Soil Stress State Research Articles

Long-Term Settlement Prediction for Over- Consolidated Soft Clay under Low Embankment

The long-term post-construction settlement of an embankment laid on a deep, soft soil foundation can give rise to a series of safety concerns and significant structural damage. The settlement is primarily attributed to the creep deformation of the soft soil following the removal of the surcharge load and the impact of traffic loads on the soft soil. In this study, a plan strain triaxial test was conducted to investigate the deformation of an undisturbed soft clay specimen subjected to static and cyclic loading. The results demonstrate that the volume creep and vertical creep are associated with the overconsolidation state of the soft soil. The Over-Consolidated Ratio (OCRq) of shear stress, can be used as a parameter to describe the state of overconsolidation of soft soil under spherical stress. Based on the vertical creep coefficient and considering the influence of stress history on the stress state of soft soil, a two-dimensional long-term settlement model under cyclic loading has been proposed.

Finite element analysis of soil-nailed vertical excavation in fine-grained soils in an urban area

This paper presents analysis and discussion of an urban vertical excavation, with the soil nailing technique, in fine-grained soils with high plasticity. The excavation was numerically evaluated using finite element analysis (FEA) using a 3D finite element model (FEM). Several field and laboratory soil investigations were conducted, interpreted and used in the FEA. The FEM model was calibrated using instrumentation and monitoring results during the soil nailing construction. Analyses were conducted focusing in horizontal displacements, nail pullout strength, nail tensile load, soil stress state, and tensile load at the nail head. The results showed that the behavior of the soil nailing retaining wall in fine-grained soils differs significantly from what is typically reported in the literature for non-cohesive soils. The high apparent cohesion of the fine-grained soil resulted in good performance, with reduced horizontal displacements on the face and lower tensile loads in the nails and at their head.

A new calculation method for the response of active-passive bridge piles considering pile-soil interface friction

The pile foundation of bridges under unregulated surcharge loads experiences a complex force pattern, which is subject to active loads and passive loads. In this paper, a more comprehensive calculation method for the response of active-passive piles was proposed, which considered the effect of pile-soil interface friction and soil stress states on the passive loads. First, the Ito theory was improved by adopting the adhesion c a and external friction angle φ a to characterize the pile-soil interface friction. Then, the soil around the pile in elastic, elastoplastic, and plastic flow states was considered to calculate the variation of the passive load. Finally, a nonlinear p-y curve was introduced to establish the calculation model of active-passive piles, and the finite difference program CPAPP was prepared for the solution. The results show that the pile response calculated by CPAPP is consistent with the field measurement, indicating that the proposed calculation method has high accuracy. When the soil is in an elastic state, the pile-soil interface friction does not affect the passive load. When the soil is in a plastic state, the passive load is closely related to soil strength parameters and pile-soil interface friction parameters.

Influence and Mechanism of the Excavation Width on Excavation Deformations in Shanghai Soft Clay

This study investigated the influence and mechanism of the excavation width on excavation deformations in Shanghai soft clay. Based on three excavations that had different final excavation depths, dissimilar retaining structures and diverse geological conditions, 40 sets of two-dimensional numerical models with different excavation widths were employed to analyze the deformation rules affected by the excavation width. Moreover, a series of simplified models with different excavation widths were employed to analyze the effect of the excavation width on excavation deformations. The results show that under the same excavation depth, both the horizontal displacements of the retaining walls and ground surface settlements increase as the excavation width increases, but the increasing rate gradually decreases. Factors such as the unloading influence depth, the overlap degree of the passive zones, the stress state of the basal soils and the development of the relative shear stress have a significant influence on excavation deformations. With increasing excavation width, the unloading influence depth gradually deepens, the overlap area of the passive zones gradually decreases, the direction of the rotation of the major principal stress gradually reduces and the relative shear stress of the distant and deep soils gradually expands. Therefore, the constraint ability of the passive zones on excavation deformation gradually reduces and excavation deformations gradually increase.

