Surcharge Load Research Articles

Upper bound limit analysis of circular tunnel in cohesive-frictional soils using isogeometric analysis based on Bézier extraction

This paper presents an isogeometric analysis (IGA) based on Bézier extraction to investigate the stability of a circular tunnel in cohesive-frictional soils subjected to uniform surcharge loading. The basic concept of IGA is to use non-uniform rational B-splines (NURBS) basis functions for both the parametrisation of the geometry in computer-aided design (CAD) and the finite approximation for analysis. In this study, IGA based on Bézier extraction is applied to model a circular tunnel's geometry discretisation. The soil is modelled to be a homogeneous material obeying the Mohr-Coulomb failure criterion. The ultimate surcharge loading and failure mechanisms of the circular tunnel are obtained via upper bound limit analysis. Based on the results of isogeometric analysis, design tables and charts are generated for various soil properties, internal friction angles and ratios of tunnel cover to tunnel diameter. The numerical results for plane strain circular tunnels are compared with those of other available solutions to prove the reliability and efficiency of the present method.

Numerical study of geosynthetic reinforced soil bridge abutment performance under static and seismic loading considering effects of bridge deck

Although the use of Geosynthetic Reinforced Soil (GRS) bridge abutments has been increasing, the seismic performance of such structures has remained a significant concern due to their unknown behavior in load-bearing and stress distribution under bridge load and seismic conditions simultaneously. This paper investigates the static and dynamic response of GRS bridge abutment. A series of numerical models representing the realistic field conditions of these structures, including two reinforced soil walls and a single span deck that restrains the top of walls, rather than equivalent surcharge load, was developed. The calibrated numerical model in FLAC program was used to evaluate the effects of horizontal restraint from the deck on the GRS wall displacements and reinforcement loads at the end of construction and under harmonic base acceleration up to 0.5 g. Results indicated that the restraint mobilized from the bridge deck presence, considerably affected the results at both the end of construction and after the dynamic load was applied. Moreover, a series of the parametric studies were performed to investigate the influences of backfill soil relative compaction, reinforcement stiffness, reinforcement length, and reinforcement vertical spacing on the response of GRS abutments at the end of construction and post dynamic state.

An Experimental Study of a Nailed Soil Slope: Effects of Surcharge Loading and Nails Characteristics

The earth nailing system is a ground improvement technique used to stabilize earth slopes. The behavior of the earth nailing system is dependent on soil and nailing characteristics, such as the spacing between nails, the orientation, length, and method of installation of nails, soil properties, slope height and angle, and surcharge loading, among others. In the present study, a three-dimensional physical model was built to simulate a soil nailed slope with a model scale of 1:10 with various soil nail characteristics. The simulated models consist of Perspex strips as facing and steel bars as a reinforcing system to stabilize the soil slope. Sand beds in the model were formed, using a sand raining system. The performance of nailed soil slope models under three important nails characteristics, i.e., length, spacing and orientation, with varying surcharge loading were studied. It was observed that there is a reduction in the lateral movement of slope and footing settlements with an increase in length. It was found that the slope face horizontal pressure is non-linear with different nail characteristics. The increase in length and inclination of the soil nails decreased the vertical, horizontal stress and footing settlement, while the increase in spacing of the nails increased the vertical and horizontal stress behind the soil mass.

Study on the earth pressure of a foundation pit adjacent to a composite foundation with rigid-flexible and long-short piles.

Model tests were performed to investigate the lateral earth pressure acting on the retaining structure adjacent to both natural ground (NG) and composite foundation (CFRLP), which were supported with rigid-flexible and long-short piles. Two testing procedures, namely, applying a load to the foundation and rotating the retaining structure along its toe, were considered. The results indicate that the additional lateral earth pressure acting on the retaining structure adjacent to the CFRLP is less than that of the NG in the depth of the reinforcement area strengthened by flexible piles. Compared with NG, the CFRLP yielded a smaller normalized height of application of the lateral earth pressure, suggesting that the CFRLP blocked the horizontal diffusion of the load and had a strong ability to transfer the surcharge load to the deep soil. When rotating the retaining structure, the lateral earth pressure acting on the upper part of the retaining structure experienced limited reduction once the displacement at the top of the retaining structure was greater than 8 mm, whereas the pressure acting on the lower part of the retaining structure continued to decrease with increasing displacement. In addition, a three-dimensional finite element model (FEM) was used to investigate the influence of the pile parameter and the wall-soil friction angle on the additional lateral earth pressure.

