Limit Equilibrium Design Research Articles

Reliability Comparative Analysis of Codes for the Design of Cantilever Sheet Pile Walls: Basis for Studying the Principles of International Standards

Abstract In geotechnical engineering, the design of cantilever sheet pile walls is one of the most important and complex tasks due to the use of different limit equilibrium design methodologies to ...

Limit Equilibrium Analysis and Design of Geosynthetic-Reinforced Fill Walls Under Special Conditions

Geosynthetics have been successfully used to reinforce fill for walls for different applications. Under certain circumstances, these walls may be designed and constructed under special conditions, for example, tiered walls, back-to-back walls, limited space fill walls and fill walls with secondary reinforcement. Most design methods available in design standards or manuals based on lateral earth pressure theories cannot or must be modified to approximately handle such special conditions. Limit equilibrium methods have been commonly used to analyse and design slopes including reinforced slopes and can be used to analyse and design geosynthetic-reinforced fill walls under these special conditions. This paper describes progressive failure from fill walls to fill slopes, presents the evidences of fill walls and slopes having similar failure modes and justifies the use of the limit equilibrium approach to analyse and design geosynthetic-reinforced fill walls under special conditions. This paper also presents a top-down limit equilibrium design procedure and demonstrates the advantages of using this procedure for designing geosynthetic-reinforced fill walls under special conditions.

Numerical analysis of collapse in a deep excavation supported by ground anchors

On 3 October 2013, a global failure occurred in one of the stabilised sections of a deep excavation in Shahrak-e gharb in Tehran. The excavation was supported by ground anchors with enlarged reinforced concrete thrust blocks and sprayed concrete facing. The failure occurred despite the system passing conventional limit equilibrium design and satisfying requirements in terms of allowable displacements. The case of Shahrak-e gharb was modelled using Abaqus in order to understand the soil deformation leading to the failure condition. It was found that a near-surface zone of weaker fill material was responsible for the failure. As the displacements during construction and the soil failure mechanisms were well matched by the numerical model, the case study was considered to be a validation case for the numerical model and a parametric study on the extent to which anchoring bypassed the weaker surface soil was performed. This study showed that proper embedment into competent soils is important as excessive strains can still develop if the anchor bond zone is insufficiently confined, potentially leading to progressive collapse.

Experimental verification of current seismic analysis methods of reinforced soil walls

The paper investigates the accuracy of two limit equilibrium design approaches used for seismic stability analyses of reinforced soil walls. Responses from a series of reinforced soil wall models, tested on shaking table, are compared to similar responses predicted using both NCMA, and AASHTO/FHWA design guidelines. First, orientation of the soil slip surface measured during subsequent base shaking was compared to the values predicted analytically. Comparison results suggested that the current design methods tend to under predict the size of the soil failure wedge at different input base acceleration amplitudes. Therefore, the current equation proposed to predict the angle of soil slip surface is modified to match measured values. Second, the horizontal and vertical seismic coefficients used in the current seismic analysis methods are compared to values inferred from shaking table accelerometer responses. Results indicated that equations suggested to calculate horizontal acceleration coefficient for design of reinforced soil walls are non-conservative for input base acceleration larger than 0.3 g. Finally, dynamic earth force magnitudes and locations, and dynamic reinforcement force increments measured from shaking table model tests are used to identify sources of conservatism and non-conservatism of the current design methodologies. Different modifications have been suggested to reduce conservativeness or increase the safety of the design guidelines.

Performance of hybrid MSE/Soil Nail walls using numerical analysis and limit equilibrium approaches

MSE/Soil Nail hybrid earth retaining walls provide a more economical design for applications in cut/fill situations than the traditionally used full height MSE and drilled shaft retaining walls. MSE/Soil Nail hybrid earth retaining walls use a soil nailed wall in the cut section and an MSE wall in the fill section. The paper demonstrates the results of instrumentation and monitoring an MSE/Soil Nail hybrid retaining wall system. Innovative 2D finite element models were used to simulate the behavior of the hybrid retaining wall system. The paper describes the philosophy of design of hybrid MSE/Soil Nail walls and the outline of circumstances which are favorable for such walls. In order to evaluate the global Factor of Safety (FOS) of the soil nail wall portion using the conventional limit-equilibrium design methods (LEM), two different methodologies to simulate the effect of the upper MSE walls were proposed. The obtained results of numerical and limit equilibrium approaches as a design methodology are presented for such type of walls, in this paper.The design of hybrid MSE/SN walls is complicated as it is a composite of two different reinforcement techniques, traditional limit equilibrium approaches cannot be used alone for design of such walls, it shall be supported by numerical methods for estimation of the global factor of safety and failure surface. The limit equilibrium approaches can be used for the estimation of internal stability and facing connection for both soil nails and mechanically stabilized walls.

