A loose, debris avalanche deposit, resulting from the 1980 eruption of Mt. St. Helens, was encountered during the foundation geotechnical investigation for Bridge 12 on the Spirit Lake Memorial Highway (SR 504) in the Mt. St. Helens National Volcanic Monument, Washington. The deposit was determined to be at risk of experiencing liquefaction and/or dynamic settlement during the design seismic event for the structure (Richter magnitude 6.5 and peak ground acceleration 0.55g). The experimental use of deep blasting was selected by the Washington State Department of Transportation and approved by the Federal Highway Administration, to densify the deposit full depth (up to 43 meters) for the purpose of mitigating the liquefaction and dynamic settlement risks. The contract for the work utilized a test section to evaluate the blast design. Revisions were made to the blast design based on the results of the test section blasting, and production blasting was completed in December of 1992. Numerous instruments were utilized to monitor and quantify the results of the blast densification.
Standard Penetration Testing (SPT) before blasting indicated average, corrected blowcounts of about eight full depth in the deposit. Post-blasting SPT blowcounts increased to above the goal of about 25 in the upper 15 meters of the deposit, and to above about 20 below 15 meters. Modified Becker Penetration Testing corroborated the SPT results. Total ground surface settlement of up to 1.5 meters was observed. Volumetric compressive strains on the order of about eight percent were determined on the basis of surface settlement and slope inclinometer measurements.
The total project costs were $599,000. On a unit cost basis, the treatment cost about $2.50 per cubic meter of densified ground. A cost savings of approximately $300,000 was realized over alternative ground improvement methods such as stone columns. Blast densification also allowed the use of cost-effective, shallow spread footings for the 60 meter, single-span bridge.
Even greater cost savings were realized when the blast densification method was used on the East Creek section of the final 6.2 km of SR 504, from Coldwater Lake Outlet to Johnston Ridge.