Numerical analysis of earth pressure of foundation pit adjacent to composite foundation

It is necessary to consider the influence of the surcharge load of the existing building and the displacement of the retaining structure when excavating the foundation pit adjacent to the existing composite foundation. FEM analysis work were conducted with two working conditions (condition of loading and condition of translation). The distribution and diffusion mechanism of lateral earth pressure acting on the retaining structure adjacent to composite foundation was investigated. It was found that, when there is no displacement of the retaining structure, the magnitude of additional lateral earth pressure acting on retaining structure adjacent to the natural ground and the composite foundation in the shallow area is almost the same. Moreover, compared with the natural ground, the composite foundation can block the horizontal diffusion of the load, resulting in the reduction of lateral earth pressure acting on the shallow area of retaining structure adjacent to the composite foundation is larger than that of adjacent to natural ground when retaining structure translation outwards.

The Analysis of Anchoring Mechanism of Rock Slope in Two Layers Based on the Nonlinear Twin-Shear Strength Criterion

The nonlinear strength criterion is introduced into the paper firstly, and then the geometric anchoring characteristic model of rock slope separated from two layers according to rock type with the fracture surface of log-spiral curve considering nonlinear twin-shear criterion subjected to seismic loads is established. Finally, some correlated influential parameters are analyzed, and the conclusions are drawn, showing that the intermediate principal stress has great influence on the total anchoring force of bolt P, so when different criteria are selected, the influence of intermediate principal stress should not be omitted. The effects of thickness ratio h1/h2 on total anchoring force increase gradually with the increase of inclination angle ϕ . The total anchoring force P decreases gradually with the increase of inclination angle ϕ and surcharge load q. The total anchoring force increases gradually with the increase of horizontal seismic acceleration coefficients kh and geometric coefficients mi and GSI.

A Simplified Analysis Method for the Deformation Response of an Existing Tunnel to Ground Surcharge Based on the Pasternak Model

Surface surcharge changes the existing equilibrium stress field of the stratum and adversely affects the existing tunnel. This paper presents a simplified analytical solution for calculating the longitudinal displacement of existing tunnels that are subjected to adjacent surcharge loading. Based on the Boussinesq solution, the distribution of the additional load matrix caused by the surface surcharge on the existing tunnel was obtained. A Euler–Bernoulli beam with a Pasternak foundation was used as a simplified model for tunnel stress analysis. Using the corrected reaction coefficient of the foundation bed, the differential equation of tunnel deformation was established, and the solution matrix of the longitudinal displacement of the tunnel was obtained by using the finite difference method. The reliability and applicability of the proposed method were verified by comparing the results with finite element simulation results, field test data, and the calculation results of three simplified elastic analysis methods with different foundation bed coefficients. On this basis, the parameters of the load–tunnel model were analyzed, and the effects of the buried depth, the size of the load, the relative positions of the load and the tunnel, and the relative stiffness of the tunnel soil on the maximum displacement of the existing tunnel were calculated. An empirical formula is proposed for calculating the maximum longitudinal displacement of the existing tunnel subjected to surface surcharge. The findings of this research can provide a basis for the theoretical verification of the deformation response of an existing tunnel subjected to adjacent surface surcharge.

Swell-shrink and hydraulic behaviour of compacted red soil-bentonite mixture

Clay liners are provided in waste landfills to prevent the leachate from percolating into underlying soil and ground water and polluting it. Hence, soils used as landfill liners must possess low hydraulic conductivity (< 10−7 cm/s). In the initial as-compacted state, liners satisfy this design criterion. However, in the field, liners are subjected to alternate wet-dry cycles and the initial microstructure of liners change, thereby affecting their hydraulic properties. The present study was conducted on a locally available red soil mixed with bentonite so that it satisfies the requirement of hydraulic conductivity in the as-compacted state. To achieve this objective, identical specimens compacted at optimum moisture content to its maximum dry density were inundated with distilled water under a surcharge load of 12.5 kPa. Upon attainment of maximum swelling, hydraulic conductivity tests were conducted at swollen state under a hydraulic gradient of 20. The soil specimens were then dried at a temperature of 45±2 °C. The weight, height and diameter of specimens were measured continuously during drying to study the shrinkage behaviour of the soil. The dried specimens were subjected to subsequent wet-dry cycles until the specimens reached an equilibrium state and the hydraulic conductivity was determined after each wetting cycle. SEM images were also captured to analyse the changes in the soil structure in the as-compacted state and at the end of different wetting cycles.

