Experimental Studies and Finite Element Modeling of Piles and Pile Groups in Dry Sand under Harmonic Excitation (original) (raw)
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A case study using finite-element software for the dynamic analysis and structural design of a machine foundation on piles in homogeneous sandy soil is reported. A parametric study was carried out to investigate the effect of the foundation geometry, the amplitude and frequency of the dynamic load, and the damping ratio. It is concluded that as the pile cap thickness increases the oscillation of displacement decreases due to material damping inherent in the concrete of the pile cap. There is a limit of the pile cap size at which its stiffness governs its dynamic response. Above this size the weight of the pile cap overrides its stiffness effect, and the additional weight leads to an increase in pile displacement. When the pile group size increases, the frequency at which maximum displacement occurs increases, and hence the system becomes more stable against resonance. In the case of changing the pile spacing, the maximum moment factor IM is always at the pile cap centre, where the l...
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In this paper, the influence of the following parameters on different modes of vibration response of model footings on sand is studied: (vertical, rocking and pitching load amplitude and frequency). To achieve the objective of this study, the model footing was selected to be rectangular footing with an aspect ratio (L/B= 1.33) i.e (L= length and B= width). A physical model was manufactured to simulate a steady-state harmonic dynamic load applied at different operating frequencies with the same load amplitude (0.5 ton). A total of (9) cases was performed taking into account different parameters. The loading frequency ranged from (0.5) to (2) Hz and the footing was rectangular with an aspect ratio (L/B = 1.33). The soil type used was dry sand having a relative density of (50%). The behavior of the soil underneath the footing was examined by measuring the strain using a shaft encoder and the amplitude of displacement using a vibration meter. It was found that the change in vibration mo...
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The main objective of the current work is to examine the dynamic axial response of three pile group under machine-based harmonic loads. Field tests are carried out under axial harmonic excitations on a group pile with a pile length of 300 cm and an diameter of 11.4 cm in order to accomplish this objective. For various eccentric moments, the frequency versus amplitude responses of the group pile are measured. The field test results of the soil-pile system show non-linear behaviour as their resonant frequencies decrease and their resonant amplitudes disproportionally increase with eccentric forces. The inverse methodology proposed by Novak (1971) is used for the theoretical study. With this methodology, the changes in stiffness, damping, and effective mass of the piles under various eccentric moments are quantified by analyzing the frequency-amplitude response curves of field results. The theoretically back-calculated soil-pile system response curves are compared with the field results, and it is observed that the analytically predicted responses closely match the field responses. It is also found that the values of estimated damping of the pile group increased, while effective mass and average stiffness values decreased with an increase in eccentric moments. Keywords Axial harmonic excitations • Soil-pile system • Field tests •
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This paper presents a new mathematical approach for the analysis of harmonically vibrating horizontal, linear, elastic uniform pile. The soil properties may vary from layer to layer. No separation is allowed at the soil-pile interface. The pile is modeled as a number of cylindrical segments connected by rigid nodes. The length of each segment is chosen such that the effects of the soil inhomogenity are accounted for. The governing differential equation for an arbitrary pile segment is obtained and solved. According to the pile support types such as pinned, fixed and free conditions, first an arbitrary appropriate value for either toe force, bending moment, rotation, or displacement is assumed. The governing differential equation is then solved from the lower pile segment to the top one. The stiffness of the whole pile-soil system will then be computed. It is shown that the slenderness ratio, the stiffness ratio and toe fixity are the governing parameters affecting the stiffness of the soil-pile system. The new analytical model, which is verified using existing numerical and analytical solutions, is more efficient than the equivalent numerical solutions for example finite eminent methods.
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The way to achieve satisfactory performance of a machine foundation is to limit its dynamic amplitude to a few micrometers. In using piles for the foundation, the interaction of the pile with the surrounding soils under vibratory loading will modify the pile stiffness and generate damping. This study presents the results of a threedimensional finite element model of a soil-pile system with viscous boundaries to determine the dynamic stiffness and damping generated by soil-pile interaction for a vertical pile subjected to a vertical harmonic loading at the pile head. The pile was embedded in a linearly elastic, homogeneous soil layer with constant material damping. A parametric study was undertaken to investigate the influence of the major factors of
Response of Circular Machine Foundation Resting on Sandy Soil to Harmonic Excitation
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This paper focuses on the effect of the machine's circular foundation on the variation of surface settlement, vertical displacement and stress with a number of cycles. A special setup was designed and manufactured to simulate the vertical vibration of a machine foundation. Six laboratory model footings were prepared on medium and dense dry sand separately. A circular steel model of (150 mm) diameter was used to represent the footing. The models were tested under dynamic load amplitude of 0.25 ton and different frequencies of 0.5, 1, and 2 Hz. It was found that the rate of increase in settlement decreased remarkably when increasing the frequency for both types of sand. While increasing the soil relative density under the same load and frequency resulted in a decrease of the settlement. Moreover, the amplitude displacement decreased when increasing frequency and relative density. The resulting stress due to the dynamic load below the foundation decreased with depth. Besides, there...
Analysis of pile groups under vertical harmonic vibration
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In this paper, a simple method is proposed for the analysis of vertically loaded pile groups under dynamic conditions. The method makes use of the closed-form stiffness matrices derived by Kausel and Roësset [Kausel E, Roësset JM. Stiffness matrices for layered soils. Bull Seismol Soc Am 1981;71(6):1743-61] to simulate the response of layered soils. These matrices are incorporated in a calculation procedure that is essentially analytical. Further computational advantages of the procedure derive from the fact that, under the simplified assumptions of free-field soil displacements and symmetry of the pile-soil interaction forces, the analysis of a pile group may be achieved simply using the solution for the single pile. Moreover, the soil layering effect is reliably accounted for. The accuracy of the method is assessed by comparing the results with those deduced from other existing theoretical solutions. The method is also used to predict the experimental measurements from dynamic tests on pile groups documented in literature.