Foundation Soil Research Articles

Experimental and Numerical Research on a Sand Cushion Geotechnical Seismic Isolation System in Strong Earthquakes and Cold Regions

Masonry buildings in high-intensity seismic and cold regions of China face the dual challenges of frost heaving and seismic hazards. To explore the potential of a sand cushion instead of the frozen soil layer to deal with these problems, a cost-effective sand cushion-based Geotechnical Seismic Isolation System (GSI-SC) was developed in this study, where a sand cushion is introduced between the structural foundation and natural soil, while the space around the foundation is backfilled with sand. Shaking table tests on a one-story masonry structure equipped and non-equipped with the GSI-SC system were undertaken to investigate its effectiveness in seismic isolation, where the input wave adopted the north–south component of the EL Centro wave recorded in 1940, and the peak input acceleration (PIA) was set as 0.1 g, 0.2 g, and 0.4 g. It is found that the GSI-SC system significantly reduced the seismic response of the structure, effectively achieving seismic isolation. For a PIA of 0.4 g, the GSI-SC system reduced the acceleration of the roof panel and the inter-story displacement of the structure by 33% and 39%, respectively. Numerical simulations were performed to evaluate the seismic response of buildings equipped and non-equipped with the GSI-SC system. The simulation results matched well with the experimental results, verifying the effectiveness of the newly developed seismic isolation system. The GSI-SC system can provide the potential to reduce frost heave and earthquake disasters for buildings in high-intensity seismic and cold regions.

Analysis of the behavior of piled foundations in unconventional soil

Building foundations using pile groups generally assume that the piles absorb the entire load of the structure. However, it is known that pile caps rest not only on the piles but also on the surrounding soil. These foundations involve great complexity in the phenomenon of load sharing, especially when they are situated on unconventional soils, such as tropical climate soils. To study this behavior, static load tests were conducted on real foundations. These tests were also used to calibrate a three-dimensional finite element numerical model (FEA-3D) using Rocscience's RS3 software. The results showed that the settlement corresponding to the allowable load of the pile groups was significantly lower, ranging from 22 % to 73 %, compared to pile caps that exert contact pressure on the soil. For pile groups, the efficiency ranged between 92 % and 109 %, while for pile caps with contact, it ranged from 132 % to 178 %. In this sense, it becomes evident that estimating the settlement of the pile group based on the conventional design, which does not consider the contact between the cap and the soil, can significantly underestimate the settlement, and provide an overly optimistic analysis of the soil-structure interaction.

Reliability-based ductility reduction factors surfaces using the generalized bojorquez ground motion intensity measure IBg

In this work, reliability-based ductility reduction factors surfaces are obtained through uniform annual failure rate (UAFR) spectra. For this aim, several nonlinear single degree of freedom (SDOF) systems that include various hysteretic behavior models, constant ductility performance and structural reliability levels are subjected to two sets of ground motions recorded on Mexico City. The first set includes 50 ground motion records located on stiff soils, while the second set correspond to 50 soft soil ground motions. In order to compute the UAFR spectra, the ground motion records were scaled at different values of a particular case of the generalized intensity measure IBg named INp for spectral acceleration. The study discusses the influence of different post-yielding stiffness on UAFR spectra obtained for the selected Bojorquez intensity measure, through the bilinear hysteretic model. Additionally, the Takeda model is used to represent structures that exhibit the effect of stiffness degradation. This study is aimed to compute the effect to obtain ductility reduction factors for various uniform annual failure rates, different degrees of post-yielding stiffness, and different ductility capacity for various nonlinear structures. The analyses of the results indicate that strength reduction (through ductility reduction factors) is mainly influenced by parameters such as ductility, failure rate, period of the system and soil conditions. Finally, reliability-based ductility reduction factor surfaces for stiff and soft soil are provided for different UAFR. It is important to say that this is the first time that ductility reduction factors are proposed based on IBg or INp as intensity measure.

