Complex Soil Structure Research Articles

Interaction-performance-driven design of negative-stiffness friction pendulum systems for aboveground structure–connected underground structure–soil system with ground motion effects

Within the transit-oriented development framework, aboveground structure–connected underground structures (ASUS) have been widely constructed in urban areas; however, abrupt stiffness changes and complex soil–structure interactions could cause structural damage or business disruption after earthquakes. In this study, to address these challenges, a novel approach is proposed that employs negative stiffness, damping elements, and friction pendulum bearings to enhance the overall performance of ASUS. An interaction-performance-driven design procedure and parameter selection methodology are developed to simultaneously upgrade multiple performance aspects of ASUS. A mechanical model of the negative stiffness amplification system-enabled friction pendulum system (NSAS-FPS) is constructed. The theoretical basis of the equivalent negative stiffness and enhanced energy dissipation effects is elucidated, and a finite element model of the NSAS-FPS-incorporated soil–ASUS interaction system is established. Extensive parametric, robustness, and correlation analyses against short/long-period ground motions are conducted to provide a comprehensive performance assessment framework. Then, an interaction-performance-driven design principle aimed at multiperformance upgrading of aboveground and underground structures is developed with proposed parameter selections and applied in a case study. These results indicate a significant improvement in vibration control for both aboveground and underground structures when utilizing NSAS-FPS compared to utilizing conventional FPSs with the same design. By following the proposed design procedure and parameters, the NSAS-FPS demonstrates enhanced efficiency in isolating energy dissipation and robustness in seismic isolation, as well as resistance against overturning, irrespective of variations in structural masses and functionalities. While the advantages of the NSAS-FPS include its ability to mitigate the effects of stochastic earthquakes, the extent of the performance improvement may decrease during long-period earthquakes. Therefore, the velocity characteristics of seismic excitations need to be carefully incorporated into the NSAS-FPS design, particularly when targeting specific demands for the isolation-layer performance within ASUS.

Soil–Structure Interactions in a Capped CBP Wall System Triggered by Localized Hydrogeological Drawdown in a Complex Geological Setting

Retaining walls are often used to construct basements and underground station boxes. This unique case study compares the field-measured contiguous bored pile (CBP) wall, surrounding geology, and hydrogeology or groundwater responses against the results using 2D and 3D numerical back analyses of a deep excavation project that experienced localized groundwater drawdown through the leaking ground anchor points. Site observations indicated that the ground anchor installation works had caused larger than expected through-the-wall leakages that subsequently triggered nearby ground and building settlements. In order to study the complex soil–structure interaction behavior, back analyses using a hybrid modeling technique of through-the-wall transient hydrogeological seepage and geomaterial stress-strain analyses was implemented. Through these soil-structure interaction back analyses, it was evidently revealed that the presence of the continuous capping beam was key in providing pile head restraints against the active earth pressures when the groundwater was depressed, as well as efficiently distributing the beneficial wall corner effects towards the middle CBP wall, leading to smaller bending moment magnitudes, characterized by their ‘S-shaped’ profiles. This behavior had been correctly diagnosed, as opposed to the ‘D-shaped’ bending moment profile usually only seen in a typical free-head cantilever wall in similar geology. The eventual results show that the wall and ground responses, i.e., deflection, bending moment, and settlement, were reasonably well predicted when compared against the instrumented field data, thus validating the reliability of the geotechnical modeling technique, key geological parameters, and hydrogeological fluctuations adopted in the 2D and 3D numerical models, as well as the beneficial contributions of the continuous capping beam, which tend to be overlooked during routine retaining wall design.

A universal soil–water characteristic curve model based on the particle size distribution and fractal theory

