Showing papers on "Lateral earth pressure published in 2023"

Three-Dimensional Active Earth Pressures for Unsaturated Backfills with Cracks Considering Steady Seepage

Abstract: Traditional analyses for active earth pressures considered soils dry or saturated by the application of a two-dimensional (2D) failure pattern. However, soils are usually unsaturated in nature, and the collapse of backfills presents a three-dimensional (3D) characteristic. The extant studies proved that the existence of cracks and seepage flow encountered in backfills would impact active earth pressures but are still limited to 2D conditions. To this end, this study developed an analytical framework to evaluate the 3D active earth pressure considering the presence of cracks and steady-infiltration effects within unsaturated backfills. Based on the kinematic approach of limit analysis, a suction-induced effective method is introduced into a 3D failure mechanism to characterize the collapse of unsaturated backfills. By means of the work rate balance equation, the most adverse location of cracks and the explicit expression of active thrust under steady seepage conditions can be obtained through the incorporation of the suction stress profile. The presented method is verified by comparison with the exact cases in previous studies and comparison with the results of numerical simulation. A systematic parametric study is conducted to reveal the impacts of width-to-height ratio, air-entry value, pore-size distribution, vertical discharge, and cracks on the active earth pressure variations. The results show that considering 3D effects is significant because it leads to a lower economic cost for the design of retaining walls; the presence of cracks and the effect of steady infiltration encountered in unsaturated backfills would increase the lateral force of earthen structures. This study presented a more realistic understanding of the service state behavior of retaining walls and a useful strategy for evaluating the 3D active earth pressure.

A Prediction Model for the Deformation of an Embedded Cantilever Retaining Wall in Sand

Abstract: This paper proposes a prediction model based on the mobilizable strength design method with an equilibrium model to predict the deflection of an embedded cantilever retaining wall in sand. The variation in the location of the pivot point within the retaining wall and the sand dilation are considered. Based on the stress–strain relationships obtained by triaxial tests, the designer can determine a specific shear strain and corresponding mobilized earth pressure for the retaining wall to achieve a global equilibrium. Then, the wall deflection, including rotation and flexure, can be derived. The locations of the pivot points obtained by this method are compared with the results predicted by the minimization approach. Finally, the prediction precision is validated via centrifuge tests and numerical model data published in prior investigations.

Limit state analysis of rigid retaining structures against seismically induced passive failure in heterogeneous soils

Abstract: Soils are not necessarily uniform and may present linearly varied or layered characteristics, for example the backfilled soils behind rigid retaining walls. In the presence of large lateral thrust imposed by arch bridge, passive soil failure is possible. A reliable prediction of passive earth pressure for the design of such wall is challenging in complicated soil strata, when adopting the conventional limit analysis method. In order to overcome the challenge for generating a kinematically admissible velocity field and a statically allowable stress field, finite element method is incorporated into limit analysis, forming finite-element upper-bound (FEUB) and finite-element lower-bound (FELB) methods. Pseudo-static, original and modified pseudo-dynamic approaches are adopted to represent seismic acceleration inputs. After generating feasible velocity and stress fields within discretized elements based on specific criteria, FEUB and FELB formulations of seismic passive earth pressure (coefficient KP) can be derived from work rate balance equation and stress equilibrium. Resorting to an interior point algorithm, optimal upper and lower bound solutions are obtained. The proposed FEUB and FELB procedures are well validated by limit equilibrium as well as lower-bound and kinematic analyses. Parametric studies are carried out to investigate the effects of influential factors on seismic KP. Notably, true solution of KP is well estimated based on less than 5% difference between FEUB and FELB solutions under such complex scenarios.

A Semi-Empirical Approach for Estimating the Lateral Earth Pressure on Rigid Retaining Walls with EPS Inclusions in Cohesionless Soils

Abstract: Placing the expanded polystyrene (i.e. EPS) as inclusions between the rigid nonyielding wall and fill enables the lateral displacement of the fill and renders the lateral earth pressure from the at-rest state to the active state. This effectively alleviates the lateral load and facilitates cost-effective designs for rigid retaining walls. This study develops a semi-empirical approach to predict the lateral earth pressure on rigid retaining walls with EPS inclusions in cohesionless soils. This approach is simple and requires only conventional soil and EPS properties for prediction. Its performance is validated by the results of different physical and numerical modeling studies with EPS inclusions of various densities and thicknesses. It is demonstrated that (i) the lateral earth pressure depends on the stiffness of the EPS inclusions (i.e. the ratio of the elastic modulus to the thickness), and (ii) EPS inclusions with a single and proper thickness can fulfill the majority of load reduction purposes. Suggestions for the choices of (i) the EPS materials in terms of density and elastic modulus, (ii) the thickness of EPS inclusions, and (iii) the structure of the EPS inclusions are proposed for the use of EPS inclusions in cohesionless soils.

