Seismic Slope Stability of Reactivated Landslides — A Performance Based Analysis

The purpose of this study is to identify and analyze the differences between the various methods of slope stability analysis with respect to the height of the water table, and earthquake excitation. This was achieved through analysis of a cross section of the 2005 Bluebird Canyon Landslide in Laguna Beach, California. The profile was analyzed with the slope stability analysis software Rocscience Slide, using eight different methods, varying the water table in two meter increments and the seismic coefficient in increments of one tenth. A total of 15 different water tables were used with 10 different seismic loadings, yielding a relatively large set of data. Additionally, spreadsheets were constructed for analysis using Bishop’s Simplified Method with the pseudo static approach for seismic loading. While all of the methods yielded results for the tests with no seismic excitation, the number of methods yielding results diminished as the seismic coefficient increased. The only two methods that gave results for all loading conditions were the Army Corp #2 method and the Ordinary Method of Slice. In general, the Army Corp #2 method gave the least conservative results, while the Ordinary Method of Slice gave the most conservative results. This was true for almost all loading conditions and water tables. Another trend was Bishop’s Simplified Method giving nearly identical results to the Jambu Corrected method, and the spreadsheet results being nearly the same as the Jambu Simplified method. As expected, lowering the water table increased the safety factor for nearly all the methods. This beneficial effect was found to diminish as the water table lowered, and as the seismic coefficient increased. The incremental effect of lowering the water table on the safety factor was found to be nearly the same for all cases except the Army Corp #2 method. In this case, lower safety factors were obtained for lowering the water table in the presence of seismic excitation. As the seismic coefficient increased, the beneficial effect of lowering the water table decreased. The effect that the seismic coefficient had on safety factor also decreased with an increase in the coefficient. Furthermore, it was found that for lower water tables, the effect that the seismic coefficient has on safety factor is relatively large, while the effect is small for full or nearly full water tables. This was found to be true for all cases except the Army Corp #2 method

Advancements in Shear Strength Interpretation, Testing, and Use for Landslide Analysis

Landslides are devastating natural disasters that result in loss of life, property damage, and community disruption. They have global impacts, causing fatalities and economic losses, particularly in mountainous regions near densely populated areas. Landslides can be caused by natural factors, including water saturation from heavy rainfall, snowmelt, and changes in groundwater levels, as well as seismic activity such as earthquakes and volcanic eruptions. Human activities, such as altering drainage patterns, destabilizing slopes, and removing vegetation, also contribute to landslides. Construction and development on slopes, over-steepening, and improper land management practices can further increase the risk of landslides. A key component in understanding the stability of slopes will be knowledge of the shear strength of the soils involved. However, to do so, it will be necessary to understand the various measuring methods of shear strength, loading conditions, and other parameters. Different methods and tests are employed to determine the shear strength of soil, depending on the specific conditions and objectives. Direct shear tests are often utilized to measure peak and fully softened shear strengths. Triaxial tests, on the other hand, are suitable for assessing both peak and fully softened shear strengths under drained or undrained conditions. Generally, the ring shear device is preferred for measurements of the residual shear strengths. However, multiple reversal direct shear tests and specifically modified direct shear tests as well as triaxial tests have also been utilized for this purpose. The cyclic simple shear test is recommended as an effective technique for replicating in-situ conditions to investigate the cyclic resistance and post-cyclic shear strengths of soils. Several correlations have been developed in the literature to estimate various shear strengths, including the fully softened and residual shear strengths of soil, as summarized in this paper. These correlations utilize parameters such as the liquid limit, plasticity index, mineralogy, clay fraction, and effective normal stress. The undrained shear strength of over-consolidated soils can be captured with the use of the Stress History and Normalized Soil Engineering Properties (SHANSEP) method. Extending this approach with the use of the normalized undrained strength ratio can result in two correlations that can capture the undrained shear strength. The paper also presents correlations for the true and base friction angles to estimate the shear strength using Hvorslev’s theory. This allows for a departure from the use of the cohesion intercept and friction angle in the Mohr-Coulomb failure envelope, both of which are dependent on the over-consolidation ratio. The power function effectively represents the cyclic strength curves in soils with the curve fitting parameters a and b defining their shape and position. A correlation between the normalized undrained strength ratio and post-cyclic effective stress ratio to assess the undrained shear strength after cyclic loading was also introduced. This correlation was shown to also capture the effects of excess pore pressure dissipation and reductions in shear strength induced by a second cyclic load.This book chapter is published as Tiwari, B., Ajmera, B. (2023). Advancements in Shear Strength Interpretation, Testing, and Use for Landslide Analysis. In: Alcántara-Ayala, I., et al. Progress in Landslide Research and Technology, Volume 2 Issue 2, 2023. Progress in Landslide Research and Technology. Springer, Cham. https://doi.org/10.1007/978-3-031-44296-4_1. Posted with permission. © 2023 The Author(s

