Publications

Reference

Mirshekari, M., Ghayoomi, M., Kaklamanos, J. (2019) “Sensitivity of Seismic Site Response to Fluctuations in Water Levels” 7 ICEGE 2019 – International Conference on Geotechnical Earthquake Engineering

Abstract

 Differences in the degree of saturation in soil layers, from the time the soil properties were measured to the time an earthquake occurs, can influence seismic site response observations. To investigate the potential influence of variations in groundwater levels, equivalent linear analyses were performed whereby the groundwater table was changed and soil properties were subsequently adjusted for each scenario. Data from the Kiban-Kyoshin (KiK-net) network of surface-downhole seismometers were used as the baseline for the validation of numerical analyses. Surface-to-rock ratios of response spectra and Fourier transfer functions were numerically calculated for different groundwater level scenarios. Unsaturated soil conditions were modeled by increasing the suction and effective stress in the vadose zone. Stronger ground motion amplifications were observed when the groundwater water level was lowered. This trend is associated with simultaneous effects of shear modulus, damping, and unit weight on seismic response of soil profiles with different groundwater levels.

Reference

Ghayoomi, M., Jarast, P., Mirshekari, M., Borghei, A. (2018) “Application of Cone Penetrometer for Unsaturated Soils inside Geotechnical Centrifuge” UNSAT 2018, Hong Kong.

Abstract

Cone Penetration Test (CPT) is a popular field investigation technique where its results can be used to evaluate soil properties by incorporating the measured cone resistance (qc), sleeve friction (fs), and pore pressure (u) in empirical and/or analytical relations. In this study, a recently developed 12.7 mm diameter miniature piezocone is employed to investigate the effect of unsaturated condition on CPT result. Penetration tests are performed on a sandy soil at 40g gravitational acceleration while steady state infiltration was used for suction control. The measured penetration resistance was used to characterize the soil in terms of classification and friction angle. The results showed that unsaturated condition influences the penetration response and ignoring that, can cause misestimating of the soil properties.

Reference

Mirshekari, M. and Ghayoomi, M. (2018) “Effect of Shaking Intensity on Seismic Response of Unsaturated Sand Layers” UNSAT 2018, Hong Kong.

Abstract

Partial saturation might lead to significant variations in terms of dynamic soil properties, which in turn, results in different seismic site response of unsaturated soil layers. The results of a set of centrifuge seismic experiments on dry and partially-saturated uniform layers of fine sand are presented. Degree of saturation was controlled during the experiments by implementing the steady state infiltration technique in the experimental program. Scaled Northridge and Kobe earthquake motions were applied to the base of specimens with the same degree of saturation to evaluate the influence of partial saturation on the response under motions with different characteristics. The results showed higher intensity amplification inside unsaturated sand layers than those captured in dry specimens, under both input motions. Due to the higher Peak Ground Acceleration of Northridge motion, the amplification factors under Northridge motion were generally lower than those obtained under Kobe motion. The difference between the acceleration amplification in dry and unsaturated sand layers appeared to be frequency dependent, showing higher amplification values for unsaturated sand within the higher frequency ranges. Seismically-induced settlements were consistently lower for unsaturated sand during the experiments under both input motions, which was due to the increased stiffness of unsaturated sand layers.

Reference

Mirshekari, M., Ghayoomi, M., and Borghei, A. (2018) “Modeling Experiments to Investigate Soil-Water Retention in Geotechnical Centrifuge” Physical Modelling in Geotechnics, London, England.

Abstract

Steady state flow and capillary ascending from an identified water table are two common approaches to simulate unsaturated soils inside geotechnical centrifuge. However, they involve challenges with regards to introducing and monitoring unsaturated state, mapping the model results to prototype values, and accurate in-flight measurements. This paper presents planning, sensor calibration procedures, testing strategies, and preliminary results of a set of centrifuge experiments to evaluate the effect of g-level on soil-water retention; while both water content and suction were measured simultaneously throughout the tests. Considering the length scaling for capillary rise, negligible g-effect was observed on suction-water content relation. Steady state infiltration, however, resulted in suction-water content coordinates along the hysteresis curve depending on the sequence of hydraulic scenarios.

Reference

Ghadirain, S., Mirshekari, M., Ghayoomi, M., Khosravi, A. (2017) “Earthquake Induced Lateral Deformation of a Pile-Supported System in Unsaturated Sand” PBD-III, Vancouver, Canada.

Abstract

Soil-pile-structure interaction and its effects on overall response of infrastructure have been an active area of research over the last several decades. However, focus has been mostly on developing methods of quantification for structures interacting with soils in dry or saturated conditions. In this study, a series of seismic centrifuge tests were conducted in an attempt to characterize the effects of degree of saturation on lateral response of soil-pile-structure systems. The scaled physical model tests were carried out on a single pile-mass system embedded in a 11-m layer of Ottawa sand with Dr of about 45%. A steady state infiltration technique was used inside a laminar box mounted atop an inflight shake table to provide uniform suction profiles through the sand layer during shaking. In this paper, the model development and construction procedure are explained followed by comparison of lateral deformation at different levels. Overall, unsaturated soil layers resulted in lower lateral deformation due to the presence of suction and higher shear modulus in soil. This difference was more significant at the superstructure level where more deformations were introduced.

Reference

Mirshekari, M. and Ghayoomi, M. (2017) “Simulating Seismic Response of Unsaturated Sand Layers inside a Geotechnical Centrifuge”, ICSMGE, Seoul, South Korea.

