to describe the relation between heat transfer coefficient and flow velocity. Both models show that water temperature increases non-linearly along fracture plane.

Pile and pile group response to tunnelling using a large diameter slurry shield – Case study in Shanghai

Abstract: The study investigates the disturbance to piles and pile groups caused by multiple nearby drives of a large diameter slurry shield-driven tunnelling machine in Shanghai. The minimum distance between the slurry shield tunnel (with diameter D = 15.43 m) and the adjacent pile groups of Metro Line 3 and Yixian Elevated Road is 1 m. The nonlinear finite element (FE) software program ABAQUS was used to analyse the movement of the pile groups caused by the process of shield tunnelling. A field investigation was conducted before the multiple crossings to study the impact (movement and excess pore water pressure) on the surrounding soil and piles caused by the tunnelling process. The field investigation is divided into two sections: (i) free-field tunnelling, and (ii) tunnelling close to trial piles pre-installed in the section. For the full-scale test, tunnelling variable, including slurry pressure and grouting pressure, are adjusted during construction to reduce the disturbance during tunnelling close to pile groups supporting two elevated bridges. The FE simulation of the multiple crossings includes two steps: (a) shield tunnelling along the south bound tunnel approaching the working shaft at the western bank of the Huangpu River, and passing separately between two adjacent pile groups of each bridge; (b) return tunnel passing again between the pile groups of the two bridges along the north bound tunnel. Three different FE models are generated: (1) free-field tunnelling process, (2) tunnelling close to trial piles, and (3) multiple tunnel passes adjacent to pile groups supporting the two elevated bridges. Most of the relevant factors in tunnelling are taken into consideration in the FE models including (a) slurry pressure, (b) grouting pressure, (c) grouting material hardening, and (d) soil-pile interaction.

A three-dimensional state-dependent model of soil-structure interface for monotonic and cyclic loadings

Abstract: An elasto-plastic model based on critical state soil mechanics and generalised plasticity was modified to capture the three-dimensional (3D) behaviour of soil–structure interfaces. The modified model assumed isotropic behaviour in different directions of a soil–structure interface. Two circular surfaces were defined on the plane of normalised shear stresses, and the cyclic interface response was determined by the distances of the normalised shear stress on this plane. With a single set of parameters, the modified model simulated the monotonic and cyclic 3D behaviour of soil–structure interfaces over a wide range of soil densities, normal pressures and normal stiffnesses. A unified description of the particle breakage under monotonic and cyclic loading conditions was incorporated, and the considerable contractive behaviour under cyclic loading can be captured by employing a more obvious translating critical state line related to particle breakage. Most of the model parameters have definite physical meanings and can be calibrated through conventional direct shear or simple shear tests. The modified model was validated against published tests of sand–steel interfaces and gravel–steel interfaces under both monotonic and cyclic loadings in 2D and 3D stress paths.

Modeling the particle breakage of rockfill materials with the cohesive crack model

Abstract: A practical combined finite–discrete element method was developed to simulate the breakage of irregularly shaped particles in granular geomaterials, e.g., rockfill. Using this method, each particle is discretized into a finite element mesh. The potential fracture paths are represented by pre-inserted cohesive interface elements (CIEs) with a progressive damage model. The Mohr–Coulomb model with a tension cut-off is employed as the damage initiation criterion to rupture the predominant failure mode occurs at the particle scale. Two series of biaxial tests were simulated for both the breakable and unbreakable particle assemblies. The two assemblies have identical configurations, with the exception that the former is inserted with CIEs and is breakable. The simulated stress–strain–dilation responses obtained for both assemblies are in agreement with experimental observations. We present a comprehensive study of the role of particle breakage on the mechanical behavior of rockfill materials at both the macroscopic and microscopic scales. The underlying mechanism of particle breakage can be explained by the force chain in the assemblies.