A soil–water characteristic curve (SWCC) is a prerequisite relationship to study water transport in unsaturated soils and has significant applications in agriculture and engineering sciences. Therefore, many SWCC models were proposed and improved the understanding of water transport. Many of these models were obtained depending on the particle size distribution (PSD) because the water content is related to the sizes and distributions of particles and pores in soils. However, the continuous PSD was divided into many fractions to calculate the water content in these models. Moreover, the particles and pores were simplified as spheres and cylinders, respectively. These simplifications are idealistic and contrary to actual complex structures in soils. To solve the above limitations of these models, we considered the Weibull distribution and fractal theory and established a universal SWCC model called PF (PSD and fractal theory) based on the continuous PSD and irregularities of the pore structures. The new proposed PF model was verified by measured data in the UNSODA database. Furthermore, its accuracy was proven by comparison with two classical models widely used. The verification and comparison results show that the PF model can well describe the measured data of various types of soils and has fewer errors than the other two models. Meanwhile, the continuous PSD in this study overcomes the limitation of artificial discretization and is convenient for subsequent calculation. In addition, the actual irregular soil geometry is described by the fractal theory, which well matches complex soil structures. Therefore, the PF model based on the PSD and the fractal theory is reasonable and reliable in predicting SWCC. In addition, this study can provide a foundation and reference for subsequent studies on hydraulic behaviors.

Inversion of reclaimed soil water content based on a combination of multi-attributes of ground penetrating radar signals

Rapid, accurate, and non-destructive acquisition of the distribution of reclaimed soil moisture information can provide data for the rapid monitoring of reclaimed soil in areas experiencing coal mining subsidence. However, the water content inversion methods based on ground penetrating radar (GPR) are mostly single-attribute analysis methods, which are easily affected by the soil structure. This paper proposes a multi-attribute joint analysis method, which can reduce the influence of complex soil structure on the prediction results. Surveys and soil sampling using GPR were performed on a subsided reclamation area in Huaibei City, Anhui Province, China. Correlation analysis of GPR attribute information and volumetric water content (VWC) showed that frequency peak (FP), average envelope amplitude (AEA), energy, instantaneous amplitude area, instantaneous frequency area, and average instantaneous frequency values were significantly related to the VWC of reclaimed soil. The applicability of different single-attribute analysis methods under the condition of reclaimed soil was compared. Results showed that the predictive effects of FP and AEA attributes were better than other attributes. Generally, the single-attribute analysis method was greatly affected by the structure of reclaimed soil, so the accuracy and reliability of this method need to be further optimized. The prediction results of the single- and multi-attribute joint analysis methods in the structure of reclaimed soil were compared and analyzed. This showed that the prediction accuracy and model reliability of the multi-attribute method are both higher than those of the single-attribute method. The multi-attribute method can overcome the problem of insufficient accuracy of water content detection of ground-seeking radar in soil with a complex structure. Finally, the multi-attribute method was used to obtain the water distribution information in the reclamation area. Results of this study provide new methods and ideas for the prediction of water content by GPR in soils with complex structure.

An Explicit Finite Difference Method for Dynamic Interaction of Damped Saturated Soil Site-Pile Foundation-Superstructure System and Its Shaking Table Analysis

The saturated soil site-pile foundation-superstructure system, with large degrees of freedom or strong nonlinear problems, often involves a large amount of calculation and low computational efficiency. In this paper, a fully explicit finite difference method is proposed for a saturated soil site-pile foundation-superstructure system. Since the proposed method has the advantages of decoupling in both time and space, it does not need to solve the equations simultaneously, which grants it high computational efficiency. At the same time, the method is implemented on the open-source software OpenSees and is used to compare and analyze the dynamic responses of the shaking table of a liquefiable soil site-pile foundation-superstructure system. After the calculation and analysis, the numerical solutions were found to be in good agreement with the experimental solutions, which verifies the proposed method and illustrates that the proposed method can reasonably simulate the seismic responses of the whole system. In addition, the proposed calculation platform in OpenSees can be used for the analysis of the liquefaction process and the possible large deformation of soil after liquefaction, as well as for analyzing the failure mode of the complex and nonlinear saturated soil sites and structures under the effects of an earthquake.

Twin earth pressure balance tunnelling – monitoring and numerical study of an urban case

In recent decades, numerical calculations have often been used to optimise, from a technical and economical perspective, the design of underground works. Using these tools allows efficient and affordable designs to be obtained. Simulation of the tunnelling procedure when using a tunnel-boring machine and in urban areas is a complex soil–structure interaction problem. Here, a three-dimensional (3D) code using finite differences is used. The reference case of this study is Lyon metro line D project in France, where a non-symmetric load is present at the surface due to the presence of a retaining wall. An accurate on-site monitoring system was set up to assess ground movements during the mechanised excavation of twin tunnels. The collected data are used as a reference to show the validity and accuracy of the developed 3D model. Two constitutive models were used to simulate the soil behaviour: plastic hardening (PH) and linear elasticity with perfect plasticity (Mohr–Coulomb shear failure criterion). The Mohr–Coulomb constitutive model shows some drawbacks, which can be eliminated by using PH. The existence of a retaining wall induces a non-symmetric load condition; however, the settlement trough predicted by advanced numerical models shows good agreement with that obtained by monitoring data.