One‐dimensional consolidation analysis of layered soil with exponential flow under continuous drainage boundary

AbstractTo comprehensively consider the influence of boundary conditions, non‐Darcy flow, load forms, and soil stratification on soil consolidation, a one‐dimensional soil consolidation equation is established. By subdividing the soil layer and employing time discretization, the nonlinear consolidation equation is linearized, resulting in an analytical solution for layered soil foundation at any given time. Subsequently, an iterative approach for time solution is employed to obtain a semi‐analytical solution. The correctness of the solution is verified by comparison with solutions based on Darcy's flow and the semi‐analytical method under traditional drainage boundary conditions. Subsequently, the influence of interface parameters, loading conditions, flow index, and other factors on consolidation characteristics is analyzed. The results indicate that higher interface parameter values for continuous drainage boundaries correspond to faster average consolidation rates for stratified soil foundations, while these parameters have little effect on the time required for complete consolidation of the soil layers. Improved boundary drainage performance amplifies the influence of exponential flow on pore water pressure and average consolidation degree. Conversely, poor boundary drainage performance diminishes the impact of exponential flow on soil consolidation, rendering it negligible. Moreover, faster loading rates accentuate the influence of the flow index on the average consolidation degree defined by pore pressure.

Local scour effects on seismic behavior of pile-group foundations in cohesionless soils: Insights from quasi-static tests

Local scour has been reported as a common phenomenon for pile-group foundations in marine, coastal, and riverine sites, while its effects on the seismic behavior of pile-group foundations are yet to be well documented. This paper reported a pair of quasi-static cyclic loading tests on 2 × 3 scoured pile-group foundation specimens, one in global scour and the other in local scour, to understand the influence of local scour on the seismic behavior, particularly in terms of soil-pile interaction features and failure mechanisms. Test results indicate a flexural failure mode for both specimens. Local scour does not change the order of limit states but postpones the occurrence of concrete cover cracking and rebar yielding due to the reduced lateral stiffness of the locally scoured piles. Moreover, local scour rarely changes the aboveground damage regions at the top of outer piles (one time the side length of squared pile section, D), but significantly aggravates and deepens the underground damage regions at outer piles (from 4 ∼ 5D for global scour to 3.7 ∼ 6D for local scour). Besides, local scour reduces the displacement ductility factor from 2.50 to 1.25 for the Easy-to-Repair limit state, resulting in adverse impacts on pile-group foundations in general.

Improving the thermal performance of vertical ground heat exchanger by modifying spiral tube geometry: A numerical study

Ground heat exchanger (GHE) is the most crucial element of a ground source heat pump (GSHP) system for building cooling and heating by the utilization of geothermal energy. Therefore, intending to enhance the performance of GHE, the present study conducts a computational investigation of the thermal performance of modified spiral tube vertical GHEs. Several modifications of uniform-pitched spiral GHE are made to increase its thermal performance. Some modifications are introduced as variable-pitched spiral tube GHE where spiral inlet pipes are densified in the lower part of GHEs by reducing pitch distance. Conversely, in some modifications, the position of the outlet straight pipe is changed. Water is considered as the working fluid and the inlet temperature of the water is maintained fixed at 300.15 K. After extensive analysis, it is evident that, when the outlet pipe is placed outside of the spiral coil, there is a 7.67 % enhancement in the thermal performance than a traditional uniform-pitched spiral tube GHE. However, modifications like variable-pitched spiral tube GHEs are not significant to improve the thermal performance due to the quick saturation of the ground soil temperature around the GHE pipes. To have a balance between heat transfer rate and pressure drop, thermal performance capability (TPC) and coefficient of performance improvement (COPimprvt) criterion were evaluated and it is found that the uniform-pitched spiral tube GHE along with the outlet pipe at the outside of the spiral provides maximum thermal performance with a maximum TPC value of 1.062 and provides the positive value of COPimprvt criterion. The positive values of COPimprvt indicate that the spiral tube GHEs are energy efficient based on heat transfer and pressure drop. Moreover, spiral GHE with high-density polyethylene (HDPE), concrete pile, and sandy clay outperform the other materials for pipe, backfill, and soil, respectively. Specifically, HDPE pipe, concrete backfill, and sandy clay as soil offer around 7 %, 5 %, and 7.8 % higher thermal performance compared to polyethylene, sand silica, and clay, respectively.

