Majid Ghayoomi

Impact of effective stress on the dynamic shear modulus of unsaturated sand.

The dynamic shear modulus of soils is needed to predict soil behavior in response to cyclic loading. Even though the effective stress has been shown to have a significant impact on the dynamic modulus of water-saturated and dry soils, its effect on the dynamic shear modulus of unsaturated soils has not been evaluated. Specifically, studies on the dynamic response of unsaturated soils have characterized variations in small-strain shear modulus (Gmax) as a function of the degree of saturation or matric suction alone. In contrast, this study evaluates the use of the suction stress characteristic curve to characterize the impact of mean effective stress (’m) on the dynamic shear modulus of unsaturated sand. A fixed-free resonant column test device was adapted with a hanging column setup so that the small-strain dynamic shear modulus could be measured for sand specimens under different confining pressures and matric suction values. Trends between the small strain shear modulus and effective stress for unsaturated sand were found to be different from those reported in the literature, where Gmax varied linearly with the square root of ’m.

Seismic Performance of Underground Reservoir Structures: Insight from Centrifuge Modeling on the Influence of Structure Stiffness

AbstractThe available simplified analytical methods for the seismic design of underground structures either assume yielding or rigid-unyielding conditions. Underground reservoir structures do not fall into either of these categories. In this paper, we present the results of three centrifuge experiments that investigate the seismic response of stiff-unyielding buried structures in medium dense, dry sand and the influence of structure stiffness and earthquake motion properties on their performance. The structure to far-field spectral ratios were observed to amplify with increased structural flexibility and decreased soil-confining pressure at the predominant frequency of the base motion. Lateral earth pressures and racking displacements for a range of structural stiffnesses were compared with procedures commonly used in design. Pre-earthquake measured lateral earth pressures compared well with expected at-rest pressures. However, none of the commonly used procedures adequately captured the structural loadin...

Measurement of Small-Strain Shear Moduli of Partially Saturated Sand During Infiltration in a Geotechnical Centrifuge

This paper describes the use of bender elements to measure changes in small strain shear modulus, Gmax, of sand layers due to the change in degree of saturation during centrifuge tests. The goal of the measurements is to verify that steady-state infiltration is an appropriate technique to control the effective stress in centrifuge physical modeling of partially saturated sands. Specifically, the suitability of infiltration is assessed by checking if the measured values of Gmax of partially saturated sand layers follow a similar trend to dry and saturated sand layers when the effective stress is defined from the suction and degree of saturation profiles during steady-state infiltration. Three pairs of bender elements were installed at different depths in a container of Ottawa sand, and the shear wave velocities of the sand were measured during steady-state infiltration into the sand layer. The applied infiltration rate was varied to obtain different uniform distributions of degrees of saturation with depth. Consistent with results from suction-controlled resonant column tests performed on the same sand, the values of Gmax measured from the bender element tests varied nonlinearly with degree of saturation with a peak value at a degree of saturation between 0.3 and 0.4. When interpreted in terms of mean effective stress, the values of Gmax from the bender element tests on partially saturated sands followed a unique trend consistent with measurements for dry and saturated sands.

Centrifuge Test to Assess the Seismic Compression of Partially Saturated Sand Layers

A new testing approach for characterization of the response of partially saturated sand layers to cyclic loading is described in this paper. The approach involves basal shaking of a soil specimen within a laminar container using a hydraulic servo-controlled shake table in a geotechnical centrifuge. Infiltration of water was used to control the profiles of matric suction and degree of saturation, and thus the effective stress state, in the partially saturated sand layer during centrifugation. At steady state infiltration, relatively uniform profiles of degree of saturation and matric suction developed with depth in the sand layer. By varying the infiltration rate, different initial unsaturated conditions were obtained for cyclic testing. Instrumentation was incorporated into the setup to measure the accelerations induced in the shake table and soil profile, surface settlements, volumetric water content profiles, and pore water pressure. Cyclic tests were performed on sand layers having degrees of saturation ranging from 0.00 to 0.55 to assess the impact of effective stress on the layer’s deformation response. A nonlinear trend was observed in the variation of surface settlement with degree of saturation, with a minimum value obtained for sand having a degree of saturation of 0.28. This trend is consistent with the relationship between small strain shear modulus and degree of saturation for this sand.

Empirical Methodology to Estimate Seismically Induced Settlement of Partially Saturated Sand

AbstractSettlement of soil layers during and after earthquake shaking is a major cause of damage to buildings and geotechnical structures. The available empirical design methods to consider seismically induced settlement focus on sands in dry or water-saturated conditions, and there is currently a gap in the basic understanding of the mechanisms of seismically induced settlements of partially saturated sands. An effective stress-based empirical methodology is proposed to estimate the seismically induced settlement of a free-field layer of sand in partially saturated conditions. This approach estimates the settlement by separately considering the volumetric strains caused by compression of void space during strong shaking (seismic compression) and dissipation of excess pore water pressures generated during earthquake shaking (postcyclic reconsolidation). A parametric evaluation of the methodology indicates that the small strain shear modulus, the parameters of the modulus reduction curve, the approach to e...

