Generation and propagation of rock blast-induced pore pressures in quick clay

Jörgen Johansson

In 2009 rock blasting for a road construction pushed a block of rock into quick clay, causing a landslide in Namsos, Norway. This slide also triggered whether vibrations other than earthquakes can cause quick clay landslides. The first study in 2012 resulted in a Norwegian standard (NS 8141:3) for a vibration limit and measurement procedure for safer blasting near quick clay. Recent projects have shown the need to understand better how tunnel blast vibrations affect quick clay and how to deal with it during road and railroad construction. In connection with a road construction project, we measured blast vibrations both in and on rock and on the ground surface and at 2 depths in clay as well as pore pressure in the clay at 5 and 10 m depths. The peak velocity of blast vibration below 90 mm/s cause a modest pore pressure increase in the quick clay of less than 3 kPa, which is smaller than the natural variation over a season. Further measurements of vibrations and pore pressures (up to 20 kPa) during blasting for a rock tunnel beneath areas with quick clay indicate the pore pressures are generated in a border zone between rock and soil, and then propagate out to surrounding areas, where they might affect slope stability. The gathered data set of vibrations and pore pressures will allow further detailed studies to understand how rock blasting affects quick clay slopes.

Jörgen Johansson Senior Engineer, Norwegian Geotechnical Institute

Jörgen graduated from KTH in his native Sweden and continued with post-graduate studies at the University of Washington in Seattle, where he worked briefly before moving to Japan for a longer period. He was part of several reconnaissance trips to observe earthquake struck areas in Japan, Taiwan and the Americas. The large fault displacements observed in the 2001 Taiwan and 2004 Niigata, Japan earthquakes became the topic of the PhD work at the University of Tokyo. In 2008 he joined the Norwegian Geotechnical Institute in Oslo, Norway, where he has been involved in a variety of earthquake and vibration projects, as well as other research and engineering tasks within onshore and offshore geotechnics.

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