Magdi El-Emam

Probabilistic seismic hazard analysis and spectral accelerations for United Arab Emirates

In recent years, the United Arab Emirates (UAE) has experienced an unprecedented growth which is coupled with the increase in seismic activity in the surroundings. Previous studies presents significant variations in their results whereas some recent studies although very detailed focus on only few cities. This study reviews the results of previous studies and presents new findings for the whole of UAE based on the improved source model and use of next generation attenuation (NGA) equations. The peak ground accelerations, spectral accelerations and deaggregation of hazard for major cities are presented. Moreover, the breakdown of the range of mapped spectral accelerations (S0.2 and S1) is proposed to form the basis for the development of site amplification factors in subsequent studies. The results of this study indicate almost similar values of ground motion compared to some recently published studies and smaller values compared to some earlier studies.

Experimental and Numerical Study of At-Rest Lateral Earth Pressure of Overconsolidated Sand

The paper presents a one-meter-height rigid facing panel, supported rigidly at the top and bottom to simulate nonyielding retaining wall system. A set of load cells is used to measure the horizontal force at the top and bottom of the facing panel, which is converted to equivalent horizontal earth pressure acting at the back of the wall. Another set of load cells is used to measure the vertical load at the bottom of the wall facing, both at the toe and the heel. Uniformly graded sand was used as backfill soil. The measured wall responses were used to calibrate a numerical model that used to predict additional wall parameters. Results indicated that the measured horizontal earth force is about three times the value calculated by classical at-rest earth pressure theory. In addition, the location of the resultant earth force is located closer to 0.4 H, which is higher compared to the theoretical value of H/3. The numerical model developed was able to predict the earth pressure distribution over the wall height. Test set up, instrumentation, soil properties, different measured responses, and numerical model procedures and results are presented together with the implication of the current results to the practical work.

Energy dissipation in engineered sand of large damping ratio

Many geotechnical problems such as seismic resistant designs and machine vibrations require the installation of dampers or isolators to control the amplitude of vibrations. Engineered fills designed with increased capability of dissipating energy can provide a more economical approach to control excessive vibrations. This study presents a novel technique to increase the damping ratio of sand without affecting its stiffness and shear strength. The increase in damping ratio is evaluated by performing resonant column tests on the engineered sand. The damping ratio of the sand is increased by adding a controlled amount of viscoelastic material to the voids. The resonant column tests indicate that the damping ratio of the sand can be increased manifold without affecting the shear modulus. The micromechanical evaluation of the results shows a good correlation between the particle surface area in contact with the pore-mixture and the damping ratio of the sand. The suitability of engineered sand as foundation material is also evaluated by performing direct shear tests. The direct shear tests on mixtures indicate similar or better shear strength parameters compared to pure sand.

Numerical prediction of plane strain properties of sandy soil from direct shear test

Abstract Direct shear test is commonly used for research and geotechnical engineering design due to its simplicity and cost effectiveness. However, analysis of many geotechnical structures such as earth pressure and slope stability problems require measurement of plane strain properties of soil. Therefore, it is usually relied on empirical relationships to predict soil properties in plane strain conditions from direct shear or triaxial tests. In this study, a two-dimensional plane strain numerical model is developed using the finite difference program, FLAC to simulate the mechanical behavior of sandy soil tested in direct shear box. Results are presented in terms of stress distribution within the soil specimen at different stages of shearing process and at different locations of the failure surface. Principal stresses and their directions are also investigated and discussed. Results indicated that, the plane strain properties of the sandy soil can be back-calculated from numerical simulation of direct shear tests with reasonable accuracy. Moreover, the numerical model was able to capture the trend in the experimental results and in most cases gave reasonable estimates of the shear strength and volume change of sandy soil. Numerical results also indicated that angle of internal friction at plane strain condition is significantly larger than the direct shear friction angle. In addition, both normal and shear stresses distributions at failure plane are diverted from being uniform at initial conditions to non-uniform during shearing process and at failure. Finally, principal stresses at failure surface are non uniform and rotated significantly during shearing process.