Cyclic Triaxial Test to Measure Strain-Dependent Shear Modulus of Unsaturated Sand

Dynamic shear modulus plays an important role in the seismic assessment of geotechnical systems. Changes in the degree of water saturation influence dynamic soil properties because of the presence of matric suction. This paper describes the modification of a suctioncontrolled cyclic triaxial apparatus to investigate the strain-dependent shear modulus of unsaturated soils. Several strainand stress-controlled cyclic triaxial tests were performed on a clean sand with various degrees of saturation. Suction in unsaturated sands increased the shear modulus in comparison with the ones in dry and saturated conditions for different shear strain levels, with a peak modulus in higher suction levels. Also, shear modulus decreased with an increase in the shear strain for specimens with similar matric suction. The normalized shear moduli of the unsaturated sand specimens followed a similar trend to the ones predicted by the available empirical shear modulus reduction functions but showed lower values. The modulus reduction ratios of unsaturated sands shifted up as a result of higher effective stress and suction-induced stiffness. These trends were consistent for both strainand stress-controlled tests. DOI: 10.1061/(ASCE) GM.1943-5622.0000917.© 2017 American Society of Civil Engineers.

Methodology to evaluate performance of pavement structure using soil moisture profile

Variation of moisture content in the subgrade layer of a flexible pavement may play a significant role in its structural performance by influencing the stiffness of the pavement system. The objective of this study is to develop and test methodologies to incorporate the soil moisture profile into flexible pavement evaluation and to determine how the changes of water table level will affect the pavement deformation. Falling weight deflectometer (FWD) data at two locations in Minnesota and Oklahoma were used to estimate the in situ measured pavement deformation with seasonal changes of water content. Then, a layer elastic analysis approach was adjusted to include the moisture dependency of the subgrade resilient modulus in calculating the deflection basin. The controlling parameters in these set of analyses were asphalt layer temperature, the degree of water saturation in unbound materials, depth to the groundwater table, and depth to bedrock. The environmental data for this study were obtained from the Long Term Pavement Performance-Seasonal Monitoring Program (LTPP-SMP) for the two pavement sections for different climatic conditions. The comparisons showed that dividing the subgrade layer into sublayers with variable, moisture-dependent modulus according to the location of moisture measurements will result in more accurate estimation of the pavement deflection. In addition, the predicted deflections indicated that considering moisture data up to the depth associated with 10% of surface vertical stress may suffice for this type of analysis. However, the overall quality of prediction is sensitive to the material type in the subgrade layer.

SIMPLIFIED EQUIVALENT LINEAR AND NONLINEAR SITE RESPONSE ANALYSIS OF PARTIALLY SATURATED SOIL LAYERS

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.

Effect of Ground Motion Characteristics on Seismic Soil-Foundation-Structure Interaction

A series of dynamic centrifuge experiments involving a soil-structure model were performed to investigate the influence of ground motion characteristics on site performance and soil-foundation-structure interaction (SFSI) on medium-dense sand. When investigating kinematic SFSI, the translational component of the foundation motion was observed to de-amplify compared to the free-field in terms of most intensity parameters primarily during intense shaking and at higher frequencies. The buildings fundamental rocking frequency was strongly influenced by the predominant frequency of the base motion. When investigating inertial SFSI, increasing the shaking intensity was observed to increase the flexible-base natural period of the structure. Spectral accelerations at the foundation level were significantly amplified near the buildings flexible-base natural period. Structural settlements were greater than those in the free-field, and their rate followed the rate of the Arias intensity time history of the base motion. More holistic ground motion parameters, such as Arias and Housner Intensities, demonstrated a strong and consistent influence on SFSI and site performance.

Impact of subsurface water on structural performance of inundated flexible pavements

ABSTRACT The assessment of the structural performance of inundated pavements is crucial to reduce the induced damage in the aftermath of flooding. The objective of this study is to present a simplified approach to investigate the impact of different subsurface water levels on the performance of pavement structures. The flooded conditions and subsequent water recession were simulated by slowly lowering the subsurface water from the pavement surface down in depth. A hydrostatic water pressure distribution was assigned above and below the water level for different cross sections with different subgrade types. Layer Elastic Analysis was performed to predict the surface deflection, stresses and strains at the bottom of asphalt layer and at the top of subgrade layer to evaluate the impact of saturated and unsaturated pavements. Finally, the influence depth for subsurface water level at which the road can withstand traffic with zero to minimum deterioration was discussed given the pavement structure and soil type. The results showed that the pavement structural capacity decreases significantly when the base and subgrade layers are fully saturated. The pavement starts to regain its strength once the subsurface water level dropped below the base course layer.