Geostress-associated settlements of a raft-foundation building due to shield tunnelling in soft ground

The stress state of soil may affect the building settlements induced by tunnelling, which, however, has not been well understood. In this study, 3D numerical analyses combined with in-situ measurements were performed to investigate the geostress-associated settlements of a raft-foundation building due to tunnelling in soft ground. Basically, two types of geostress fields were investigated: the first type considered the effect of additional stress generated in the foundation soil (FAS) due to building weight; while in the second type, a sequential twin tunnelling was presumed, and the effect of additional soil stress induced by the first tunnel (TAS) on the building response to the second tunnel was considered. The results indicated that FAS may aggravate the stress release of the foundation soil, and thus gave rise to a larger building settlement or inclination. In the sequential tunnelling process, the effect of TAS can be more complex: when the first tunnel lowered the stress of foundation soil, TAS effect of the first tunnel may help reduce the building settlements induced by the second tunnel; otherwise, it may aggravate building settlements. In addition to TAS effect, the sheltering effect was also found to play an important part in twin tunnelling.

Analysis of hole wall stability in saturated clay using undrained solution for cylindrical cavity contraction

To investigate the stress state of soil during the instability of the hole wall (hole shrinkage) in saturated clay, the theory of cylindrical cavity contraction in an infinite medium is applied, utilizing the modified Cam-Clay (MCC) model. By employing intermediate variables, the issue of cylindrical cavity contraction is reformulated into a first-order ordinary differential equation in Lagrangian form, with three effective stress components and pore pressure as the unknowns. In MATLAB, the stress conditions at the elastoplastic boundary serve as the initial values for solving the initial value problem of the differential equation system. After determining the correlation between the stress distribution around the cavity and the diameter reduction ratio, the analysis proceeds to evaluate the internal support pressure required for stabilizing the hole wall according to various stability criteria. This pressure is then integrated into the hole wall stability coefficient formula within the soil pressure model, facilitating the evaluation of hole wall stability. The effectiveness and accuracy of the proposed semi-analytical elastic–plastic solutions were subsequently confirmed through comparison with finite element analysis results, underscoring its relevance for addressing hole wall stability challenges.

Liquefaction-induced large deformation method with automatic time-step mapping and interfacial interpolation improvement: Case study of the San Fernando dam

This study constructed the u-p equation for saturated soil in the meshfree global weak form within the arbitrary Lagrangian-Eulerian (ALE) framework to solve the problem of liquefaction-induced large deformation induced by strong seismic loading. The interpolation improvement in the near-interface zone was achieved using domain truncation optimization and the local nodal refinement algorithm, and the relative error of the shape function was proposed as the judgment criterion for radial basis function (RBF) field variable mapping. Finally, a high precision meshfree liquefaction-induced large deformation method (MFLLDM) with automatic time step mapping and interfacial zone interpolation improvement was established. The MFLLDM achieved the efficient dynamic updating of soil stress state and pore pressure information in space during the deformation process and more accurately depicted the localized large shear deformation characteristics of the soil in the near-interface zone. The proposed method was integrated into a custom-built large-scale finite element method (FEM) computing system (GEODYNA), based on object-oriented and super element program design technology. This enabled the efficient coupled analysis of MFLLDM in the liquefied-induced large deformation zone and FEM in the small deformation zone. Numerical consolidation examples were used to validate the accuracy, convergence, and applicability of the soil elasto-plastic model for MFLLDM. The method was then used to simulate the liquefaction damage of the San Fernando dam, which reproduced the development process of large slip deformation and high-water pressure ratio for the upstream dam slope.