Development of Temperature-Controlled Direct Shear Box

An increase in the temperature of permafrost that is caused by global warming can lead to a significant decrease in shear strength. Seasonal freeze-thaw (F-T) cycles can also adversely affect the shear strength of soils. This can result in damages to infrastructure, negative impacts on the economy, and a decline in the quality of life. Thus, it is crucial to understand the shear strength of permafrost and seasonally frozen-thawed soils. Several studies have utilized various instruments to observe the behavior of soils under such conditions, including a temperature- controlled triaxial system to apply F-T cycles or a traditional direct shear apparatus placed within a temperature-controlled room. Since most commercial geotechnical labs do not have a temperature-controlled room or a temperature-controlled triaxial system, this article presents the design of a new cost-effective direct shear box that was developed to allow temperature-controlled testing in a traditional direct shear device. The modifications to the direct shear box comply with ASTM D3080/3080M, Standard Test Method for Direct Shear Test of Soils under Consolidated Drained Conditions. Like the standard direct shear box, it consists of two halves and a direct shear cap, but each of these components is hollow to allow for the circulation of glycol. The chiller is capable of imposing temperatures within the range of −40°C to +40°C on the sample being tested. It is also possible to freeze and thaw specimens at a desired normal stress while monitoring the associated heave and compression. The freezing mechanism applied to the soil sample affects the distribution of ice within the pore spaces, necessitating that samples be frozen from all sides if a uniform distribution of ice is necessary. Shear strength parameters from the newly designed temperature-controlled direct shear box matched well with those from the traditional shear box. In addition, the feasibility of temperature-controlled direct shear testing was evaluated at different temperatures, strain rates, and normal stresses.This is a manuscript of an article published as Ahari, Hossein Emami, and Beena Ajmera. "Development of Temperature-Controlled Direct Shear Box." Journal of Testing and Evaluation 51, no. 6 (2023). DOI: 10.1520/JTE20220574. Copyright 2023 by ASTM International. Posted with permission

Development of a Temperature-Controlled Direct Shear Box for Frozen Samples

Global surface temperatures rose by 1°C in 2020 relative to the average temperatures between 1951 and 1980. A reduction in the extent of the almost 24% of the Northern Hemisphere covered by permafrost will impact the stability of new and existing civil infrastructure in the area. Frozen soil is a multi-phase system consisting of ice, unfrozen water, air, and soil particles. Fluctuations in temperature change the proportions of the constituents significantly impacting the strength of the soil mass. The strength behavior of fine-grained soils will differ substantially from that of frictional soils since fine-grained soils can have high ice contents due to their high-water absorption capacities. However, the lack of commercially available equipment has deterred research efforts in understanding the impact of climate change on the shear strength properties of fine-grained soils. This paper describes the development of a new direct shear box that allows for temperature-controlled testing. Modifications to the temperature-controlled shear box as deemed necessary from validation tests are also described. For example, comparisons of the temperature measurements within the soil sample to the chilled glycol circulating in the modified direct shear box indicated that additional insulation had to be provided to the apparatus. The freezing mechanism applied to the soil sample affected the distribution of ice within the pore spaces and illustrated the need to freeze the sample from all directions. This was incorporated into the modifications that were designed in this study. Shear strength parameters from the newly designed temperature-controlled direct shear box matched well with those from the traditional shear box.This is a manuscript of a proceeding published as Emami Ahari, Hossein, and Beena Ajmera. "Development of a Temperature-Controlled Direct Shear Box for Frozen Samples." In Geo-Congress 2023, pp. 262-272. This material may be found at DOI: 10.1061/9780784484678.027. This material may be downloaded for personal use only. Any other use requires prior permission of the American Society of Civil Engineers. Copyright 2023 American Society of Civil Engineers. Posted with permission

An Advanced Edge-Detection Method for Noncontact Structural Displacement Monitoring

