Ben Leshchinsky

Effects of Geocell Confinement on Strength and Deformation Behavior of Gravel

AbstractIn past years, railroad transportation has been of growing interest because of its efficiency and advancement in railway technologies. However, many issues arise because of the variability in subsurface conditions along the sizeable lengths of track that exist. One very important issue is the need for significant upkeep and maintenance for railways passing over areas of poor soil conditions as a result of continuous deformation and a lack of stiffness from the foundation. One general solution for lack of substructure integrity has been confinement, applied through a variety of reinforcement types, including geocell. To investigate the effectiveness of geocell confinement on substructure integrity, a series of embankment model tests with different configurations of geocell placement (one layer and two layers of geocell) were constructed and loaded monotonically and cyclically for comparison with unreinforced, control tests. On the completion of these tests, the model embankments were simulated nume...

Centrifuge Modeling of Slope Instability

This paper demonstrates the use of a centrifuge modeling technique in studying slope instability. The slope models were prepared from sand, and sand mixed with 15 and 30% fines by weight, compacted at optimum water content. The validity of the modeling technique was confirmed using slope models of different heights, inclinations, and soil types. The soil behavior was studied under triaxial and plane strain conditions, and the extended Mohr-Coulomb failure criterion was found relevant for expressing the strength of unsaturated compacted soil based on the angle of internal friction and apparent cohesion. The Bishops circular mechanism, together with the extended Mohr-Coulomb failure criterion, was able to simulate the slope failure reasonably well. The rainfall of different intensities was then induced on the 60° stable slopes of sand with 15% fines. It was found that the failure of slope under rainfall may be interpreted as a reduction in apparent cohesion. The centrifuge tests also allowed the rainfall intensity-duration threshold curve (local curve) to be generated for the test slopes, and the accumulated rainfall corresponded well to some of the reported field observations.

Active Earth Pressures for Unsaturated Retaining Structures

AbstractAlthough commonly ignored in classical earth pressure methods, precipitation-induced failures in retaining structures highlight a need for explicit consideration of negative pore pressures or matric suction. Changes in the degree of saturation in the backfill can significantly affect active earth pressures. This paper presents an analytical framework for calculating the thrust of active earth pressures under unsaturated steady flow conditions. The proposed method addresses limitations associated with the collapse mechanism and effective stress representation in alternative analytical methods. The formulation was derived by implementing an analytical solution for one-dimensional (1D) steady flow into a limit equilibrium–based effective stress analysis. A closed-form solution for the thrust was obtained by employing a log spiral surface, considering the effect of tension cracks, backslope inclination, and batter for c′-ϕ′ soils with variable backfill saturation. A unified effective stress representa...

Impact of Cohesion on Seismic Design of Geosynthetic-Reinforced Earth Structures

AbstractThis paper calculates the thrust of lateral earth pressures exerted by unstable slopes comprised of c-ϕ soil and subjected to seismic (pseudostatic) loading conditions. Although the proposed method can be used for seismic stability analysis of geosynthetic-reinforced earth structures (GRESs), the formulation and results are also applicable to many other relevant earth retention systems. To study the impact of cohesion, the authors develop formulations to determine the seismic active earth pressure coefficient for c-ϕ soils resulting in a closed-form solution. The pseudostatic formulation considers the effect of tensile cracks, backslope inclination, and batter while assuming a log spiral failure surface. Two formulas are presented. One is for the conventional inclination of thrust. The second formulation considers a more feasible inclination of the thrust when it is likely to act against facing units with large setbacks constructed for large batter walls. The authors perform parametric studies and...

Bearing Capacity of Footings Placed Adjacent to c′-ϕ′ Slopes

AbstractConventional bearing capacity analyses for shallow foundations placed on slopes use a modified set of bearing capacity factors based on soil properties, footing geometry, and slope configuration, but are restricted to purely cohesionless or purely cohesive soils. This approach is adequate for establishing bearing capacity on engineered fills with controlled foundation properties, yet does not adequately address design for bearing capacity on soils that have both cohesion and internal frictional resistance—a common scenario for native soils. This role becomes increasingly important in design for mechanically stabilized earth walls, which are often placed on slopes of native c′-ϕ′ soils in which the bearing capacity can often be the critical design constraint. Prior approaches to bearing capacity on horizontal ground for c′-ϕ′ soils utilize principles of limited state plasticity in their formulation, yet the most commonly applied bearing capacity approaches on slopes use semiempirical formulations t...

Limit Equilibrium and Limit Analysis: Comparison of Benchmark Slope Stability Problems

AbstractConventional stability analyses use various limit equilibrium (LE) methods to determine the minimum, critical factor of safety and its associated failure mechanism. These methods generally assume that collapse will follow predefined geometric constraints that are dependent on certain input criteria—an effective approach for simple geotechnical problems, but challenging in consideration of complex problems. An effective solution lies in the use of upper bound limit analysis (LA) in conjunction with a discretization procedure known as discontinuity layout optimization (DLO). Use of DLO-LA can be an effective tool for establishing a critical failure mechanism and its stability without the constraints or assumptions required in LE analyses. This study compares the use of LE (Spencer method with dynamic programming optimization, Morgenstern-Price, Spencer) and LA for several examples that focus on complex geotechnical scenarios to illustrate agreement and differences between the analyses, as well as si...