Behaviour of reinforced embankments on soft rate-sensitive soils

The behaviour of reinforced embankments constructed over rate-sensitive soft foundation soils is studied. Factors such as the viscoplastic properties and hydraulic conductivity of the soil, reinforcement stiffness and construction rate are examined. The time-dependent responses of excess pore pressures, reinforcement strains and foundation deformations are investigated. The short-term embankment stability is of particular interest. The construction of reinforced embankments to the height determined based on a limit equilibrium design is simulated to examine the assumptions made in the conventional undrained analysis. It is shown that creep and stress-relaxation of viscoplastic soils after the end of embankment construction may be significant for rate-sensitive soils. The embankment stability is shown to be critical during creep and stress-relaxation of foundation soils after construction. The undrained shear strength measured in laboratory triaxial tests using currently recommended strain rates without an appropriate correction may lead to unsafe design for rate-sensitive soils. For such soils, the increase in reinforcement strain shortly after the completion of construction can be higher than that developed during the construction. Excess pore pressures increase after construction owing to the viscoplastic behaviour of the foundation soil. Reinforcement is shown to have the potential to both increase stability and decrease long-term creep deformations.

Behaviour of reinforced embankments on soft rate-sensitive soils

The behaviour of reinforced embankments constructed over rate-sensitive soft foundation soils is studied. Factors such as the viscoplastic properties and hydraulic conductivity of the soil, reinforcement stiffness and construction rate are examined. The time-dependent responses of excess pore pressures, reinforcement strains and foundation deformations are investigated. The short-term embankment stability is of particular interest. The construction of reinforced embankments to the height determined based on a limit equilibrium design is simulated to examine the assumptions made in the conventional undrained analysis. It is shown that creep and stress-relaxation of viscoplastic soils after the end of embankment construction may be significant for rate-sensitive soils. The embankment stability is shown to be critical during creep and stress-relaxation of foundation soils after construction. The undrained shear strength measured in laboratory triaxial tests using currently recommended strain rates without an appropriate correction may lead to unsafe design for rate-sensitive soils. For such soils, the increase in reinforcement strain shortly after the completion of construction can be higher than that developed during the construction. Excess pore pressures increase after construction owing to the viscoplastic behaviour of the foundation soil. Reinforcement is shown to have the potential to both increase stability and decrease long-term creep deformations.

Soil Reinforcement Loads in Geosynthetic Walls at Working Stress Conditions

Knowing the load in geosynthetic reinforcement layers in full-scale walls is an important step to improving internal stability design methods. Interpretation of empirical reinforcement load data enables analytical models to be properly calibrated. High-quality empirical data also provides a baseline against which new design methods can be validated. In this paper, loads in soil reinforcement layers from 16 full-scale geosynthetic wall case histories were estimated from strain measurements and converted to load through the stiffness of the reinforcement material. The paper summarizes these estimated peak loads, describes general trends in the data, and compares these reinforcement loads to predictions using current design practice applied to the wall case histories. It was found that reinforcement loads derived from strain measurements are, in general, much lower than would be predicted based on current limit equilibrium design methods that use classical earth pressure theory. The low reinforcement strains and loads measured to date in geosynthetic walls point to the desirability of using peak soil shear strength rather than constant volume shear strength for design. This approach will help to reduce design conservatism and will be consistent with the philosophy of preventing failure of a major component of the reinforced soil system, the soil. Once the soil has failed, for all practical purposes, the wall has failed as well.

Modelling system uncertainty in the design of embedded retaining walls

The purpose of this paper is to explore some of the aspects of embedded retaining wall design, in particular the interrelationship between system uncertainty and the use of factors of safety in the various design procedures. System uncertainty reflects the mismatch between idealized retaining wall behaviour and what is observed in practice. This feature is modelled using the concept of fuzzy sets where the degree of support for the lateral stress state at failure is considered and related to current limit equilibrium design methods, measured lateral pressures and the results of finite element analyses. The proposed model is shown to have some advantages over current design practice.

EMBEDDED RETAINING WALL DESIGN PROCEDURES: SYSTEM UNCERTAINTY AND FUZZY SAFETY MEASURES

Abstract This paper explores some of the aspects of embedded retaining wall design, in particular the interrelationship between system uncertainty and the use of factors of safety in the various design procedures. System uncertainty reflects the mismatch between idealised retaining wall behaviour and what is observed in practice. This feature is modelled using the concept of fuzzy sets where the degree of support for the lateral stress state at failure is considered and related to current limit equilibrium design methods, measured lateral pressures and the results of finite element analyses. The proposed model is shown to have some advantages over current design practice.

