4.2 Amusement Park on Port Island

4.2.1 Location and Treatment

The Kobe Portopialand Amusement Park is located along the southern boundary of Port Island Phase I, as shown in Figure 3.4 and 3.11. The ground surface, which is about 1 m higher than the surrounding area, is typically about 3 to 6.5 m above the Mean Water Level, and thus the fill is largely submerged. Within the amusement park area the fill was treated in 1979 by the vibro-rod method over its full 19 m thickness using a 2.6 m square pattern. Post-treatment SPT blow counts typically ranged between about 20 and 30, with averages of 21 and 23 at the rod locations and between rod locations, respectively. Figure 4.1 shows typical SPT blow count data with depth corresponding to the pre-treatment (one set) and post-treatment (two sets - one at the center of a rod location and the other between rod locations) conditions. It was reported that the ground surface settled of the order of 30 to 40 cm during treatment.

Figure 4.1: Pre- and post-treatment standard penetration resistance values at the Portopialand Amusement Park site on Port Island (after Fudo Construction Co., Ltd., 1995).

4.2.2 Observations Around the Park Perimeter

A narrow parking lot is located along the south side of the park, between a retaining wall for the park to the north at an elevation about 2 to 3 m higher than the parking lot, and the original Port Island Phase I sea wall that parallels the park wall to the south. The concrete pavement slabs separated along the long axis of the parking lot and soil ejecta (predominantly sand) covered the entire parking lot, as may be seen in Figure 4.2. Water marks on the entrance kiosk showed that the parking area filled to a depth of roughly 1 m (Figure 4.3).

Figure 4.2: Parking lot between the Phase I seawall on the south and the Portopialand Amusement Park on the north.

Figure 4.3: Parking lot kiosk showing high water level caused by sand liquefaction.

The east side of the park is separated from the adjacent road by a strip of unpaved, fenced-off ground. At the southern edge of the east side there is a small water supply building that is fenced on all sides. Liquefaction ejecta covered much of the enclosed area around this building, and differential settlements and ground cracking occurred around the building. At the northern end of the east side there was a large sand boil within the fenced off area. No other evidence of liquefaction was observed in the fenced-off area along the east side. The adjacent street surface had a hummocky appearance, and on the other side of the street differential settlements of 10 to 50 cm were observed between a building and its adjacent sidewalk and fence.

The north side of the park is next to a road and elevated railway. Differential settlements of 20 to 50 cm occurred between the supporting piers for the railway and the adjacent ground surface. Sand boils were also observed in this area.

The west side of the park is next to the grassy area of a public park. Large sand boils and many ground cracks were seen in the grassy area near the amusement park (Figure 4.4). A differential settlement of roughly 30 cm was observed between a ramp to the elevated railway and the surrounding ground near the northwest corner of the amusement park (Figure 4.5).

Figure 4.4: Sand boils and ground cracks in the grassy area near the amusement park.

Figure 4.5: Differential settlement and sand boils outside the northwest corner of the amusement park.

4.2.3 Observations Inside the Amusement Park

The amusement park contains a large number of buildings and elevated supports for rides. All structures inside the park reportedly were constructed on shallow foundations. The large Ferris wheel, perhaps the largest structure in the park, was reported to be founded on spread footings tied together by grade beams. At the time of the reconnaissance, no specific survey data about the park was yet available.

Clear evidence of liquefaction inside the amusement park was limited to a small area near the southwest corner, where ground cracking and sand boils were observed as shown in Figure 4.6. Additional cracking and ejecta were observed along most of the southern boundary of the park (Figure 4.7), although some of this may have been the result of a broken water line. Over the remainder of the park the only apparent damage consisted of occasional cracks and separations in pavement materials that were generally less than 5 mm wide, but up to 25 mm wide in some spots, with Figure 4.8 being an example. Cracks and separations generally were smallest near the northern boundary and became progressively larger toward the south. Many large areas in the park had no visible cracks, separations, or differential settlements, as illustrated in Figure 4.9.

Figure 4.6: Sand boil at the southwest corner of the amusement park.

Figure 4.7: Cracks and ejecta along the southern boundary of the amusement park.

Figure 4.8: Small separation between pavement and concrete in the amusement park.

Figure 4.9: Undamaged area in the central portion of the Portopialand amusement park.

No differential settlements were observed at any of the large footings for the Ferris wheel or roller coaster. The roller coaster was operated during the field reconnaissance and appeared to function satisfactorily. No signs of structural distress were seen in any of the park facilities, and all gas lines were reported to be intact.

4.2.4 Discussion

Foundations within the park performed well. Without survey data, however, it is impossible to determine whether the park settled or not. The absence of visible structural damage inside the park does suggest that any areal settlements that may have occurred did not result in unacceptable differential settlements. However, outside the park on many other parts of the island, the liquefaction-induced settlement was remarkably uniform, and many large paved areas away from pile supported structures sustained no visible damage. Differential settlement between the park and the surrounding area was not clearly observed, although only the eastern boundary could be inspected readily.

The significance of the observed liquefaction inside the park along its southern boundary depends on the southern limit of the ground improvement. Liquefaction evidence was mostly within a distance of 10 to 20 m from the southern fence line, which is roughly one half to one times the thickness of the liquefiable fill. Given the extensive liquefaction observed in the parking lot on the south side of the park, ground cracks and soil ejecta could be expected to extend some distance into the treated area. Correlation of the ground performance with level of improvement, together with accurate settlement and lateral displacement data both within the park and in the surrounding areas should provide valuable insights for design of ground improvement projects in the future.

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