Modelling of soil mechanical stability and hydraulic permeability of the interface between coated biopore and matrix pore regions

Exudates and finer particles often coat the surface of biopores, increasing the mechanical stability and altering the physicochemical properties (e.g. wettability or sorption) of the surrounding. Properties of the biopore surface govern the macropore-matrix mass exchange processes during preferential flow in the soil macropores. However, the relationships between mechanical and hydraulic properties of coated biopore regions are not fully understood nor expressed by numerical models. Correlations between soil hydraulic and mechanical properties could perhaps be established by quantifying the water flow in defined pore structures of the biopore. The objective was to develop a model-based approach for studying the effects of clay-organic coatings on hydro-mechanical properties of biopore walls. The technical challenge was to develop a one-way directed solution (i.e., structural impact on fluid flow) between discrete element method (DEM) and a finite difference Stokes solver (FDMSS) to perform hydro-mechanical simulations of a coated biopore structure. Increasing the coating cohesion by DEM simulation to account for concentrating organo-mineral cementing agents, increased the overall stiffness as well as crack propagation through the soil matrix. As a result, biopore structure displayed higher yield strength and higher permeability through the interface between coating and matrix. Coating cohesion was positively correlated with Young's modulus and permeability displaying non-linear relationships. The one-way directed solution between DEM and FDMSS Stokes solver could be further extended to describe dynamic hydro-mechanical properties of more complex soil structures.

Design and analysis of geosynthetic-reinforced and floating column-supported embankments

ABSTRACT Geosynthetic-reinforced column-supported system is an economic and effective solution to support embankments constructed on soft soils. In this solution, both end-bearing and floating columns are commonly used in practice. For deep soil foundation depths, floating columns are more economical than end-bearing piles. The design of a floating column foundation involves complex soil–structure interactions and there are still no clear uniform guidelines available for the design of embankments supported by floating columns. The main focus of this paper is to present a design method for the geosynthetic-reinforced floating column-supported (GRFCs) embankments. The main features of the proposed method are combining the bearing capacity theory for the floating columns, the arching theory for fill soils, the tensioned membrane theory for the geosynthetic, and considering interaction models between geosynthetic, soil, and piles. Using the proposed method, the influences of the pile geometry, soft clay, geosynthetic, and embankment fills properties were investigated. It was observed that the geosynthetic membrane inclusion enhances the load transfer mechanism and reduces significantly the differential settlements of floating pile-supported embankments. The floating columns with a higher ultimate bearing capacity cause more soil arching. In general, the soil shear strength properties and column geometry (length, diameter, column spacing) have a strong influence on the GRFCs embankment behaviour. Finally, the proposed method is compared with the BS 8006–1 and EBGEO design standards considering several experimental and numerical models to investigate its validity. The results showed that the proposed method is able of very good prediction performance and allows conducting the design optimization of GRFCs embankment.

Calculation of Earth Surface Potential and Neutral Current Caused by HVDC Considering Three-Dimensional Complex Soil Structure

In ground return mode of high-voltage direct current (HVDC) transmission, direct current is injected into the soil through a ground electrode, causing ground potential rise in different areas and direct current at the neutral point of transformers. Earth surface potential (ESP) and neutral current are closely related to the 3-D structure of soil, but few studies treat the soil three dimensionally when calculating these two parameters. In this article, a 3-D resistor network (RN) is developed to construct the soil model, and a soil–grid unified circuit model is proposed to simplify the calculation of neutral current. The RN method is verified by calculating ESP of a horizontal multilayer soil model and contrasting the result with that of CDEGS software. The feasibility and accuracy of the soil–grid unified circuit model is demonstrated through comparing neutral currents of two substations with the results calculated by finite-element method (FEM). The influence of resistivity changes in a water-bearing fault on ESP and neutral current is specifically analyzed using RN method and FEM to prove the necessity of considering the 3-D complex soil structure. The results of this article contribute to the simulation of DC distribution of wide-area HVDC transmission in complex soil structures.