Assessing the Improvement of Geotechnical Properties of Clayey Soil Using a Substrate Cement Mortar Material, from Ilgin, Konya City, Turkey

The increase in needs and the decrease in places with good, usable foundation soil have made the construction of engineering structures on problematic soils mandatory. Problematic soils generally do not have sufficient bearing capacity and soil strength, which may create an environment prone to high settlement or liquefaction. To investigate potential ways to enhance the compaction, undesirable or problematic plasticity, and soil’s strength characteristics, various geotechnical tests have been carried out on clayey soils and their mixtures treated with a Substrate Cement Mortar (0, 4, 8, and 12% by weight). These tests involved Atterberg limits, dry density, compaction, and shear strength. The obtained results show that the addition of substrate cement materials increased the dry density of soil treated with SCM, which can significantly enhance the soil properties. Meanwhile, adding SCM decreased the soil's plasticity limits. Furthermore, adding 12% Substrate Cement Mortar resulted in the lowest optimum moisture content and the maximum dry unit weight. Soil treated with 12% also exhibits maximum cohesion and shear strength. This research led to the conclusion that the utilization of Substrate Cement Mortar additive is an inexpensive product and has significantly improved the soil’s geotechnical parameters.

Non-linear critical speed in high-speed ballasted railways with ground reinforcement

When constructing or upgrading high-speed railway lines on soft soils, reinforcing the soil foundation becomes crucial. This not only involves small track displacements but also increases the critical speed, thereby ensuring safe and efficient regular operations. Nevertheless, existing studies on the critical speed phenomenon in high-speed railway tracks with ground reinforcement are limited, and they all assume a linear elastic behavior for both the soils and reinforcement materials. The main novelty of this research is that it presents the first study on the effect of non-linear soil behavior on the critical speed of high-speed railway ballasted tracks with soil reinforcement and introduces a novel simplified approach based on an equivalent homogeneous soil layer by replacing the soil layer containing inclusions/columns. To accomplish this, a full 3D non-linear numerical model was developed and experimentally validated. The findings from a detailed and comprehensive parametric analysis demonstrate a significant influence of non-linear behavior on the critical speed, resulting in reductions of up to 30% compared to the linear elastic scenario. Furthermore, it is found that the soil-reinforcement stiffness contrast, the area replacement ratio, and the plasticity index of the soil foundation play a crucial role in the critical speed. Other aspects such as the installation pattern and the thickness of soft soil have a relatively lower impact on the critical speed.

Research on the Reinforcement Characteristics of Thick Cushion Layer and Rigid Pile Composite Foundation

The rigid pile composite foundation method is the most commonly used method for strengthening weak soil foundations. In this method, piles usually need to pass through weak soil layers, and the pile end falls on a bearing layer with good bearing capacity. Under existing technical conditions, the thicker the weak soil layer, the longer the pile body, and the more difficult it is to ensure the construction quality of the pile. In response to this issue, some scholars have adopted the rigid pile composite foundation method with a thick cushion layer for reinforcement treatment. This article uses PLAXIS 3D (V20.04.00.790) software to establish a finite element model of rigid pile composite foundation with a thick cushion layer and simulate the process of foundation reinforcement. The influence of parameters such as thickness, compression modulus, and shear strength index of the cushion layer on foundation settlement and pile–soil stress distribution is studied, and the reasonable range of these parameters is analyzed under the condition of considering reinforcement effect. Through comparative analysis, it can be concluded that for deep and weak soil areas, the thickness of the cushion layer can range from 0.5 to 2.6. The thickness and compressive modulus of the cushion layer have a significant impact on the settlement of the foundation, the pile–soil stress ratio, and the stress of the pile body, while the shear strength index of the cushion layer has a relatively small impact on these parameters. Reasonably selecting the geometric and mechanical parameters of the cushion layer can effectively reduce stress concentration at the pile top and better play the role of the soil between piles.

Field Observation and Settlement Prediction Study of a Soft Soil Embankment under Rolling Dynamic Compaction

Rolling dynamic compaction (RDC) has been found to be useful for compaction soils and is now widely used globally. Because RDC is not often used in soft soils with poor engineering properties, field monitoring was used to study the soft clay embankment responses under RDC conditions in this study. Analysis of the monitoring data revealed that the response of the soil occurred mainly in the first 20 passes. Field monitoring revealed a strong correlation between settlement, horizontal displacement, and pore water pressure. The depth of impact of RDC on the soft soil embankment was between 3 and 3.5 m. Although settlement prediction is an important issue for construction, there is a lack of prediction methods for RDC-induced soil settlement. In this study, we used three different machine learning algorithms: random forest regression (RFR), multilayer perceptron (MLP), and extreme gradient boosting (XGBoost) to predict the total settlement and uneven settlement induced by RDC on the soft soil embankment. The three prediction models were compared, and the predictive accuracy of these models was assessed. This study analyzes and summarizes the effect of RDC application on a soft clay embankment and explores the machine learning method used for settlement prediction based on monitoring data, which provides some methods and ideas for research on the application of RDC on soft soil foundations.

