Majd Ahmad

57679245000

Publications - 6

Modeling the Stiffening Behavior of Sand Subjected to Dynamic Loading

Majd Ahmad R. P. Ray

Publication Name: Geosciences Switzerland

Publication Date: 2024-01-01

Volume: 14

Issue: 1

Page Range: Unknown

Description:

In geotechnical engineering, dynamic soil models are used to predict soil behavior under different loading conditions. This is crucial for many dynamic geotechnical problems related to earthquakes, train loading and machine foundation design. Researchers agree that under dry or drained conditions, cohesionless soils increase in stiffness with each loading cycle. Soil models that simulate the dynamic behaviors of soils are often coupled with the Masing criteria. Such models neglect the impact of stiffening during cyclic loading, leading to an underestimation in the shear modulus (G). This study investigates the stiffening behavior by conducting laboratory tests on three types of Danube sands using the Resonant Column-Torsional Simple Shear device (RC-TOSS). The increase in the dynamic shear modulus with an increasing number of cycles is substantial, especially for samples with low density. Sometimes, the dynamic shear modulus doubles when loaded at high stress levels for more than 50 cycles. A new model is introduced to simulate the stiffening behavior of dry sand when subjected to cyclic torsional loading. Modifications are proposed for the Ramberg–Osgood and Hardin–Drnevich models and for the Masing criteria to overcome the limitations that accompany these models due to the influence of stiffening caused by repetitive loading being ignored. This model can be implemented in finite element and finite difference software to solve dynamic geotechnical problems.

Open Access: Yes

DOI: 10.3390/geosciences14010026

Damping of Dry Sand in Resonant Column-Torsional Simple Shear Device

Majd Ahmad R. P. Ray

Publication Name: Sustainability Switzerland

Publication Date: 2023-07-01

Volume: 15

Issue: 14

Page Range: Unknown

Description:

The damping ratio values of three different Danube sands were measured in the Resonant Column-Torsional Simple Shear device (RC-TOSS). The distinctive configuration of the RC-TOSS device employed in this investigation enabled the performance of both tests using a single sample. This research estimates and compares the damping ratio values measured with three distinct methods (two of which are in the RC test): The Free Vibration Decay (FVD), the Steady-State Vibration (SSV) methods, and the method of calculating the damping ratio from the hysteretic loops generated in the TOSS test. Both dense and loose samples were tested up to a peak-to-peak amplitude shear strain of 1%. The device provides measurements over a wide range of shear strain amplitudes. The results support the employment of the SSV methods at low strains (below 0.005%), while the FVD method gives a better estimate at higher strains (above 0.03%). The two methods and the TOSS results are in agreement with each other between 0.005% and 0.03%. The effect of the number of cycles on the damping ratio was investigated where a significant decrease was observed in the damping ratio with an increasing number of cycles. A parameter is introduced to describe the rate of this decrease, which should be considered during the structural design to reduce maintenance and life-cycle costs and enhance sustainability.

Open Access: Yes

DOI: 10.3390/su151411060

The Dynamic Properties of Sand under Torsion: A Literature Review

Majd Ahmad R. P. Ray

Publication Name: Geotechnics

Publication Date: 2023-06-01

Volume: 3

Issue: 2

Page Range: 480-514

Description:

Resonant column (RC) and the torsional simple shear (TOSS) tests have shown proven competency in acquiring precise and repeatable measurements regarding the shear modulus and damping ratio of soil. For most dynamic geotechnical problems, the shear modulus represents the stiffness of the soil, while the damping ratio describes energy dissipation. Many studies in the last few decades focused on developing the relevant equipment and investigating the effect of different soil properties on the dynamic behavior of soil. Researchers have introduced correlations to approximate this behavior without conducting dynamic torsional testing. Soil models (e.g., Ramberg-Osgood and Hardin-Drnevich) can simulate shear stress-strain curves after finding the curve-fitting parameters. Due to the complexity of dynamic behavior and its dependency on various factors in soils, the RO and HD equations help model the behavior more simply. This paper presents a literature review and evaluation of the studies, correlations, soil models, and parameters affecting the dynamic behavior of dry sand under torsion.

Open Access: Yes

DOI: 10.3390/geotechnics3020027

Modelling of the Torsional Simple Shear Test with Randomized Tresca Model Properties in Midas GTS NX

Majd Ahmad R. P. Ray

Publication Name: Geotechnical and Geological Engineering

Publication Date: 2023-05-01

Volume: 41

Issue: 3

Page Range: 1937-1946

Description:

The dynamic properties of soil obtained from the torsional simple shear test (TOSS) are assumed to be uniform throughout the specimen. For some exceptional soils, this may hold true, but for the most majority of soils that we examine, it is obviously not the case, and this level of non-uniformity depends on the conditions in which the soil was formed. In this paper, we discuss a method of modelling inherently non-uniform soil specimens by representing them with elements that have an elasto-plastic simple Tresca material model with different properties (Elastic young modulus and yield stresses). The combination of properties that can simulate the nonlinear behaviour of the soil is found and calibrated using a model of the TOSS test built in the finite element software Midas GTS NX. Furthermore, the influence of rigid inclusions in the soil is studied and the results show an increase in stiffness with the increasing percentage of inclusion in the soil.

Open Access: Yes

DOI: 10.1007/s10706-023-02382-z

Comparison between Ramberg-Osgood and Hardin-Drnevich soil models in Midas GTS NX

Majd Ahmad R. P. Ray

Publication Name: Pollack Periodica

Publication Date: 2021-12-31

Volume: 16

Issue: 3

Page Range: 52-57

Description:

This paper studies the two widely used material models for predicting the dynamic behavior of soils, the Ramberg-Osgood and Hadrin-Drnevich models. Resonant column and torsional simple shear test results on dry sand were used to calibrate and evaluate the model built in the finite element software Midas GTS NX. Both material models are already implemented by the software. This study estimates the ability and efficiency of both soil models coupled with the Masing criteria to predict the behavior of soil when subjected to irregular loading patterns, (e.g., earthquakes), and measure the two most important dynamic properties, the dynamic shear modulus, and the damping ratio.

Open Access: Yes

DOI: 10.1556/606.2021.00353

Review of materials used for ballast reinforcement

Majd Ahmad

Publication Name: Acta Technica Jaurinensis

Publication Date: 2021-08-25

Volume: 14

Issue: 3

Page Range: 315-338

Description:

This mini review summarizes the most recent research in ballast reinforcement. Several materials are being used for the purpose of improving the ballast layer in railways, including geosynthetics, rubber sheets and binding agents. Such methods of reinforcement have proven to be beneficial for increasing the strength, stiffness, and resilience of the ballast layer in addition to reducing settlement, breakage, degradation, and maintenance cost and frequency. Latest studies try to find the best types, placement, and combination of geosynthetics to achieve the highest strength and resistance, in addition to obtaining the optimum percentage of binding agents and methods of applying them in order to discover the most effective binder that achieves the most improvement to the mechanical properties of the layer for a reasonable price. An overview of the recent tests conducted to study the reinforced ballast layer and their results is presented in this paper, as well as an overall evaluation of the implementation of these reinforcement methods in railways.

Open Access: Yes

DOI: 10.14513/actatechjaur.00610