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FIGURE 18.71 Soil deep mixing construction. (Drawing courtesy of Japan's Public Works Research Institute.)

breaking up the soil and making it soft and permeable. The wings rotate 20 cycles per minute as they descend into the soil. Then, the stirring wings are pulled up while injecting cement milk into the soil. The milk is composed of an equal weight of cement and water. About 100 to 150 kg of cement milk is injected per cubic yard of soil. The wings rotate at 40 cycles per minute as they ascend. Loose soil comes up during this procedure and is removed with an excavator. The stirring wings descend and ascend at 1 m/min. There is a cement plant for each machine and a separate pump for each rod. The cement milk flows through flexible hoses from the cement plant to the top of the rods where it is injected into the soil. When the piles are completed the machine crawls to the next location on a plywood mat and drills another pile. Display panels in the machine cab give information about depth, rotation, and the amount of cement milk being pumped.

The deep mixing method is also very effective in preventing soil liquefaction. In fact, this method was used to prevent liquefaction for some flood prevention works next to this site. In this case the piles are placed in a lattice pattern to contain the liquefiable soil.

The deep mixing method was found to be the most effective method for creating strong, highly ductile ground that does not liquefy or settle. It is also the most expensive method, but the lack of noise and vibration makes it ideal for city environments.

18.5.1.3 Sand Compaction Pile Method: Ohgishima Island, Tokyo

Sand compaction piles are a popular way of preventing liquefaction of loose alluvium. However, the noise and vibration make it unacceptable at some locations.

This project was on a man-made island in Tokyo Bay (Figure 18.72). It is the location of an LNG tank farm. Soil remediation was required at the toe of an embankment that covers these tanks.

FIGURE 18.72 Sand compaction pile method. (Drawing courtesy of Japan's Public Works Research Institute.) Copyright 2005 by CRC Press

Because it was a remote site, the loud noise and vibrations were not a problem. The sand compaction pile method uses a modified pile-driving machine to vibrate a steel pipe into the ground. When the penetration reaches the proper depth, sand is carried to the top by a hopper and forced to the bottom of the pipe with compressed air. Then, the pipe is raised and lowered in the hole as sand is repeatedly shot to the bottom of the hole. The result is a pile of compacted sand and an area between the piles of compacted soil. The equipment is similar to the gravel drain method. The pile driver has a steel casing with a lid that can be opened and closed at the bottom. Sand is brought in by dump truck to the site. A front-end loader pours the sand into the hopper. The hopper carries the sand to the top of the pipe where it is poured into the air compression chamber. The steel casing with the lid closed is vibrated to the required depth. In this project the depth was 17 m and the pile diameter was 0.7 m. When the pile is completed, the panels the truck sits on are moved back and the pile driver is moved back to drive a new pile. The pile is driven at about 2 min/m and a pile is completed in about 30 min. Before this project began, the soil had an average N value of 10 to 12. The areas that have been completed now have an N value of 15 to 20.

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