Assessment of the Dynamic Behaviour of Saturated Soil Subjected to Cyclic Loading from Offshore Monopile Wind Turbine Foundations

Abstract: The fatigue life of offshore wind turbines strongly depends on the dynamic behaviour of the structures including the underlying soil. To diminish dynamic amplification and avoid resonance, the eigenfrequency related to the lowest eigenmode of the wind turbine should not coalesce with excitation frequencies related to strong wind, wave and ice loading. Typically, lateral response of monopile foundations is analysed using a beam on a nonlinear Winkler foundation model with soil–pile interaction recommended by the design regulations. However, as it will be shown in this paper, the guideline approaches consequently underestimate the eigenfrequency compared to full-scale measurements. This discrepancy leads the authors to investigate the influence of pore water pressure by utilising a numerical approach and consider the soil medium as a two-phase system consisting of a solid skeleton and a single pore fluid. In the paper, free vibration tests are analysed to evaluate the eigenfrequencies of offshore monopile wind turbine foundations. Since the stiffness of foundation and subsoil strongly affects the modal parameters, the stiffness of saturated soil due to pore water flow generated by cyclic motion of monopiles is investigated using the concept of a Kelvin model. It is found that the permeability of the subsoil has strong influence on the stiffness of the wind turbine that may to some extent explain deviations between experimental and computational eigenfrequencies.

Numerical simulation of desiccation cracking in a thin clay layer using 3D discrete element modeling

Abstract: Desiccation cracking of clay soil is of critical importance in many applications, such as industrial waste containment, hydraulic barriers, road embankments, and agricultural operations. The factors that influence cracking are known qualitatively, but it is not clear how to predict the initiation and propagation of cracks. This study presents a discrete element approach to modeling desiccation cracking in thin clay layers, considering material property changes. First, an aggregate shrinkage model based on the aggregate structure of clay was proposed, and the drying shrinkage of clay soil was modeled by imposing drying shrinkage kinetics for each aggregate at the micro-scale. Second, the clay soil was represented by an assembly of aggregates linked by bonds, and desiccation cracking of the clay layer was modeled using a three-dimensional discrete element code (PFC3D), with the aid of the embedded programming language FISH. When the clay layer is sufficiently thin, the water content gradient along the section can be neglected; thus, the shrinkage kinetics are the same for all of the grains of clay. In the model based on the discrete element method (DEM), the bond strength and contact stiffness changed during drying. Their changes were determined by matching the simulation results with the experimental data. Third, the DEM approach was validated by reproducing experimental desiccation tests performed on a thin clay layer in a disk shape. The geometric parameters of surface cracks were quantified using image analysis techniques and were compared with experimental observations. Fourth, some factors of influence, such as the sample thickness, the properties of the soil–base interface, micro-mechanical parameters, and shrinkage parameters, were investigated using the DEM model. The results obtained from the DEM analyses were compared with the results of prior research in this field of study. The approach used in this study is very promising for simulating desiccation cracking in thin clay soil because the model captures the initiation and propagation mechanism of desiccation cracks. Although this study was carried out on surface cracking in a thin clay layer, the extension of this methodology is of potential benefit not only for predicting three-dimensional desiccation cracking in real clay liners but also for modeling cracking in other materials with properties that vary with water content or temperature, such as concrete and rock.

Simulation of roof shear failure in coal mine roadways using an innovative UDEC Trigon approach

Abstract: Shear failure is a common failure mechanism in underground coal mine roadways. This paper presents an innovative numerical approach to simulate shear failure of a coal mine roadway roof. The distinct element code, UDEC, incorporating a proposed Trigon logic is employed for the study. Using this approach, shear failure in the mine roof characterized by fractured initiation and propagation is successfully captured. The results suggest that shear failure of the roadway roof initiates at the roadway corners and then progressively propagates deeper into the roof, finally forming a large scale roof failure. The numerical results confirmed the time sequence of marked microseismic activity, significant stress changes and accelerated displacement during the process of a roof fall. The effect of rock bolting in the control of roof shear failure in a roadway is evaluated using the UDEC Trigon approach. It is found that the installation of rock bolts constrains rock dilation, reduces failure of rock bridges and maintains rock strength thereby leading to a significant decrease in roof sag.

