Piled foundation dynamics: Considering inertial and underground railway excitation
Understanding the fundamental dynamics of piled foundations is important for practitioners, as ground-borne vibration can result in adverse disturbances, particularly in urban areas. Through the development of computationally efficient models, this dissertation examines the vibration of piles due to (1) inertial excitation at the pile heads and (2) incident waves from underground railways. The latter excitation mechanism also considers the building superstructure above the foundation. The numerical models account for the dynamic motion in three-dimensional space and are formulated in the frequency domain.
A new iterative model is developed using the boundary-element method (BEM) to study the effects due to wave scattering in inertially excited pile-groups. When different inertial loads are applied, the converged solutions of the iterative BEM model strongly agree with the interaction factors predicted using a conventional BEM model. A comprehensive parametric study on the inertial response of piles provides new insight into the wave-scattering effect and how various material and geometric parameters influence it, particularly at high excitation frequencies.
The iterative approach is then used to combine the BEM model of a piled foundation and the semi-analytical model of an underground railway tunnel. The result is an efficient coupled tunnel-foundation model, which fully accounts for the source-receiver interaction that has been neglected in existing models. It is discovered that this interaction is most significant when the separation distance between the tunnel and foundation is less than the shear wavelengths in the soil. For the first time, two counteracting mechanisms that govern the added-foundation effect, which is the modification of the ground (greenfield) response when a foundation is constructed near an underground railway, are also highlighted in a thorough parametric study.
It is demonstrated that a simple dashpot model is representative of the essential dynamics of a modern tall building, with respect to its base impedance, over the frequencies associated with the perception of vibration. When the simplified building model is combined with the coupled tunnel-foundation model, the differences between the train-induced vibration of a building that is supported on deep piles and shallow footings are likely to be imperceptible by the occupants. Furthermore, the mean vibrational energy entering through the foundations of a tall building is found to be dominated by the added-foundation effect. The effectiveness of full and partial base isolation to mitigate the vibration disturbances in a building is also explored using power-flow techniques. Finally, a virtual case study concerning the construction of a building next to a pre-existing underground railway tunnel shows how the developed models can be used in practice to evaluate the performance of different designs. The conclusions are expected to establish new guidelines for designing foundations and buildings near underground railways.