Kianoosh Hatami

Numerical Study of Reinforced Soil Segmental Walls Using Three Different Constitutive Soil Models

A numerical finite-difference method (FLAC) model was used to investigate the influence of constitutive soil model on predicted response of two full-scale reinforced soil walls during construction and surcharge loading. One wall was reinforced with a relatively extensible polymeric geogrid and the other with a relatively stiff welded wire mesh. The backfill sand was modeled using three different constitutive soil models varying as follows with respect to increasing complexity: linear elastic-plastic Mohr-Coulomb, modified Duncan-Chang hyperbolic model, and Lades single hardening model. Calculated results were compared against toe footing loads, foundation pressures, facing displacements, connection loads, and reinforcement strains. In general, predictions were within measurement accuracy for the end-of-construction and surcharge load levels corresponding to working stress conditions. However, the modified Duncan-Chang model which explicitly considers plane strain boundary conditions is a good compromise between prediction accuracy and availability of parameters from conventional triaxial compression testing. The results of this investigation give confidence that numerical FLAC models using this simple soil constitutive model are adequate to predict the performance of reinforced soil walls under typical operational conditions provided that the soil reinforcement, interfaces, boundaries, construction sequence, and soil compaction are modeled correctly. Further improvement of predictions using more sophisticated soil models is not guaranteed.

Influence of toe restraint on reinforced soil segmental walls

A verified fast Lagrangian analysis of continua (FLAC) numerical model is used to investigate the influence of horizontal toe stiffness on the performance of reinforced soil segmental retaining walls under working stress (operational) conditions. Results of full-scale shear testing of the interface between the bottom of a typical modular block and concrete or crushed stone levelling pads are used to back-calculate toe stiffness values. The results of numerical simulations demonstrate that toe resistance at the base of a reinforced soil segmental retaining wall can generate a significant portion of the resistance to horizontal earth loads in these systems. This partially explains why reinforcement loads under working stress conditions are typically overestimated using current limit equilibrium-based design methods. Other parameters investigated are wall height, interface shear stiffness between blocks, wall facing batter, reinforcement stiffness, and reinforcement spacing. Computed reinforcement loads are ...

Sensor-Enabled Geosynthetics: Use of Conducting Carbon Networks as Geosynthetic Sensors

A novel technique is developed based on the piezoresistivity of electrically filled polymers to measure the tensile strain in modified geosynthetics without the need for conventional instrumentation (e.g., strain gauges). This paper reports the development of the technique and the results obtained on high-density polyethylene and polypropylene (PP) geogrid specimens filled with carbon black and carbon nanotubes (NTs). It was found that except for NT-filled PP specimens all other composites exhibited significant strain sensitivity in their conductivity. The proof-of-concept study reported in this paper has two important features: (1) strain sensitivity of electrical conductivity was demonstrated in polyolefins used to manufacture geosynthetics; and (2) this strain sensitivity was obtained and demonstrated over the range of strain values that are important in geosynthetic engineering applications.

Laboratory performance of reduced-scale reinforced embankments at different moisture contents

Abstract The paper describes instrumentation, testing, and detailed results of three 1-m high reinforced embankment models that were built in the laboratory at three different gravitational water contents (GWC). Each embankment model was subjected to a strip load near its crest until failure. The embankment models were constructed using lean clay at the GWC values ranging between OMC−2% and OMC+2% (OMC: optimum moisture content). Each embankment model included a single reinforcement layer which was placed 180 mm below the embankment surface. The location of the reinforcement layer was selected based on preliminary embankment tests and numerical simulations to ensure that it would intercept the failure surface that developed underneath the strip footing near the embankment slope. The embankments were instrumented with a total of 67 sensors to measure the soil GWC, matric suction and excess pore pressure, reinforcement strains, earth pressure and deformations of the embankment models, and the test box during the tests. The test setup and data described in this paper are part of a long-term study to validate a set of moisture reduction factors (MRF) introduced by the authors in their recent studies which involved pullout and interface shear tests on the same soil and reinforcement materials. Specifically, the data and discussions in this paper provide a basis for further in-depth analysis to verify or modify the authors’ proposed MRF values for actual embankment configurations. Furthermore, the test descriptions and results in this study will be used to validate numerical models to examine the influence of the soil matric suction and moisture content on the soil–geotextile reinforcement interface strength and internal stability of reinforced soil structures. The ultimate goal of the study is to develop a better understanding of the influence of matric suction on the performance of reinforced earthen structures constructed with marginal soils, and improved methodologies for their design.

Reinforcement Pullout Capacity in Mechanically Stabilized Earth Walls with Marginal-Quality Soils

Pullout capacity of geotextile reinforcement is an important consideration in the analysis of internal stability of reinforced soil structures, especially those constructed with marginal soils. Precipitation, ground water infiltration, and seasonal variations of water content during the construction process or service life of the structure could result in significant reductions in the matric suction and lead to a reduction in the strength of the soil–geotextile interface. Consequently, the reinforced soil structure may experience unacceptable deformations or even failure during its construction or postconstruction periods. The loss of matric suction in the soil influences both the shear strength of the soil and the soil–reinforcement interface. However, the focus of this study was merely on the latter. Nine pullout tests and 18 interface shear tests were performed to measure the pullout resistance of a reinforcement geotextile in a marginal soil that was compacted at different gravimetric water contents (GWCs). The marginal soil was selected to meet the limiting requirements of the National Concrete Masonry Association guidelines for segmental retaining walls with respect to fines content, gradation, and plasticity. The range of GWC values investigated varied from the dry to the wet side of the optimum moisture content of the soil. The matric suction in the soil was measured to evaluate its influence on the soil–reinforcement interface shear strength. A moisture reduction factor is proposed to account for the reduction in the soil–geotextile interface strength as a result of the loss in matric suction.

