4.7 Conclusions

The earthquake provided a severe test for several improved ground sites in the Kobe area. In all cases the site stratigraphy included dumped hydraulic fill placed over soft alluvial clay. The treatment method was to construct sand compaction piles using either the decomposed granite fill itself densified by vibro-rod probes or the casing method with introduction of additional material from above ground. The post-treatment SPT N-values were typically 20 to 25 blows per ft as opposed to about 10 to 15 blows per ft before treatment. The treatment itself resulted in ground surface settlements of 20 to as much as 40 cm, with the largest values being reported for the Portopialand Park area. It may be noted that the surface settlements of untreated ground caused by the earthquake were of the order of 50 cm.

Overall, the observations show that improved ground sites on land sustained significantly less deformation and damage than did the adjacent untreated ground. The offshore Nishinomiya breakwater failed for reasons that have not yet been determined. The amount of differential settlement and the damage to pavements, floor slabs, utilities, and other facilities was variable, ranging from very little over the major part of the amusement park, to substantial within parts of the warehouse area. The larger settlements in the warehouse area appear attributable to the presence of an old buried quay wall which restricted the ground improvement beneath some of the structures.

Further analyses are needed to better correlate the amounts of settlement that were observed to (1) the level of ground improvement, (2) the extent of the treatment zone beyond the structure or facility to be protected, (3) the liquefaction and lateral spreading of the ground adjacent to the treated zone, and (4) the input ground motions.

Chapter Five
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