P-52

SEISMIC DESIGN OF EARTHWORKS AND FOUNDATIONS FOR THE HAYWARDS INTERCHANGE,

Janet Duxfield, Pathmanathan Brabhaharan, Siva Arumugam Opus International Consultants, Wellington,

The proposed Haywards Interchange is proposed at the intersection of State Highway 58 with State Highway 2, in the Hutt Valley north of Wellington.

The interchange is proposed in an area of high seismicity, and within 100 m of the active Wellington Fault, the characteristic rupture of which can cause a Magnitude 7.5 earthquake with displacements of 4 m horizontal and 1 m vertical. The alluvial fan area is underlain by some liquefiable deposits, some steep slopes and compressible peat deposits. The area was investigated to characterise the ground and groundwater conditions and the Wellington fault location.

The interchange bridges were designed to be resilient to liquefaction, fault rupture and the high seismicity, using reinforced earth walls and reinforced soil approach embankments. A displacement performance based design approach was developed and applied to the design of the interchange. Piled abutments were proposed to ensure reliable performance given the close proximity of the active Wellington Fault and the high seismicity. A combination of anchored soldier piled walls and reinforced soil embankments were designed to support the widening of state highway 58 towards the interchange, with the walls being designed for controlled displacement in earthquakes.

Compressible peat deposits of limited thickness were addressed through ground improvement measures comprising surcharge preloading designed to reduce post-construction settlements.

The Transport Agency decided to procure the construction as a design and construction package and principal's requirements were developed and tenderers guided through the interactive tendering process, to ensure adequate seismic performance of the interchange.