Failure criterion for graphene in biaxial loading—a molecular dynamics study

Molecular dynamics simulations are carried out in order to develop a failure criterion for infinite/bulk graphene in biaxial tension. Stresses along the principal edge configurations of graphene (i.e. armchair and zigzag directions) are normalized to the corresponding uniaxial ultimate strength values. The combinations of normalized stresses resulting in the failure of graphene are used to define failure envelopes (limiting stress ratio surfaces). Results indicate that a bilinear failure envelope can be used to represent the tensile strength of graphene in biaxial loading at different temperatures with reasonable accuracy. A circular failure envelope is also introduced for practical applications. Both failure envelopes define temperature-independent upper limits for the feasible combinations of normalized stresses for a graphene sheet in biaxial loading. Predicted failure modes of graphene under biaxial loading are also shown and discussed.

Influence of Inadequate Compaction near Facing on Construction Response of Wrapped-Face Mechanically Stabilized Earth Walls

Standard practice for the compaction of backfill soil near the facing of a mechanically stabilized earth (MSE) wall or embankment is to use lightweight compaction equipment to prevent excessive facing deformation. Complications caused by compaction with heavy equipment near the facing could also include misalignment or structural damage of the wall facing and overstressing of the reinforcement layers. However, inadequate compaction near the facing could result in later settlement or appearance of voids behind the facing. Little research has been reported in the literature to quantify the effects of loosely compacted soil behind the facing on the stability and serviceability of MSE walls at the end of construction. The influence of inadequate compaction effort near the facing on the construction performance of idealized wrapped-face MSE wall models was investigated by using a numerical simulation approach. It was shown that inadequate backfill compaction within 1 m of the wall facing could increase the wall lateral displacement by about 40% and the reinforcement strains by about 90% compared with the response of an otherwise identical (i.e., control) wall model constructed with uniform compaction throughout the backfill. This effect was found to be more significant for higher-quality backfills with greater friction angle values and less stiff reinforcement materials. Results of this study on idealized wrapped-face wall models highlight the importance of proper soil compaction and quality control near the facing of MSE walls and offer example quantitative increases that could be expected in the out-of-alignment and reinforcement loads in these MSE structures.

Atomic-Scale Simulation of Sensor-Enabled Geosynthetics for Health-Monitoring of Reinforced Soil Slopes and Embankments

Safety and serviceability assessment of reinforced soil slopes and embankments includes monitoring their reinforcement strains during their service life to ensure that they are within the allowable limits. Strain gauges are used as a common method to measure and monitor strains in geosynthetic reinforcement. However, the use of strain gauges has several shortcomings which are discussed in the paper. Recently, a novel technique has been developed at the University of Oklahoma which is based on the piezoresistivity of carbon black (CB)and carbon nanotube (CNT)-filled polymers. This method is devised to eliminate the need for conventional instrumentation to measure tensile strain in modified geosynthetics. In this technique, a polymeric material (e.g. polyethylene, PE) and an electrically conductive filler (e.g. CB or CNT), are blended to fabricate Sensor-Enabled Geosynthetics (SEG) with piezoelectric characteristics. In this study, Molecular Dynamics (MD) simulations are used to examine the effect of the filler (CB in this case) on the stress-strain behavior of SEG samples. Dreiding and OPLS force fields are adopted for the simulations which are carried out in the LAMMPS environment. Results of the study indicate that adding small quantities of CB to otherwise pure PE samples may not adversely affect their tensile strength properties.

Shear strength of unsaturated soil-geotextile interfaces.

Geotextiles are widely used in various applications including earthen structures, which are often built in unsaturated soil conditions. The design of these earthen structures is often dominated by the shear strength of the interface between soil and reinforcement (i.e. geotextile) layers. However, the unsaturated soil-geotextile interface interactions are not completely understood. This paper examines the shearing behavior of unsaturated soil-geotextile interfaces. Direct shear test results are used to define failure envelopes for unsaturated soil and soil-goetextile interfaces. Experimental results reveal a nonlinear relationship between the soil-geotextile interface strength and matric suction. The paper demonstrates that this non-linear failure envelope can be modeled using the Soil Water Characteristic Curve (SWCC) and saturated effective stress-strength parameters. The paper also compares the shearing behavior of unsaturated soil and unsaturated soil-geotextile interfaces.

Influence of Soil Strength on Optimal Embedment Depth of Geosynthetic Reinforcement Layer in Granular-Reinforced Foundations

The effect of soil friction angle value on the optimal embedment depth of a single geosynthetic reinforcement layer in granular foundations was investigated with a numeric simulation approach. The numeric model was validated against the measured results of reduced-scale plane strain model foundations tested with two footing width sizes. The model foundations were constructed with gravel-sized round aggregates and were subjected to the concentrated vertical load of shallow footings analogous to a track-ballast system. After the validation of the numeric model, a series of parametric analyses was carried out with an idealized, full-scale foundation model to investigate the dependency of the optimal reinforcement embedment depth on the soil friction angle value. The optimal reinforcement depth in field-scale reinforced foundations under strip footings is between 10% and 15% of footing width, depending on the site soil friction angle value. The optimal reinforcement depth is found to be slightly closer to the...