Congratulations to Chris McGann for recently being awarded the New Zealand Geotechnical Society Geomechanics Award. He led a paper “Development of an empirical correlation for predicting shear wave velocity of Christchurch soils from cone penetration test data,” (published in Soil Dynamics & Earthquake Engineering, 75, 66-75, 2015) . The paper was authored with Brendon A. Bradley (University of Canterbury); Merrick L. Taylor (Arup); Liam M. Wotherspoon (University of Auckland); and Misko Cubrinovski (University of Canterbury).
We decided to dig deeper and while we felt we had experienced a fair amount of soil looseness since 2011, decided to start with a more scientific understanding the importance of ‘soil stiffness’.
Why is understanding ‘soil stiffness’ important when it comes to understanding how the earth shakes and moves.
The magnitude and distribution of stiffness in the soil profile below a site plays a critical role in how earthquake ground motions coming from the underlying bedrock are amplified or de-amplified at the ground surface. In terms of magnitude, softer soils will tend to amplify lower frequency parts of the motion, while stiffer soils will amplify higher frequencies. In terms of distribution, the presence of large abrupt changes in stiffness between layers will also strongly influence the site effects. The shear wave velocity profile of a site provides the information necessary to account for these site specific effects in engineering analysis as shear wave velocity is directly proportional to small strain shear stiffness.
There’s an unprecedented dataset in the Christchurch region and this is a low-cost experimental method – in what way is this important globally? Who would be interested in this, and in what parts of the world?
The correlation between cone penetration test (CPT) data and shear wave velocity developed in this work combined with the unprecedentedly large and spatially dense CPT data set made available through the New Zealand Geotechnical Database project enables an assessment of the spatial variability of shear wave velocity across the region that hasn’t ever been possible before at this scale and resolution anywhere in the world. Establishing a sensible way to account for the inherent variability of soils in engineering analysis is an important topic of research, and researchers all over the world are interested in the insights that the Christchurch dataset can provide.
Can you describe a place/context out in the field in the Christchurch region which demonstrates your work in a practical way?
The two strong motion stations in Lyttelton provide a textbook example of the importance of local site effects. One station is sited on rock, while the other is sited on soft soils. The ground motions from the February 2011 earthquake recorded at these sites are dramatically different, with the softer site showing a large amplification at lower frequencies relative to the rock site. Because the position and distance of these stations relative to the earthquake source are essentially identical, any differences in the recorded ground motions can be attributed to site effects and the dramatic differences observed for the February event highlight the importance of the soil stiffness in the surficial ground motions.