Local Site Effects on Seismic Ground Response of Dubai-Sharjah Metropolitan Area

This paper investigated the influence of local geotechnical and geological soil conditions on the intensity of ground shaking of Dubai-Sharjah metropolitan area (DSMA). Acceleration-time histories of Dubai and Sharjah were chosen according to spectral shape and similarity in magnitude and distance to a target response spectrum that were obtained from the results of Seismic Hazard Analysis (SHA). Subsurface geotechnical data of 72 different sites, located in the area under consideration, were used in this study. The effect of local site conditions on ground response during earthquake has been evaluated by performing equivalent linear analysis. Dynamic properties of selected soil profiles were evaluated using empirical relations between Standard Penetration Test (STP) N-values and shear wave velocity (Vs). Modulus degradation and damping curves for sand and rock were also employed in the analysis. Results indicated that surficial deposits in the area amplified earthquake ground motion. The peak amplification was observed over relatively narrow frequency range of 1.5-5 Hz (0.2-0.8s period) that was found to represent the predominant frequency range of the site classes under consideration. Amplification factors at PGA and at both short and long periods (0.2s and 1.0s), which are site class dependent, were compared to the current code of practice.

Behaviour of Strip Footing in the Vicinity of Non-Yielding Wall

The paper presents two-1/3 scale nonyielding wall-strip footing models that were constructed and tested at the sand box test facility at the American University of Sharjah (AUS). The objective of the testing program is to quantify the responses of both strip footing and nonyielding wall constructed closer to each other. To achieve this objective, different design construction parameters have been considered including: (1) the strip footing width (B), (2) footing distance from non-yielding wall back (a), and, (3) strip footing embedment depth below the backfill surface (Df). Test results indicated that the location of the strip footing relative to the wall is a major factor in dictating the load bearing capacity of the strip footing, and the maximum lateral deflection of the non-yielding wall. Meanwhile, the vertically loaded strip footing imposed substantial vertical and horizontal forces at the wall top and bottom boundaries. The strip footing load bearing capacity increased as the distance from the wall increased, the footing width decreased, and the embedment depth increased. Data collected from the responses of scaled-physical model test were used to recognize deficiencies in current design approaches for similar soil structure interaction problems.

Response of Strip Footing Adjacent to Nonyielding Basement Wall

A series of 1/3-scale experimental model tests were performed at the American University of Sharjah (AUS) to assess the performance of strip footing constructed adjacent to nonyielding basement wall. Different construction design parameters have been considered including the strip footing width (B), its distance from nonyielding wall back (a), and footing embedment depth below the backfill surface (Df). The effects of nonyielding wall proximity to the strip footing were investigated. The experimental results show that the lateral deflection of the nonyielding wall is a major factor in dictating the load carrying capacity of the strip footing. The vertically loaded strip footing imposed significant vertical and horizontal forces at the wall top and bottom boundaries. The strip footing load carrying capacity increased as the footing width decreased, the distance from the wall increased and the embedment depth increased. Meanwhile, the vertically loaded strip footing imposed significant vertical and horizontal forces at the wall top and bottom boundaries. The data gathered from this program is used to identify deficiencies in current design methodologies for similar soil structure interaction problems.

Modulus and damping from shaking table tests of reinforced soil walls

The paper presents rational procedures to estimate normalised shear modulus and damping ratio from measured responses of reinforced soil model walls tested in a 1-g shaking table. Displacement measured at the top of the model walls and input base acceleration time histories are used to produce equivalent hysteretic responses of the model walls. Equivalent hysteretic loops at different strain amplitudes are developed and used to calculate normalised modulus degradation curves and damping ratio for the tested model walls. Model wall responses are also analysed using the single degree of freedom (SDOF) model to determine damping ratio and phase difference, and to verify the validity of SDOF model for application with reinforced soil walls. Results indicate that the normalised shear modulus significantly decreased at the early stage of the base shaking (i.e. γs < 0.03%). For example, about 68% and 80% reductions in shear modulus occurred at γs = 0.005% and 0.01% respectively. In addition, a 90% reduction in the normalised shear modulus realised at relatively strong input base acceleration (i.e. for γs > 0.03%). Minimum damping ratio of 5% to 10% for both model walls was obtained, increased to 15% to 20% at strain amplitude γs = 0.3%, and reached higher values thereafter. Finally, dynamic properties determined from the proposed method are compared with experimental and empirical relationships proposed in literature as well as resonant column test results.