LSTM-based deformation forecasting for additional stress estimation of existing tunnel structure induced by adjacent shield tunneling

A redistribution of stress is caused by the excavation of a new tunnel within the soil surrounding the existing shield tunnel structure. This stress redistribution results in alterations to the soil and water loads around the existing tunnel structure, ultimately impacting its stress and deformation. The deformation response of existing tunnels is primarily focused on in most existing studies, with limited research conducted on the coupling system of new tunnel excavation, soil stress state, and deformation and stress of existing tunnels. This paper proposed an innovative methodology to predict deformation in existing tunnel structures and estimate additional stress during the excavation of a new shield tunnel. The reliability of the suggested calculation method was verified using measured internal force and convergence data of the structures. The results demonstrated that the Long Short-Term Memory (LSTM) model could more effectively establish the temporal correlation between the excavation parameters of new tunnels and the deformation of existing tunnels. Compared to the Recurrent Neural Network (RNN) model, the R2 of the LSTM model increased by 11.81% in the prediction of the tunnel waist and 9.38% in the prediction of the tunnel crown. The Light gradient boosting machine (LightGBM) algorithm was used to analyze the crucial factors of deformation of the existing tunnel structures during shield tunneling, and the results showed that the relative position and the average earth pressure were the essential factors. Furthermore, a calculation method for the additional stress of existing tunnel structures was proposed based on the load structure model. The method assumed that the additional load was distributed rectangularly at the top and bottom of the existing tunnel, and triangularly at the waist of the tunnel. The comparison between the calculated results and the measured results indicated that the phased characteristics of the internal force changes in the existing tunnel structure during the construction of a new tunnel could be accurately reflected by this method.

Evaluation of the coefficient of lateral stress at rest of granular materials under repetitive loading conditions

Although the internal stress state of soils can be affected by repetitive loading, there are few studies evaluating the lateral stress (or K0) of soils under repetitive loading. This study investigates the changes in K0 and directional shear wave velocity (Vs) in samples of two granular materials with different particle shapes during repetitive loading. A modified oedometer cell equipped with bender elements and a diaphragm transducer was developed to measure the variations in the lateral stress and the shear wave velocity, under repetitive loading on the loading and unloading paths. The study produced the following results: (1) Repetitive loading on the loading path resulted in an increase in the K0 of test samples as a function of cyclic loading number (i), and (2) Repetitive loading on the unloading path resulted in a decrease in K0 according to i. The shear wave velocity ratio (i.e. Vs(HH)/Vs(VH), where the first and second letters in parentheses corresponds to the directions of wave propagation and particle motion, respectively, and V and H corresponds to the vertical and horizontal directions, respectively) according to i supports the experimental observations of this study. However, when the tested material was in lightly over-consolidated state, there was an increase in K0 during repetitive loading, indicating that it was the initial K0, rather than the loading path, which is responsible for the change in K0. The power model can capture the variation in the K0 of samples according to i. Notably, the K0 = 1 line acts as the boundary between the increase and decrease in K0 under repetitive loading.

Dual Benefits of Hydrogel Remediation of Cadmium-Contaminated Water or Soil and Promotion of Vegetable Growth under Cadmium Stress.

This study aims to solve the problem of cadmium heavy metal ion pollution caused by the abuse of chemical fertilizers and activities such as mining, which pose a serious threat to the plant growth environment. We successfully synthesized DMAPAA (N-(3-(Dimethyl amino) propyl) acrylamide)/DMAPAAQ (N, N-Dimethyl amino propyl acrylamide, methyl chloride quaternary) hydrogels via free radical polymerization. Subsequently, we conducted experiments on this hydrogel for growing vegetables under cadmium stress conditions in aqueous solutions and soil. The cadmium capture capacity of DMAPAA/DMAPAAQ hydrogels under different cadmium ion concentrations and pH values was evaluated by using inductively coupled plasma optical emission spectrometry (ICP). The research results show that under the condition of pH = 7.3, the cadmium capture capacity of DMAPAA/DMAPAAQ hydrogels is the greatest. We used the Langmuir model to fit the adsorption data, and the correlation coefficient was as high as 0.96, indicating that the model fits well. The application of the hydrogels promoted the growth of vegetables in soil under cadmium stress conditions. The results showed that when the added amount of hydrogel was 4%, the dry weight of the vegetables was the largest. In addition, when the added amount of cadmium was 500 mg/kg and the added amount of hydrogel was 4%, the absorption of cadmium by the vegetables decreased to an undetectable level. In summary, the hydrogel successfully synthesized in this study can be effectively used to immobilize cadmium ions in soil while positively promoting the growth and yield of vegetables. This achievement has practical significance for solving the problem of heavy metal ion pollution.