Effect of rebar self-stress on behavior of autoclaved aerated simply supported R/C thin beams subject to uniform transverse dead load

Autoclaved aerated concrete (AAC) beams typically incur counter-arch deflection with stored stress induced by the high-temperature and pressure steam curing. This study investigates the effects of the resulting self-stress on the flexural performance of steel rebar reinforced AAC beams. Accordingly, a simplified experimental model was designed to decouple the bar self-stress from other load mechanisms that exist in full-scale structures. Surcharge loading was applied, and the self-stress of steel rebar was measured by the releasing method. The cracking pattern of the AAC test beams and the associated load-deflection curves were analyzed. The values of self-stress in different locations of the beam were obtained, and the influence of steel bar self-stress on the cracking load was calculated. The test results show that the cracking moment resisted by the self-stress accounted for 65.7% of the theoretical cracking moment. Numerical simulation results confirmed that asymmetric reinforced AAC beams incur a counter-arch phenomenon, and further quantified the corresponding effect of self-stress on the cracking load. Therefore, accurately accounting for the rebar self-stress can more accurately define the cracking load capacity of AAC beams and should be considered in design, which should make AAC beams a stronger contender in diverse field applications.

One-dimensional large-strain model for soft soil consolidation induced by vacuum-assisted prefabricated horizontal drain

Prefabricated horizontal drain (PHD) assisted by vacuum pressure has been used in practice to accelerate the consolidation process of dredged soft mud. However, very few studies have investigated the large-strain nonlinear consolidation caused by vacuum-assisted PHD. Therefore, based on the piecewise-linear method, this study proposes a one-dimensional consolidation model (called VCS herein) for the soft soil consolidation induced by vacuum-assisted PHD. The proposed VCS model has the capability to consider the large strain, PHD assisted by time-dependent vacuum pressure, soil self-weight, time-dependent surcharge loading and nonlinear changes of the geotechnical parameters and is thus superior to the existing models in the literature. The VCS was validated by comparing with the analytical and semi-analytical solutions reported in the literature. In addition, model test of PHD-vacuum preloading consolidation was designed and performed to further verify the proposed model, and excellent agreement was observed. Based on this model, a parametric study was performed to investigate the effect of several relevant parameters on the vacuum preloading consolidation. Moreover, the differences between PHD-vacuum consolidation and PVD-vacuum radial consolidation were investigated. The results indicate that the initial soil height, vacuum pressure and boundary condition considerably influence the performance of vacuum preloading (e.g., time rate of consolidation and final settlement), whereas the time-dependent vacuum preloading affects only the time rate of the vacuum preloading consolidation and not the final settlement magnitude. For the same soil layer, the final settlement of PHD-vacuum consolidation and PVD-vacuum radial consolidation is relatively close. However, the consolidation rate of PHD-vacuum consolidation is obviously faster than that of PVD-vacuum consolidation.

Determination of Passive Earth Pressure Coefficients for a Sloping Cohesive Backfill under Uniform Surcharge Loading

Abstract Compared to the Rankine and Coulomb passive earth pressure theories, the logarithmic spiral theory considers curved failure surfaces, which is its greatest advantage. To improve the logari...

Mapping soil arching-induced shear and volumetric strains in dense sands

Arching is regarded as a ubiquitous phenomenon in geomaterials where a new stress field is created following partial, induced displacements in the geomaterial. In the past, less attention has been paid to the deformations associated with the arching effect. The Digital Image Correlation (DIC) technique was implemented to map the shear and volumetric strain fields where arching occurred in a dense sand layer placed in a trapdoor apparatus. Model tests were conducted under active and passive arching along with surcharge loading. It was observed that shear bands comprising of two sections with different orientation angles were formed. The orientation angles of the shear bands were associated with the dilation angle of the sand (the deformation related angle of the soil) rather than its internal angle of friction (the stress related angle of the soil). The magnitude of shear strains increased with trapdoor displacement. It was also found that strain localization was associated with dilation. Contraction zones were also detected in the sand layer specifically at the surface and on the trapdoor element. In addition, it was realized the shear bands developed in the passive arching condition possessed greater width and inclination angles compared to bands of the active arching situation. The local surcharge used in the experiments markedly affected the strain localization and volumetric strain patterns and magnitudes.

