Document library

Geographic area covered: The Kapiti Coast, extending from Pukerua Bay to Otaki, including Paekakariki, Raumati, Paraparaumu, Waikanae, Te Horo, and Otaki Beach. 

This report presents the findings of a liquefaction hazard study conducted for the Kapiti Coast region, prepared by Works Consultancy Services Ltd for the Wellington Regional Council in May 1993. The study is part of a broader regional assessment of seismic risks and focuses on identifying areas susceptible to liquefaction and related ground damage during earthquakes. 
 
The Kapiti Coast study area spans from Pukerua Bay to Otaki and includes several coastal and inland communities. The terrain consists of sand dunes, floodplains, and low-lying plains bordered by the Tasman Sea and the Tararua Range. The underlying geology features Triassic Wellington Greywacke overlain by Holocene and Pleistocene sediments such as alluvium, dune sand, peat, and gravels. 
 
The study involved compiling existing borehole and cone penetration test (CPT) data, conducting two new boreholes and three CPTs, and performing laboratory tests including Atterberg limits and particle size distribution. These investigations helped characterize the subsurface conditions and assess soil susceptibility to liquefaction. 
 
Two earthquake scenarios were considered: Scenario 1, a distant magnitude 7 event (similar to the 1848 Marlborough earthquake) with a return period of 20–80 years; and Scenario 2, a local magnitude 7.5 event on the Wellington Fault with a recurrence interval of approximately 600 years and a 10% chance of occurring in the next 30 years. Ground shaking intensities and peak ground accelerations were estimated for each scenario. 
 
Liquefaction susceptibility was assessed using methods by Ambraseys (1988), Seed & Idriss (1982), and Sugawara (1989), based on standard penetration test (SPT) and CPT data. Soils most at risk include loose sands, silty sands, sandy gravels, and low-plasticity silts. Particle size analyses confirmed that many local soils fall within the “easily liquefiable” range. 
 
Under Scenario 1, most soils were found to be resistant to liquefaction, though historical evidence suggests possible liquefaction in Waikanae and Otaki during past earthquakes. Scenario 2 presents a higher risk, with four of eight key boreholes (PAE, RAU, PA3, WA2) indicating potential for liquefaction. The risk varies locally and is not strictly correlated with geological trends. 
 
Ground damage types include subsidence (estimated up to 150 mm in some areas), slope failures (especially along sand dunes), and lateral spreading (notably within 100 m of rivers and coastal banks). Additionally, very loose dry dune sands may densify during shaking, causing damage similar to liquefaction. 
 
The report includes liquefaction susceptibility maps, potential maps, and ground damage maps, classifying areas by risk level. However, it emphasizes that the assessment is regional and indicative only, not suitable for site-specific decisions. Boundaries and damage estimates are approximate, and further site-specific geotechnical investigations are recommended for development or infrastructure planning. 
 
In conclusion, the Kapiti Coast has a variable but real risk of liquefaction and ground damage during major earthquakes, particularly under a local Wellington Fault event. The study provides valuable regional insights but highlights the need for detailed local assessments.