Analytical analysis for horizontal displacement of pile anchor support structure considering prestress

The pile-anchor supporting structure is widely used in foundation pit engineering, it is particularly necessary for calculating lateral displacement of the pile-anchor supporting structure. The soil stress state is transformed by the additional stress caused by the prestress, thus the lateral displacement can be decreased own to pile-anchor supporting structure bearing active earth pressure had changed. Prestress as a concentrated force is decomposed into the horizontal component and vertical component, calculating the additional stress caused by the prestress and getting the formula of active earth pressure of considering the additional stress, setting up calculation model of the pile-anchor supporting structure. For typical examples of foundation pit engineering, the parameters are substituted into a program written based on derived formulas, and the elastic deformation superposition method is used to calculate the horizontal displacement of the pile anchor support structure. The comparison and verification is carried out by Finite element analysis. The influence of cohesion and friction angle on pile displacement, bending moment, and shear force has been studied. This study found that cohesion and friction angle have a significant impact on the displacement of pile anchor support structures.

Triaxial compression test of coarse-grained soil in waste dump under different consolidation stresses conditions

In order to explore the mechanical response characteristics of coarse-grained soil in waste dump under different consolidation stress conditions, the isobaric consolidation drained triaxial test and K0 consolidation drainage triaxial test under different confining pressures are performed, and the test data of stress-strain and volume deformation of soil are compared and analysed. The results show that the initial stress of soil under K0 consolidation condition is not zero. The shear stress-strain curves of soil with different consolidation conditions show hardening characteristics. When confining pressure increases, the shear stress of isobaric consolidated soil increases rapidly, and its failure shear stress increases significantly compared with K0 consolidated soil. Shear expansion of coarse-grained soil under low confining pressure and shear shrinkage under high confining pressure. Under low confining pressure, K0 consolidated soil is more prone to shear expansion, and the amount of shear expansion is greater than that of isobaric consolidated soil. Under medium and high confining pressure, the volume compression change of isobaric consolidated soil is greater than that of K0 consolidated soil. The initial tangent Poisson’s ratio of coarse-grained soil under different consolidation stress conditions decreases with the increase of confining pressure, and changes in a power function with the confining pressure. The cohesion of soil under isobaric consolidation stress condition, while the difference of internal friction angle of soil under different consolidation stress condition is small. The strength and deformation characteristics of soil are closely related to the initial stress state of soil. The triaxial test is used to obtain the shear strength index of soil, and the isobaric consolidation condition should not be used instead of K0 consolidation condition.

Numerical Simulation of Karst Collapse in Qingling Town, Wuhan

With background information on the geological conditions of Fenghuo village, Qingling town, Wuhan, based on Terzaghi consolidation theory and GMS software, a hydrodynamic-mechanics coupling model was established. On the basis of simulating the groundwater flow field by GMS software, a mechanical calculation program was developed, and the coupling simulation of hydrodynamic mechanics was achieved. The results showed that the stress state, vertical deformation, and damage of soil are closely related to the underground water flow. The soil pressure value and effective stress value are relatively low from July to August every year. It is the result of buoyancy produced by groundwater. Soil settlement generally occurs in the dry season. When the load is not considered, the deformation value of soil settlement is generally less than 6 mm. In the wet season from July to August every year, the soil will float up, and the floating value is less than 4 mm. The groundwater table changes frequently, and the destruction around the cave body eventually leads to karst collapse.