Integrated impacts of vegetation and soil type on slope stability: A case study of Kheyrud Forest, Iran

In this study, an integrated 2D numerical model is applied to investigate the mechanical effects of the vegetation and soil type on slope stability. The developed model can assess the mechanical aspects of vegetation in slope stabilization. For this purpose, a case study of the Kheyrud forest located in northern Iran is considered as a real case site. Different scenarios including; three soil types (fine grain with low-, medium-, and high-strength) and two vegetation types (Maple and Common-ash) are assessed in the stability analysis (based on safety factor). The results confirm that the considered vegetation can prevent shallow landslide occasions, but has a limited impact on deep landslide events. The ratio of root zone depth to the depth of slide is the most important parameter in the contribution of vegetation in the increasing of the slope stability. The simulation results show an improvement in the safety factor up to 25% when the mechanical aspects of vegetation are considered. This improvement leads to stability for slopes with 10% more gradient. The results also demonstrate the reinforcement effect of the roots (stability improvement up to 25%) and negative effect of the surcharge load (Reduction of stability up to 5% at a surcharge of 1.2 kPa) on the slope stability. The extent of these impacts varies by soil properties, tree characteristics, and slope geometry so that the overall result can vary. The sensitivity analysis demonstrated an increase in the soil cohesion grows the negative effect of tree weight. Also, it is observed that the increase of soil unit weight follows a decrease in the stabilization role of vegetation. The role of the soil's friction angle is insignificant. The findings can serve as a foundation for enhancing the scientific understanding of the relationship between the stabilization role of vegetation and soil, vegetation, and slope characteristics. Generally, in the stabilization of a slope by artificial planting forests, the native shrubs or trees with more reinforcing effects and low surcharge are more effective.

Simulation Analysis on Drainage Consolidation Characteristics of Hydraulic Fill Soft Soil

Based on the treatment of dredger fill soft soil foundation in Eastern Shantou, Guangdong Province, consolidation theory and numerical simulation based on composite consolidation coefficient are used to study the consolidation characteristics of dredger fill soft soil, and the results are compared with the field monitoring results. The calculation results show that the theoretical consolidation degree of the site is 93.4%, which is basically consistent with the final consolidation degree of field monitoring. In the first two stages of surcharge loading, field monitoring is faster than theoretical calculation. The field monitoring value of the final settlement is 1342 mm, which is about 10% less than the theoretical value. The final average consolidation degree of the two models is 97.9%, which is 4.7% more than the field monitoring value. The results show that the pore water pressure law is obvious, the dissipation rate is about 16% higher than the field monitoring value, and the consolidation effect is more obvious. The numerical simulation of the composite coefficient model has certain applicability and can be used to guide the design and construction of similar projects.

Optimum design of cantilever retaining walls under seismic loads using a hybrid TLBO algorithm

The main purpose of this study is to investigate the performance of the proposed hybrid teaching-learning based optimization algorithm on the optimum design of reinforced concrete (RC) cantilever retaining walls. For this purpose, three different design examples are optimized with 100 independent runs considering continuous and discrete variables. In order to determine the algorithm performance, the optimization results were compared with the outcomes of the nine powerful meta-heuristic algorithms applied to this problem, previously: the big bang-big crunch (BB-BC), the biogeography based optimization (BBO), the flower pollination (FPA), the grey wolf optimization (GWO), the harmony search (HS), the particle swarm optimization (PSO), the teaching-learning based optimization (TLBO), the jaya (JA), and Rao-3 algorithms. Moreover, Rao-1 and Rao-2 algorithms are applied to this design problem for the first time. The objective function is defined as minimizing the total material and labor costs including concrete, steel, and formwork per unit length of the cantilever retaining walls subjected to the requirements of the American Concrete Institute (ACI 318-05). Furthermore, the effects of peak ground acceleration value on minimum total cost is investigated using various stem height, surcharge loads, and backfill slope angle. Finally, the most robust results were obtained by HTLBO with 50 populations. Consequently the optimization results show that, depending on the increase in PGA value, the optimum cost of RC cantilever retaining walls increases smoothly with the stem height but increases rapidly with the surcharge loads and backfill slope angle.

Determination of Critical Slope Face in c – ϕ Soil under Seismic Condition Using Method of Stress Characteristics

This study proposes a plasticity-based approach to ensure the static and seismic stability of a finite soil slope supporting a uniformly distributed surcharge on the horizontal top surface. Most of the available investigations based on the limit equilibrium and the limit analysis method mainly rely on the assumed slip surface to determine the stability of a slope with a given geometry. The present analysis employs the method of stress characteristics coupled with the original pseudodynamic approach to trace the actual slip surface of a soil slope under the seismic condition. The results are presented in terms of the critical slope face (CSF) corresponding to a global factor of safety of 1.0. The obtained CSF can be used as a reference to determine the stability of slopes with different geometries. The current approach supersedes the available theories developed to analyze slopes by presenting a more general solution without assuming any predefined slip surface. The proposed idea is endorsed with a detailed parametric study that demonstrates the influence of various parameters such as cohesion and the angle of internal friction of soil, surcharge loading, and seismic wave properties on the stability of a finite slope. As a notable outcome, this investigation promotes a bilinear or concave slope face, which may be an efficient and economical alternative to the traditional linear slope face.