Application of FEM and Artificial Intelligence Techniques (LRM, RFM & ANN) in Predicting the Ultimate Bearing Capacity of Reinforced Soil Foundation

In this research paper, the behavior of shallow footing with square and rectangular shapes over geosynthetic reinforced soil was studied. A novel geogrid called “3D tube-geogrid” was utilized for this work. The impact of various reinforcement parameters, including the depth of the final layer (z), length (l), inclination (α), filler material used inside the geogrid tube, relative soil density, and the tensile stiffness of the geogrid (EA), were analyzed by running numerical simulations using PLAXIS 3D V20 software. The simulated data were used to quantify the relationship between the ultimate bearing capacity of the soil and the reinforcement parameters. Several artificial intelligence (AI) techniques, such as linear regression analysis, a random forest model, and an artificial neural network (ANN), were employed on the generated dataset. To evaluate the preciseness of these techniques, various statistical indicators, such as the squared correlation coefficient (R2), mean absolute percentage error (MAPE), mean squared error (MSE), and root-mean-square error (RMSE), were calculated, and error percentages of 20.98%, 12.5%, and 6.4% were obtained for the linear regression, random forest, and ANN, respectively. The numerical study determined the optimal values of the reinforcement parameters length, z/B, inclination, and filling material to be 4B, 3, 0°, and aggregate, respectively.

The Role of Spatial Distribution of Geotechnical Soil Parameters in Site Investigation

Abstract The correct investigation of the foundation soil is essential for the optimal and efficient design of any structure. The interaction of a structure with the foundation soil can only be evaluated through the physical and mechanical characteristics of the intercepted geotechnical layers on the entire zone of influence. For this reason, the role of technical documentation that includes and summarizes field investigations and laboratory tests is particularly important. An important and sometimes complicated component in providing useful design information is the division into geotechnical or computational layers. This can be done at different levels, starting from the physical characteristics such as color, grain size distribution, plasticity and consistency and can continue with the evaluation of the mechanical characteristics of compressibility and shear strength. The aim of the paper is to create a graphical representation of the geotechnical parameters using a spatial interpolation technique (Kriging method). The creation of 2D maps using SURFER software assists geotechnical engineers in the correct interpretation of the geotechnical parameters. This interpolation technique for division into layers is also useful in quarries and borrows pits, when soil is used as construction material.

Analytical modeling and experiments on soft soil consolidation with vacuum preloading‐airbag pressurization

AbstractThis study aims to provide substantial theoretical support for employing vacuum preloading‐airbag pressurization (VP‐AP) to enhance soft soil foundations. By integrating the airbags and prefabricated vertical drains (PVDs) within the analytical framework, the consolidation models are developed to accommodate double smear zones and well resistance. Analytical solutions for various airbag pressurization scenarios are derived under the equal‐strain assumption. A model test is subsequently conducted to investigate the settlement process of soft soil using the VP‐AP method. The validity of the proposed theoretical model is corroborated through comparison with existing analytical solutions and experimental data. The accuracy of the predictive model is further substantiated by the model test results. Lastly, this research offers an in‐depth analysis of soil consolidation behavior under diverse influencing factors. The findings indicate that the VP‐AP method demonstrates superior efficacy over conventional vacuum‐surcharge preloading when the airbag pressure matches the external load pressure. Additionally, an increase in the radius of airbags enhances the soil consolidation efficiency. The analytical model presented in this study elucidates the consolidation mechanisms of the VP‐AP method, with experimental research confirming the efficacy of this approach and the predictive model established herein.

