Farshid Vahedifard

Impact of Toe Resistance in Reinforced Masonry Block Walls: Design Dilemma

Reinforced masonry block retaining walls are comprised of a narrow column of stacked blocks at their exposed end. This column is placed on a nonstructural leveling pad to facilitate the placement of facing units. Theoretically, this column can generate very large toe resistance to sliding. A recent publication indicates that an accepted design methodology implicitly counts on this resistance in assessing the reinforcement load. Although not calculated in this design, it unconditionally considers that over 60% of the resultant horizontal force in a 12-m-high wall is carried by the toe, which is made up of 0.3-m-deep blocks. This paper elucidates this issue by explicitly identifying the magnitude of toe resistance and critically reviews whether such high resistance is universally suitable for design. It shows that high toe resistance may not be feasible for most foundation soils. The high impact of toe resistance on the reinforcement force poses a design dilemma as to the reliability of this resistance, eve...

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...

Effective Stress-Based Limit-Equilibrium Analysis for Homogeneous Unsaturated Slopes

AbstractWith a suction stress–based effective stress representation, stability analysis of unsaturated engineered and natural slopes can be performed effectively in the same manner as the classical limit-equilibrium (LE) methodologies. This paper presents an analytical framework for effective stress LE analysis of unsaturated homogeneous slopes under steady one-directional (vertical) flow. The proposed log spiral failure surface–based LE method involves only two additional hydromechanical parameters for unsaturated soil, approximating the inverse of the air-entry pressure and pore-size distribution. Both parameters are used to describe seepage and effective stress variations in unsaturated soils. Unlike most other LE formulations, the method is statically determinate. A parametric study was performed, and stability charts for general use are presented. The impact of infiltration and evaporation on the stability of slopes for four hypothetical soil types was studied. It is shown that the apparent cohesion ...

Relationship between the Seismic Coefficient and the Unfactored Geosynthetic Force in Reinforced Earth Structures

AbstractThis paper presents an integrated analytical method for calculating the resultant unfactored geosynthetic force in reinforced earth structures under seismic loading conditions. The method utilizes a pseudostatic limit equilibrium approach for assessing the internal stability of a reinforced earth structure, assuming a potential rotational failure along a log spiral trace. A closed-form solution is presented for determining the sum of all horizontal forces mobilized in the geosynthetic reinforcement along their intersection with the critical log spiral surface. This mobilized sum is then redistributed among the individual layers to determine the unfactored reinforcement forces that are needed to resist the applied seismic acceleration. Parametric studies were utilized, and the results are presented in a series of design charts for different conditions. Such charts can be used to determine the required tensile strength of the reinforcement for a given seismic coefficient. Alternatively, for a given ...

A Comparison of Density-Based and Modulus-Based In Situ Test Measurements for Compaction Control

This paper presents and compares the results from a series of in situ density-based and modulus-based compaction control tests that were conducted during construction of a coarse-grained soil embankment. To simulate current construction practices as closely as possible, these in situ tests were performed on an embankment that was constructed and compacted by a vibratory smooth drum roller in a series of lifts. During construction of the test embankment, the compaction process was monitored using the nuclear density gauge device and a number of alternative modulus-based devices, including the lightweight deflectometer, the dynamic cone penetrometer, and the soil stiffness gauge. Comparison of the in situ test results illustrates that point-to-point variability in measured values is quite common for each of these test devices, to varying degrees for the different devices that were examined. Consistent increases in measured soil properties from pass-to-pass of the compactor are considered critical for proper control of the compaction process, with some devices faring better than others in this area of performance. The measured modulus values correlated poorly to the nuclear density gauge dry unit weights, and also correlated poorly with other measured moduli when the results from different devices were compared. This lack of agreement was likely caused by a variety of factors including: variations in the magnitude of strain and rate of strain application between the different modulus-based devices, variations in the tested volume between the different devices, and variations in the local moisture content and matrix suction conditions. Finally, the effect of soil moisture content was shown to be critically important when interpreting the results from modulus-based tests, and the utility of multiple regression analyses was explored for including this effect.

Unified Method for Estimating the Ultimate Bearing Capacity of Shallow Foundations in Variably Saturated Soils under Steady Flow

AbstractThis study presents a unified method for estimating the ultimate bearing capacity of shallow foundations resting on variably saturated soils under a steady flow. The effective stress approach originally employed for predicting the bearing capacity of saturated soils is extended by incorporating a suction stress-based representation. A closed-form equation is used to define the suction stress characteristic curve, capturing changes in the effective stress because of varying soil saturation and matric suction. The proposed method uses the classic effective shear strength parameters and two fitting parameters to represent the soil water characteristic curve. The method can be used for different soil types, various degrees of saturation, and different surface flux boundary conditions. The results from the proposed formulation were compared, and good agreement was observed against three sets of experimental results from model footing and plate load tests. A parametric study was performed, and the ultim...

Drought threatens California's levees.

Comment on “Tectonic control of Yarlung Tsangpo Gorge revealed by a buried canyon in Southern Tibet” Peter K. Zeitler, Peter O. Koons, Bernard Hallet,

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.

Sustainable Improvement of Clays Using Low-Carbon Nontraditional Additive

Nontraditional low-carbon additives are widely used in the sustainable treatment of problematic soils for construction and pavement materials. This study investigated the mechanical and microstructural properties of white kaolin (low strength clay) and green bentonite (high swelling clay) treated with a low-carbon sodium silicate-based liquid additive. The mechanical tests included unconfined compressive strength (UCS), direct shear and one-dimensional compression tests. Microscale assessments, including a field emission scanning electron microscopic (FESEM) test, nitrogen-based Brunauer, Emmett, and Teller (N2-BET) surface area analysis and particle size analysis (PSA), were performed on the treated specimens to investigate the modification of soil structure, including soil fabric and interparticle forces. The performance of the proposed additive is demonstrated by the improvement of shear strength and compressibility of both tested soils. The optimum additive content was found to be 6%, and a significant improvement occurred in the first 7 days of curing. The mechanical property improvement is attributed to the formation of cementitious products and, subsequently, the modification of the soil structure. These cementitious products filled the pores and bonded the soil particles, resulting in an increase in interparticle forces. The sodium silicate-based additive can offer a low-carbon alternative to traditional additives such as cement and lime, which is significant from the engineering and environmental perspectives.