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.
Field testing to evaluate stone column performance in a seismic area
For construction of a 16 mgd sewage treatment plant on predominantly deep, soft, cohesive soils in an area of highest seismic susceptibility, soil improvement with stone columns was considered as one foundation solution. While the deformation behaviour of subsoil improved with stone columns, under static vertical load, has been documented, little if any information is available on the performance of such improved soils under seismic loading conditions. For this reason, large scale field tests were performed to demonstrate that (a) in the process of stone column installation, sand lenses in the predominantly cohesive subsoil are sufficiently densified with respect to liquefaction potential; (b) the combined mass of stone columns and native, intervening soil develops suficient shear strength to resist safely horizontal forces resulting from a ground acceleration of 0·25 g; (c) the stone column pattern which satisfied the shear and density requirements also provides an adequate load-settlement relationship. ...
Centrifuge modelling for seismic retrofit design of an immersed tube tunnel
A centrifuge testing program was conducted to assess the earthquake performance of countermeasure retrofit techniques for an existing concrete tunnel embedded in a liquefiable sandy deposit. In a series of three separate heavily instrumented model tests, a tunnel section-foundation system was studied first without retrofitting, and then with a densified-wall and a gravel-wall liquefaction countermeasure techniques. The test results provided valuable insights into the underlying mechanism and effectiveness of each countermeasure and served as basis for calibration and validation of computational modelling of the George Massey Tunnel seismic retrofit design. Test results showed that the implemented countermeasures significantly reduced the tunnel movements. The gravel-wall technique was found to be particularly effective in reducing the extent of liquefaction as well as reinforcing the tunnel-foundation system against earthquake induced deformations. The centrifuge test results and the calibrated numerical ...