The Usage of the Harmony Search Algorithm for the Optimal Design Problem of Reinforced Concrete Retaining Walls

In this paper, the Harmony Search (HS) algorithm is utilized to perform single and multivariate parametric studies to acquire the optimization of both size and cost of reinforced concrete (RC) retaining walls embedded in pure frictional soils. The geotechnical properties of the backfill and foundation soil such as shear strength angle, unit weight, and the ultimate bearing pressure of the soil have been used to create different cases for evaluating the effects of site properties on the size and cost of the wall. The change of depth of excavation and surcharge loading condition is fictionalized for generating different environmental conditions for all envisaged soil profiles to predict possible rates of influences. The unit cost of the concrete has also been evaluated as a variant to show the economic constraints on the selection of structural materials. The results of the analyses represent the integrated influences of different significant parameters on the achievement of minimum cost-dimension optimization. Besides, a well-known commercial geotechnical engineering software is used to compare the appropriateness of the suggested designs in terms of both the attainment of geotechnical stability and the structural requirements. Consequently, this study can guide both researchers and designers to select the proper and optimal sections of RC-retaining wall systems with simultaneous analyses of parameters that are influenced by the design process. Furthermore, the optimization results indicate that a significant cost reduction may be achieved when compared with the traditional pre-design method.

Numerical modeling of effect of surcharge load on the stability of nailed soil slopes

This study aimed to examine the behavior of a nailed soil slope under different conditions of the surcharge load based on numerical modeling. The numerical analysis process was performed using FLAC3D difference element software. In this regard, effect of three different types of surcharge loads under different conditions was investigated for factor of safety of slope, maximum nail force, horizontal and vertical displacements of slope edge using static analysis. The surcharge loads included a distributed load of different values at a distance of 2 m from the slope edge, a particular line load at different distances from the slope edge, and different values of a point load in the middle of the slope edge. The results showed that the maximum nail force increased with increasing the distributed load. In addition, by increasing the distance of the line load from the slope edge, the maximum nail force decreased. Moreover, the horizontal displacement of the slope edge was found to increase with increasing distributed and point loads, whereas it decreased with increasing line load distance from the slope edge. According to the results, it was found that the distributed loading of 100 kPa at a distance of 2 m from the slope edge was the most critical one. The factor of safety value for this loading was equal to 1.27 and less than the allowable limit (1.35). In addition, the horizontal displacement in this type of loading was equal to 22.8 mm and more than the allowable limit (18 mm).

Numerical investigation of pile-head load effects on the negative skin friction development of a single pile in consolidating ground

This study conducted parametric analyses to investigate the influence of structural loading (pile-head loading) on the development of negative skin friction in friction single piles and friction-end-bearing single piles in consolidating ground. Numerical effective stress-based mechanical-flow analysis models were built in ABAQUS software. The appropriateness of the modeling approach was verified by applying it to simulate a field model test described in the literature. For small surcharges, pile-head loading considerably reduces the depth of the neutral plane and therefore drag load for the friction pile. Under the same pile-head loads, the reduction in total drag load for the friction-end-bearing pile is smaller than that for the friction pile. However, when the friction-end-bearing pile is subjected to larger pile-head loads, the degree of total drag load reduction is comparable to that of the friction pile. Considerable pile-head displacement occurs under the combined action of pile-head and surcharge loadings. In contrast to the friction pile, the friction-end-bearing pile, because of the greater stiffness and strength of its bearing stratum, exhibits a significant reduction in negative skin friction, no bearing failure, and a smaller pile-head displacement when it is subjected to large pile-head loading.

Analytic study of stress and displacement for shallow twin tunnels subjected to surcharge loads

This paper presents an analytical solution for the stress and displacement for shallow twin tunnels subjected to surcharge loads. Multi-interactions, including the interaction between liners and geomaterial, the interaction between twin tunnels, and the interaction between surcharge loads and twin tunnels, are considered in the proposed solution. The solution is elaborated using an iterative three-step procedure based on the Schwarz alternating method. The complex variable method and the Flamant solution and its modification are employed in the derivation. Verification of the analytical solution is obtained through comparisons to a corresponding numerical solution and an existing case study. The close correspondence indicates the accuracy and generality of the proposed solution. A parametric investigation is performed to investigate the influence of surcharge load (including load position, range and distribution), elastic parameters of geomaterial, and liner geometry (including tunnel radius and liner thickness). Based on the parametric investigation, several advices for surcharge loads and liners are provided to help tunnel designers to avoid possible damages for shallow twin tunnels.