Relationship between the Intermediate Soil State and Settlement Control Measures during Tunnel Construction Undercrossing the Existing Station

Settlement control of existing stations has consistently been a key issue in tunnel construction. Intermediate soil has a significant influence on the settlement of the existing station as a connector between the new tunnel and the existing station which should be considered when selecting settlement control measures. To clarify the relationship between the state of the intermediate soil and the settlement and control measures of the existing station, this study investigated 49 new tunnel projects under existing stations and elucidated the relationship between the settlement and the settlement control measures of the existing station and the intermediate soil. The relationship between the failure of the intermediate soil and the settlement of the existing station was further analyzed by numerical simulation. Lastly, a simple mechanical model of intermediate soil failure was constructed to determine the stress state of the intermediate soil in constructing a tunnel under the existing station to guide the formulation of settlement control measures for the existing station. Hence, when the intermediate soil is completely destroyed, active control measures, such as jack or grouting lifting, should be implemented; when the intermediate soil is partially damaged, passive control measures should be undertaken. Thus, this study can provide a reference for settlement control of tunnels under existing stations.

Study on dynamic strength and liquefaction mechanism of silt soil in Castor earthquake prone areas under different consolidation ratios

Under the Castor earthquake, there is a risk of liquefaction instability of saturated tailings, and the evolution of dynamic pore pressure can indirectly reflect its instability process. Before applying dynamic loads, the static stress state of soil is one of the main factors affecting the development of soil dynamic strength and dynamic pore pressure, and there are significant differences in soil dynamic strength under different consolidation ratios. This paper conducted dynamic triaxial tests on saturated tailings silt with different consolidation ratios, and analyzed the dynamic strength variation and liquefaction mechanism of the samples using the discrete element method (PFC3D). The results showed that 1) as the Kc′ gradually increased, and there was a critical consolidation ratio Kc′ during the development of the dynamic strength of the sample. The specific value of Kc′ was related to the properties and stress state of saturated sand. The Kc′ in this research was about 1.9. When Kc < 1.9, dynamic strength was increased with the increase in Kc; when Kc > 1.9, dynamic strength was decreased with the Kc. 2) Under the impact of cyclic load, when samples were normally consolidated (Kc =1), the pore water pressure would tend to be equal to the confining pressure to cause soil liquefaction. In the case of eccentric consolidation (Kc > 1), the pore water pressure would be less than the confining pressure, thus, the soil liquefaction would not be induced, and the pore pressure value would decrease with the increase of consolidation ratio. This paper provides engineering guidance value for the study of dynamic strength and liquefaction mechanism of tailings sand and silt in Castor earthquake prone areas under different consolidation ratios.

Investigation of the time-dependent bearing capacity of a jacked pile in saturated structured clays

Natural clay setup significantly contributes to the improvement of the bearing performance of a jacked pile. This paper presents a semianalytical solution to predict the time-dependent bearing capacity of a single pile in natural clays. The proposed solution properly incorporates the initial structure and stress-induced structure degradation of soils and the subsequent thixotropic and reconsolidation of disturbed soils. Pile penetration is simulated using the cavity expansion method (CEM) based on the Modified Structured Cam Clay (MSCC) model. The governing equations computing stress components during consolidation are established using a variable relaxation gradient function and are numerically solved by combining initial boundary conditions. A functional relationship between the ultimate strength and stress state of soils is presented according to critical state soil mechanics theory. The predicted results are validated against the results from the existing analytical approach, published centrifuge model tests, and laboratory model tests in structured clays. The excellent agreement between the measured and predicted values demonstrates that the method can properly predict the time-dependent bearing capacity of jacked piles in structured clays. Considering the impact of soil structure on soil setup plays a significant role in quantitatively evaluating the time-dependent bearing capacity of piles in natural clays.