A non-contact vision sensor system for monitoring structural displacements with advanced Zernike subpixel edge detection technique is suggested in this paper. Edge detection can detect features of objects effectively without using templates. Subpixel techniques provide more accurate and cost-effective results when compared to integer pixel methods. Built on these two techniques, a new version sensor method was developed to detect the vibrations of structures in this study. Satisfactory agreements were found between the displacements measured by the vision sensor system and those recorded by the Multipurpose Testing System (MTS). A field test was then carried out on a street sign using the proposed vision system. Satisfactory results were obtained using the new version of the sensor system at many points simultaneously without any manually marked targets. Moreover, the system was able to provide natural frequencies and mode shapes of the target instantaneously, which could be used to accurately locate damage

Maximum shear modulus and shear modulus reduction curves of silt-clay mixtures

Dynamic analyses of soils require the shear modulus reduction curves of the materials involved. Previous research has demonstrated that the dynamic response of soils is not only dependent on the plasticity index of the soil mass, but also on the clay mineralogy. However, limited studies have systematically evaluated the impact of these parameters on the variation of shear modulus with strain. In this study, cyclic simple shear and bender element tests are conducted on nearly twenty laboratory prepared mixtures of the montmorillonite, kaolinite, and quartz. The maximum shear modulus is found to reduce with an increase in the plasticity index with more substantial reductions observed in soils with kaolinite than those with montmorillonite. This paper also presents a method to estimate the maximum shear modulus from the results from cyclic simple shear tests. The maximum shear modulus values obtained using this approach are compared with the results from the bender element tests. Modulus reduction curves obtained from cyclic simple shear tests are presented and used to examine the influence of plasticity characteristics and clay mineralogy on the variation in the secant shear modulus with strain.This presentation is published as Ajmera, B., Tiwari, B., Phan, Q.H., Maximum shear modulus and shear modulus reduction curves of silt-clay mixtures. Proceedings of the 20th International Conference on Soil Mechanics and Geotechnical Engineering– Rahman and Jaksa (Eds) © 2022 Australian Geomechanics Society, Sydney, Australia, ISBN 978-0-9946261-4-1. Posted with Permission. This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE.<br

Shear and Elastic Moduli of Fine-Grained Soils: Impact of Consolidation Pressure and Plasticity Characteristics

Montmorillonite, kaolinite, and quartz were used to prepare 20 samples in the laboratory to have liquid limits ranging from 8 to 486 and plasticity indices ranging from 4 to 431. The samples were normally consolidated to pressures ranging from 25 kPa to 100 kPa and then subjected to bender element tests to determine the maximum Young’s modulus of elasticity and maximum shear modulus. Both the maximum Young’s modulus of elasticity and the maximum shear modulus were observed to depend on the consolidation pressure, the clay mineralogy, and the plasticity characteristics. In particular, an increase in the consolidation pressure would result in an increase in the modulus of elasticity and the shear modulus. At a constant plasticity index, a soil containing montmorillonite had a higher moduli value than a soil containing kaolinite. An increase in the plasticity index corresponded to a decrease in the value of the maximum Young’s modulus of elasticity and the maximum shear modulus. Little variations were observed in the values of the maximum Young’s modulus of elasticity and the maximum shear modulus when the plasticity index exceeded 250 regardless of the consolidation pressure or the clay mineralogy. Figures that can be used to estimate the maximum Young’s modulus of elasticity and the maximum shear modulus as a function of the clay mineralogy, plasticity characteristics, and consolidation pressure are provided.This is a manuscript of a proceeding published as Ajmera, Beena, Binod Tiwari, and Quoc-Hung Phan. "Shear and Elastic Moduli of Fine-Grained Soils: Impact of Consolidation Pressure and Plasticity Characteristics." In Geo-Congress 2023, pp. 323-330. This material may be found at DOI: 10.1061/9780784484678.033.  This material may be downloaded for personal use only. Any other use requires prior permission of the American Society of Civil Engineers. Copyright 2023 American Society of Civil Engineers. Posted with permission

Statistical Study of the Geology, Topography, and Pore Fluid Salinity Controls on the Large Slope Failures Observed in North Dakota

