Chapter Four: Performance of Improved Ground Sites
Ground improvement has been used at several filled sites in the Kobe area, primarily on Port and Rokko Islands. Both islands consist of hydraulic fills overlying soft alluvial clays. Information about the setting, soil conditions and ground motions is given in Section 3.2. A typical soil profile for Port Island with relevant soil properties, as given by Nakakita and Watanabe (1981), is shown in Fig. 3.6. As the intensity of ground motions at some of the improved sites was higher than has been experienced in past earthquakes, examination of their behavior provides valuable data concerning the effectiveness of ground improvement methods for mitigation of liquefaction and ground failure.
4.1 Ground Improvement Methods
Sand drains in the young alluvial clay layer and preloading were used at a number of locations on Port and Rokko Islands for settlement control, and most of the high-rise structures, elevated rail lines, bridges, and many of the other buildings were founded on piles. In general the pile supported structures performed well. The installation of the sand drains was reported to give an increase in the SPT N-value of the sand hydraulic fill of the order of 2 to 3 blows per foot. Additional densification of the hydraulic fill was undertaken only at a few locations. It is the response of this densified ground that is of particular interest here.
Most of the densification was done using either the vibro-rod method developed by Saito (1977) or the sand compaction pile method developed by Murayama (1958). Both of these methods are described by Mitchell (1981). In the vibro-rod method a vibratory pile hammer is used to penetrate the ground repeatedly by a specially designed probe. In the sand compaction pile method a casing pipe is driven to the desired depth using a vibrator at the top. A sand charge is introduced into the pipe, and the pipe is withdrawn part way while compressed air is blown into the casing to hold the sand in place. The casing is then vibrated down to compact the sand pile and enlarge its diameter. The process is repeated until the pipe reaches the ground surface. For both the vibro-rod and sand compaction pile methods the spacing between columns is ordinarily 1.5 to 3.0 m depending on the loading conditions and fines content of the soil being treated. For construction on land a sand compaction pile diameter of 700 mm is used, and the pile diameter by the vibro-rod method is about 600 mm.