Abstract:
A huge tsunami that followed the East Japan Great Earthquake on March 11, 2011 ran up the area's rivers as far as 49km (24 miles) upstream from their mouths and caused innumerable damage along the way. The Hei River in Iwate Prefecture was one of those that saw the major calamity. In order to lower the risk of future tsunami run-up into the river and also to mitigate future disasters of a similar kind, the prefectural government is building a 164m (540 ft) wide concrete tidal barrage at the mouth of the river. The barrage structure is made of five large concrete columns with four gates between them to stop 10.4m high tsunamis above the datum water level. Their load bearing layer is sound granite, but it is sloped and overlain with dense sand, silt, and stiff clay layers. Since the granite layer is sloped and the structure is required to withstand a strong horizontal loading from future tsunamis, a grouped pipe pile foundation with their pile toes embedded into the granite layer wasdesigned. The contractor proposed an alternate pile arrangement with a smaller number of piles with a larger diameter (1,000mm, i.e., 39.4in) at a distance further apart (2.5D on center), achieving a shorter construction period and major cost saving compared to the original design. To efficiently install piles into the granite, the Gyropress Method was utilized which rotated and simultaneously pressed in pipe piles with a cutting shoe welded to the toe of each pile. The amount of penetration into the granite layer was equal to the pile's diameter (1.0D). The Gyropress machine coupled with Skip Lock attachments was the key to the success of the project. This presentation will compare the original and the final designs and study the advantages of the rotary press-in piling method.