A geospatial database of over 24,000 slope failures across the state of North Dakota was developed. The database revealed that nearly 1,500 slopes failures were larger than 100,000 m2 in area posing significant safety concerns and engineering issues. To better understand the influence of various conditioning factors, a statistical study was undertaken in this paper. The slope failures were found to be concentrated in western North Dakota primarily occurring in the Sentinel Butte Formation. Statistical analyses suggest that this is likely due to the steeper topography and the abundance of Sentinel Butte Formation in this region in comparison to eastern North Dakota. The number of slope failures were log-normally distributed with the slope inclination. The mean slope inclination on which slope failures occurred was found to be 9.5° with a standard deviation of 5.7°. Interestingly, despite the presence of high salt concentrations in the soils underlying the majority of the state, the slope failures were found to be concentrated in areas with low salt concentrations. This may be attributed to the lower shear strengths of the soils with lower pore fluid salinity than that of soils with greater concentrations of salinity in the pore fluid.This is a manuscript of a proceeding published as Ajmera, Beena, Aaron Lee M. Daigh, and Kamal Raj Upadhaya. "Statistical Study of the Geology, Topography, and Pore Fluid Salinity Controls on the Large Slope Failures Observed in North Dakota." In Geo-Congress 2023, pp. 491-499. This material may be found at DOI: 10.1061/9780784484654.049. This material may be downloaded for personal use only. Any other use requires prior permission of the American Society of Civil Engineers. Copyright 2023 American Society of Civil Engineers. Posted with permission

True and Base Friction Angles of Clays

Hvorslev’s theory states that the true friction angle and the base friction angle (or Hvorslev’s constant) are fundamental properties of soils that remain constant regardless of the overconsolidation ratio. The use of the true and base friction angles allows for the determination of the traditional shear strength parameters without the need to conduct testing for different stress histories. Despite the importance of these parameters, there is little study available on understanding these parameters for different types of soils. In this study, laboratory prepared mixtures of montmorillonite, kaolinite, and quartz were overconsolidated in an oedometer before their strengths were measured in a constant volume direct simple shear device. The samples were tested at overconsolidation ratios ranging from 1 (normally consolidated) to 32. The results obtained were used to determine the values of the true and base friction angles. Values of the base friction angle were found to depend on the dominating clay mineral. In montmorillonite-dominated soils, the base friction angle remained nearly constant regardless of the plasticity index. However, it was found to increase with an increase in the plasticity index in kaolinite-dominated soils. For both montmorillonite- and kaolinite-dominated soils, the true friction angle would decrease with an increase in the plasticity index for soils with plasticity indices less than 80. An increase in the plasticity index beyond 80 did not result in significant changes in the true friction angle.This is a manuscript of a proceeding published as Tiwari, Binod, Beena Ajmera, Mohammed Al-Behadili, and Mohammed Mohammed. "True and Base Friction Angles of Clays." In Geo-Congress 2022, pp. 56-64. This material may be found at DOI: 10.1061/9780784484036.006.  This material may be downloaded for personal use only. Any other use requires prior permission of the American Society of Civil Engineers. Copyright 2022 American Society of Civil Engineers. Posted with permission

Experimental evaluation of shear strength fluctuations of clayey soils due to multiple cyclic loads

Often seismic loading causes instability and/or failure of geotechnical structures such as foundations, slopes, and embankments in the form of settlements, landslides, and liquefaction. Numerous researchers who have worked on cyclic behavior and strength loss of clayey soils after earthquake loading have focused on the strength reductions immediately after a single cyclic event. Little work is found on the changes in the shear strength of clayey soils after multiple cyclic events representing the main shock and aftershocks in an earthquake sequence. To do so, cyclic direct simple shear tests were conducted on kaolinite specimens. The shear strengths of this soil were measured immediately after a sequence of cyclic loads. The results obtained were used to develop cyclic strength curves and evaluate the strength degradation in the clayey soils. The degree of consolidation between cyclic loads was found to impact the cyclic resistance available during the following cyclic load as well as the resulting strength degradation.This presentation is published as Ajmera, B., Tiwari, B., Iqbal, M. S., and Meeks, T., (2022) Experimental evaluation of shear strength fluctuations of clayey soils due to multiple cyclic loads. Proceedings of the 20th International Conference on Soil Mechanics and Geotechnical Engineering– Rahman and Jaksa (Eds) © 2022 Australian Geomechanics Society, May 1st to May 5th 2022 in Sydney, Australia., ISBN 978-0-9946261-4-1. Posted with permission. This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE