Three-Dimensional Modeling of Spatial Reinforcement of Soil Nails in a Field Slope under Surcharge Loads

Soil nailing has been one of the most popular techniques for improving the stability of slopes, in which rows of nails and a structural grillage system connecting nail heads are commonly applied. In order to examine the spatial-reinforcement effect of soil nails in slopes, a three-dimensional (3D) numerical model has been developed and used to back-analyze a field test slope under surcharge loading. Incremental elastoplastic analyses have been performed to study the internal deformation within the slope and the development of nail forces during the application of top surcharge loads. Different treatments of the grillage constraints at nail heads have been studied. It is shown that the numerical predictions compare favorably with the field test measurements. Both the numerical and the field test results suggest that soil nails are capable of increasing the overall stability of a loose fill slope for the loading conditions considered in this study. The axial force mobilization in the two rows of soil nails presents a strong dependence on the relative distance with the central section. With the surcharge loads increased near the bearing capacity of the slope, a grillage system connecting all the nail heads can affect the stabilizing mechanism to a notable extent

Three-Dimensional Stability Analyses of Soil-Nailed Slopes by Finite Element Method

Modern computer capabilities enable complex slope stability problems to be analyzed using the finite element method (FEM), including three-dimensional slopes. This research presents the results of analyzing two- and three-dimensional, unreinforced and soil-nailed, reinforced slopes. Previously performed two-dimensional slopes were modeled as three-dimensional slopes in an effort to validate the modeling technique. The Shear Strength Reduction (SSR) Method was used throughout this study to determine the Factor of Safety (FOS). Both two- and three-dimensional FEM models compared well with conventional, two-dimensional Limit-Equilibrium (L-E) results. Overall, results show that the FEM is an extremely diverse and robust alternative to conventional, L-E slope stability analyses, especially when complex site geometries or conditions exist.;When modeling two-dimensional, unreinforced soil slopes using FEM, the most efficient and accurate element type that provides an acceptable failure mechanism is the CPE4 (4-noded bilinear quadrilateral) element in conjunction with either the Mohr-Coulomb or Drucker-Prager soil failure yielding criteria. When modeling three-dimensional, unreinforced soil slopes using FEM, the most efficient and accurate element type that provides an acceptable failure mechanism is the C3D8 (8-noded linear brick) element in conjunction with the Mohr-Coulomb soil failure yielding criteria. Although the Drucker-Prager soil yielding criteria assumptions seem to offer more potential for three-dimensional applications, this study found no significant benefit from its use.;For unreinforced slopes, three-dimensional, unit-width FEM models provide identical results to FEM slope models with depth, when end conditions are not considered. For soil-nailed reinforced slopes, three-dimensional, unit-width FEM models yield FOS values marginally higher than two-dimensional FEM and L-E models for all slope angles. Unit-width FEM models provide designers with a valuable tool for performing parametric studies and preliminary design. Three-dimensional FEM models of soil-nailed reinforced slopes can be used to effectively determine the soil nail orientation that yields the highest FOS, also known as the optimum soil nail orientation. For slopes with a level backfill, the optimum soil nail orientation (in degrees measured downward from horizontal) can be first approximated using the equation 58°- 0.6beta, where beta is the slope angle in degrees. Three-dimensional FEM models can also be used to effectively determine the most efficient soil nail length. For slope heights of about 10 meters, the most efficient soil nail length can be first approximated using a soil nail length to slope height ratio equal to 1.0. A maximum vertical soil nail spacing of 2.4 meters and 1.9 meters is recommended for soil-nailed slopes with slope angles less than or equal to 60° and greater than 60°, respectively.;A maximum horizontal soil nail spacing less than 1.9 meters is recommended for soil-nailed slopes for all slope angles. When performing three-dimensional FEM modeling of reinforced slopes, the FOS value is relatively insensitive to a soil\u27s Elastic Modulus, Unit Weight, and Poisson\u27s Ratio value. The most significant soil parameters are the Angle of Friction and the Cohesion. Further, the results of this study indicate that regardless of the soil type, there is no significant increase in the FOS of soil-nailed reinforced slopes with slope angles less than 80°.;Three-dimensional FEM models can be used effectively to evaluate unreinforced and soil-nailed reinforced slopes that have a surcharge load and can be used to estimate slope deformation, even prior to failure. In addition, pre-tensioned soil nails can be modeled using an FEM approach. Fully modeled, three-dimensional FEM slope stability analyses yield FOS values that are slightly lower than either traditional two-dimensional L-E models or three-dimensional FEM unit-width models that use a worst case section technique. Overall, three-dimensional FEM slope stability modeling is superior to other methods due to its capabilities and versatility