Seismic Performance of Deposit Slopes with Underlying Bedrock before and after Reinforcement by Stabilizing Piles

The seismic performance of stabilizing piles used to reinforce underlying bedrock in a deposit slope is a complex soil–structure interaction problem. Two centrifuge shaking table model testswere conducted to ascertain the dynamic responses of the underlying bedrock deposit slopes without and with the use of stabilizing piles during an earthquake. Multi-stage seismic waves with various peak accelerations were applied from the bottom of each model. The differencesin the response accelerations between the deposit and bedrock increase significantly with the increase in amplitude of the input seismic waves. The presence of the rock-socketed stabilizing piles can bridge the uncoordinated movement of the bedrock and the overlying deposit to some extent. The resultant force arising from a distributed load increment on the piles caused by an earthquake is mainly concentrated in the upper part. With increases in the peak ground acceleration of the input motion, the resistance of the bedrock in front of the stabilizing piles increases and the peak resistance under the bedrock surface of the stabilizing piles gradually moves downwards.This finding indicates that the strong seismic motion significantly changes the embedded working state of the stabilizing pile.

Distribution characteristics of soil particle size in alpine steppe and alpine meadow in the source region of the Yangtze River, China.

To explore the heterogeneity of spatial distribution and particle size distribution (PSD) in the main underlying surface soil in the source area of the Yangtze River, we used the fractal theory to describe the soil particle size distribution of alpine steppe and alpine meadow, and analyzed the difference of fractal dimension characteristics of soil of two different underlying surface and its relationship with soil particle composition. The results showed that soil particle size in the study area was mainly concentrated in 100-800 μm. The single fractal dimension (DV) of alpine steppe soil was 2.429-2.508, and that of alpine meadow soil was 2.697-2.743. The soil texture of alpine steppe was coarse, while that of alpine meadow soil was fine. Soil texture was the finest in the 20-30 cm depth, and the coarsest in the 0-10 cm layer. The multi-fractal dimension (capacity dimension D0, information entropy dimension D1, correlation dimension D2) of alpine steppe (0.896-0.961, 0.828-0.887, 0.725-0.819) was higher than that of alpine meadow (0.890-0.914, 0.693-0.744, 0.540-0.603). Compared with the alpine meadow, the alpine steppe had wider soil PSD, more complex soil structure, and higher soil heterogeneity. DV was positively correlated to clay and silt contents, and negatively correlated to sand content. D1 and D2 were negatively correlated with clay and silt contents, and positively correlated with sand content. Sand content was the main factor accounting for the non-uniform distribution of PSD and the changes of fractal dimension.

Coupling surface flow and subsurface flow in complex soil structures using mimetic finite differences

We explore the coupling of surface and subsurface flows on fully unstructured meshes that conform to complex soil structures. To accommodate the distorted meshes that inevitably result from explicit representation of complex soil structures, we leverage the structure of the Mimetic Finite Difference (MFD) spatial discretization scheme to couple surface and subsurface flows. The MFD method achieves second-order accuracy and maintains local mass conservation on distorted meshes. We couple the diffusion wave approximation for surface flows to the Richards equation for subsurface flow, ensuring continuity of both pressure and flux between the surface and subsurface. The MFD method is particularly convenient for this coupling because it uses face-based constraints in the subsurface system that can be expressed as face-pressure unknowns. Those unknowns are coincident with surface cell-based unknowns, thus allowing the discrete surface system to be directly substituted into the subsurface system and solved implicitly as a global system. Robust representation of the transition between wet and dry surface conditions requires upwinding of the relative permeability and is facilitated by globalization in the nonlinear solver. The approach and its implementation in the Advanced Terrestrial Simulator (ATS) are evaluated by comparison to previously published benchmarks. Using runoff from soils with patchy groundcover (duff) as an example, we show that the new method converges significantly faster in mesh convergence tests than the commonly used two-point flux approximation.