Model Behavior of Prestressed Tie‐back

Tie‐back walls are frequently used presently for the support of deep excavations, yet soil‐structure interaction under “service” conditions is not really understood. This study is an attempt to analyze the problem using the results of experimental work by the writer. Soil and wall movements, variations in anchor prestress, as well as sand stresses on the wall were measured on a small scale physical model. Numerical parametric analyses of soil‐structure interaction based on a Winkler model were also made by varying soil stiffness, soil properties, and anchor prestress values to fit the model tests. Results were subsequently extrapolated to real size. It was shown that the soilwall‐anchor interaction was profoundly influenced by an increase in the anchor prestress, which generally led to a large reduction of the associated tie‐back displacements. This should be attributed to the force equilibrium on the wall, which was shown to differ markedly from the one used by conventional limit equilibrium design.

Observed behaviour of a geotextile-reinforced embankment constructed on peat : Rowe, R K; MacLean, M D; Barsvary, A K Can Geotech JV21, N2, May 1984, P289–304

The design, instrumentation, and field performance of two instrumented sections of a geotextile-reinforced embankment are described. The 1-1.5 m high embankment was constructed in three stages on top of a peat deposit that extended to depths of up to 7.6 m. The peat was highly compressible with average water contents of 445% and 785% at the two instrumented sections. A polypropylene, monofilament woven fabric (Permealiner 1195) was used to reinforce the embankment over the less compressible section of the deposit. At stage I, 1.37 m of fill resulted in settlements of approximately I m but only 1% transverse strain in the geotextile. At stage 11, a total of 3.87 m of fill had been added with total settlements of approximately 3 m and transverse geotextile strain of 21%. Stage I11 construction involved bringing the embankment to final grade, with a total fill and pavement thickness of 4.2 m. Settlements at stage I11 were relatively small. A strong, twisted, slit film, polypropylene woven fabric (Geolon 1250) was used to reinforce the more compressible section of the deposit. At stage I, 2.74 m of fill resulted in settlements of approximately 3.1 m and transverse geotextile strains of 3%. At stage 11, a total of 6.1 m of fill had been added resulting in settlements of approximately 4.6 m. At stage 111, the embankment was brought to grade by reducing the fill thickness and constructing the pavement. The final fill and pavement thickness was 5.7 m. It is concluded that the use of a single layer of even a very strong geotextile was insufficient to prevent large shear deformations in these deep, compressible, peat deposits. The procedures used in the design represented the state-of-the-art at that time; however, they did not provide a good indication of how the embankment would perform in the field. It is recommended that simplified limit equilibrium design procedures should be viewed with considerable caution when designing geotextile-reinforced embankments on peat.

The observed behaviour of a geotextile-reinforced embankment constructed on peat

The design, instrumentation, and field performance of two instrumented sections of a geotextile-reinforced embankment are described. The 1–1.5 m high embankment was constructed in three stages on top of a peat deposit that extended to depths of up to 7.6 m. The peat was highly compressible with average water contents of 445% and 785% at the two instrumented sections.A polypropylene, monofilament woven fabric (Permealiner 1195) was used to reinforce the embankment over the less compressible section of the deposit. At stage I, 1.37 m of fill resulted in settlements of approximately 1 m but only 1% transverse strain in the geotextile. At stage II, a total of 3.87 m of fill had been added with total settlements of approximately 3 m and transverse geotextile strain of 21%. Stage III construction involved bringing the embankment to final grade, with a total fill and pavement thickness of 4.2 m. Settlements at stage III were relatively small.A strong, twisted, slit film, polypropylene woven fabric (Geolon 1250) was used to reinforce the more compressible section of the deposit. At stage I, 2.74 m of fill resulted in settlements of approximately 3.1 m and transverse geotextile strains of 3%. At stage II, a total of 6.1 m of fill had been added resulting in settlements of approximately 4.6 m. At stage III, the embankment was brought to grade by reducing the fill thickness and constructing the pavement. The final fill and pavement thickness was 5.7 m.It is concluded that the use of a single layer of even a very strong geotextile was insufficient to prevent large shear deformations in these deep, compressible, peat deposits. The procedures used in the design represented the state-of-the-art at that time; however, they did not provide a good indication of how the embankment would perform in the field. It is recommended that simplified limit equilibrium design procedures should be viewed with considerable caution when designing geotextile-reinforced embankments on peat. Keywords: embankment, muskeg, peat, geotextile, settlement, pore pressures, field observation, instrumentation, soil reinforcement.

Design Of Retaining Walls : SystemUncertainty & Fuzzy Safety Measures

This paper explores some of the aspects of embedded retaining wall design, in particular the interrelationship between system uncertainty and the use of factors of safety in the various design procedures. System uncertainty reflects the mismatch between idealised retaining wall behaviour and what is observed in practice. This feature is modelled using the concept of fuzzy sets where the degree of support for the lateral stress state at failure is considered and related to current limit equilibrium design methods, measured lateral pressures and the results of finite element analyses. The proposed model is shown to have some advantages over current design practice.