Cui, Ge
(2020)
Physical and numerical modelling of ground-borne vibrations in sand.
PhD thesis, University of Nottingham.
Abstract
The problems caused by urban congestion and population growth result in a growing need to construct railway networks in urban areas due to the advantages of safety, efficiency and reliability over alternative mass transportation options. However, railway systems can cause environmental issues due to the transit of vehicles, including railway noise and vibration. The problem of ground-borne vibration has gained increasing attention because people are becoming less likely to tolerate environmental disturbance and sensitive buildings and equipment could be significantly affected by nearby groundborne vibrations. Therefore, it is necessary to investigate those effects and provide a better understanding of ground-borne vibrations from railways. This research work was primarily concerned with the transmission of vibration from railways via a single friction pile through the soil-pile interface and the ground.
In this study, dynamic centrifuge tests were conducted to study the dynamic pile behaviour, the soil-pile interaction and ground-borne vibration propagation characteristics generated from the vertical oscillation of a single pile in sand. The use of the centrifuge is attributed to its great advantage of reproducing the full scale stress field within a small scale model, thus correctly modelling soil behaviour and soil-pile interface characteristics. However, the boundary effects in centrifuge models on vibration propagation have not been fully understood. In addition to the boundary effects, centrifuge and shaker induced vibrations and the embedded piezoelectric accelerometors can also affect the measured vibration results in a centrifuge test; these were also considered in this research project.
The main aims of this study were to provide insights into the soil-pile interaction and ground-borne vibrations in sand from the vertical oscillation of a single pile and to examine the use of centrifuge modelling as a tool for investigating this problem. To achieve these aims, centrifuge tests were performed to simulate the ground-borne vibration by applying small vertical harmonic loads to the top of a single rough pile (with sand stuck to the pile surface) embedded in sand. The effects of vibration absorbing material used in the centrifuge tests, Duxseal, on the soil stress field and soil stiffness were investigated experimentally by measuring the shear wave velocity of the soil using centrifuge based air hammer tests and triaxial based bender elememt tests. A series of numerical models were developed in FLAC3D to further the understanding of soil-pile interaction under vertical harmonic loads and to examine the effects of Duxseal and piezoelectric accelerometers on the centrifuge measurements.
A series of combinations of dead loads and dynamic loads were used to investigate their effects on pile and soil behaviours. Three dead loads with a safety factor (ratio=ultimate bearing capacity of the pile/dead load) of approximately 8, 4 and 2 were employed in this study to investigate the effect of the static load amplitude on the dynamic pile and soil behaviour. The dynamic loads with two different amplitudes were applied by an electromagnetic shaker for each dead load. Centrifuge test results demonstrated that both dynamic pile and soil behaviour were in the linear range when the safety factor was greater than 2. Under all the combined forces applied in this study, the soil and the pile experienced the same displacement at the interface and the dynamic load applied to the pile top did not result in accumulated pile displacement. Therefore, the soil-pile interface can be treated as a perfectly bonded interface for the setup used in this study. Under the three dead loads (safety factor>2), the pile top response amplitude indicated the pile top response decreased with the increasing dead load.
It was also found that the pile was compressible and the displacement of the pile element under vertical harmonic loading was purely caused by the axial elastic deformation. The axial load within the pile decreased with depth, thus resulting in a decreasing axial elastic deformation of the pile and soil particle displacement along depth at the soil-pile interface. It was found that the vibration level at the same horizontal distance from the pile center decreased almost linearly with depth.
The damping ratio of soil along depth was calculated by an empirical equation and the logarithmic decrement method using data measured by the accelerometers. It was found that within very small shear strain (less than 1.0 × 10−6 ), the material damping had a slight effect on the vibration attenuation at three depths. The vibration attenuation along radial distance from the pile center was evaluated by comparing the ratio of vibration responses measured by two accelerometers with the same embedded depth. It was found that test results agreed well with the theoretical estimation at most frequencies. The vibration attenuation curves at three depths (3.36 m, 10 m and 16.8 m at prototype scale) were calculated. It was found that within the radial distance of 15 m (prototype scale), the vibration attenuates very quickly. The curves tend to be flat when the radial distance from the pile is greater than 27 m (prototype scale).
The original purpose of the use of Duxseal was to absorb vibrations and reduce reflections from the rigid boundaries of the centrifuge container so that more accurate model response can be measured in the centrifuge. However, it was found that Duxseal can change the lateral stress in the soil and the soil stiffness due to soil behaviour being stress dependent. It is suggested that side Duxseal should not be used to investigate the ground-borne vibration because the soil-pile interaction and measured ground-borne vibrations were affected and the Duxseal was not able to eliminate reflections.
The effects of piezoelectric accelerometers embedded in the soil were examined by comparing the results from two FLAC3D models: one with piezoelectric accelerometers embedded in the soil and one without piezoelectric accelerometers. It was observed that calculated results with and without the piezoelectric accelerometers were close to each other at most frequencies. This indicated that the presence of piezoelectric accelerometers did not significantly change the model response.
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