Blast Densification of a Thick, Loose Debris Flow at Mt. St. Helen's., Washington

by Joseph E. Hachey, Golder Associates Inc, Redmond, United States,
Robert L. Plum, (M.ASCE), Golder Associates Inc, Redmond, United States,
R. John Byrne, (M.ASCE), Golder Associates Inc, Redmond, United States,
Alan P. Kilian, (M.ASCE), Golder Associates Inc, Redmond, United States,
David V. Jenkins, Golder Associates Inc, Redmond, United States,

Document Type: Proceeding Paper

Part of: Vertical and Horizontal Deformations of Foundations and Embankments


Numerous debris flows occurred during the 1980 eruption of Mt. St. Helen's in Washington State. The proposed location of Bridge 12 over Coldwater Creek along State Route 504 is underlain by 40 meters of debris flow composed of very loose silty sand and gravel with boulders and cobbles. It was determined that, under the design seismic event (magnitude, M = 6.5; peak ground acceleration amax = 0.55 g), the soils would be susceptible to liquefaction, which could result in the proposed bridge being seriously damaged or destroyed, even if supported on deep piles. A blast densification program was conducted to mitigate ground settlement. The project included a test section and considerable instrumentation including both SPT and Becker penetration testing, surface and subsurface settlement devices, inclinometers, piezometers, geophysical surveys, and ground vibration measurements. The deep blast densification was successful and significantly increased the density of the deposit, reduced the liquefaction potential, and allowed the bridge to be supported on spread footings. The instrumentation indicated that blast-induced liquefaction occurred throughout the 40-meter depth of the deposit. Up to 1.5 meters of settlement occurred. Penetration resistance increased from pre-blast values of 7 blows/305 mm to a post-blast resistance of more than 20 blows/305 mm.

Subject Headings: Soil liquefaction | Soil settlement | Penetration tests | Blasting effects | Soil surveys | Solids flow | Debris | Washington | United States

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