Contour Connection Method for automated identification and classification of landslide deposits

Landslides are a common hazard worldwide that result in major economic, environmental and social impacts. Despite their devastating effects, inventorying existing landslides, often the regions at highest risk of reoccurrence, is challenging, time-consuming, and expensive. Current landslide mapping techniques include field inventorying, photogrammetric approaches, and use of bare-earth (BE) lidar digital terrain models (DTMs) to highlight regions of instability. However, many techniques do not have sufficient resolution, detail, and accuracy for mapping across landscape scale with the exception of using BE DTMs, which can reveal the landscape beneath vegetation and other obstructions, highlighting landslide features, including scarps, deposits, fans and more. Current approaches to landslide inventorying with lidar to create BE DTMs include manual digitizing, statistical or machine learning approaches, and use of alternate sensors (e.g., hyperspectral imaging) with lidar.This paper outlines a novel algorithm to automatically and consistently detect landslide deposits on a landscape scale. The proposed method is named as the Contour Connection Method (CCM) and is primarily based on bare earth lidar data requiring minimal user input such as the landslide scarp and deposit gradients. The CCM algorithm functions by applying contours and nodes to a map, and using vectors connecting the nodes to evaluate gradient and associated landslide features based on the user defined input criteria. Furthermore, in addition to the detection capabilities, CCM also provides an opportunity to be potentially used to classify different landscape features. This is possible because each landslide feature has a distinct set of metadata - specifically, density of connection vectors on each contour - that provides a unique signature for each landslide. In this paper, demonstrations of using CCM are presented by applying the algorithm to the region surrounding the Oso landslide in Washington (March 2014), as well as two 14,000ha DTMs in Oregon, which were used as a comparison of CCM and manually delineated landslide deposits. The results show the capability of the CCM with limited data requirements and the agreement with manual delineation but achieving the results at a much faster time. CCM presents a new way of detecting landslide deposits using Bare Earth LiDAR.CCM requires simple input data to perform automated landslide deposit detection.Each landslide has an associated signature that can be used for classification.CCM presents a supplement to manual landslide inventorying.

Enhancing Ballast Performance using Geocell Confinement

Enhancing Ballast Performance through Geocell Confinement Ben Adam Leshchinsky In past years, railroad transportation has been of growing interest due to its efficiency and advancement in railway technologies. However, many issues arise due to the variability in subsurface conditions along the sizeable lengths of track that exist. One very important issue is the need for significant upkeep and maintenance for railways passing over areas of poor soil conditions due to continuous deformation and a lack of stiffness from the ballasted foundation. One general solution for lack of substructure integrity has been confinement, applied through a variety of reinforcement types, including geocell. To investigate the effectiveness of geocell confinement on ballasted substructure integrity, a series of embankment model tests with different configurations of geocell placement (one layer and two layers of geocell) were constructed and loaded monotonically and cyclically for comparison to unreinforced, control tests. Upon the completion of these tests, the model embankments were simulated numerically using finite element procedures. The results were then used as validation for a parametric study, observing the effects of less competent geocell material, ballast and foundation conditions and their implications. Further numerical simulations were then performed on railroad embankments reinforced with and without geocell to model realistic railroad conditions and the effects of confinement on performance. The tests and numerical simulations demonstrate that geocell confinement effectively increased stiffness and strength of a ballast embankment, while reducing vertical settlement and lateral spreading. Additionally, the parametric study shows that the use of geocell provides a composite, “mattressing” effect that distributes subgrade stress more uniformly than without reinforcement, increasing bearing capacity and reducing settlement, especially on soft foundations or when using weaker ballast. The results suggested that in some site conditions, use of geocell might be an economical alternative to frequent maintenance and/or lower train speeds. Additionally, it implies that geocell might be cost-effective when used in combination withed degraded, weaker ballasts, i.e. inferior local or recycled materials. The use of geocell in ballast stabilization could prove to a sustainable solution for a common and expensive problem.

Yumokjeong Landslide: an investigation of progressive failure of a hillslope using the finite element method

The presented case study describes the progressive movement of a shallow landslide in Central South Korea after excavation of its toe and subsequent, sustained above-average rainfall. Measured soil data as well as real-time measurements of slope movement and precipitation enabled calibration of a numerical model based on the finite element method (FEM) to capture the destabilization and associated movement of the slope. The numerical model employed the Bishop’s effective stress approach to define the state of stresses in unsaturated soils, as well as transient seepage analyses to introduce the non-uniform effects of actual rainfall data into the analysis. The resulting model provided insight into the complex behavior of a progressively failing slope. It demonstrates that use of numerical methods like FEM that allow for coupled transient unsaturated seepage-stress analyses presents a means of evaluating the progressive failure of slopes under rainfall. Furthermore, it demonstrated that for certain slopes under partially saturated conditions, sustained above-average rainfall may cause shallow landsliding when driven by stimuli like excavation. Additionally, the kinematics of failing slopes may not always follow classical slope stability approaches (sliding block method, circular method of slices, log-spiral, etc.) prior and during slope failure. The given kinematics of a slope failure may change with movement, redistribution of principal stresses, and dissipation of suction stress. Therefore, analyses like FEM present a unique tool to evaluate the deformation behavior of failing slopes on a case-by-case basis.

Comparison of Limit Equilibrium and Limit Analysis for Complex Slopes

Conventional slope stability analysis uses various Limit Equilibrium (LE) methods to determine the minimum factor of safety and its associated critical failure mechanism. These methods frequently assume that collapse will follow a particular assumed geometry, which are effective for simple geotechnical problems, yet may encounter difficulties when considering complex problems. For such complex problems, one effective solution lies in the use of Limit Analysis (LA) based on the upper-bound of plasticity in conjunction with a discretization procedure known as Discontinuity Layout Optimization (DLO), which can be an effective means of determining a critical failure mechanism without the limitation of an assumed slip surface. This paper compares the use of LE (Spencer Method with dynamic programming optimization) and LA for several well-known examples of complex slopes. It is shown LA generally provides slightly lower factors of safety than rigorous LE and manages to handle complexities more effectively without assuming the geometry of the slip surface. It is demonstrated that using the DLO with LA can be applied to various extreme value problems beyond classical slope problems.