Displacement-Controlled Analysis of Embedded Cantilever Retaining Walls

Abstract: An uneconomical design of embedded cantilever retaining (ECR) walls unnecessarily restricts the lateral wall movement, whereas unwarranted wall displacement induced ground settlement can jeopardize the functionality of neighboring structures. Thus, correct estimation of the wall displacement under working conditions is imperative for a safe and economical design. This paper presents an analytical method for the displacement-controlled analysis of ECR walls in cohesionless soils, which can be used to calculate the required embedment depth for a prescribed wall displacement and retaining heights. Alternatively, when the retaining height and the embedment depth of an ECR wall are given, the lateral wall displacement can also be calculated. A displacement-dependent earth pressure mobilization model is proposed to derive the mobilized soil stresses along the wall height. The required embedment depth of the wall is determined by assuming rigid rotation of the wall about a point near the toe and satisfying the horizontal force and moment equilibriums. Analytical formulations are provided to determine the bending moment distribution and the ground settlement. The effect of construction by excavation is also taken into the analysis. The results show that the required embedment depth and the maximum bending moment increase exponentially with decreasing wall displacement. The depth of the pivot point is located at around 0.9 times the embedment depth. The validity of the proposed method is demonstrated by comparing the calculated results with those of the available numerical and experimental studies. The proposed method can provide first-hand design solutions of ECR walls without performing rigorous numerical and experimental studies.

Upper bound analysis of longitudinally inclined EPB shield tunnel face stability in dense sand strata

Abstract: Earth pressure balance (EPB) shield machine frequently drives along a route with a longitudinal inclination angle (δ) during tunnel construction. The tunnel face stability is a crucial issue for controlling the safety of tunneling. In this paper, the finite difference method (FDM) is used to investigate the face stability of longitudinally inclined shield tunnel in dense sand strata. Based on the numerical results of soil failure pattern in front of tunnel face, a novel two-dimensional (2D) failure mechanism is conceived following the upper bound theorem. It is assumed that the failure zone consists of an upper loosen zone, an arc-shaped shear band, and a triangular rigid body, which can calculate the active limit face support pressure pu. A scale factor for pu is introduced to quantify the three-dimensional (3D) behavior from the 2D calculation, with respect to different analysis approaches, e.g., numerical simulation, limit analysis, and limit equilibrium method. By incorporating the measure of scale factor, the calculation accuracy of the proposed 2D failure mechanism can be improved to provide reasonable prediction for longitudinally inclined tunnel.

Numerical simulation on the performance of GRS walls with freeze-thaw cycles consideration

Abstract: In practice, little attention has been paid directly to freeze-thaw (FT) cycles during the design and analysis of geogrid-reinforced soil (GRS) walls due to lacking relevant literature. This study investigates the pavement vertical deformation (s), panel lateral deformation (d), lateral earth pressure (σh), and geogrid strain (ε) of a field GRS wall using an ABAQUS-based numerical model considering variations of the recorded five-year ambient temperature (TR). Numerical results show that the s distribution follows a convex shape instead of the initial concave shape after FT cycles and can be divided into high, transition, and stable deformation zones. FT action alters both location and amplitude of the maximum d within the first two cycles, making the d distribution evolve from a J-shaped curve into an S-shaped one. During freezing, the developments of s and d are coordinated and can be described using a unified model; σh is larger than the Rankine active earth pressure; ε state depends on the interplay of two factors resulting from d and frost heave force. Furthermore, the hysteresis of s, d,σh, and ε with TR was discussed and several beneficial suggestions were proposed for GRS walls to avoid such FT destruction.