Optimization of Soil Nailing under Some Uncertainties

Soil Nailing is generally practiced in Malaysian slopes for highway and hillside development projects as a stabilization method for very steep cut slopes. Due to its ease of construction and is relatively maintenance free as an effective slope stabilization method, soil nail slopes with a height greater than 20 m is gradually being used for slopes in Malaysia. In this study, “Slope A” which is an existing cut slope with reinforcement of soil nailing is being reanalyzed in order to minimize the cost. Therefore, parametric studies are conducted using this existing project model to study the effects of certain factors such as slope geometry, water table level, soil parameters and factor of safety on the slope stability. Also, a few areas such as the soil nailing configuration (arrangement and length) and other parameters are taken into account to determine the optimization in terms of project cost. A software, named SLOPE/W is used by applying soil nails to improve the slope stability and to propose the most economical slope condition. The results obtained from the study will determine the most economical slope which optimize in terms of reduction in total length of soil nailing

Optimization of Soil Nailing under Some Uncertainties

Soil Nailing is generally practiced in Malaysian slopes for highway and hillside development projects as a stabilization method for very steep cut slopes. Due to its ease of construction and is relatively maintenance free as an effective slope stabilization method, soil nail slopes with a height greater than 20 m is gradually being used for slopes in Malaysia. In this study, “Slope A” which is an existing cut slope with reinforcement of soil nailing is being reanalyzed in order to minimize the cost. Therefore, parametric studies are conducted using this existing project model to study the effects of certain factors such as slope geometry, water table level, soil parameters and factor of safety on the slope stability. Also, a few areas such as the soil nailing configuration (arrangement and length) and other parameters are taken into account to determine the optimization in terms of project cost. A software, named SLOPE/W is used by applying soil nails to improve the slope stability and to propose the most economical slope condition. The results obtained from the study will determine the most economical slope which optimize in terms of reduction in total length of soil nailing

CIVIL ENGINEERING, SCIENCE AND TECHNOLOGY CHALLENGES: GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING

The book is based on scientific and technological advances in various Geotechnical and Geoenvironmental Engineering areas of Civil Engineering. It nurtures therefore the exchange of discoveries among research workforces worldwide including those focusing on the vast variety of facets of the fundamentals and applications within the Geotechnical and Geoenvironmental Engineering area. To offer novel and rapid developments, this book contains original contributions covering theoretical, physical experimental, and/or field works that incite and promote new understandings while elevating advancement in the Geotechnical and Geoenvironmental Engineering fields. Works in closing the gap between the theories and applications, which are beneficial to both academicians and practicing engineers, are particularly of interest to this book that paves the intellectual route to navigate new areas and frontiers of scholarly studies in Geotechnical and Geoenvironmental Engineering area

An Experimental and Numerical Study on Creep Behavior of Soil Nail Walls in High Plasticity Clays

The use of soil nails for earth retaining structures have increased tremendously. Since soil nail walls are now considered as permanent structures there is a need to study and characterize creep behavior of soil nail walls. FWHA restricts the construction of soil nail walls in high plasticity soils. This is because it is thought that clays with high plasticity fines show creep tendencies which will adversely affect the stability of the retaining wall in the long term. This thesis presents the results from laboratory testing on remolded high plasticity clays from Beaumont, Texas. Direct shear tests were conducted to understand the variation in strength with water content. Consolidated undrained tests were done to obtain undrained parameters of clay. Unconsolidated undrained and consolidated drained tests were also conducted to understand the drained behavior and variation of strength with depth. Creep tests on samples with two different saturations was conducted to understand the viscous behavior of clay under different saturation conditions. The deformation and stresses in nails of an actual soil nail wall constructed in Beaumont was monitored during construction and one year after construction. Numerical modelling of this wall using FLAC-3D was done with input parameters obtained from laboratory testing. A failure that occurred during construction because of over excavation was simulated. The creep behavior was modelled using Burger model. The stability of the retaining wall for a period of 75 years was predicted using this model. Field experience and results from numerical modelling and lab tests recommend that creep behavior of soil nail walls in high plasticity soils need not be necessarily considered in the design

