A new article co-authored by Siavash Zamiran has been published by Journal of Rock Mechanics and Geotechnical Engineering. The article discusses a wellbore stability model using a chemo-thermo-poroelastic approach. For reading the full please visit ScienceDirect website or get access to a pre-proof version from Researchgate.
Drilling through chemically-active shale formations is of special importance due to time-dependent drilling fluid-shale interactions. The physical models presented so far include sophisticated input parameters, requiring advanced experimental facilities, which are costly and in most cases unavailable. In this paper, sufficiently-accurate, yet highly practical, models are presented containing parameters easily-derived from well-known data sources. For ion diffusivity coefficient, the chemical potential was formulated based on the functionality of water activity to solute concentration for common solute species in field. The reflection coefficient and solute diffusion coefficient within shale membrane were predicted and compared with experimental measurements. For thermally-induced fluid flow, a model was utilized to predict thermo-osmosis coefficient based on the energy of hydrogen-bond that attained a reasonably-accurate estimation from petrophysical data, e.g. porosity, specific surface area (SSA), and cation exchange capacity (CEC). The coupled chemo-thermo-poroelastic governing equations were developed and solved using an implicit finite difference scheme. Mogi-Coulomb failure criterion was adopted for mud weight required to avoid compressive shear failure and a tensile cut-off failure index for mud weight required to prevent tensile fracturing. Results showed a close agreement between the suggested model and experimental data from pressure transmission tests. Results from a numerical example for a vertical wellbore indicated that failure in shale formations was time-dependent and a failure at wellbore wall after 85 min of mud-shale interactions was predicted. It was concluded that instability might not firstly occur at wellbore wall as most of the conventional elastic models predict; perhaps it occurs at other points inside the formation. The effect of the temperature gradient between wellbore and formation on limits of mud window confirmed that the upper limit was more sensitive to the temperature gradient than the lower limit.
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The article "Seismic motion response and fragility analyses of cantilever retaining walls with cohesive backfill" has been published in Journal of Soils and Foundations in February 2018. In this article, the seismic motion response of a cantilever retaining wall with cohesive and cohesionless backfill materials was evaluated using fully dynamic analysis based on finite difference method. The dynamic analysis was validated based on experimental test results and then compared to analytical and empirical correlations based on Newmark sliding block method. Seven different earthquake events and the backfills with low to high levels of cohesion were considered. Nonlinear regression analyses were carried out to provide correlations between free-field peak ground acceleration (PGA) and maximum relative displacement of the retaining wall. These results were compared to results from empirical and analytical methods. Furthermore, fragility analyses were conducted to determine the probability of damage to the retaining wall for different free-field PGAs and backfill cohesions. It is demonstrated to what extent a small amount of cohesion in backfill material can influence displacement of the retaining wall and probability of damage in seismic conditions.
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Current seismic design criteria for retaining wall structures suggested by different organizations are based on estimating seismic earth pressure of the wall using analytical solutions (e.g. AASHTO 2012; Campos 2008). Different guidelines including AASHTO and Caltrans consider pseudo-static analytical solutions to identify seismic earth pressure (AASHTO 2012; Campos 2008). The first analytical attempt as a pseudo-static method to evaluate seismic earth pressure of retaining walls was suggested by Okabe (1926) and the method was verified in retaining walls with unsaturated and cohesionless soil material by Mononobe and Matsuo (1929) using shake table test results. The method developed by these investigators is known as Mononobe-Okabe (MO) method and is still widely used to determine seismic earth pressure of retaining walls. MO procedure is an extension of Coulomb theory and is based on limit equilibrium method and assumes an occurrence of a failure wedge in the backfill. MO method considers the earthquake acceleration is uniform in the backfill and is applied to the center of gravity of the failure wedge.
There are also many studies that evaluated the total seismic earth thrust (Pae) experimentally (Agusti and Sitar 2013; Al-Homoud and Whitman 1999; Atik and Sitar 2009; Nazarian and Hadjian 1979; Prakash 1981; Seed and Whitman 1970) and numerically (Atik and Sitar 2009; Bui et al. 2014; Elgamal and Alampalli 1992; Green et al. 2008; Green and Ebeling 2003; Psarropoulos et al. 2005; Scotto di Santolo and Evangelista 2011; Wilson and Elgamal 2010; Woodward and Griffiths 1996). Specifically, Seed and Whitman (1970), hereafter abbreviated as S&W, conducted different centrifuge tests on retaining walls with cohesionless backfill materials and provided a simple equation for determining Pae, which linearly correlates with horizontal earthquake peak ground acceleration (PGA). Their experimental-based estimation has been used in design guidelines for evaluating Pae, e.g., US Army Corps of Engineers (Whitman and Liao 1985). It is worth mentioning that the earthquake acceleration intensities for the mentioned numerical and experimental studies were limited to PGA ground motions of 0.2g to 0.4g. In most of these studies, the cohesion factor of backfills and hysteretic behavior of soil were also neglected.