Passive Earth Pressure in Narrow Cohesive-Frictional Backfills

Abstract: A narrow backfill zone is formed when retaining walls are built near existing stabilized structures (e.g., rock faces). In such circumstances, the classical passive earth pressure coefficient is no longer applicable, and a correction factor is required for the design. This paper aims to develop analytical solutions for estimating the passive earth pressure problem of narrow cohesive-frictional backfills behind retaining walls. The novel arched differential element method considers both effects of the horizontal shear stress in backfills and the soil arching, and it is employed to estimate the passive earth pressure distribution along with wall depth. The solutions are compared against those published experimental data, analytical approaches, and finite-element limit analysis solutions. The factors influencing the distribution of passive earth pressure are also undertaken using a series of parametric studies. To implement the derived solutions into a routine design, a modified practical design equation is presented following the standard Coulomb’s solutions. This work provides a theoretical guideline for the initial design of retaining walls with narrow soils, and it should be of great interest to practitioners.

Soil arching evolution caused by shield tunneling in deep saturated ground

Abstract: Shield tunneling is accompanied by variations of stress and displacement in soil, which could induce the soil arching effect. Although there have been many studies on the soil arching effect caused by shield tunneling, the research on deep-buried and saturated conditions is still lacking. In this study, a finite element model was firstly built and verified by field data. Then, two hypothetical finite element models were established with two different strata to investigate the stress transfer mechanisms, the development law of excess pore water pressure, and the soil arching evolution of deep-buried tunnel. The methods to determine the arch zone and loosened zone were proposed and verified, which were adopted to compare the differences of soil arching effect between saturated sandy and clayey soil layers. The results show that the heights of the arch zone and loosened zone in the saturated sandy soil layer are both higher than those in the saturated clayey soil layer after the shield tunneling. The subsequent strata consolidation resulted in a 36% reduction in the arch zone height in clayey soil, which means that time effect for the soil arching is significant. Compared with Terzaghi's loosening earth pressure theory, in deep ground, the theoretical overburden pressure is only 5.0% larger than the numerical results for sandy soil but 18.3% less than that for clayey soil. The numerical results can bring new sights about the soil arching evolution caused by shield tunneling and the corresponding tunnel overburden pressure in deep saturated ground.

Numerical Evaluation of a Semi-Integral Bridge Abutment Under Cyclic Thermal Movements

Abstract: This paper presents a two-dimensional Finite Element (FE) simulation of the interaction between a semi-integral stub-type concrete bridge abutment and a granular backfill under cycles of temperature-induced lateral displacements. A numerical model was proposed utilizing an elastoplastic soil constitutive model considering the characteristics of a semi-integral bridge abutment located near the City of Palestine, Texas, USA. The numerical model was validated against data collected in the field from pressure cells installed on the soil side of the abutment of the semi-integral bridge. The long-term response of the backfill-abutment system representing a 50-year period was investigated numerically for annual cycles of expansion–contraction of the bridge. According to the results of the investigation, it is observed that annual cyclic lateral movements of the bridge abutment led to a rapid increase of lateral earth pressures upon the abutment wall. The locus of maximum lateral earth pressures occurred on the upper third of the abutment, which disagrees with the conventional earth pressure distributions often assumed in design guidelines for integral bridge abutments. The magnitude of the settlement trough that formed under annual cycles is deemed sufficient to negatively affect bridge performance soon after start of the bridge operation. Results predicted that cumulative shear strains prevailed in the region of the backfill soil far from the abutment wall. On the other hand, cumulative compressive volumetric strains (densification) dominated in the vicinity of the soil-abutment interface. While stabilization of lateral earth pressures on the soil-abutment interface was predicted to occur with the balance between both densification and shearing effects, settlements adjacent to the soil-abutment interface were predicted to persist as a consequence of the continued growth of cumulative shear strains (ratcheting) in the portion of the soil away from the abutment wall.