Analysis of horizontal deformations to allow the optimisation of geogrid reinforced structures

Geogrid reinforced structures have been successfully used for over 25 years. However their design procedures have remained largely focused on ultimate failure mechanisms, originally developed for steel reinforcements. These are widely considered over conservative in determining realistic reinforcement and lateral earth stresses. The poor understanding of deformation performance led many design codes to restrict acceptable soils to selected sand and gravel fills, where deformation is not as concerning. Within UK construction there is a drive to reduce wastage, improve efficiency and reduce associated greenhouse gas emissions. For geogrid reinforced structures this could mean increasing reinforcement spacing and reusing weaker locally sourced soils. Both of these strategies increase deformation, raising concern about the lack of understanding and reliable guidance. As a result they fail to fulfil their efficiency potential. This Engineering Doctorate improved the understanding of horizontal deformation by analysing performance using laboratory testing, laser scanning industry structures and numerical modelling. Full-scale models were used to demonstrate a reduction in deformation by decreasing reinforcement spacing. Their results were combined with primary and secondary case studies to create a diverse database. This was used to validate a finite element model, differentiating between two often used construction methods. Its systematic analysis was extended to consider the deformation consequences of using low shear strength granular fills. The observations offered intend to reduce uncertainty and mitigate excessive deformations, which facilitates the further optimisation of designs

Interaction behaviour of soil and geosynthetic reinforcement

The research described in this thesis concerns the use of transparent soil in physical modelling to better understand theoretical and analytical analyses of a geotechnical engineering problem. One of the more recent evolutions in the field of geotechnics is the use of geosynthetic materials as reinforcement to improve the shear resistance of soil, and ultimately provide reinforcement to earth structures. Their application in engineering earthworks has increased significantly in recent years. When designing reinforced earth structures, a vital aspect is to understand the interaction between the reinforcement and the compacted soil as this governs the overall stability. The main function of the reinforcement is to redistribute the stresses within the soil structure in order to enhance the internal stability of the reinforced soil structure. The reinforcement undergoes tensile strain as it transfers loads from unstable to stable zones of the soil. The most common example of soil-geogrid interaction research is to investigate pull-out capacity. The lack of knowledge of interaction mechanics between soil and reinforcement has considerable impact on the ability to implement rigorous analytical solutions, or to assign suitable parameters for interface elements in numerical modelling. By using classical pull-out, previous researchers have indicated that the interface factors vary between 0.6 - 0.8 (FHWA-NHI-00-043, 2001); hence, it is likely that many designs over predict the possible resistance that may be generated. Furthermore, in the absence of field validation, there is uncertainty as to how representative small scale pull-out tests reflect the likely behaviour that would prevail in the prototype structure. The transparent soil utilised here is representative of coarse soil and allows nonintrusive measurement of soil displacement on a plane highlighted by a sheet of laser light, captured by a digital camera. This enables the measurement of the displacement of the soil on the target plane by using the image process technique “Particle Image Velocimetry”. This technique allows the observation of the interaction between soil and geogrid, and the shear and pull-out boundary which is mobilised around the geogrid. The principal aim of this research is to investigate the detailed interaction between granular soil and geosynthetics, and to provide a better understanding of the interaction both analytically and numerically. To achieve this aim, this research is separated into two key areas: 1. Analytical modelling of the interaction between soil and geogrid to assess the degree of uncertainty inherent in the methods; 2. Advanced visualisation element tests using transparent soil technology and Particle Image Velocimetry (PIV) to directly observation of the patterns of strain between the soil and reinforcing material

ISGSR 2011 - Proceedings of the 3rd International Symposium on Geotechnical Safety and Risk

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