Liquefaction Potential Mitigation Using Rapid Impact Compaction

Mission Bay is a 1.21-square-kilometer (300-acre) reclaimed site in San Francisco, California, that is currently being redeveloped. The composition of the fill varies and includes loose sand, loose to medium dense silty and clayey sand, and loose to medium dense sandy gravel with clay. The fill is liquefiable throughout much of Mission Bay. The groundwater is typically 1 to 3 meters (3 to 10 feet) below the ground surface. This paper will present case studies of the mitigation of liquefaction potential at two sites using Rapid Impact Compaction (RIC). RIC is a method of soil improvement using a track-mounted machine that imparts energy by dropping an approximately 6,800 kilogram (7.5-ton) weight from a controlled height, about 1 meter, onto a patented foot. The energy is delivered at a rate of 40 to 60 blows per minute. Drop height, number of blows, and penetration per blow are monitored and/or controlled by an on-board data acquisition system. Case studies will include results of pre-treatment borings and/or CPTs, results of post-treatment CPTs, and a comparison of before and after liquefaction potential. In addition, the results of vibration monitoring performed during RIC will be discussed.