Numerical investigation on the spewing mechanism of earth pressure balance shield in a high‐pressure water‐rich sand stratum

Abstract: The spewing of a screw conveyor easily occurs from the earth pressure balance (called EPB) shield in a water-rich sand stratum. This may lead to the collapse of the tunnel face and even serious subsidence of the ground surface. To understand the spewing mechanism of the shield screw conveyor and explore the critical hydraulic condition of soil spewing in a shield–soil chamber, a simplified theoretical model for the spewing of the screw conveyor was developed based on the equation of groundwater flow in the screw conveyor under turbulent state. Thus, coupling Darcy's law with Brinkman's equation, this model was implemented within the COMSOL Multiphysics framework. The underground water flow in the shield screw conveyor was simulated so as to obtain its velocity and flow rate. Numerical simulations show that the water pressure distribution is concentrated in the lower part of the soil chamber after the groundwater enters the soil chamber. When the groundwater enters the screw conveyor, its pressure gradually decreases along the direction of the screw conveyor. When the water flow reaches the stratum–shield interface, the flow velocity changes markedly: first increases and concentrates at the entrance of the lower soil chamber, plummets and stabilizes gradually, and increases again at the exit. The soil chamber and screw conveyor are significantly depressurized. It is also found that the soil permeability coefficient can be reduced to k < 2.6 × 10−4 cm/s through appropriate soil improvement, which can effectively prevent the occurrence of spewing disasters.

Experimental Study on Influence of Initial Relative Dry Density on K0 of Coarse-Grained Soil

Abstract: The coefficient of earth pressure at rest, K0, is a significant mechanical parameter, and the investigation of the K0 of coarse-grained soil has important theoretical significance and applicational value in geotechnical engineering. However, there are few studies on the influence of the initial dry density (ρd) on the K0 of coarse-grained soil due to the limitations of the related test instruments or methods. A series of K0 tests for two types of coarse-grained soils (rockfill soil and sandy-gravel soil) were conducted based on a newly developed large-scale apparatus to reveal the relationship between the K0 and ρd of coarse-grained soil. The test results showed that the K0 of coarse-grained soil decreases with an increase in vertical stress, and this trend tends to be gentle with respect to the increase in vertical stress. The results also implied that there was a negative linear relationship between the K0 of coarse-grained soil and ρd. Furthermore, a comparative analysis between rockfill soil and sandy-gravel soil indicated that the relative equation proposed by the authors was appropriate for any type of coarse-grained soil with any ρd. Moreover, an empirical equation that can accurately describe the effective relationship of σv and ρd with the K0 for coarse-grained soil was proposed, and the accuracy of the empirical equations were verified by the K0 test results concerning sand-gravel soil. Finally, based on the published research findings, the empirical equations’ applicability to any coarse-grained soil was verified.

Influence of Small Radius Curved Shield Tunneling Excavation on Displacement of Surrounding Soil

Abstract: In contrast to straight tunnels, the mechanisms of displacement of surrounding soil induced by shield excavation of small radius curved tunnels are more complex. Based on field monitoring data of surface settlement and horizontal displacement of a small radius curved shield tunnel in a section of Zhengzhou Metro Line 3, a numerical model using three-dimensional a finite element method is established to evaluate factors of the displacement of surrounding soil. The results verify the validity of numerical simulation by comparison with field monitoring data and the influence of unbalanced additional thrust at tail jacks, curvature radius of a tunnel and tail grouting pressure on surface settlement and horizontal displacement of surrounding soil. Maximum surface settlement and horizontal displacement of surrounding soil at the outer side and inner side of curved tunnel axes are positively related to thrust ratio, while negatively related to curvature radius and grouting pressure. The ultimate objective of this study is to ascertain factors of displacement of surrounding soil induced by small radius shield excavation and provide a theoretical basis and technical support for the design and construction of similar tunnel.

New Method for Computing Slip-Line Fields and Earth-Pressure Coefficients in Cohesionless Backfills

Abstract: A simplified numerical method was proposed for computing the slip-line field, as well as the associated earth-pressure coefficient, in a cohesionless backfill with an inclined surface lying behind an inclined rough wall. The potential failure zone, in either active or passive cases, was divided into the Rankine zone, which was rigorously obtained by the theory of plasticity, and the transition zone being further divided into a series of triangular slices. Within the transition zone, the theoretical relationship between the inclination of the interslice force and that of the slip surface was established by satisfying the Mohr–Coulomb failure criterion, and equations involving the forces on a typical slice were formulated in accordance with the force and moment equilibrium conditions. An iterative procedure was presented for computing the lateral forces on the wall by adjusting the inclination of the slip surface until the stress condition on the upper boundary of the transition zone was satisfied, resulting in the two families of slip lines. Several examples demonstrate the slip-line field configurations, and the computed earth-pressure coefficients were found to agree fairly well with those of other numerical methods.