Date of Award
Doctor of Philosophy (PhD)
Civil Engineering and Engineering Mechanics
John J. Emery
A finite element method that utilizes boundary conditions from wave propagation considerations is given for predicting the seismic response of a pile embedded in soil. The response of the system and the stress distribution in the soil adjacent to and beneath the pile are resolved, and these provide a means of appraising the behaviour of a soil-pile system during an earthquake. The three directional components of the earthquake excitation are considered. The soil-pile system is idealized as an axisymmetric structure subjected to nonsymmetric loading to simplify the computations.
Records of ground motions during recent earthquakes have clearly demonstrated the significance of local soil conditions on the amplitude and frequency characteristics of seismic motions. For large epicentral distances, the usual assumption of energy transfer by means of vertically propagating shear waves is valid. However, for sites nearer to the source the direction of shear wave propagation may be inclined and surface waves also contribute to the ground motions. These aspects of the seismic motions are considered.
The spatial variations in seismic motions are computed using wave propagation theory and assuming that the earthquake energy is transferred through the soil layers by shear waves and Rayleigh waves. Spatial variations in motion are compared for various epicentral distances and wave propagation assumptions. It is shown that the surface wave makes a significant contribution to the response at near sites, while the effect of inclining the shear wave propagation is of secondary importance. The method and programs are general so that they can be used for a variety of problems. Dimensions of the soil-pile system are adopted so that "free field" conditions can be assumed at the boundary. The seismic motion record for each boundary node of the discretized structure is computed and used as the input for the full finite element dynamic analysis which utilizes a step-by-step procedure.
It was found that the single pile foundation considerably reduced the responses transferred to the structure. The method of analysis was used to consider the pore pressures developed in the case of saturated soils around a single pile. The behaviour of a pile-saturated sand system during the San Fernando Earthquake showed that the sand around the pile liquefied. When a pile-saturated clay system was subjected to the same earthquake, there was no sign of liquefaction. These results are in qualitative agreement with field observations.
Nair, Gopinathan Parameswaran, "Response of Soil-Pile Systems to Seismic Waves" (1975). Open Access Dissertations and Theses. Paper 3105.