Blasting Densifies Volcanic Debris (Available only in Focus on Geo/Environmental Special Issue)

by Thomas C. Badger, Sr. Engrg. Geologist; Washington State Dept. of Transportation, Olympia, WA,

Serial Information: Civil Engineering—ASCE, 1996, Vol. 66, Issue 3, Pg. 8A-12A

Document Type: Feature article


The relatively new ground improvement technology of blast densification overcame soil problems posed by volcanic ash at Mount St. Helens National Volcanic Monument. In 1993 WSDOT began extending the access road 11 km from Coldwater Lake to the top of Johnson Ridge. The alignment includes a 60 m long bridge and 600 m of embankments over a 3.5 km stretch of loose, saturated deposits within the active seismic zone. The U.S. Forest Service, which administers the Monument, confined all construction within the footprint of the new road. Blast densification, which cost half that of other techniques, was chosen to improve bearing capacity for the bridge foundations. Explosives detonated in bore holes induced high pore water pressures that liquefied the soil mass, rearranging the soil particles into a denser configuration and causing significant ground settlement and dewatering. An aging period of several weeks further densified the mass. A second area required blast densification before the embankments could be built, as it was unexpectedly found to be filled with a secondary, alluvial flow of very fine ash. In both areas, explosives were placed at six levels in two grids of holes about 5 m apart and detonated sequentially. Problems with blast densification include disposal of large quantities of water, siltation and damage to adjacent structures from ground vibration. However, WSDOT now includes blast densification as a viable ground improvement option.

Subject Headings: Blasting effects | Volcanic deposits | Soil settlement | Water pressure | Soil water | Soil stabilization | Soil pressure

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