BLAST DENSIFICATION TO PREVENT SOIL LIQUEFACTION IN VOLCANIC AVALANCHE DEBRIS FOR CONSTRUCTION OF THE SOUTH COLDWATER CREEK BRIDGE, MT. ST. HELENS

摘要

This project involved deep blast densification of recent volcanic avalanche debris deposits consisting of a 43 meter thickness of loose clean to slightly silty sand, gravel, cobbles and boulders deposited as a result of the 1980 eruption of Mt. St. Helens, Washington, U.S.A. This project was carried out to enhance the resistance to liquefaction or dynamic settlement of soil deposits under the bridge abutments for construction of the 59.4 meter long South Coldwater Creek Bridge. As this bridge is located close to the Mt. St. Helens fault, it will in all likelihood be subject to earthquakes in the order of 6.5 with expected accelerations of 0.55g. Without soil densification, the bridge would suffer severe structural damage in the event of a significant earthquake.Although explosive densification work has been carried out for foundation soil improvements under various structures, this is the first time the technique has been applied to this type of soil deposit.Standard penetration tests(SPT) blow counts prior to densification indicated the presence of loose soil conditions with (SPT) values of 8 or less. The target of the densification work was to achieve an increase in the Nl value to about 25 for the upper 15 meters and about 20 below the 15 meter depth. Following blast densification, the upper zone was found to have increased in density by more than double, while the lower zone had increased to approximately triple the original Nl values.The blasting technique used involved detonation of multiple explosive decks using the electrical sequential blasting system. The explosive used was prilled TNT.