Monitoring the construction of a large-diameter caisson in sand

Large-diameter open caissons are a widely used construction solution for deep foundations, underground storage and attenuation tanks, pumping stations, and launch and reception shafts for tunnel boring machines. The sinking phase presents a number of challenges during construction, including maintaining caisson verticality, controlling the rate of sinking and minimising soil resistance through the use of lubricating fluids. This paper describes the instrumentation and monitoring of a large-diameter caisson on a UK construction site. The caisson was instrumented for the measurement of settlement and tilt, soil–structure interaction contact stresses and structural performance. A key objective for the monitoring project was to provide real-time feedback to the site engineering team to inform the construction process. The monitored data revealed the occurrence of complex soil–structure interactions during sinking that are not readily captured by existing prescriptive design approaches. This case history provides valuable information for the development of an improved basis for design as well as an important frame of reference for future monitoring projects.

Short-term changes in soil pore size distribution: Impact of land use

Changes in land use affect the pore size distribution (PSD) of the soil, and hence important soil functions such as gas exchange, water availability and plant growth. The objective of this study was to investigate potentially damaging and restorative soil management practices on soil pore structure. We quantified the rate of change in PSD six years after changes in land use taking advantage of the Highfield land-use change experiment at Rothamsted Research. This experiment includes short-term soil degradation and restoration scenarios established simultaneously within long-term contrasting treatments that had reached steady-state equilibrium. The land-use change scenarios comprised conversion to grassland of previously arable or bare fallow soil, and conversion of grassland to arable and bare fallow soils. In the laboratory, we exposed intact soil cores (100 cm3) to matric potentials ranging from −10 hPa to -1.5 MPa. Based on equivalent soil mass, the plant available water capacity decreased after conversion from grassland, whereas no change was observed after conversion to grassland. Structural void ratio decreased after termination of grassland and introduction of grassland in bare fallow soil, while no change was seen when changing arable to grassland. Consequently, it was faster to degrade than to restore a complex soil structure. The study illustrates that introducing grassland in degraded soil may result in short-term increase in soil density.

Scaling properties of binary and greyscale images in the context of X-ray soil tomography

Characterization of the complex soil structure is one the cornerstones of soil science and pore space detection is a crucial step in this process. Synthetic soil image construction has been proved to be an efficient resource for validating different binarization methods given that, unlike in real world, ground truth information is known. In this work, we introduce an improved Truncated Multifractal Method (TMM), to better simulate synthetic computed tomography (CT) soil images and then we generate 150 synthetic images with three different porosities (7%, 12% and 17%), both in greyscale and in binary scale (pore spaces). Synthetic images are then compared with two sets of 260 slides of real CT soil images, in order to validate the goodness of the method. All images are subjected to multifractal analysis where we show a detailed comparative analysis of parameters such as lacunarity, characteristic length and multifractal spectrum, that are calculated both for the whole set of synthetic (greyscale and binary) and for the sets of real CT soil images. With respect to lacunarity, a not previously reported inverse relationship between binary and grey lacunarity is found for this range of porosities. Moreover, we have also reported a new relationship between lacunarity and characteristic length. Similar multifractal results, that we obtain for real CT and synthetic soil images, prove that TMM is a reasonable solution to create simulated CT soil images. Finally, a segmentation test was carried out, using TMM synthetic greyscale soil images and its binary counterpart as ground-truth information, evaluating global (Otsu) and local (Combining Singularity-CA) binarization methods, where we report better performance for the last.

Determination of hydraulic conductivity and shear strength properties of unsaturated residual soil from flysch rock mass

Slopes in flysch deposits areas wide within Europe are highly prone to landslide occurrence. Depending on the material properties and climate conditions, instabilities in a form of earthflows, shallow and deep-seated landslides were observed in these formations. Typically, slope instabilities occurred after prolonged periods of rainfall. The Rječina River Valley, Croatia, built in flysch formation, is well known by several landslides occurred in the past. The weathering process of flysch rock mass and local climate conditions resulted in a specific engineering geological profile of the valley, with the unsaturated residual soil covering the bedrock. Although the behaviour of residual soil is important for a landslide triggering both through the rainfall infiltration process and (unsaturated) shear strength, hydro-mechanical properties of this material in unsaturated conditions were not investigated in the past. This paper summarizes the results of different laboratory tests performed on intact samples for hydro-mechanical characterization of the residual soil from flysch rock mass. It was found that the unique shear strength envelope could be used to determine failure conditions both for saturated and unsaturated conditions. The results obtained from strain-controlled and wetting tests performed in conventional and modified direct shear apparatuses indicated high values of the apparent cohesion that the near-surface soil can experience due to the increase of matric suction. The hysteresis effects and hydraulic paths to which soil was exposed to in the past were found to affect the soil behaviour, while the soil formation process results with a complex soil structure that imposes the necessity of using intact soil samples for proper hydraulic characterization of the soil.