System reliability analysis of geogrid reinforced retaining wall using random finite element method

Stabilizing earthwalls is extremely important in geotechnical engineering projects and has become an integral part of transportation infrastructures. Meanwhile, geogrid reinforced retaining walls have been the attention of designers due to their advantages. On the other hand, the soil heterogeneity and the requirement of investigating the internal (geogrid rupture and geogrid pullout) and external (global and lateral displacement) stability modes of these structures have necessitated performing system reliability analysis. In this study, finite element in conjunction with random fields is used to evaluate the reliability of these walls. For this purpose, a finite element program is coded in MATLAB to obtain internal and external safety factors considering staged construction. The deterministic program is extended to a stochastic framework to account for spatial variability of soil parameters in retained backfill, foundation soil, and reinforced fill. In the last part of this study, reliability indices of stability modes are calculated to obtain the series system reliability index, utilizing the Sequential Compounding Method (SCM). The results indicated that the effect of soil heterogeneity is more significant on internal stability modes compared to external stability modes especially geogrid pullout. The results of the system reliability analysis demonstrated more critical conditions thus the reliability index of the system was less than the reliability index of individual components.

Study on heavy metal leaching from steel slag in coastal soft soil and its environmental impact

ABSTRACT To explore the environmental impact of steel slag deposited within a coastal soft soil foundation, the steel slag produced by a steel plant in Fangchenggang city was chosen as a case study. Seawater was collected from the coastal foundation, and water with different levels of salinity was obtained by low-temperature evaporation, filtration, sterilisation, and dilution. Leaching tests of steel slag in artificial seawater with different levels of salinity were carried out according to the Toxicity Characteristic Leaching Procedure (TCLP). The results showed that with the increase in the leaching time and salinity of the solution, the concentration of heavy metals in solution displayed an upward trend; however, the rate of increase decreased once the concentration reached a certain level. From an environmental perspective, it is not advisable to use steel slag as a filler near drinking water, and steel slag should not be used as a foundation material in the high-salinity coastal soft soil layer.

Kinematic response of floating single piles in saturated and nearly-saturated soil to P-waves

This paper presents closed-form expressions for the analysis of floating single piles subjected to vertically propagating harmonic P-waves. The foundation soil is treated as poroelastic medium, and can be either fully-saturated or nearly-saturated. We show that the existence of non-continuous air phase in the form of air bubbles dissolved in the pore water will have noteworthy effect on the vertical kinematic response of piles. Through parametric analyses we demonstrate that hydromechanical analysis techniques that consider soil as fully saturated may significantly over-estimate pile head displacements for frequencies relevant to seismic P-waves. In addition, we quantify differences in the response between end-bearing and floating piles, and show that these differences are more prominent when soil is not fully saturated. Although the proposed expressions have been derived while considering elastic soil and pile response, findings relevant to the effect of the degree of soil saturation to the vertical kinematic response of piles are generally applicable to any coupled stress-flow analysis method.

Viscosity of Clayey Soils: Experimental Studies

Due to the high rate of the development of housing, transportation and hydraulic engineering construction in the last hundred years, the study of the phenomenon of creep of clay soils has become a subject of scientific research. In the study, experimental investigations of clay soil were conducted using a simple shear device in kinematic loading mode, aimed at examining the influence of shear rate on the viscosity coefficient of the clay soil and its strength characteristics. The tests were performed at four different shear rates and three different vertical load values. Based on the results of experimental and theoretical studies, the viscosity coefficients of clay soil were obtained, and a new rheological equation was proposed, which simultaneously takes into account the influence of Coulomb friction, structural cohesion, cohesion of water–colloidal bonds and viscous resistance of the soil. It has been shown that the shear rate has a significant impact on the viscosity coefficient of clay soil, and the viscosity coefficient itself is a variable quantity, depending both on the magnitude of the applied load and the duration of its application. The obtained results can be used for further improvement of methods for calculating the settlement of structures over time, as well as for predicting the time until the bearing capacity of foundation soils is exhausted.