Guidelines by AASHTO and state Departments of Transportations require the use of granular materials as backfill for retaining wall constructions as backfills with fine and cohesive material are sensitive to swell, shrinkage, and degree of saturation (AASHTO 2012; Campos 2008; Murinko 2010). However, according to field observations in several cases, backfill materials have a various amount of cohesion (Kapuskar 2005). Kapuskar (2005) conducted field observations of more than 100 retaining wall and abutment backfills used in 20 different bridge sites in the State of California. It was concluded that out of 20 bridge sites, 15 of them had sandy backfills with low plasticity fines that had cohesion up to 95 kPa.
Seismic response of retaining walls considering backfill cohesion has been taken into account analytically (Das and Puri 1996; Prakash and Saran 1966; Shukla et al. 2009; Shukla and Bathurst 2012; Vahedifard et al. 2014). Most of these approaches were developed based on an extension of MO method with consideration of backfill cohesion, wall adhesion, and tension cracks in cohesive backfill materials. The MO-based methods have restrictions to be used for backfills with different soil layers and complex geometries. Therefore, analytical methods based on trial wedge procedure has been proposed for backfills with various layers of soil or complex geometries (Anderson et al. 2008).
In addition to analytical solutions, some experimental and numerical investigations have also been conducted to evaluate the effects of backfill cohesion on seismic response of retaining walls there are some limited experimental and numerical studies are available that assessed the effect of cohesion on seismic response of retaining walls (Agusti and Sitar 2013; Mikola et al. 2014; Wilson and Elgamal 2010, 2015, Zamiran and Osouli 2014, 2015). The limitations of these studies are: 1) the wall response with a variation of backfill cohesion was not considered; 2) the Pae, its point of action, and induced moment under full seismic analyses were not considered; 3) the representative hysteretic damping and shear reduction of the backfill materials have not been considered. Also, these studies focused on the effect of either single soil cohesion parameter or single PGA.
In this study, seismic response of retaining walls is evaluated for cantilever walls with cohesive sandy backfill materials via fully dynamic analysis (FDA). A constitutive model accounting the hysteretic behavior of soil during dynamic loading excitation is utilized. A validated numerical approach based on centrifuge test results is used to conduct the FDA. The effect of three earthquake ground motions and backfill with various cohesions on seismic earth pressures, total seismic earth thrust coefficient (Kae), incremental seismic earth thrust coefficient (ΔKae), the location point of action of Pae, and wall moment variations during the shaking event are studied. The results of FDA are compared to estimations based on current analytical solutions. Finally, recommendations are provided for considering the effects of backfill cohesion in seismic response of cantilever retaining walls.
The news of GeoConfluence Research Scholarship earned by Siavash Zamiran is reflected in SIUE news webpage.
The content of the news is provided in the following:
SIUE’s Zamiran Earns Inaugural Geoconfluence Research Scholarship
14 November 2016, 1:18 PM
Southern Illinois University Edwardsville research assistant Siavash Zamiran has been awarded the inaugural GeoConfluence Research Scholarship from the St. Louis Chapter of the Geo-Institute of American Society of Civil Engineers (ASCE). He was recognized during GeoConfluence, the association’s annual conference, on Friday, Nov. 4 in St. Charles, Mo.
Zamiran is a doctoral candidate in the SIUE School of Engineering’s Cooperative PhD Program in Engineering Science with Southern Illinois University Carbondale. The scholarship will support his dissertation research, “Seismic Investigations of Retaining Wall Structures.”
“This scholarship funding will be used for developing a program that assists engineers through the nation to design retaining walls based on earthquake characteristics in their specific seismic zone” said Zamiran. “My future professional goal is to continue engineering research with experimental and numerical studies, and increase my publications in conferences and scientific journals. I would like to be active in different areas of geotechnical engineering in both academic and industrial environments.”
The GeoConfluence Research Scholarship promotes research in the field of geotechnical/geoenvironmental engineering, particularly that which will benefit the state of practice in the Midwest and can be implemented by regional engineers.
Zamiran’s extensive research activity during his doctoral studies includes the study of retaining walls’ behavior during earthquakes, subsidence and stability evaluation of Illinois coal mines, and the study of levees due to flooding and soil erosion, among other projects.
The scholarship includes up to $3,000 to be used for specific educational tools and equipment, materials and travel needed to fulfill a thesis/dissertation. As part of the award, Zamiran will present his research results at a future GeoConfluence conference.
Photo: (L-R) Siavash Zamiran, doctoral candidate and GeoConfluence Research Scholarship recipient, Pravin Jha, chairman of the GeoConfluence Research Scholarship Subcommittee, Kord Wissmann, president of Geo-Institute, and Sandeep Goud Burra, scholarship recipient.
Foundations on claystone with swell potential may experience upward movement and failure. In this case study, the cause of 5.8 cm upward movement of drilled-in piers is analyzed using survey data, extensometer readings, and moisture content monitoring of claystone at the site. Swell laboratory tests were conducted to characterize the swelling characteristics of the weathered rock. A swell potential analysis for deep foundations is presented, which accounts for the group behavior of the piers. The interaction of the group piers with the swelling rock was considered in analyzing the initiation of the upward movement. Furthermore, a novel inverse analysis method was presented to integrate the swell laboratory test results and numerical modeling to identify the representative swell pressures acting on group piers as well as upward movement of the pier system. The behavior of the pier group foundation in swelling rock under various pier spacing and superstructure pressures are discussed.
The results has been recently published in the journal of Geotechnical and Geological Engineering. For more details please click here.
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