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.
Analysis and settlement evaluation of an end-bearing granular pile with non-linear deformation modulus
Abstract Ground improvement with granular piles increases the load-carrying capacity, reduces the settlement of foundations built on the reinforced ground and is also a good alternative to concrete pile. Granular piles or stone columns are composed of granular material, such as crushed stone or coarse dense sand. An analytical approach based on the continuum approach is presented for the non-linear behaviour of the granular pile. The formulation for pile element displacement is done considering the non-homogeneity of the granular pile as it reflects the true behaviour and also accounts for the changes in the state of the granular pile due to installation, stiffening and improvement effects. The present study shows that the settlement influence factor for an end-bearing granular pile decreases with increase in the relative stiffness of the bearing stratum. The settlement influence factor decreases with increase in linear and non-linear non-homogeneity parameters for all values of relative length. For a shorter pile, the rate of decrease of the settlement influence factor is greater in comparison to that for a longer pile. Shear stress at the soil–granular pile interface reduces in the upper compressible portion of the granular pile and increases in the lower stiffer portion of the granular pile due to the non-homogeneity of an end-bearing granular pile.
EFFECT OF STORAGE CAPACITY ON VERTICAL DRAIN PERFORMANCE IN LIQUEFIABLE SAND DEPOSITS
The use of vertical drains to improve the performance of potentially liquefiable ground during earthquakes has received increased attention over the last two decades. This paper briefly describes the formulation of a finite element code, FEQDrain, developed to analyze the development of excess pore pressure in a layered soil profile, accounting for vertical and horizontal drainage. The code includes equations describing a vertical drain with a non-constant “equivalent hydraulic conductivity” which more accurately describes the flow properties of perforated pipes and wick drains, and head losses due to horizontal flow into the drain. It can also model the presence of a reservoir directly connected to the drain, allowing the accumulation of the water discharged by the drain element as well as head losses in the reservoir itself. In contrast, most analyses used in practice assume the water level in the drain is at the ground surface and therefore all flow into the drain escapes. The accumulation of water within the drain, however, can lead to a significant retardation of flow into the drain, causing an increase in the predicted pore pressures developed during the earthquake. INTRODUCTION Over the past several decades, research into seismically induced liquefaction, largely in the United States and Japan, has followed several different paths. Early investigations focused on the causes of liquefaction and determination of the 1 Assistant Professor, Civil and Environmental Engineering Dept, University of California, Berkeley, CA 94720 2 Graduate Student Researcher, Civil and Environmental Engineering Dept., University of California, Berkeley, CA 94720 3 Principal, Geotechnics America, Inc., P.O. Box 2324, Peachtree City, GA 30269