Predicting bimodal soil–water characteristic curves and permeability functions using physically based parameters

Abstract: Experimental evidence shows that a gap-graded soil or a widely-graded granular material may have a bimodal soil–water characteristic curve (SWCC) and a bimodal permeability function. A bimodal SWCC or a bimodal permeability function originates from a dual-porosity structure. To date, the prediction of bimodal SWCCs for gap-graded soils is still a difficult task. In this paper, a bimodal SWCC model is proposed to describe the drying process of granular soils considering a dual-porosity structure. The new SWCC model shows powerful capability in fitting the SWCCs for soils varying from gravel to silt. Regression analysis is conducted to establish empirical relations between the model parameters and the indexes of soil grain-size distribution (GSD). Based on these relations, the new model predicts well both the bimodal SWCCs for gap-graded soils and the unimodal SWCCs for well-graded soils and uniform soils. A bimodal permeability function is also proposed and linked to the new SWCC model. In the absence of experimental SWCCs and permeability functions, the new model can be used to obtain preliminary SWCCs and permeability functions for granular soils. It should be mentioned that the prediction of the SWCC from the GSD is still empirical and does not address the cyclic wetting/drying process. Measurement of the SWCC should be performed wherever an accurate SWCC is required.

On the behavior of suction buckets in sand under tensile loads

Abstract: Suction buckets can be used instead of driven piles for the support of jacket or tripod foundations for offshore wind energy converters (OWECs). However, due to the relatively small self weight of offshore wind structures, considerable tensile loads can occur for instance during storms; these loads usually govern the dimensioning of the buckets. Under rapid tensile loading, suction pressures are induced inside the bucket, which can considerably increase the tensile capacity. This paper presents results of numerical simulations based on a coupled pore fluid diffusion and stress analysis which allow for the description of the partly drained load-bearing behavior as well as the quantification of the tensile resistance. It is shown that a high pull-out rate leads to a large increase of the tensile capacity. The maximum capacity is reached when the soil behaves fully undrained or when cavitation of the pore water occurs. In this regard, the main influence parameters are the bucket geometry, the soil permeability, the pull-out rate (loading rate) and, regarding cavitation, also the water depth. It is shown that the mobilization of suction pressures requires a large heave of the bucket, which might be inadmissible with respect to serviceability requirements. Simulations in which variable tension loads are applied with a specific loading rate and then kept constant are also presented. It is found that an accumulation of heave over time occurs when the load exceeds the drained capacity of a bucket. This indicates that cyclic loading of buckets with tensile loads often exceeding the drained capacity might lead to excessive heave.

Case study and back analysis of a residential building damaged by expansive soils

Abstract: This paper presents a case study of a residential house damaged by expansive soils. The field investigation revealed that the damage was most likely caused by excessive lawn watering and leaks of sewer pipe and/or stormwater pipe, which resulted in non-uniform soil moisture conditions. Three-dimensional back analysis of this distressed structure indicated that stresses were most critical at a re-entrant corner and that steel reinforcing bars in the beam in this area had yielded. The results of the back analysis also indicated that a stronger footing was required to limit differential deflection to an acceptable level and reduce stress in the footing. The case study has clearly shown that a leaking underground water pipe and/or excessive watering of a garden could cause more severe distortion to a single storey masonry veneer house than could be expected from seasonal moisture change and the deeper moisture re-distribution caused by the imposition of the house on seasonally dry reactive soil. Moreover it has been demonstrated that it would be extremely costly to design a footing for extreme, or abnormal, moisture changes.

A simple implementation of RITSS and its application in large deformation analysis

Abstract: Large deformation finite element (LDFE) analysis is being applied increasingly in geomechanics as it allows numerical interpretation of problems in which the structural element moves a relatively long distance through the soil. The ‘remeshing and interpolation technique with small strain’ (RITSS) method for LDFE analysis, in which the soil domain is periodically remeshed with the stress and material properties interpolated from the old to the new within the standard Lagrangian finite element framework, has been successfully applied to a number of practical applications. It allows any standard finite element theory to be used in the Lagrangian analysis, and because the mesh topography and connectivity are not influenced by the previous deforming increment, large deformations are possible. The major barrier of the RITSS method is that the remeshing and interpolation requires specialised and user-dependent computer code. This has limited its application to specialists and hindered its routine application in engineering practice. This paper proposes a simpler, more practical method to implement RITSS for geotechnical applications. By utilising the ABAQUS in-built procedures for interpolation and remeshing, it avoids any need for user-defined code (although a piece of Python script can be used to automate the iteration instead of operating the ABAQUS user interface). A series of four example problems benchmarking this new approach show it to be robust and numerically accurate.

