CASE STUDIES OF STONE COLUMNS IMPROVEMENT IN SEISMIC AREAS
Soil improvement with stone columns is often implemented in seismic areas in particular as a countermeasure against liquefaction. The efficiency of stone columns can be explained by the compaction effect during their execution, by the reduction of the seismic stress on the soil, and by their draining effect as well. A historical evolution of the first realisations is presented, followed by an inventory of the observed performance of such systems in seismic conditions. Then, recent applications of stone columns combined with pile foundations are presented. Key-Words: seismic area, stone columns, liquefaction, case study, pile foundation.
NUMERICAL STUDY OF THE SOIL-STRUCTURE INTERACTION DURING STRONG EARTHQUAKES
The non linear dynamic soil-structure interaction of three reinforcement concrete building frames (3, 6, and 12 levels) subjected to strong earthquakes is studied using the FEM. The soil behaviour is modelled with hypoplastic and visco-hypoplastic relations which are able to model the response of cohesionless and cohesive soils under alternating and dynamic loading, for drained and undrained conditions. The behaviour of reinforced concrete is modelled using an elastic-plastic constitutive law for the plain concrete and elastic-ideal plastic for the steel bars. It is assumed that the buildings are founded on a layered ground from Treasure Island, San Francisco Bay (Seed et al., 1991). An acceleration record at Yerba Buena Island during the Loma Prieta 1989 earthquake is used as bedrock excitation. In order to quantify the influence of the soil-structure interaction, the ground motion at the foundation depth due to the bedrock excitation is first calculated without considering the structure (“free field acceleration”). This ground motion is then applied to the base of the buildings to evaluate the internal forces and deflections of the structure. In a third analysis, the dynamic response of both structure and soil due to the same bedrock excitation is considered. Based on the comparison of the numerical results it is concluded that the proposed numerical model is a powerful tool to quantify the influence of soil-structure interaction on the response of buildings during strong earthquakes.
Growth from Solutions
Growth from solutions is the crystallization of a compound whose chemical composition differs markedly from that of the initial liquid phase. Commonly used solvents are water, multicomponent aqueous or nonaqueous solutions, and melts of some chemical compounds. Differentiations are made based on growth temperatures and on the chemical nature of the solvent. In growth from low-temperature aqueous solutions, the temperatures generally do not exceed 80–90°C; in growth from superheated aqueous solutions (hydrothermal method), they reach 800°C, and in growth from salt melts (molten-salt, flux, or fluxed melt growth), they usually do not exceed 1200–1300°C, but sometimes reach 1500°C.