Numerical Modelling of Displacement Pile Resistance in Sand Ground. Part 2: Discrete Model, at Rest Stage, Load Test

The article presents a continuation of research results on numerical modelling of displacement pile prototype ground (sand) resistance. Numerical modelling of ground resistance consists of three stages: restitution of initial soil stress state, the restoration of stress state developed during pile installation and the modelling of pile resistance against applied load (pile load test). Restitution of initial ground state induced in artificially created sand deposit after compaction procedures was realized by creating stress history and defining experimentally determined over consolidation stresses. Installation effects were restored using a well-known empirical approach based on relation between CPT test data and radial stress increase. Hardening soil model and its parameters were employed for modelling pile test substantiated in the first paper (Part 1) of the research. The discrete model of pile, soil layers, and pile-soil contact was created. Modelling of pile loading-unloading resistance was applied using an incremental loading procedure. Numerical analysis results validated with displacement pile prototype testing results.

Experimental study to enhance prediction of jet-grout column diameter and efficiency in sand

One of the most significant factors affecting the efficiency of jet-grouting operation is the diameter of the constructed column, depending on soil properties, stress state and operating parameters, including grout pressure (P), grout flow rate (Q), monitor rotational speed (ω) and lifting step (Δs). Increasing treatment efficiency results in a larger diameter with specific energy applied per unit length of the column. In this study, 35 full-scale and small-scale single-fluid jet-grouting elements were constructed in the field and laboratory to investigate the effects of the operating parameters on the diameter and treatment efficiency of the columns constructed in silty sand. Furthermore, measured values and obtained trends were controlled by a comprehensive equation commonly used to predict the diameter of jet-grouting columns. The results indicate that an increase in the time interval per step (Δt) and grout pressure (P), as well as a decrease in the lifting step (Δs), causes a larger column diameter and lower efficiency. However, a decline in rotational speed (ω) improves both diameter and efficiency. Subsequently, the well-known equation has been modified by considering field measurements in this study to predict more-accurate values and trends for both full-scale and small-scale jet-grouting elements.

Study Method of Pile near Cohesionless Slope under Reversed Lateral Load Considering Sand Strength State and Lateral Deflection of Pile

A p-y curve method of pile near cohesionless soil slope under reversed lateral load is proposed. This method takes into account the failure mode of soil around the pile under reversed lateral load and the interaction mode between pile and soil, and derives the ultimate soil resistance of the pile. Considering the change of the original stress state of soil due to the lateral deflection of the pile foundation, the influence of the relative density, the initial mean effective stress and the lateral deflection of pile foundation on the internal friction angle of the sand is evaluated to further accurately calculate the soil resistance value at each depth. The prediction results of this method are well verified by comparing with the FE results and centrifuge test results. Finally, the influence of the process of lateral deflection of pile on the strength state of soil around the pile and the bearing capacity of pile is studied.

Soil stress state at the roadhead

Choosing the optimum tool shape for soil cutting is an urgent task for shaftsinking and tunnelling as well as in soil treatment for agriculture. The roadhead shape at the available load determines the stress-strain state of soil and development depth in the bottomhole area. By changing the roadhead contour, the penetration efficiency can be increased, while the feed force and tool wear can be reduced.Purpose: The aim of the work is to create the roadhead calculation algorithm and determine the stress-strain state of soil near it with a minimum number of boundary conditions, no discretization of the entire region into elements and approximation between nodes.Design/methodology: Test tasks for the proposed methodology for the roadhead and the bottom-hole area. The numerical model of the boundary conditions is based on the integral formulation of the Kelvin elasticity theory problem and uses the potential method as fictitious loads and discontinuous displacements, that makes it possible to minimize the problem with regard to symmetry, obtain a more compact system of equations and give accurate solutions at any domain point.Research findings: The assessment of the stress state of the roadhead of various shape and the bottom-hole zone of soil; loss of soil strength calculated according to the Mohr–Coulomb theory.Originality/value: The proposed method for solving the boundary value problem is recommended as an express method for estimating the roadhead stress state of an arbitrary shape in infinite regions with regard to the symmetry and implementation in Excel.