Abstract

Dynamic properties of partially saturated soils may differ from those in dry condition, which may lead to different seismic site response. A set of degree of saturation-controlled centrifuge seismic models were tested to assess the seismic site response of partially saturated sands. Steady state infiltration technique was used to control and obtain uniform profiles of degree of saturation along the specimen depth. The results demonstrate higher surficial acceleration amplification in partially saturated sand than the ones in dry layers. The amplification factors, however, were found to be depth-dependent showing lower values for partially saturated sand. Partial saturation led to higher intensity amplification reflected by higher values of Arias intensity in the unsaturated layers. The changes of frequency content were negligible as a result of low suction range in the sand. Seismic settlement of partially saturated sand with higher stiffness was lower than the one in dry condition.

Reference

 Mirshekari, M. and Ghayoomi, M. (2016) “Challenges in Seismic Modelling of Soil-Structure Systems with Unsaturated Soils using Geotechnical Centrifuge”, Geo-SEI 2016, Phoenix, Arizona.

Reference

Mirshekari, M. and Ghayoomi, M. (2015) “Seismic Site Response of Partially Saturated Soil Layers”, EERI Annual Meeting, Boston, MA.

Reference

Mirshekari, M. and Ghayoomi, M. (2015) “Simplified Equivalent Linear and Nonlinear Site Response Analysis of Partially Saturated Soil Layers”, Geo-Congress 2015 (IFCEE), San Antonio, TX, 1-10.

Abstract

Dynamic properties of soils including small-strain shear modulus (Gmax), shear modulus reduction function (G/Gmax), and damping (D) are affected by changes in the degree of saturation. Inter-particle suction forces in partially saturated soils result in higher effective stress values, which in turn, vary the dynamic soil properties. These alterations could lead to different wave propagation mechanisms, acceleration amplification patterns, and seismically induced settlements. This paper aims to identify the challenges involved in nonlinear seismic site response analysis of partially saturated soils by looking at the response of 10-m sand and silt layers with different constant suction profiles. A set of frequency domain equivalent linear and nonlinear site response analysis under scaled Northridge earthquake motion was performed. A modified version of Bishop’s effective stress equation for partially saturated soils has been utilized to calculate the dynamic soil properties (i.e. Gmax, G/Gmax, and D). Specifically, surface-to-base intensity amplifications (Peak Ground Amplifications and Arias Intensities), spectral accelerations, and lateral deformation profiles of the sand and silt layers with different suction profiles were generated and compared. The insight gained from this study was used to plan and design more complex nonlinear Finite Element site response analysis.

Reference

Ghayoomi, M. and Mirshekari, M. (2014) “Equivalent Linear Site Response Analysis of Partially Saturated Sand Layers” UNSAT 2014, Sydney, Australia.

Abstract

Suction can change the dynamic material properties of soil including the small-strain and straindependent shear modulus and damping. These changes influence the seismic site response of unsaturated soil layers by affecting the wave propagation mechanisms, acceleration amplification and de-amplification, and seismically induced settlements. The higher shear modulus of unsaturated soils due to the presence of interparticle suction forces could result in a different site response. To study this behavior, a set of equivalent linear site response analysis were performed using DEEPSOIL program. A 30-m layer of sand with different constant suction profiles was modeled under Northridge earthquake motion with different scale factors. Bishop’s single-parameter effective stress approach was implemented to incorporate suction into the effective stress equation. The site response was different in soils with various suction levels especially in shallow ground.

Reference

Fakher, A., Mahmoudpour, S., Mirshekari, M., and Meidani, M. (2013) “Study on Drivability of Offshore Piles for the Clayey Soils in Persian Gulf” Fifth National Conference of Offshore Industries, Tehran, Iran.

Reference

Mirshekari, M,. Behnia, C., and Fakhretaha, S. (2013) “Investigation on Finite Element Modeling of Rammed Aggregate Piers: Use of Various Constitutive Models” ISEC-7 2013, Honolulu, HI.

Reference

Mirshekari, M., Behnia, C., and Fakhretaha, S. (2012) “Investigation on Finite Element Modeling of Group of Stone Columns” GEOMAT 2012, Kuala Lampur, Malaysia.

Abstract

In recent years, stone columns have been widely used to increase load bearing capacity as well as settlement reduction of soft soils. 2D finite element methods are usually used for analysis of load-displacement in the soil reinforced by stone columns. Plain strain modeling of stone columns is frequently carried out by idealization of equivalent continuous stone strips. This might result in changing the failure modes of stone columns. For instance, bulging failure mode, one of the important failure modes in stone columns, is neglected in the stiff axe of equivalent stone strip. The usual method is used for defining the equivalent stone strip, does not consider the failure mode variations. In this paper a new method is purposed for 2D modeling of stone columns. Plaxis 2D is used and the analysis is carried out considering various diameters and spacing of stone columns. Floated and rigid based stone columns have been studied applying the proposed method. Comparing the results with those obtained by 3D modeling analysis shows the accuracy of the proposed method.

Keywords

Stone Columns, Finite element, Plain Strain, Equivalent Stone Strip

Reference

Salimi, M., Mirshekari, M., and Fakhretaha, S. (2012) “Dynamic Analysis of Earth Dams Using Elasto-Pelastic Constitutive Models” ACCMES 2013, Tokyo, Japan.

Reference

Mirdamadi, A., Golalipour, A., Mirshekari, M., and Shekarchizadeh, M. (2008) “Utilization of Paper sludge for producing Metakaolin pozzolan” Second National Conference of Env. Eng., Tehran, Iran.