Settlement and stress analysis in stabilized slab- and pile-supported embankment based on double-equal settlement plane

PurposeThe soft stabilized slab and pile-supported (SSPS) embankment is an improvement technique to increase the efficiency of resources in road construction. To capture the effects of stabilized slabs on the stress transfer mechanism, the differential settlements and the lateral displacement of the embankment completely. A theoretical model of SSPS is proposed by considering the effect of soil arching and the interaction between the embankment fill, stabilized soil, pile, foundation soil and bearing stratum.Design/methodology/approachIn the theoretical model, the stress and strain coordination relationship of the system was analyzed in view of the minimum potential energy theory and equal settlement plane theory. Subsequently, the theoretical method was applied to field tests for comparison. Finally, the influence of the elastic modulus and the thickness of the stabilized slab on the stress concentration ratio and foundation settlement were examined.FindingsIn addition to the experimental findings, the method has been revealed to be reasonable and feasible, considering its ability to effectively exploit the stabilized slab effect and improve the bearing capacity of soil and piles. An economical and reasonable arrangement scheme for the thickness and strength of stabilized slabs was obtained. The results reveal that the optimum elastic modulus was chosen as 28 MPa–60 MPa, and the optimum thickness of the stabilized slab was selected as 1.5 m–2.1 m using the parameters of field tests, which can provide guidance to engineering design.Originality/valueAn optimization calculation method is established to analyze the load transfer mechanics of the SSPS embankment based on a double-equal settlement plane. The model’s rationality was analyzed by comparing the settlement and stress concentration ratios in the field tests. Subsequently, the influence of the elastic modulus and the thickness of the stabilized slab on the stress concentration ratio and settlement were examined. An economical and reasonable arrangement scheme for the thickness and elastic modulus of stabilized slabs was obtained, which can provide a novel approach for engineering design.

APPROACHES TO ASSESSING THE STABILITY OF BANK PROTECTION STRUCTURES OF WATERBODIES: ANALYSIS OF CONSTRUCTIONS AND MODELS FOR THEIR CALCULATION

The article analyzes the theoretical foundations for determining the stress state in a soil mass and the design of fastening the bank slope of reservoirs. Scientific research and theoretical principles on determining the forces that act on a bank protection structure have been systematized. Methodological approaches to the static calculation of bank protection for indirectly vertical structures are proposed, taking into account the relationship between the load on the structure and its deformation. The purpose of the research is to ensure the stability and reliability of bank protection structures and to substantiate directions for improving technical solutions in modern conditions. The work analyzes the use of various types of slope fastenings for bank protection structures in accordance with the requirements of State construction standards. It is proposed to focus research on sheet piling shore fastenings, as a modern and progressive technology for bank protection. The "soil massif - fastening structure" system is considered as a calculation model in the form of a one-sided type, which is an elastic element, which makes it possible to apply the modern apparatus of the theory of elasticity in considering this problem. This makes it possible to accept a linear relationship between stress and strain and obtain sufficient accuracy, which is confirmed by the available results of domestic and foreign research. For calculations of deformations, and assessment of the strength and stability of soil massifs and foundations, it is proposed to pay direct attention to the characteristics of the mechanical properties of soils, while three stages of foundation deformation are considered. The formulated differential equations of the equilibrium of the soil massif make it possible to solve a wide range of issues related to the limit equilibrium and to obtain the calculated parameters of the pressure of earth masses on the retaining walls of shore fortifications of the oblique-vertical type. The results of the research analysis are recommended for use in determining the main loads on hydraulic structures, substantiating technical solutions for the development and improvement of slope and slope-vertical types of bank protection of reservoirs.

Finite Element Analysis of Geogrid-Incorporated Flexible Pavement with Soft Subgrade

Improving the durability of flexible pavements and constructing new roads on weak soil foundations present significant challenges, prompting designers to explore alternative methods to prolong pavement lifespan. Geosynthetics have emerged as a promising solution for soil stabilization, with various materials developed for this purpose. The current study concentrates on using the finite element (FE) method to examine the effectiveness of geogrid-incorporated flexible pavements on soft soil substrates. A three-dimensional layered pavement is constructed with an FE model, incorporating subgrade layers of varying strengths based on their California bearing ratio (CBR) values, with a geogrid layer implemented to enhance subgrade stability. Additionally, attention is also given to investigating the effect of base course thickness. The findings reveal that the geogrid layer primarily influences the formation of plastic strains in the subgrade rather than resilient strains, effectively reducing vertical compressive strain by approximately 40%. With increasing CBR values, there is a reduction in vertical strain, although the influence zone extends up to a depth of 300 mm within the subgrade. At the surface of the subgrade, vertical strain decreases by around 17%, 39%, and 49% as the CBR values increase from 1% to 3%, 5%, and 8%, respectively.