Experimental and numerical study of arching soil effect in coarse sand

Abstract: The arching effect in soils consists on the redistribution of stresses from a yielding portion of the soil mass to more rigid regions. This article presents the results of an experimental and numerical study of the arching effect in soils. The experimental study is based on Terzaghi’s trapdoor test. The displacement field of the soil is estimated using the Digital Image Correlation (DIC) technique and some displacement transducers. The stress redistribution is assessed using several load cells. The numerical study compares two elastoplastic constitutive models of increasing complexity and examines their ability to reproduce this phenomenon. The goal is to select the constitutive model that better reproduces this phenomenon to be later used for more complex situations.

Dynamic response of a circular lined tunnel with an imperfect interface subjected to cylindrical P-waves

Abstract: The analytic solutions for the dynamic response of a circular lined tunnel with an imperfect interface subjected to cylindrical P-waves are presented in the paper The wave function expansion method was used and the imperfect interface was modeled with a spring model The interface separating the liner from the surrounding rock was considered to be homogeneous imperfect The dynamic stress concentration factors (DSCF) of the rock and liner were evaluated and discussed The effects of incident wave’s frequency, bonding conditions and distance between the wave source and the tunnel were examined The results showed that the low-frequency incident wave leads to a higher DSCF than the high-frequency incident wave The bonding conditions have a great effect on the dynamic response of the lined tunnel When the bond is extremely weak, the resonance scattering phenomenon can be observed When the distance between the wave source and the tunnel, depending on frequency of the incident wave, is considered as large, the cylindrical wave can be treated as a plane wave Limiting cases were considered and good agreement with the solutions available in the literature was obtained

Prediction of ultimate axial load-carrying capacity of piles using a support vector machine based on CPT data

Abstract: The support vector machine (SVM) is a relatively new artificial intelligence technique which is increasingly being applied to geotechnical problems and is yielding encouraging results. In this paper SVM models are developed for predicting the ultimate axial load-carrying capacity of piles based on cone penetration test (CPT) data. A data set of 108 samples is used to develop the SVM models. These data were obtained from the literature containing pile load tests and each sample contains information regarding pile geometry, full-scale static pile load tests and CPT results. Moreover, a sensitivity analysis is carried out to examine the relative significance of each input variable with respect to ultimate strength prediction. Finally, a statistical analysis is conducted to make comparisons between predictions obtained from the SVM models and three traditional CPT-based methods for determining pile capacity. The comparison confirms that the SVM models developed in this paper outperform the traditional methods.

Numerical investigation of the failure of a building in Shanghai, China

Abstract: The overturning failure of a 13 storey residential building in Shanghai, China, has been investigated by plane strain finite element analysis (FEA). The results of the FEA indicate that ultimate failure of the building was probably initiated by the formation of tensile cracking in the reinforced concrete piles located under the side of the building adjacent to an excavation. This eventually led to complete structural failure of the piles located along the excavation side, which probably caused further settlement of the building, leading eventually to a toppling failure resulting in overturning of the entire building. Excessive tensile stress in the piles was probably caused by the combination of excavation of soil at one side of the building and the temporary dumping of the excavated soil on the opposite side of the building. It is likely that the effect of temporary dumping of the excavated soil adjacent to the building was either not considered or not properly taken into account in the foundation design nor the construction operations. A simple but important lesson to be draw from this failure is the need for engineers who design foundations in soft soil regions to consider not only the final loading conditions, but also any temporary and transient loading conditions during the construction process.

A new and simple stress-dependent water retention model for unsaturated soil

Abstract: The water retention curve (WRC) is an important hydraulic property of unsaturated soil needed for seepage analysis. Experimental evidence shows that the WRC is affected by various factors such as net stress and soil type. Many attempts have been made to describe the effects of net stress by including the void ratio in a water retention model. But the void ratio (i.e., soil density) is not the only parameter altered by the application of net stress. The pore structure, including the pore size distribution, pore shape and pore orientation, is also changed. Thus the influence of net stress on the WRC should not be treated as equivalent to density effects. In this study, it is verified that the inclusion of the void ratio cannot adequately capture the effects of net stress on the water retention behaviour. A new and simple water retention model is thus developed by considering the stress effects on the void ratio as well as the pore structure. The model is then applied to simulate the WRCs of three different soils tested over a wide range of stress conditions, including isotropic and anisotropic stress conditions. The results show that better predictions of experimental data can be made by incorporating the effects of net stress on both the void ratio and the pore structure.