Deep Foundation Pit Excavations Adjacent to Disconnected Piled Rafts: A Review on Risk Control Practice

Foundation pit excavation engineering is an old subject full of decision making. Yet, it still deserves further research due to the associated high failure cost and the complexity of the geological conditions and/or the surrounding existing infrastructure around it. This article overviews the risk control practice of foundation pit excavation projects in close proximity to existing disconnected piled raft. More focus is given to geotechnical aspects. The review begins with achievements to ensure excavation performance requirements, and follows to discuss the complex soil structure interaction involved among the fundamental components: the retaining wall, mat, piles, cushion, and the soil. After bringing consensus points to practicing engineers and decision makers, it then suggests possible future research directions.

Percolation theory applied to soil tomography

This study provides insights on the significance of network features of soil macropores on the transport of solutes and colloids, and in the filtrating capacity of the soil. We applied percolation theory and network analysis to the pore network extracted from X-ray computed tomography (imaged porosity) in intact columns sampled from topsoils with different tillage treatments. Moreover, we developed a procedure to extract the backbone, which is the part of the percolation cluster that controls the direct flow between two boundaries in near saturated conditions, using the ImageJ open source imaging software. We also calculated the percolation threshold of each soil, the probability at which the soil starts to percolate (for the resolution considered).Some backbone characteristics (pore volume, wall surface, circularity, fractal dimension, number of loops and tortuosity) showed significant differences between the treatments. Tilled conventional and organic with high earthworm activity exhibited more complex backbones than no-till soil. Backbone volume, surface, fractal dimension and number of loops are correlated with the surface area of pore walls stained by fluorescent microspheres (MS) used as a colloidal tracer. We also found significant correlations between the tortuosity and the number of end-points of the backbone and the transport model parameters for the microspheres and bromide. These findings reinforce the phenomenology between transport in porous media and percolation theory. Moreover, the properties based on percolation theory allow a complete characterization of the complex soil structure and the development of more accurate transport models.

Load-sharing mechanism of non-displacement piled-raft foundation in sand

Piled-raft foundations are recognised for sharing the total load by both the piles and the raft. The economic benefits of this system lead to its popularity in practice; however, research in this field has continued to lag behind owing to the difficulties associated with modelling and availability of field data. This is due to the fact that the load-sharing mechanism of the piled-raft foundation is a complex soil–structure interaction problem, which is governed by the pile/raft/geometry/soil condition and by their by-product parameters. This paper presents the results of an experimental investigation on a prototype model of a piled-raft foundation in sand. The effects of the governing parameters on the load-sharing mechanism are examined. The results of this investigation show that pile spacing and settlement govern the load-sharing mechanism between the piles and the raft. A widely accepted analytical model in the literature was modified in this study to take into account the effect of settlements and pile spacing on the load-sharing mechanism in a piled-raft system. The theory developed is validated by the current experimental results and those available in the literature. The design procedure is presented as a guide in the design of the piled-raft foundation in sand.

On consistent nonlinear analysis of soil–structure interaction problems

Abstract Nonlinear soil–linear structure computational strategy is commonly accepted in the community of geotechnical engineers using advanced finite element software for solving complex soil–structure interaction problems. However, further design procedure of the structural elements is carried out using increased values of the computed elastic stress resultants. It is absolutely not clear whether this method is conservative and, therefore, whether safe or not. To tackle this problem, a fully consistent nonlinear analysis of a deep excavation protected by the diaphragm wall is analysed here. The subsoil is modelled using the Hardening Soil model, while reinforced concrete is modelled using the modified Lee–Fenves model enhanced by the Eurocode 2 (EC2)-compatible creep module, developed by the author. It is shown that the commonly used nonlinear soil–linear structure computational strategy may yield insufficient amount of reinforcement from the ultimate limit state (ULS) and serviceability limit state (SLS) points of view. A consistent and conservative method of combining fully nonlinear analysis and the rules imposed by the EC2 is proposed.